Dear Customer,
I would like to take this moment to congratulate you on your purchase of the “Metal Plating
Bible”. It is without a doubt, the most practical learning guide for the beginner or intermediate
plating enthusiast. There’s a lot of information crammed into the next 120 or so pages. You may
be tempted to skip around, but if you do, you may risk missing vital pieces of information that
will substantially aid you in your metal plating efforts. So do yourself a favor and read this
eBook from front to back.
Also, due to the technical nature of Metal Plating, there will likely be many words that you might
not understand. Because of this inevitable fact, we have included a glossary at the very end of
this digital book. If you ever need to refer to it, just go to the last few pages where you can read
it.
Ok, there’s nothing more to say, go ahead get started. The following 120 pages contain all the
information you’ll ever need to do your own high quality metal plating. Enjoy!
Table of Contents
CHAPTER 1
<<Introduction to Electroplating>>
(page 4)
Chapter 2
<<Electroplating - Process Description and Electroplating of Different Metals>>
(page 10)
Chapter 3
<<Preparation of Electroplating Kits for Different Metals & Solution Formulations>>
(page 20)
Chapter 4
<<Nickel Plating in Detail>>
(page 37)
Chapter 5
<<Painting and Lacquering – Electrocoating – Process Description and Solution Formulation>>
(page 42)
Chapter 6
<<Conventional Painting and Lacquering –
Process Description and Solution Formulation>>
(page 44)
Chapter 7
<<Powder Coating>>
(page 47)
Chapter 8
<<Conversion coating – Chromating,
Phosphating, Anodizing>>
(page 48)
Chapter 9
<<Avoiding Contamination, Corrosion and Surface Preparation>>
(page 72)
Chapter 10
<<Controlling Thickness and Most Used Tools>>
(page 89)
Chapter 11
<<Metal Plating for Polymers>>
(page 95)
Chapter 12
<<Plating on plastics>>
(page 99)
Chapter 13
<<Plating in Semiconductor Industry – Lead Free Plating>>
(page 106)
Chapter 14
<<Safety Hazards>>
(page 109)
Chapter 15
<<Miscellaneous Topics>>
(page 114)
Chapter 16
<<PLATING BATH COMPOSITIONS AND OPERATING CONDITIONS>>
(page 117)
CHAPTER 1
<<Introduction to Electroplating>>
Electroplating is the process of coating a metal object with another metal, using electrical
current passed through a chemical solution. It is the process that produces a thin, metallic
coating on the surface of another metal. The purpose of Electroplating is to improve appearance
of the material, protection against corrosion and in certain special processes like printing.
The process of Electroplating involves placing the metal to be plated, (“Metal A”) in the solution
of the metal (“Metal B”), with which it has to be plated. The metal to be coated (Metal A) is made
the cathode in an electrolytic cell and the anode is made up of another conductor mostly the
“Metal B” (metal with which Metal A will get coated). When electric current is applied, the
electrode reaction occurring on the cathode is the reduction of the metal ions to metal. E.g., gold
ions can be discharged form a gold solution to form a thin gold coating on a less expensive
metal to produce "custom" jewelry.
To further illustrate the Electroplating process, let us assume that an object made of one of the
copper (Metal A) has to be plated with nickel (Metal B).
The Setup:
•
Step 1:
o
•
Attach a wire to the copper object (Metal A) while the other end of the wire
should be attached to the negative pole of a battery (or a power supply). To the
positive pole of the battery (or power supply) we connect another wire with its
one end connected to a rod made of nickel (Metal B).
Step 2:
o Next we fill the electrolytic cell with a solution of the metal salt to be plated. In our
present example the nickel chloride salt dissociates in water into positively
charged nickel cations and negatively charged chloride anions. As the copper
object to be plated is negatively charged it attracts the positively charged nickel
cations, and electrons flow from the copper object to the cations to neutralize
them (to reduce them) to metallic form.
o
Meanwhile the negatively charged chloride anions are attracted to the positively
charged nickel rod (known as the anode of the electrolytic cell). At the anode
electrons are removed from the nickel metal, oxidizing it to the nickel cations.
This illustrates that the nickel dissolves as ions into the solution. That is how
replacement nickel is supplied to the solution for plating and we retain a solution
of nickel chloride in the cell.
Fig. GRAPHICAL REPRESENTATIONS OF THE ELECTROPLATING PROCESS
Nickel chloride is used here to exemplify the process of electroplating as it is simple to
understand. It is not recommended; however, that nickel is used for, say, school science
demonstrations because some individuals are quite allergic to it. It is also not recommended
that chloride salts be used because they are amenable to release chlorine gas. For school or
amateur type demonstration it is recommended to plate copper coins with zinc or nickel coins
with copper.
Another common example is the shining work on cars -- bumpers, door handles and
manufacturer logo. Much of this begins as a piece of zinc, steel or plastic. The manufacturer
uses a copper electroplate, then a nickel electroplate and then chromium depositing over one
another. The result is a surface brighter and more corrosion resistant than bare metal or plastic.
Electroplates are applied by immersing the object to be coated in a tank containing the proper
chemicals dissolved in water. If nickel is being applied, nickel metal is one of the components of
that solution.
Now imagine that the part to be plated is attached to a negative electrical lead (like that on your
car battery). Once it is attached to the negative electrical lead it is called a cathode.
The other electrical lead, the positive (+) is in the solution. When current is turned on, the
negatively charged part to be plated attracts positively charged metal from the solution
(opposites attract). This continues as long as current is on, and the coating or deposit becomes
thicker and thicker. But most electroplates are not very thick. One thousandth of an inch (0.001
inch) is regarded as pretty thick.
Since metal is being taken from the solution, it must be replenished. Often this is done by
hanging pieces of the metal nickel, if nickel is being plated, for example, in the solution. The
chunks of metal are called ANODES, and the positive electrical lead is then attached to them.
They dissolve in the solution as metal is taken away by plating. So at this point we have metal
being removed from the anode and deposited on the cathode, which are the parts to be plated.
Since a car battery is not a good source of power for this application, electroplaters use
electrical current supplied by their power companies. But they must have DIRECT current (DC),
while the power company supplies ALTERNATING current (AC). To convert AC to DC
electroplaters use a RECTIFIER. Its function is to convert AC to DC.
Purpose / Objective of doing Electroplating
1)
Protection from corrosion
Copper, nickel, and chromium on steel and zinc die castings
Zinc or cadmium on steel
2)
Appearance
Copper, nickel, and chromium on steel
Nickel and gold on brass
Silver on brass
3)
Superior hardness and better wear resistance
Chromium on steel
Electroless nickel on steel
(This hardness improvement is gained without sacrifice of ductility; the plating
allows a hard surface while maintaining a softer ductile core.)
4)
Lower contact resistance and increased reliability of Electrical contacts
Gold on Brass or copper
5)
Improved solderability and/or weldability
Tin on Brass
Electroless nickel on steel
6)
Better base for other finishes
Nickel under Gold or chromium
(The nickel inhibits the migration of the gold into the brass basis)
7)
Improved lubricity under pressure
Silver on Bronze
8)
To strengthen the base and render it more temperature resistant
Copper, Nickel and chromium on plastics
9)
To act as a stop-off in Heat treating
Copper on Steel for carburizing
Bronze on steel for nitriding
Chapter 2
<<Electroplating - Process Description and Electroplating of Different Metals>>
Process description
Virtually all metals and some metal alloys can be electroplated to produce a coating on a
substrate. The substrate is usually made from a metal, though selective non-metals may also be
coated. Electroplating is also referred to as electro deposition, and both terms are in common
usage.
In aqueous solution, metallic salts ionize to form positively charged metal ions and negatively
charged acid radical ions. For example, copper sulphate in solution ionizes as follows:
CuSO4→Cu2+ + SO42The ions exist independently of one another in solution but balance out electronically, i.e. the
number of negative and positive charges is equal.
Application of a potential from a direct current source by the immersion of two electrodes into
the solution causes the ions to migrate. Positively charged ions migrate to the cathode whilst
negatively charged ions migrate to the anode. In the example of copper sulphate solution
above, the copper ions migrate to the cathode and accept electrons from the cathode causing
the copper atoms formed to adhere to the cathode.
Cu2+ + 2e- → Cu(metal)
If the anode is copper, the negatively charged sulphate ions give up electrons at the anode to
produce copper sulphate, which then ionizes to restore the equilibrium.
Most metal electro deposition occurs via this route. In theory the solutions are maintained at
their optimum concentrations, though it is necessary to ensure that there is an adequate supply
of the anode metal source.
Some deposition solutions, such as those used for chromium, gold or other precious metals,
use insoluble anodes. Since no metal source is present to maintain the solution in equilibrium,
the solution becomes progressively depleted in metal salt and to maintain optimum solution
concentration frequent additions of the salt must be made.
It is possible to electro deposit metal from a single salt solution, though this is rarely used in
practice. Most solutions consist of several salts, which have different functions. For instance,
chlorides are added to nickel plating solutions to promote anode corrosion and boric acid is
added to act as a buffer to maintain pH equilibrium. Most metal cyanides are insoluble in water
and must be dissolved in sodium or potassium cyanide solution. Other salts are used to
promote conductivity.
In practice, these solutions are referred to as the basic solution. It is possible to deposit metal
from basic solutions but the deposits produced are generally unacceptable to users, as they are
dull, not very adhesive and crystal formation occurs. For example, in the early part of the
century, nickel plated deposits were dull and required mechanical polishing prior to deposition of
chromium to produce a bright reflective finish; silver deposits were similarly treated.
Basic solutions are generally made up by the user, usually from the necessary salts, in crystal
or powder form, which are purchased from the chemical manufacturer/supplier. Some solutions,
particularly those for gold and other precious metals may be purchased pre-made.
Modern electroplating solutions contain many complex organic or metallic organic chemicals,
referred to as brighteners or addition agents. The purposes of these agents are numerous and
include faster plating speeds, higher tolerance of contaminants in plating solutions, production
of mirror-bright deposits, increase/decrease of hardness, changes to crystal structure of the
deposit, and decrease of internal stress. These addition agents are the results of the research
and development efforts of supply houses. Therefore the chemical composition of the agents is
usually confidential.
These addition agents are co-deposited with the metal. The optimum concentration is
maintained in the solution by frequent additions, often by dosing meters, based on an
ampere/hour basis. Quite low concentrations may be present in the solution but fairly high
maintenance additions are made frequently during production. For example the concentration in
basic solution may be 3 ml/l but the rate used may be 200 ml/l per 1000-ampere hour. The
solutions supplied generally contain between 3-25% of the chemical. Therefore in this example,
the concentration in the actual working solution may be between 0.009-0.075 percent.
Plant installations for electroplating consist of several tanks assembled together in sequence
and the articles for processing are transferred from tank to tank on racks or in barrels, by
manual or mechanical means, and in the case of the latter, often by computer control. Some
plants are of the return automatic type where the tanks are arranged in a double row and are
typically to be found in a manufacturing organization where high volume output of similar type
articles is required.
What to Avoid during the Electroplating Process
1) Sharp edges and right angles should be avoided. Every sharp, protecting edge will
draw extra current and build up with extra plate. Conversely the part will receive very
little plate in the acute angle. All sharp edges and right angles should be rounded to
the greatest degree design allows.
2) Holes should be either counter sunk or counterbored, because build up on the sharp
edges may exceed tolerance allowance.
3) Deep recesses should be avoided. The recess will receive the lesser thickness of
plate then the adjoining area, and either require heavier average thickness on the
overall part to meet the minimum specification or receive too little plate if the average
area is used to compute thickness.
4) If the article is a threaded fastener or threaded screw machine part, special care
should be taken to build up of at least four times the plating thickness on the pitch
diameter.
5) Formed tubular articles will often trap and carry over solution if drainage holes are
not provided in the design.
6) Larger parts to be rack plated must be provided with some way to rack the part in
hole, lug or rim. Since the contact part will be poorly plated, the electrical contact
should be in a non-significant area.
7) Blind holes, rolled edges, seams and other crevices will trap solution unless special
plating techniques are used.
8) Bolted assemblies should be avoided because of possible unplated areas subject to
subsequent corrosion.
9) Dissimilar metals are difficult to plate because cleaning methods vary for different
basis materials.
Good Habits for Electroplating Process
In the design to be plated, it is well to utilize all the advantages that may be incorporated
to permit as uniform a distribution of plating thickness as possible and still retain the
basic design desired. Each sharp corner (recessed or protruding) should be provided
with as large a radius as possible. Figure 1 illustrates the distribution of nickel-plating
thickness on a formed part. Note the lack of adequate radii at the ends of the central
recessed section. The ratio of 9.0 illustrates the effect of these sharp corners.
If reasonably good distribution of plating thickness is desired, every effort should be
made to avoid recesses, to fillet all sharp corners, and to use convex instead of concave
surfaces wherever possible. This will improve distribution of plating thickness and, by so
doing; will provide a finish having better corrosion resistance at a reduced cost of plating.
<Electroplating of Different Metals>
Platinum Electroplating
Platinum is rare, scarce, and very costly and is considered one of the most precious metals.
Platinum electroplating is used to coat electrodes that are used in the refining of oil, and in the
manufacturing of fertilizers, acids, and explosives. The automotive industry uses platinum
plated catalytic converters to treat automobile exhaust emission. In the medical industry,
platinum plate is used on instruments such as catheters and connectors for surgical
equipment. The electrical and electronics industries use platinum plating for low voltage and
low energy contacts. In electroplating, platinum is often used to coat titanium, niobium, or
stainless steel anodes. It is also used in the jewelry industry.
Platinum is considered a premium protective finish over sterling silver and nickel base metal.
Platinum’s luster is much purer than silver or gold, enhancing the brilliance of gemstones and
diamonds. Platinum electroplate coatings typically range from 0.5 to 5 microns depending on
the application. It is applied utilizing a rack fixture that is submerged in a chloroplatinic acid or
a sulfate based platinum solution. The finished product can range in color from tin white to a
matte gray finish depending on the base metal finish, activation process, and the thickness of
the platinum coating.
Platinum electroplating is accomplished by placing the electrode tips into a solution of
platinum chloride and applying a small current such that the platinum in solution is reduced,
causing platinum deposition at the tip of the metal electrode. We can plate the electrodes
using a solution of hydrogen hexa-chloro-platinate (8% PtCl4 by weight) with a multi-channel,
constant-current plating device.
Rhodium Plating
Rhodium is white in color and a precious metal, of the platinum group. Rhodium is the hardest
of all of the precious metals. It provides the most wear resistant finish possible for the most
demanding environments. It is one of the most suited metals for plating of parts such as sliding
electrical contacts that require protection from corrosion or galling. Rhodium provides a bright,
attractive finish that is non-tarnishing. Under-plating of nickel should be used when parts are of
corrosion or heat resistant steels. When under-plated with nickel it provides a mirror surface that
is highly reflective.
Surfaces other than nickel, silver, gold, or platinum should be either nickel-plated or nickel over
copper plated.
Rhodium plating is widely used on high voltage switch gear, silverware, silver models, medals,
white gold jewelry and top end furniture fittings to prevent tarnishing / corrosion as well as due
to its hardness it makes the surface scratch resistant.
As rhodium is a relatively inert metal, it cannot be stripped from the more active base material
without damaging the less active substrate in the case, if an item is damaged and require
repairs. Also, rhodium is plated from an acid solution which has poor throwing power. It cannot
generally be used on items with deep cavities without some consideration to masking, jigging,
pumping and shielding. As it is an expensive process, the areas that do not require coating can
be masked with special masking tapes and paints.
Of the platinum group metals, rhodium has found wide acceptance in decorative precious
metals applications. Rhodium has several desirable properties – it has a brilliant white color,
high reflectivity, and hardness, which makes it very popular with the jewelry and faux jewelry
industries.
Rhodium can provide excellent tarnish protection for sterling silver and silver plated flatware and
hollowware from quite thin deposits. Typically, rhodium electroplate is deposited on precious
and faux jewelry, sterling and silver plate to a thickness of 0.05 to 0.125 microns (2 to 5 micro
inches). This thickness of rhodium is produced in about 20 to 60 seconds from phosphate,
sulfate or phosphate-sulfate baths.
Rhodium on Steel
For rhodium to get plated on steel, it is needed to activate in order for the rhodium to adhere to
the surface. Rhodium plating on the steel does not conceal surface flaws or blemishes. Many
electroplaters typically use either a woods nickel strike or an acid gold strike to cover the steel
with a thin layer of metal to achieve adhesion. For best results it is advisable to choose a good
pre-plate of bright nickel, 5-10 microns in thickness. This will help level out any minor surface
waves that may exist in the material, and should not interfere with any build tolerances on the
surface. This is followed by a final layer of rhodium and at least .50-1.0 microns of rhodium are
recommended.
Silver Plating
Silver plating offers the highest electrical conductivity of all metals. It is a semi-precious metal
that gets oxidize rapidly. Silver plating is best suited for engineering purposes, as for soldering,
electrical contact characteristics, high electrical and thermal conductivity, thermo-compression
bonding, provides wear resistance to load-bearing surfaces, and spectral reflectivity, good
corrosion resistance, and other electrical applications.
Silver Plating - Grades
A. With supplementary tarnish-resistant treatment.
B. Without supplementary tarnish-resistant treatment
Silver Plating – a Useful tool for Corrosion Protection
For applications where corrosion protection is important, the use of silver plating with an
electrodeposited nickel undercoat is advantageous.
Silver Plating – Under-plate Recommendations
Silver plating on steel, zinc and zinc-based alloys should have an undercoat of nickel over
copper.
Silver plating on copper and copper alloys should have a nickel undercoat. Copper and copper
alloy material on which a nickel undercoat is not used, and other base metals where a copper
undercoat is employed, should not be used for continuous service at a temperature in excess of
300 degrees F (149 degrees C). Adhesion of the silver plating is adversely affected because of
the formation of a weak silver and copper inter-metallic layer.
Silver Deposit and Tarnish
Tarnishing is a natural process that occurs on the surface of silver jewelry. Tarnish starts as a
light yellow discoloration of silver; it then starts to change to darker shades of brown as the
tarnish gets to be more severe. In extreme cases the tarnishing of silver could look very dark
and almost black.
Tarnishing occurs due to certain climatic conditions and also due to certain ingredients that are
present in some materials. One such chemical that causes silver to tarnish is hydrogen sulfide
(H2S) and things that contain this chemical will cause silver to tarnish quickly. Materials that
stimulate silver tarnish are wood, felt, rubber bands, food items like eggs, onions etc.
High humidity in the climate also hastens the silver tarnish process.
The factors that can cause silver tarnish are wide and varied, and is evident that the process of
silver tarnishing can hardly be avoided because silver tarnish is a natural process and occurs
with silver of all purities.
Silver jewelry is generally made from silver that is around 92.50% pure and this is done to
increase the hardness of silver. Sterling silver as it is normally referred to (925=92.50% purity),
is an alloy of silver and other metals.
The tarnishing of sterling silver has nothing to do with the percentage of silver in the alloy. It
would be safe to assume that all silver will tarnish. It is possible to clean tarnished silver jewelry
and the procedure followed will depend on the degree of tarnish that is present on the silver
jewelry.
Brass Plating
Brass plating is primarily used as a decorative finish. However, the process is also used for
some engineering applications, such as brass-plated steel wire promotes adhesion to rubber
in steel-belted tires and as an anti-galling coating. Brass is also plated on the surfaces of
bearing materials.
For bright decorative brass finishing, material is first plated with bright nickel, followed by a
brass flash plate for 35-90 sec. Such finishes are used in wire goods, decorative lamps,
furniture hardware and builder’s hardware.
Heavy brass deposits (0.0003-0.0006 inch) are used for finishes that will be buffed, burnished,
antiqued and/or oxidized. Some of the brass plating is removed with antiquing and oxidizing
processes and, therefore, the minimum thickness for such processes is 0.0003 inch.
Heavy brass deposits are not as bright as brass plated over bright nickel. To obtain bright
finishes with heavy brass deposits, they must be buffed or burnished. Addition agents can
refine the grain of the brass so that the amount of burnishing or buffing is greatly reduced.
Gold Plating
Gold is unique for its yellow color. It is a precious metal and does not oxidize in air, so its
electrical conductivity stays uniform over long periods of time. It is ideally suited for
electroplating applications. Gold plating offers good corrosion resistance, good solderability, and
when alloyed with cobalt, it has very good wear resistance. Gold is commonly used in electrical
switch contacts, connector pins and barrels, and other applications where intermittent electrical
contact occurs.
Gold Plating - gold electroplating specification
Specification: Gold Plating, Electro-deposition
•
Type I 99.7 % gold minimum; hardness grades A, B, or C. Gold plating used for generalpurpose, high-reliability electrical contacts, solderability, and wire wrap connections.
•
Type II 99.0 % gold minimum; hardness grade B, C, or D. A general-purpose, wearresistant gold. It will not withstand high-temperature applications because the hardening
agents in the gold plating will oxidize.
•
Type III 99.9 % gold minimum; hardness grade “A” only. Gold plating for semiconductor
components, nuclear engineering, thermo-compression bonding, and high-temperature
application.
Gold Plating - purity and coating thickness
Co-deposited impurities can make soldering more difficult, and for this reason gold plating with
high purity is preferred. Soldering requirements are best achieved when gold coatings range
between
0.00005
and
0.0001
inch
(50
and
100
micro
inches)
of
thickness.
Gold Plating - hardness grades
1. 90 knoop, maximum
2. 91-129 knoop, inclusive
3. 130-200 knoop, inclusive
4. 201 knoop, minimum
Gold over silver is not recommended for electronics hardware.
Gold Plating - underplate recommendations
When gold is applied to a copper rich surface such as brass, bronze, or beryllium, copper metal
ions from these base metals will diffuse into the gold layer and degrade its hardness and nonoxidizing properties. An anti-diffusion under-plate such as nickel (electroless or sulfamate)
should be applied to prevent this. We recommend electroless nickel under gold where part
flexure of deformation is not expected and a bright finish is desirable. Where part flexure or
deformation is expected, we recommend sulfamate nickel as the under-plate because of its
higher ductility.
Palladium Plating
Palladium is white in color, harder than cobalt gold, and is precious, it also retains the nonoxidizing property so is used in electrical connector applications.
Palladium electrodeposits have better ductility, which provides superior contact bending
tolerance, lower porosity, and superior resistance to corrosion than hard gold. This makes
palladium an excellent candidate for applications such as reed switches or relay contacts.
However, palladium has a generally lower wear resistance in sliding contact, such as pin/socket
interfaces, than gold. Palladium mated against pure Palladium has less wear resistance than
palladium mated against gold or a palladium surface with a thin overlay of gold.
When under-plated with a flash of soft gold, palladium also demonstrates excellent solderability.
Electroless Nickel Plating
Nickel is a silver white, hard metal with satin to bright luster. It can be plated uniformly in
recesses, blind holes and cavities, does not build up on edges, and has very high wear
endurance. Higher phosphorus variations provide superior corrosion resistance. Nickel is often
applied as a base layer for its leveling, smoothing and barrier characteristics which provide
resistance to attack of some base metals by electrolytic metals such as, cyanide copper or
silver. Nickel is a hard metal with generally poor ductility that is not recommended for
applications where a part of flexure is required.
Electroless Nickel plating - corrosion inhibitor
As a corrosion inhibitor, nickel is used to protect iron, copper, or zinc alloys against corrosive
attack in rural, industrial or marine atmospheres depending upon the thickness of the nickel
deposit.
Nickel, with its leveling and pore-filling characteristic, is also an excellent undercoat for the
precious metals by reducing the total amount of the precious metal required to achieve
performance specifications.
Electroless Nickel plating - engineering purposes
Electroless nickel plating intended for engineering purposes is used for wear resistance,
abrasion resistance and such corrosion protection of parts as the specified thickness of the
nickel plating provides. Heavy deposits of the electroless nickel plating may be used for build up
of worn or undersized parts, or for salvage purposes, and to provide protection against corrosive
chemical environments.
Watts Nickel per QQ-N-290A - Nickel plating
Watts nickel is an electrolytic system that provides very bright, decorative finishes. It also
provides corrosion resistance according to thickness, good abrasion resistance, and a low
coefficient of thermal expansion. It has a relatively low tensile strength and hardness and
relatively high internal stress and therefore is not recommended in engineering applications
where part deformation or flexure may occur.
Teflon Electroless Nickel Plating
The co-deposit of electroless nickel & Teflon contains15% Teflon particles dispersed in an
electroless nickel matrix. It is a hard, ductile finish, brown in color, which has superior antifriction characteristics. This is ideally suited to high cycle mechanical sliding applications. In
addition this is a good electrical conductor and provides corrosion resistance.
Sulfamate Nickel Plating
Sulfamate nickel provides the lowest hardness, lowest internal stress and highest ductility of all
the nickel plating systems. The finish is dull; and is used as an engineering finish and not a
decorative finish. Sulfamate nickel has excellent solderability good corrosion resistance. The
high ductility of sulfamate nickel makes this product an excellent candidate for applications
where part flexure or deformation, such as crimping, will occur.
Note: As nickel finishes become brighter, they become harder and less ductile. Bright nickel
finishes are not recommended if parts are intended for flexure applications or will be bent or
crimped in manufacturing operations subsequent to electroless nickel plating.
Tin Plating & Tin Alloys
Tin is a silver-colored, ductile metal whose major application is to impart solderability to
otherwise unsolderable base metals. Tin has generally good covering characteristics over a
wide range of shapes. It is an electrolytic process.
Tin and its salts are reported to be non-toxic and non-carcinogenic and are approved for food
container and food contact applications.
Tin plating - soft, ductile finish
Tin does not tarnish easily and can serve as a low cost decorative finish, although care must be
exercised in subsequent part handling as tin is a soft, ductile finish that can scratch or mar
easily.
Alloying tin with lead to reduce it’s melting point for soldering and to prevent "whiskering". See
specific descriptions below for details on each of the various tin systems.
Tin plating - corrosions protection & conductivity
Tin is a good electrical conductor and has historically been utilized for its combined corrosion
protection and conductivity in aerospace avionics radio frequency applications.
•
Primarily used to facilitate solderability to base metals that have poor solderability.
•
A ductile, bright finish. Can serve as a low cost decorative finish.
•
Bright acid tin’s ductility will help prevent galling of base metals in friction contact
applications.
•
It applies well to most base metals; will act as a stop-off barrier in nitriding high strength
steels; has a bright appearance; and provides some corrosion resistance.
Tin plating - corrosion properties
For indoor environments, tin provides anti-corrosion properties to copper and copper alloys, and
ferrous metals. Note that tin is not an optimal choice for corrosion protection where outdoor
environments are expected.
Tin should be not less than 99.5% pure except where alloyed for special purposes.
Tin plating –Alkaline brightening system
Alkaline brightening system provides excellent solderability, corrosion resistance with a .02%
bismuth content to stabilize the structure of the metal deposit and stop "whiskering" in extreme
temperatures. Finish is dull in appearance, very soft and is easily marrable.
60/40 & 90/10 Tin Lead
The co-deposition of lead with the tin reduces the risk of whisker growth. Metal filaments, or
whiskers, sometimes grow spontaneously from the surface of electrodeposited metals such as
tin, cadmium, and zinc within a period that may vary from weeks or months to years. These
whiskers are about 0.0001 inch (2.5 µm) in diameter, can grow up to 3/8 inch (10 mm) long and
can have a current carrying capacity of as much as 10 mA.
Tin plating - melting points
Tin is also alloyed with lead to reduce the melting point of tin. This provides flexibility in selecting
soldering temperatures that will not impart too much thermal energy to delicate assemblies.
60/40 melting point range is 361°- 374°F
90/10 melting point range is 450°- 464°F
Copper Plating
Copper is the second most common metal plated, behind nickel. It provides a soft, red, ductile,
solderable surface. Copper is an excellent electrical conductor. However, it is not often used as
a final plate, because it tarnishes easily.
As copper has excellent leveling properties and very high throwing efficiency, it makes an
excellent undercoat for most other metals. In addition, because copper is ductile, it polishes
easily to a high shine so that it supports a bright, shiny finishing metal above it.
Copper is able to fill sharp corners and surface imperfections, allowing smooth and uniform
coverage of the base metal. The throwing and leveling properties of copper insure that pinholes
and subsequent blistering of finish metals will be avoided.
Copper makes an excellent undercoat on aluminum, which is a base metal that most other
electrodeposited metals will not attach to.
Copper is the only metal that can be electroplated onto zinc die casts.
Generally copper is applied as an under-plate in thickness between 100 and 200 micro inches.
Chapter 3
<<Preparation of Electroplating Kits for Different Metals & Solution Formulations>>
Parameters for Plating of different Metals
Nickel Plating
From a rusted component to a Nickel plated lasting showpiece in three hours.
For decorative work, the time required for Nickel plating will range from 1/2 hour up to 3
hours according to the thickness of the Nickel plate required. The plating solution is
reusable. 1kg of the Nickel salts (as supplied in the workshop kit) will last for over 13,000
sq cm of .001 (25 micron) Nickel plating without the need to replenish or replace. The salts
will store for many years in liquid or solid form. Nickel plating can be carried out at any
temperature from 15 degree centigrade up to 35 degree centigrade, although slightly better
results are obtained at 24 degree centigrade or higher.
The comprehensive instructions supplied with each Nickel plating kit will guide you through
all the stages of the plating process from the initial preparation of the parts to be Nickel
plated through to the finished product with hints and tips to get the very best from your kit.
The Nickel plating process can be used as a substitute for Zinc or Cadmium plating. The
finish is dictated by the preparation. For a matt finish wire brushings is sufficient. If the
surfaces are polished prior to being electroplated they can be buffed up to achieve the
beautiful sheen of the original.
Kit Contents
1kg of Nickel plating salts
2 Nickel plating anodes
150g of degreasing salts
150g fine pumice powder
30m of copper wire
1 set of comparator papers
1 set of instructions
5 pairs of polythene gloves
Makes approx 6 liters
gives 1,000 sq inches of plating
makes 5 liters of degreaser
to scour the parts before plating
to suspend the objects
to check the condition of the solution
comprehensive instructions and tips
to avoid contaminating the degreased
parts
Extra items you will need
1. A plastic bowl or bucket or small fish tank
2. 12v battery or a 12vdc battery charger
3. 1 or more standard automotive type 12v bulbs
Gold Plating
1. For articles made of karat gold, gold-filled, rolled-gold plate, nickel, copper, and brass.
•
•
Preparation
o
Buff and polish item to be plated.
o
Steam clean and boil out.
o
Rinse.
o
Electroclean.
o
Rinse again.
Plate
o
•
Gold plate.
Completion
o
Rinse.
o
Soda rub.
o
Rinse again.
o
Alcohol dip.
2. For articles made of white metals or contaminated with soft solder - including most costume
jewelry.
•
•
•
Preparation
o
Buff and polish item to be plated
o
Steam clean and boil out.
o
Rinse
o
Electroclean
o
Rinse again
Plate
o
Copper plate (an under plate that prepares the item for receiving gold plate)
o
Rinse
o
Gold plate
Completion
o
Rinse
o
Soda rub
o
Rinse again
o
Alcohol dip
Rhodium Plating
1. On articles made of karat gold and platinum
•
•
Preparation
o
Buff and polish item to be plated.
o
Steam clean and boil out.
o
Rinse.
o
Electroclean In rhodium plating use 14K or platinum holding wire instead of
copper)
o
Rinse again.
Plate
o
Rhodium plate.
•
Completion
o
Rinse.
o
Alcohol Dip.
2. On articles of silver, palladium, gold-filled, rolled-gold, white metal, or contaminated with soft
solder - including most costume jewelry.
•
Preparation- as above.
•
Plate
•
o
Gold plate (flash under plate) 10-15 seconds to prepare item for receiving
rhodium plate.
o
Rinse.
o
Rhodium plate.
Completion - as above.
Silver Plating
Bowls and hollowware can be plated on the inside by filling bowls with solution and suspending
silver anode inside. Negative plating lead wire is attached to item being plated. Positive lead
wire is attached to anode.
Procedure
•
Plating solution at room temperature.
•
Immerse silver anode connected to positive lead wire.
•
Attach negative lead wire alligator clamp to handling wire on item to be plated.
•
Dial 2V and turn switch to "On Position".
•
Immerse article 30 seconds or until completely covered with silver. Article will usually
emerge with a dull milky color.
•
Brighten color by buffing with a bristle brush.
It is important to realize what type of silver plating finish is required on the part to be plated.
If a highly polished surface is required, the part must be buffed to a high shine before
plating. If a matt finish is required, then the part should be blasted or buffed with something
like a Britex wheel.
The steps to achieve a brilliant silver finish are:
1. Buff & Polish
2. Degrease & Rinse
3. Acid Activate (with battery acid, not supplied)
4. Plate with Flash Copper (Steel, Pot Metal & Pewter only)
5. Plate with Silver
6. Acid Activate
7. Treat with Silver Conditioner (prevents tarnishing)
Potassium Hydroxide is used to raise the PH of the solution when required. The system
must be maintained at a PH of 8.8-9.5.
Figure Representing Silver Electroplating
Cadmium Plating:
Cadmium plating generally is performed in alkaline cyanide baths that are prepared by
dissolving cadmium oxide in a sodium cyanide solution. However, because of the hazards
associated with cyanide, noncyanide cadmium plating solutions are being used more widely.
The primary noncyanide plating solutions are neutral sulfate, acid fluoborate, and acid sulfate.
The cadmium 7/96 Metallurgical Industry 12.20-7 concentration in plating baths ranges from 3.7
to 94 g/L (0.5 to 12.6 oz/gal) depending on the type of solution. Current densities range from 22
to 970 A/m2 (2 to 90 A/ft2).
Copper Plating
Copper cyanide plating is widely used in many plating operations as a strike. However, its use
for thick deposits is decreasing. For copper cyanide plating, cuprous cyanide must be mixed
with either potassium or sodium to form soluble copper compounds in aqueous solutions.
Copper cyanide plating baths typically contain 30 g/L (4.0 oz/gal) of copper cyanide and either
59 g/L (7.8 oz/gal) of potassium cyanide or 48 g/L (6.4 oz/gal) of sodium cyanide.
Current densities range from 54 to 430 A/m2 (5 to 40 A/ft2). Cathode efficiencies range from 30
to 60 percent. Other types of baths used in copper plating include copper pyrophosphate and
copper sulfate baths. Copper pyrophosphate plating, which is used for plating on plastics and
printed circuits, requires more control and maintenance of the plating baths than copper cyanide
plating does.
However, copper pyrophosphate solutions are relatively nontoxic. Copper pyrophosphate
plating baths typically contain 53 to 84 g/L (7.0 to 11.2 oz/gal) of copper pyrophosphate and 200
to 350 g/L (27 to 47 oz/gal) of potassium pyrophosphate. Current densities range from 110 to
860 A/m2 (10 to 80 A/ft2).
Copper sulfate baths, which are more economical to prepare and operate than copper pyrophosphate baths, are used for plating printed circuits, electronics, rotogravure, and plastics, and
for electroforming and decorative uses. In this type of bath copper and sulfate and sulfuric acid
form the ionized species in solution. Copper sulphate plating baths typically contain 195 to 248
g/L (26 to 33 oz/gal) of copper sulphate and 11 to 75 g/L (1.5 to 10 oz/gal) of sulfuric acid.
Current densities range from 215 to 1,080 A/m2 (20 to 100 A/ft2).
Zinc Plating
The value of zinc as a rust-proof finish for iron and steel has long been appreciated. Zinc
plating is being used on an increasing scale, particularly for components which would formerly
have been cadmium plated.
ZINC FAST
Zinc Fast is a complete system that has made it possible for the small operators to achieve
fully professional results without any previous experience in electro-plating. "Zinc Fast XL"
gives excellent value for money and, most important, allows the user full Control of the
operation.
How fast is "Zincfast XL"?
A 10 MicrQn (UN) Average coating is achieved in less than 20 minutes. Passivation takes
appx.30 seconds.
The "WORKSHOP XL" Kit contains all the necessary chemicals for 12 liters. of Plating
Solution and 12 liters of Passivating Solution. The Degreasant, the Copper Wire for
suspending the parts, the pH papers and high Purity Anodes are all included in the Kit.
To electro-plate you will need a l2 Volt battery, Plastic containers for the Plating and
Passivating tanks 'and l2 Volt light bulbs for regulating the Amperage as used with the Nickel
Plating System.
Kit Contents
•
1 x litre Zinc Fast concentrate
•
1 x2.4kg 625CDP
•
1 x2.4kg 625CDP
•
1 x300g 3AB
•
1 x500cc IB brightener
•
1 x250cc MB brightener
•
1 x250cc wetter
•
1 x250cc 3CRP passivation
•
1 x250cc nitric acid
•
1 x250cc hydrochloric acid
•
1 x150g activax degreasant
•
1 x200g sodium hydroxide
•
1 xcomparator papers
•
1 xcomparator papers (set2)
•
5 pairs of gloves
•
3 xpure zinc anodes
•
1 reel of copper wire
•
1 instruction booklet
You will also need plastic containers
• 1 metal container
• 12vdc car battery
• 12vdc light bulbs
<Solution Formulations>
The formulations of the more common electroplating solutions used for industrial purposes are
given in Tables 2.1 to 2.14. The operating temperature, where above ambient, and the pH are
also indicated.
For reasons of commercial confidentiality, the brighteners/addition agents cannot be named and
are indicated by their generic group only. They are only a small percentage of the materials,
which are added to the bath in aqueous solutions. The brightener solutions typically contain
between 3 to 25% of the brightener compound.
Table 2.1
Copper electroplating solution formulation
Acid bath
Copper sulphate
170-200 g/l
Sulphuric acid
45-50 g/l
Temperature
20-40°C
Addition agents (sulphur containing compounds) Low concentration
and wetting agents. For example benzotriazole
and thiourea.
Cyanide bath
Copper cyanide
15-75 g/l
Potassium cyanide
25-125 g/l
Potassium hydroxide
0-30 g/l
Temperature
50-75 °C
Brighteners/addition agents are not usually required, but traces of cobalt or nickel may be employed
Rochelle bath
Copper cyanide (71% copper)
20-30 g/l
Potassium cyanide
25-50 g/l
Potassium carbonate
15-25 g/l
Potassium hydrogen tartrate
30-40 g/l
Temperature
50-60°C
Pyrophosphate bath
Copper pyrophosphate (42.3 % copper)
50-85 g/l
Potassium pyrophosphate
200-300 g/l
Ammonium hydroxide (sg 0.880)
3-10 g/l
PH
8.6-9.2
Temperature
50-55°C
Sulphur containing compounds may be used as addition agents. For example benzotriazole and
thiourea.
Table 2.2
Cadmium electroplating solution formulations
Cyanide Bath
Cadmium oxide
15-30 g/l
Sodium cyanide
40-90 g/l
Sodium hydroxide
5-15 g/l
Addition agents are available but usually 1 g/l
prohibited
Fluoroborate bath
Cadmium fluoroborate
200-240 g/l
Ammonium fluoroborate
50-60 g/l
Boric acid
15-25 g/l
Liquorice (addition agent)
1 g/l
Primary brighteners, have a sulphonic acid (=CO-SO2) active group in the molecule. Below
table gives some typical examples of primary brighteners. Usually the alkali salt, in particular
sodium, of the acid, is used as a water soluble salt. Typical concentrations in the nickel solution
vary between 0.5-4 g/l and are dependent upon the type used.
Secondary brighteners have various active groups in the molecule; below table gives some
typical examples. The concentrations used in the formulations can vary.
CHAPTER 4
<<NICKEL PLATING IN DETAIL>>
Types of Nickel Plating Solutions
Sulfate Solutions. The most common nickel plating bath is the sulfate bath known as the Watts
bath. Typical composition and operating conditions are shown in Table I. The large amount of
nickel sulfate provides the necessary concentration of nickel ions. Nickel chloride improves
anode corrosion and increases conductivity. Boric acid is used as a weak buffer to maintain pH.
The Watts bath has four major advantages: 1) Simple and easy to use; 2) Easily available in
high purity grades and relatively inexpensive; 3) Less aggressive to plant equipment than nickel
chloride solutions; and 4) Deposits plated from these solutions are less brittle and show lower
internal stress than those plated from nickel chloride electrolytes.
High Chloride Solutions - Chloride baths have an advantage over sulfate baths in deposition
speed; not necessarily in current density, but in improved current distribution.
All-Chloride Solutions - The advantages of all-chloride nickel plating solutions include the
following: 1) Low voltage; 2) Good polishing characteristics; 3) Heavy coatings can be
deposited; 4) Low pitting; 5) Improved cathode efficiency; and 6) No need to cool the plating
solution.
See Table I for composition and operating parameters.
However, there are disadvantages to this bath as well: 1) Highly corrosive; 2) Nickel chloride is
sometimes less pure than nickel sulfate (particularly important in bright nickel plating); 3)
Mechanical properties of the deposit are not as good as those from the Watts bath.
Fluoborate Solutions - In nickel fluorborate baths, the electrolyte is maintained at a pH of 2.0-3.5
using fluoroboric acid. Metal content is maintained at up to 120 g/liter of nickel, which is much
higher than in a Watt's bath. Because of this, higher current densities are necessary.
Nickel coatings deposited from this type of bath have properties similar to those deposited from
Watt's baths; however, these coatings are usually specified for heavy nickel applications and
electroforming.
Anode dissolution in a nickel fluoborate bath not containing chloride is better than in a nickel
sulfate solution with nickel chloride.
Disadvantages of fluoborate baths include the following:
1) High cost of chemicals;
2) Throwing power less than that of sulfate solutions.
Sulfamate Solutions - This bath is based on the nickel salt of sulfamic acid, and the pH is
adjusted using sulfamic acid, nickel oxide or carbonate. When intensive agitation is used in
solutions with a high nickel concentration, current densities up to 500 asf can be achieved.
Nickel coatings from this type of bath usually have very low stress values and high elongations.
Another advantage is that it is possible to operate the sulfamate bath without difficulties related
to anode dissolution at low chloride levels or even without chloride. The principle advantage of
this bath is that it can be operated at nickel concentrations of 180-200 g/liter. This allows for the
use of high current densities without losing the properties of the coating.
Current Fluctuations while plating with a nickel sulfamate solution
This bath is based on the nickel salt of sulfamic acid, and the pH is adjusted using sulfamic
acid, nickel oxide or carbonate. When intensive agitation is used in solutions with a high
nickel concentration, current densities up to 500 asf can be achieved.
Nickel coatings from this type of bath usually have very low stress values and high
elongations. Another advantage is that it is possible to operate the sulfamate bath without
difficulties related to anode dissolution at low chloride levels or even without chloride. The
principle advantage of this bath is that it can be operated at nickel concentrations of 180-200
g/liter. This allows for the use of high current densities without losing the properties of the
coating.
There can be a lot of reasons, stopped barrel, bad power supply, current to the barrel, dirty
barrels that do not allow solution transfer and barrel danglers that are not riding with the load. All
these can occur intermittently. Also, check to see if the parts are not over etched before the
going into the nickel bath. We doubt that there is really an effective practical way to reduce the
throwing power significantly, but the theoretical factors are:
•
Low solution concentrations increase throwing power (by starving the HCD areas), so
high concentration should reduce it.
•
Low temperatures increase throwing power the same way, so high temperature should
reduce it.
•
Good agitation reduces throwing power.
Types of Nickel Plating
Bright Nickel - Bright nickel plating baths are used in the automotive, electrical, appliance,
hardware and other industries. It’s most important function is as an undercoating for chromium
plating, helping finishers achieve a smooth bright finish as well as a significant amount of
corrosion protection.
Bright nickel plating baths use combinations of organic agents to achieve bright nickel deposits.
There are two classes of these organic additives. The first class is the aromatic sulfonic acids,
sulfonamides and sulfonamides that contain the functional group =C-SO2. Saccharin is a widely
used example of this type of brightener. Nickel deposits plated using these additives are mirror
bright initially; however as the nickel builds, brightness diminishes. This first class of brighteners
incorporates sulfur into the bright nickel, reducing corrosion resistance.
Brighteners in the second class, also called levelers, have inorganic metal ions and organic
compounds. These may include butynediol, coumarin, ethylene cyanohydrin and formaldehyde.
These are used as leveling agents because they increase surface smoothness, as the nickel
deposit thickness increases. See More on Brighteners and Levelers in Chapter 11.
Semi-Bright Nickel - At first, coumarin was used to obtain a high-leveling, ductile, semi-bright
and sulfur-free nickel deposit from a Watts nickel bath. However, coumarin-free solutions are
now available. A semi-bright nickel finish is semi-lustrous, as the name implies. However, it was
specifically developed for its ease of polishing and buffing. Or, if subsequently bright nickel
plated, buffing can be eliminated. Brightness and smoothness are dependent on operating
conditions.
The reason semi-bright nickel finishes are so easily buffed and/or polished is that the structure
of the deposit is columnar, whereas the structure of a bright nickel finish is plate-like (lamellar).
However, the structure can be changed with additives, a change in pH, current density or even
an increase in solution agitation. This is not a problem unless it affects properties such as
internal stress.
Internal stress can be compressive or tensile. Compressive stress is where the deposit expands
to relieve the stress. Tensile stress is where the deposit contracts. Highly compressed deposits
can result in blisters, warping or cause the deposit to separate from the substrate. Deposits with
high tensile stress can also cause warping in addition to cracking and reduction in fatigue
strength.
Watt baths and high-chloride type baths can produce high tensile stress. During bright-nickel
plating, stress-reducing additives are used, but these co-deposit sulfur materials that affect the
physical and/or engineering properties of the deposit. Saccharin is often used as a stress
reducing agent. Nickel sulfamate baths can deposit pure low-stressed finishes without using
additives.
Other Types of Nickel - To obtain other types of finishes such as satin nickel, organic additives
are used and deposition conditions are altered. Deposits from a Watts bath are usually 7-10 mm
thick, with the appearance dependent on the temperature and/or pH. At higher temperatures
and a pH of 4.5-5.0, nickel deposits are matte. At 122F and a pH of 2.5-3.5, deposits are bright.
Black nickel plating is lustrous and has a black or dark gray color. Plating is done with little or no
agitation. Occasionally it is necessary to remove hydrogen gas (bubbles) from the part's surface
using wetting agents. The pH of the bath ranges from 5-6, and the temperature varies from
ambient to 140F. Current density remains at approximately 0.5 A/dm2.
The coatings average 2 mm thick and corrosion resistance is limited, therefore they are usually
lacquered or coated with oil or grease. If the black nickel must have good corrosion resistance,
an undercoating such as bright or dull nickel, zinc or cadmium is necessary.
Barrel Nickel Plating
Barrel plating solutions are relatively similar to rack plating solutions; however, operating
conditions may differ, although not radically. The pH is usually maintained at about 4, unless
plating zinc die-casting, in which case a pH higher than 4 may be necessary. However, anode
corrosion is better at a lower pH, and anode area is limited. The anode area should be as large
as possible to avoid the liberation of oxygen and chlorine.
Temperatures can vary for barrel nickel plating from 86-104F for some solutions and 104-140F
for others. Current density can also vary. For a typical barrel, approximately 24-32 inches long
and 16 inches in diameter, the load is 300-600 amps per load or between 1-1.5 A/dm2. Other
considerations are the barrel loading, surface area and coating thickness.
There are some special considerations for barrel plating: 1) Parts must be able to move about
freely in the barrel; 2) Precise surface preparation is essential, including thorough rinsing; and 3)
When the electrolytes are used to full capacity, low-current-density treatment should be used
continuously.
Properties of Nickel Deposits
Thickness - Corrosion resistance is often intimately related to the thickness of the coating;
however, the functional requirements of the coating are also important. Micrometer readings are
used most often to determine coating thickness.
Hardness - Certain addition agents, such as saccharin or napththalene sulfonic acid, can
increase the hardness of a nickel deposit. Wetting agents may also increase hardness. Nickel
deposits plated from Watts nickel baths, sulfamate or fluoborate baths can rise to 650 HV (HV is
Vickers hardness). Heavy nickel baths produce deposits with hardness between 250-350 HV.
Hardness is not only a result of addition agents but is also affected by the plating bath
composition, temperature, current density and other operating conditions.
Ductility - Ductility can be measured using a tensile testing machine; however this test is specific
to measuring plated thin foils.
Nickel Plating without use of Cyanide
This can be done using the NICKELSOL process. The NICKELSOL process is a hydrogen
peroxide-sulfuric acid formulation designed to strip nickel and copper from aluminum, plastic
and stainless steel.
The NICKELSOL process can replace nitric acid strippers, which cause the evolution of harmful
NOx fumes.
The NICKELSOL process does not contain cyanide or chelating agents and treatment of the
subsequent rinse water is reduced to simple neutralization and precipitation.
The NICKELSOL process offers the following advantages:
•
•
The bath can be regenerated indefinitely, eliminating frequent dumping and the related
waste treatment cost
The economical recovery of the dissolved nickel and copper is made possible by
crystallization.
•
The system, in most cases, is readily adaptable to most existing automatic, semiautomatic and manual operations
•
Simple control and maintenance
The NICKELSOL Process may be used in almost any industrial application where the
removal of nickel and copper from base surfaces of aluminum, plastic and stainless steel is
required. The bath composition can be adjusted to meet the specific operating requirements.
Chapter 5
<<Painting and Lacquering – Electrocoating – Process Description and Solution
Formulation>>
Process description
The development of electrophoretic coating started in the USA for the painting of automotive
bodies. Whilst still often referred to as "electrophoresis", it is now known that the deposition
mechanism has more to do with the electrolysis of water and de-stabilization of polymer
particles than with the simple movement of a polymer in an electrical field. Systems are
available for both anodic and cathodic coating. In recent years, anodic coatings have given way
to cathodic coatings.
The solutions used are 70-90% water and the remainder consists of resin, pigments, additives
and small quantities of organic solvent.
The resin systems used may be acrylic,
phenolic/acrylic, epoxy, epoxy/polyester or polybutadiene. It is important that whichever resin is
used, it must possess a reactive chemical group which will form a salt with an acid or a base.
The choice of resin system is therefore dependent upon the use for which the coating is
required.
The application of a potential to a solution causes the electrolytic breakdown of water at the
anode and cathode. The secondary products developed begin the process of coagulation of the
resin in solution. In anodic systems hydrogen ions are the secondary product whilst in cathodic
systems hydroxyl ions are the secondary product. The reactions involved are:
Anodic deposition:
Reaction at anode: 2H2O → 4H+ + O2 + 4ePolymer-COO(soluble) + H+ → Polymer-COOH(insoluble)
Cathode deposition:
Reaction at cathode: 2H2O + 2e-→ H2 + 2OHPo1ymer-N+R2H(soluble) + OH- → Po1ymer-NR2(insoluble) + H2O
The hydrogen and oxygen released produce foam on the wet film. This acts as an electrical
resistance, hence limiting the film thickness.
When a direct current potential is applied, the current seeks the path of least resistance and
products nearest the electrode are coated first. As the electrical resistance increases, the
current seeks paths of least resistance, thus virtually all areas can be coated, even internal
sections.
Clear lacquer coats (unpigmented) can be applied as top coat protection, for example to
Silverware.
Prior to application, the products must first be cleaned using an aqueous alkaline solution. It is
then usual to apply a phosphate coating (see Section 2.3.2). After treatment in the paint bath,
the product is rinsed to remove the surface material and then heated to produce a continuous
coating.
The solution remaining on the surface of the product on removal from the bath, known as cream
coat, is richer in resin than the basic solution. It is rinsed and the rinse water subject to ultrafiltration. This method of filtration uses membranes of various constructions. The material
passing through the membrane (permeate) is used to rinse the product, preferably during its
removal from the paint bath. This is followed by rinsing in further tanks of the permeate or by
spraying, followed by a final rinse in de-ionized water. The material held back during filtration,
which has a higher concentration of resin materials, is returned to the paint bath to avoid excess
loss and assist in maintaining the optimum concentration.
Solution formulation
Materials for manufacture of solutions consist of a suitable resin, pigment, additive, and a small
quantity of organic solvent in an aqueous solution. All formulations are proprietary and supplied
in a concentrated form for dilution with de-ionized water. Typically, in use, they have a solids
content between 8-14 %.
Anodic systems
Cathodic systems
Styrene-maleic anhydride copolymers or acrylic acid-acrylic ester
copolymers.
Resin based on aminoalkylesters of acrylic acid or systems with
tertiary suiphonium ions, epoxides and secondary amines.
Releases
It is claimed that discharges from electrocoating installations are very low with over 95% of
solution being recycled by ultra-filtration. The filtration elements used in conventional filtration
systems for the removal of foreign particles are disposed of by registered disposal contractors
when no longer serviceable.
\
Chapter 6
<<Conventional Painting and Lacquering – Process Description and Solution
Formulation>>
Process description
Conventional paints are synthetic (organic) chemical materials in suitable solvents (organic or
water), which dry by the evaporation of the solvent, generally by the application of heat.
Lacquers are similar except they are generally unpigmented or slightly tinted.
Methods of application are numerous and include spray, dip, flow coating and barreling.
The solids content of the as-used material (that which remains on the product after drying) is
usually less than 50% and as a consequence the remainder must be driven off by natural or
forced evaporation methods to enable the coating to fulfil its decorative or protective
requirements.
Solution formulation
Solvent and resin losses from the use of 1 litre of organic solvent/resin paint may be as follows:
Total material at start of process
1000 ml (50% solids)
Amount of material lost to over spray
Coating applied to article
(25%) 250 ml
750 ml
Solvent losses (50%):
From overspray
From article
Total
125 ml
375 ml
500 ml
Resin loss:
From overspray
125 ml
Chapter 7
<<Powder Coating>>
Powder coatings are mixtures of resins and pigments blended together and supplied in fine
powder form. The materials used are usually thermoplastics or thermoset powders, for example
polythene, nylon, PVC, mixed epoxy polymers, polyesters, acrylics and polyurethanes.
Electrostatic spray guns are generally used for the application of powders to components,
although some use is made of fluidized bed principles. The component for coating is at earth
potential, and is usually supported on some form of conveyor system. The powder is emitted
from the gun. At the point of emission it is electrically charged and attracted to the component.
The essential requirements for such a system are:
•
Charged powder particle
•
An electrical field
The particles are charged by a phenomenon known as "Corona" discharge. If a voltage is
applied to a needlepoint, the current flowing to the workpiece will be negligible at first. As soon
as the high tension (HT) reaches about 20 kV, a current will commence to flow - this is the
Corona discharge and is the voltage at which the air in the vicinity of the needle breaks down
and becomes ionized. As the voltage is increased still further, the current flowing between the
needle and the workpiece will rapidly increase. Thus, the area between the workpiece and the
gun consists of an electric field, a cloud of particles and ionized air molecules.
Particles are attracted to areas nearest to the gun first; as the covering builds up the covered
area becomes insulating and so deposition occurs on more distant areas. The process is selflimiting in terms of the thickness of the coating. The powder particles are attracted to the
workpiece and remain adherent, by electrostatic forces, for sufficient time to enable the
workpiece to be transferred for heating, where the particles melt to form a continuous coating.
Whilst some powder is lost during application, any overspray material is collected, using cyclone
recovery systems for in-house re-application. Where re-use may not be economical, or color
contamination is a problem, it is often sold for less critical applications (e.g. automotive chassis
use). If not sold it is disposed of via landfill.
Chapter 8
<<Conversion coating – Chromating, Phosphating, Anodizing>>
Conversion coatings are produced by the chemical treatment of a metallic surface to produce a
superficial layer of compound on the metal surface. Often these coatings are given their own
Terminology:
•
•
•
•
Passivating or Chromating
Phosphating
Anodizing
Galvanizing
<Passivating or Chromating>
Process description
Passivating may be applied direct to a manufactured product for the following reasons:
•
To extend its corrosion resistance, for example stainless steel.
•
To benefit the adhesion of a subsequent coating, for example prior to painting of
aluminium or zinc-based die castings.
•
To previously applied coatings of metal, in particular cadmium and zinc
electrodeposits and galvanizing. Passivated coatings increase the corrosion
resistance of these coatings and prevent the oxidation of the coating (white rust
formation).
Various degrees of passivation are available. These are usually designated by the color
obtained:
•
Bright Colorless to pale blue
•
Full
Yellow, iridescent color
Olive drab (khaki)
Black (Produced directly from a passivate solution or by dyeing of
an olive drab film)
Generally, passivation solutions for the treatment of zinc and cadmium consist of an aqueous
solution of inorganic chemicals, traditionally based on chromates or dichromates. Recently
some organic based materials have been developed to comply with environmental legislation,
and the use of trivalent chromium in place of hexavalent chromium salts is becoming more
common. All solutions are proprietary developments of `supply companies' from whom the
product is obtained in either liquid or solid form for use in an acidic media. In addition to
chromium salts, activators are also present in very low concentrations, such as acetate,
formate, chloride, nitrate, phosphate and sulphamate ions.
The solutions are used at room temperature and fume extraction is not normally required.
Discharges are due to drag-out into the water rinses. Solutions are replaced periodically and the
spent solution treated for reduction of hexavalent chromium prior to discharge. The amount
discharged is dependant upon the method used, such as rack or barrel. Aluminium and its
alloys may be passivated prior to painting, as an alternative to the more costly process of
anodizing. The solutions used are similar to those used for zinc and are based on acidic
hexavalent chromium. They are used at slightly elevated temperatures.
Solution Formulation
<Phosphating>
Phosphate coatings consist of layers of crystalline, water insoluble metal phosphates of varying
crystal size. The crystal size is dependent on the type of phosphate used and the surface
condition of the product being treated. Most metal phosphates are insoluble in water but soluble
in mineral acids. This forms the basis of the phosphate coating reaction. Commercial
phosphating solutions are carefully balanced solutions of metal phosphates dissolved in
phosphoric acid. When a reactive metal is immersed in the solution, light pickling takes place at
the liquid/metal interface. When metal from the substrate is dissolved, hydrogen is evolved and
the phosphate coating deposited. As the coating is formed in place at the metal surface, it
incorporates metal ions dissolved from the surface of the product. The deposit formed is a
conversion coating and differs from electrodeposited coatings, which are added to or
superimposed on the metal.
Phosphate coatings fall into three main types:
•
•
•
Iron phosphate: An amorphous coating suitable where a coating film in the order of
300-700 mg/m2 is required.
Zinc phosphate: Lightweight (1.0-4.5 g/m2), medium weight (4.5-10 g/m2) and heavy
weight (10-3 0 g/m2)
Manganese phosphate: For coatings of 10-30 g/m2.
Solutions for phosphating are based on the tribasic acid, ortho-phosphonic acid H3P04 and give
rise, on neutralization, to three series of salts:
•
•
•
Primary salt NaH2PO4.Zn(H2P04)2
Secondary salt Na2HPO4.ZnHPO4
Tertiary salt Na3PO4.Zn3(P04)2
An example of a phosphating reaction is:
Fe + 3Zn(H2P04)2 →FeHPO4 + Zn3(P04)2 + 3H3P04 + H2
While, a very simple process, the theoretical equations, by which, it occurs are complex.
Phosphating from a simple phosphoric bath is time consuming; hence other chemicals may be
added to reduce process times. Referred to as accelerators, they may be divided into two
classes:
•
•
Additions of heavy metals, particularly small quantities of copper and nickel in the
form of a soluble salt at a concentration of 0.002-0.010%.
Additions of oxidizing agents, particularly nitrates, nitrites, chlorates and some
organic nitro compounds.
Modification of the coating crystal structure may be made by the deposition of a mixed element
layer, such as calcium or manganese.
The proprietary processes are usually stated, for example calcium modified zinc phosphate and
nitrate accelerated zinc phosphate.
The product should be in a clean, rust free condition prior to treatment; therefore most
installations include pre-treatment stages. Post treatment is advisable to impart the best
corrosion resistance properties. Post treatments are based upon chromic acid or alkaline metal
chromates or dichromates with a chromium concentration of between 0.10-0.5 g/l.
A typical phosphating operation may be:
•
•
•
•
•
•
•
•
Cleaning 2-5 minutes
Rinse
Rinse
Phosphating 2-30 minutes (Dependent on type and weight of coating)
Rinse
Rinse
Chromate rinse 15-60 seconds
Dry
The method of application may be either by immersion or spraying. There are some processes
which both clean and phosphate in a single operation.
Solution formulation
All phosphate preparations are proprietary but consist of iron, zinc or manganese phosphate in
phosphoric acid with low concentrations of other metals such as iron or copper and calcium.
They are usually supplied as a liquid concentration and between 20-100 ml/l are used. The size
of bath used varies depending upon the object to be processed; the smallest size in use is
approximately 1000 liters. Table 2.19 gives the typical formulations of some phosphating
solutions. Note that the concentrations in this table are those in the formulation, not those in the
actual treatment bath (in contrast to most other tables in this document). Table 2.20 gives
examples of concentrations of species in baths in use, from German industry.
<Anodizing>
Process description
Anodizing is an electrolytic process designed to produce an oxide film integral with the surface
of the metal. In theory anodizing can be applied to a number of metals such as zinc, magnesium
and titanium, though its only commercial application at present is as a treatment for aluminium.
Both of these processes are usually used prior to sulphuric acid anodizing. There is also a
usage of phosphoric acid in electropolishing which is used in a variety of processes.
There is also a growing volume of matt chemical polishing. This is carried out in a bath
containing approximately the following formulation:
•
•
•
Phosphoric acid 80% v/v
Sulphuric acid 20% v/v
Operating temperature 90-105°C
Aluminium coil coaters use an electropolishing solution of a similar formulation.
The final stage for almost all anodizing processes is sealing, which is preceded by dyeing in
many decorative applications. The dyestuffs are complex organic materials, usually at low
concentrations of around 2g/l, with concentrations of 6 g/l for black dye.
Sealing is usually accomplished with boiling water, but sealing effectiveness can be improved
by rinsing with nickel acetate. Actual concentrations of nickel acetate used vary widely, but 5-15
g/l is common.
At the cleaning stage, the chemicals used are alkaline in nature and sometimes pH adjustment
may be sufficient before release. If an acid rinse is in the process line then virtually automatic
pH control may be obtained. If the acid is used for pickling it may contain heavy metals, and so
precipitation will be required followed by settlement or filtration prior to discharge. Similar
treatment may also be needed if there is any significant release of metals from the substrate
being cleaned during the process.
The requirements for effluent treatment or discharges from the phosphating rinse are dependent
upon the local water authority and may require one of the following:
•
Simple neutralization
•
Neutralization and removal of suspended solids
•
Neutralization plus removal of phosphates
Where chromate treatment is used, no rinsing is generally undertaken; hence no waste
treatment is necessary. Sludges from phosphating can be a problem during production. They
can settle out at the bottom of the process tank and also coat the heating coils, immersion
heaters etc. Periodically the solution must be pumped to a storage tank and the sludge removed
and disposed of by registered contractors.
Anodizing Releases
The major discharges from the anodizing industry are sodium sulphate and aluminium. There
will be very minor discharges of other metals from a variety of low concentration sources. In
addition there will be some discharge of oxides of nitrogen to air, and discharge of nitrates and
phosphates to water from chemical brighteners.
The following releases are based upon information provided by a supplier to the anodizing
industry. The amount of phosphoric acid consumed and therefore ultimately going to effluent is
1500-2000 tones/annum, Nitric acid is removed from exhaust fumes by scrubbing and ultimately
discharged to effluent, the total amount discharge is approximately 500 tones/annum.
Approximately 100 tones/annum nitrous oxide fumes are produced, of this 25% will be scrubbed
and discharged to water, the remainder being discharged to air. There is currently no significant
volume of phosphoric acid recycling.
Mechanical plating
Mechanically deposited coatings of cadmium, tin, tin/zinc and zinc can be cold welded onto
ferrous metals, individually or in combination.
Deposits are produced by impingement, cold welding and compaction of metal powder or
granules onto cleaned and suitably activated ferrous substrates. The structure of the deposit is
typified by the presence of agglomerated particles and voids in the coating. The protective and
functional properties of the coatings are similar to electrodeposits of equivalent film thicknesses.
The main advantage of this process over electroplating is that coated parts can be produced
which are free from hydrogen embrittlement; the process is especially suited to coating severely
cold-worked parts, heat-treated or surface-hardened components and components
manufactured from high tensile steels.
In common with barrel electroplating, limits exist with regard to component size, weight and
shape.
After degreasing and cleaning, components to be coated are loaded into a barrel with the
appropriate quantity of glass beads, water and promoter chemicals in order to condition the
surface of the components. The metal to be deposited is then added in powder form, the
quantity being dependent on the surface area of the components and the coating thickness
required. Rotation of the barrel at the appropriate speed results in the generation of impact
forces by the glass beads or the components, with the subsequent cold welding of metal
granules on the substrate. After the prescribed time, the components are separated from the
glass beads and dried.
A specialized version of mechanical plating operates under the proprietary name of
Sherardizing. While not strictly a mechanical process, it can be compared to carburizing, with
which it has similarities. In carburizing, heating with a carbon-bearing media causes the carbon
to be absorbed into the surface. In sherardizing a similar phenomenon takes place but zinc is
absorbed in the surface. In reality, the process could be called a mechanical diffusion process.
After the necessary cleaning and pre-treatment, the articles are loaded into a container with the
pre-determined quantity of zinc dust, which is dependant upon the thickness required, and inert
filler which prevents mechanical damage and ensures even distribution of the zinc dust. The
sealed container is then loaded into a furnace and the temperature is raised to the required
level. The temperature used is normally between 350-450°C, and is chosen so as to not affect
the physical properties of the material being processed. When the operation is complete, the
sealed container is removed and cooled. The parts are separated from the inert filler, which
after screening can be re-cycled. The residual zinc dust is discarded and disposed of to landfill.
Articles processed in this way are often given a post treatment to further increase their corrosion
resistance and life span by such treatments as passivating, phosphating or blackening/oiling.
Similar to galvanizing, this process gives very good uniformity of coating over contoured and
recessed articles.
<Galvanizing>
Process description
Galvanizing is the most widely used of the major methods for the coating of iron and steel with
zinc, particularly for corrosion resistance in the `as produced' state or as a pre-coating for paint
finishes. The protection afforded to iron and steel is not due solely to the barrier effect of zinc
forming a continuous coating over the whole area, but largely due to its behavior as the anode
in electro chemical reactions. The result is that zinc corrodes in preference to the underlying
substrate. In this context, the zinc coating acts as a sacrificial coating. Corrosion of zinc results
in the development of a tenacious carbonate film, which resists further attack. Zinc/ aluminium
alloys can also be used for coatings in a similar way.
Zinc coated sheet is used for many presswork applications. The zinc coating has the ability to
`roll over' the cut edges during the press operation, and often gives the required protection
without further treatment.
The usage of zinc is in the order of 100,000 tones per annum in the United Kingdom, of which
some 45% is used for continuous strip and sheet, 15% for wire and tube and the remainder for
general component processing.
When a clean and fluxed component is dipped into molten zinc at a temperature of round
450CC, a series of zinc-iron alloys are formed by reaction of the zinc with the component
surface. At the normal galvanizing temperature of 450°C, the reaction between the iron and zinc
is usually parabolic with time i.e. the reaction is rapid at first then slows down. Hence the zinc
layer reaches a certain thickness quite rapidly, after this there is no significant increase in the
thickness. An exception is with high silicon steel where the reaction is linear with time and
hence very high thicknesses can be produced.
Cleaning and acid rinsing are essential treatments prior to a specific treatment known as
Fluxing. Fluxing is categorized by three descriptions:
•
•
•
Old-Dry - The components are rinsed in hydrochloric acid and dried without rinsing. The
acid salts on the surface act as the flux when the components are treated in the
molten zinc bath.
Dry - After acid rinsing, the components are treated in a flux bath and dried prior to
transfer to the zinc bath. Typically the flux could be based on zinc ammonium chloride
of about 30% concentration.
Wet - The components are transferred after rinsing directly to the zinc bath, which has a
blanket of molten flux floating on the surface of the zinc. The blanket typically zinc
ammonium chloride together with foaming agents to thicken the blanket and lower the
surface tension.
There are three types of process used in the galvanizing industry:
•
•
•
General hot dip galvanizing
Continuous hot dip galvanizing
Continuous electroplating processes.
In General hot dip galvanizing, the components to be coated, after flux treatment are dipped into
a bath of molten zinc. Larger items are lowered into the bath by crane; smaller items are
immersed in perforated steel baskets. The duration of the immersion varies from a few minutes
to 30 minutes. After treatment the items are removed from the bath and excess zinc is removed
- this may be returned to the bath or may be sent for reclamation. Fumes can be generated
during the treatment, so the baths either have an extraction system or are located in a ventilated
enclosure. The ventilation air is cleaned by bag filters.
The zinc used is generally of a good commercial standard (98.5%) and contains just over 1%
lead, as lead is soluble to about 1% in molten zinc. Excess lead separates out at the bottom and
is usefully employed to prevent the dross (a pasty zinc iron alloy of ratio 25:1) from sticking to
the bottom of the bath, and hence aiding in its periodic removal. Aluminium is often present in
very small quantities (0.005%) to prevent surface oxidation, improve the surface brightness and
give a smoother coating.
Following removal from the zinc bath the components are quenched to cool and remove any
residual flux, where a blanket flux has been used, to prevent staining and to facilitate easier
handling.
In Continuous hot dip galvanizing, steel sheet/strip material is surface cleaned, then fed through
a heat treatment furnace with a reducing atmosphere for cleaning and annealing. It is then fed
directly into the galvanizing bath without contact with air to prevent re-oxidation, therefore
eliminating the need for fluxing. Since the substrate is already at temperature, most of the heat
required for the galvanizing bath is supplied from the substrate. Coils are automatically welded
together before entering the system to give a fully continuous process. The speed at which the
strip passes through the zinc bath means that the coating consists mainly of zinc metal rather
than of zinc-iron alloys. After treatment, gas `knives' are used to remove excess zinc. The strip
is then cooled gradually, quenched in water and dried. Any further finishing to give the desired
surface properties and appearance is then carried out; the strip is cut to the required length and
then recoiled. Coils of finished galvanized steel are very valuable and are always protected
against oxidation by a chromate rinse layer. An oil film, plastic wrap or interleaved paper, or a
combination of all. They are stored under cover, usually with controlled temperature and
humidity.
Releases
The aqueous discharges from the pre-treatment sections are similar to those experienced with
other metal finishing operations.
The dross removed is collected and sent for reclamation, since it is rarely economical for
processors to carry out reclamation themselves.
Zinc ash is formed by the disturbance of the surface of the liquid during the operation; as a
result the zinc oxidizes and particles of zinc are entrapped. The ash is therefore a mixture of
zinc oxide and varying quantities of entrapped zinc which may be as high as 80%. Oxidation
also occurs during idle periods and further increases the production of ash. The ash is
periodically removed and subjected to various methods for zinc reclamation; these include the
cylinder method, the static crucible method and the rotary crucible method. In practice it is
possible to obtain a yield of about 50% by the above methods.
Both types of hot dip galvanizing involve the use of air extraction systems, with bag filtration of
the ventilation air, and recovery of zinc from the bag filters.
Run-off loss from continuous hot dip treated steel is considered to be negligible in view of the
post-treatment handling and storage of these materials.
More detailed consideration of the releases of zinc from the galvanizing processes can be found
in the draft risk assessment report.
Vacuum deposition
Physical vapor deposition
In the physical vapor deposition (PVD) process material is vaporized and transmitted in the
vapor phase through a vacuum or low-pressure environment to a substrate where it condenses.
PVD processes are used to deposit films of compound materials by the reaction of the material
with the ambient gas environment or with a co-deposited material. Film thicknesses can vary (11000 nm) and layers can be built up to form multilayer coatings and thick deposits.
Vacuum evaporation is a PVD process in which material from a thermal vaporization source
reaches the substrate without collision in the gas phase. As such there is no scattering and the
process is by line of sight. Typically vacuum evaporation takes place in the pressure range of
10-3 to 10-7 Pa. Vacuum evaporation is widely used to form optical interference coatings, minor
coatings, decorative coatings, barrier films and electrically conducting films as well as corrosion
protection coatings. Examples of products processed include: minors, lamp reflectors, costume
jewellery, and toys; examples of the coatings produced are anti reflective oxide coatings on
spectacle lenses and sun glasses, barrier films on flexible packaging materials and abrasive
and wear resistant coatings.
Sputter deposition is the deposition of particles vaporized from a surface. It is a non-thermal
process in which the surface atoms are physically ejected by an energetic bombarding particle,
usually an ion accelerated from a plasma stream. It is performed in a vacuum or low-pressure
gas (<0.7 Pa). The process is widely used to deposit thin films on semi-conductor materials,
coating of architectural glass, reflective coatings on compact discs, magnetic films, dry film
lubricants, and some decorative applications.
Ion plating uses concurrent or periodic energetic particle bombardment of the depositing film to
modify or control the composition and properties of the depositing film. The material may be
deposited by evaporation, sputtering or other vaporizing sources. The particles used for
bombardment may be ions of inert or reactive gas, or ions of the depositing material. Ion plating
may be carried out in a plasma or vacuum environment. It is used to deposit hard coatings onto
surfaces, adherent metal coatings onto surfaces and optical coatings with high densities.
Clean room conditions are a pre-requisite for trouble free operation, so it is not suitable for
normal factory environments. It is not, at present, a main stream metal finishing technology, and
has made minimal impact on the more conventional processes. There are few by-products
released during the actual processing.
Similar to other metal finishing processes, it is an essential requirement that articles for
processing are chemically clean. Conventional aqueous based cleaners are rarely used, solvent
type cleaners are preferred leaving a dry article. Often when processing plastic substrates it is
only necessary to remove particles retained by static forces by use of an anti-static air gun. In
some circumstances, articles may be pre-coated using organic lacquers which not only give a
gloss finish to the article, but also give some leveling to the surface (vacuum deposited coatings
being very thin replicate the surface of the articles). Where the articles need to be handled after
processing, a topcoat of clear or tinted lacquer or a clear oxide coating may be applied. This is
carried out as a second stage operation whilst still in the vacuum equipment.
Discharges from the process are negligible. Releases may occur during maintenance of
equipment. Material on supporting equipment may be removed by dissolution of the coating in
suitable chemical solutions or by the use of abrasive techniques. For example, aluminium is
readily soluble in sodium hydroxide solution, the residual solution requires treatment before
discharge by conventional waste treatment methods and the sludge produced is disposed of via
registered contractors.
Chemical vapor deposition
Chemical vapor deposition (CVD) is a technique to produce coatings on a variety of substrates
where specific operational protection is required. These may be components that see arduous
conditions in diverse industries including power generation (nuclear, gas and steam), transport,
textile and general engineering.
Chemical vapor deposition is carried out at elevated temperatures (>800°C) and generally
involves the transport of volatile species to the surface of the component being coated. The
volatile species then undergoes a chemical reaction and deposition can then occur. The
following equations represent the common reactions that take place:
2MX(g) + H2(g) →M(g) + 2HX(g)
2MX(g)
→ M(g) + X2(g)
M(l)X + M(2) → M(1)(s) + M(2)X(g)
(1)
(2)
(3)
Notes: M and M(1) are the depositing material, M(2) is the substrate material and X is a halide
such as chloride, iodide or fluoride.
The MX species is generated in a maimer that is convenient with respect to its physical
properties. For example where M is aluminium or titanium, then the vapor pressure of these
compounds is sufficiently high to be able to generate these compounds external to the
hermetically sealed retort and using only moderate heating to the line (up to 200°C) pass them
into the retort along with any inert gas and/or hydrogen. MX compounds with a low vapor
pressure (e.g. CrCl3) are generated in the coating reactor by the reaction of the metal with
HX.X2 or a salt such as NH4X.
Practical experience shows that reactions (1) and (3) commonly occur during CVD and the
waste gases tend to be hydrogen, hydrogen halides and inert gases. Flow rates of the exhaust
gas are not high (e.g. 10 liters/minute) and the high solubility of the acidic hydrogen halides in
water means that a water scrubbing tower is a convenient and effective way of removing these
compounds from the gas stream Passing exhaust gases up a tower countercurrent to a mist of
alkaline water (e.g.. Water containing dissolved sodium bicarbonate) is adequate for this
purpose. The resulting scrubbing solution is kept alkaline with extra additions of sodium
bicarbonate as it reacts with the hydrogen halide as:
HX + NaHCO3→ NaX + H20 + CO2
This produces easily disposable liquor. Any heavy metals that are produced during the process
(or indeed that are not consumed) will normally condense in the cooler zone of the retort and be
collected for disposal at the end of the run.
Vitreous enameling
Process description
Vitreous enamel is also known as porcelain enamel, especially in the USA. Vitreous enamel is
the fusion of an inorganic coating (glass) to metal to produce a hard coating, which is
permanently bonded to the metal substrate.
It has all of the properties of glass - hardness, temperature, chemical and abrasion resistance,
durability, and color stability. It is widely used where these properties are an advantage, for
example in kitchen equipment and bathroom fittings. In these applications it is usually applied to
steel or cast iron. The steel required for this application has specialist properties to make it
suitable for the process. It is also used in architectural applications where its durability, fire
resistance and graffiti resistance are finding increasing uses. Its chemical resistance makes it a
suitable coating for agricultural and sewage storage tanks. A combination of its chemical and
heat resistance properties make it suitable for use in elements for flue gas desulphurization
plants and heat exchangers for power stations. High technology applications such as printed
circuit boards, heating elements and aerospace equipment are growth areas. Other applications
include in jewellery and ornamental goods. Vitreous enamel materials can be produced in a
range of colors and decorated by screen-printing, transfers or painting.
The process of vitreous enameling starts with the production of the glass, normally of the
borosilicate type, which is smelted to form a `frit'. This is formed by quenching the glass rapidly
in water forming a granular or flake form. This is ground in a ball mill with high density alumina
media. For wet applications (by spraying or dipping) it is ground with water into a suspension
with clays and salts to produce the appropriate rheology. It can also be applied electrostatically
as a dry powder.
In the wet process the enamel is dried to remove the majority of the water and then fired in a
furnace at temperatures of about 800°C for steel substrates, and at lower temperatures for
aluminium and copper substrates. The frit then fuses forming a metallurgical bond with the
substrate.
For jewellery applications onto copper and its alloys or precious metals, the enamel is often
applied as a dry powder and held in place with a gum such as Gum Tragacanth. After firing it
may be polished.
Vitreous enamel may be applied as a single coat called the direct-on process, or by the prior
application of a ground coat. Coloring pigments are complex metal-alumina-silicates formed by
calcinating transition metal oxides with alumina and silica. For deep colors up to 8% pigment
may be used.
Prior to the enameling operation, it is important to ensure that the substrate is chemically clean
and conditioned to promote the formation of the metallurgical bond and to achieve good
adherence of the enamel to the substrate. The pre-treatment necessary will be dependant on
whether a ground coat or direct-on process is used for enameling.
If the ground coat process is used than a hot alkaline soak may be sufficient, particularly if the
ground coat is highly reactive. However, it is more usual to use acid pickling followed by
deposition of a thin layer of nickel applied by either electroless electrolytic plating. A typical
process sequence may be:
•
•
•
•
•
•
•
•
•
Hot alkaline soak
Rinse
Sulphuric acid
Rinse
Nickel Deposition
Rinse
Rinse
Enameling Application
Dry and Fire
Solution Formation
The frit may be purchased ready for use.
Electroless plating (Autocatalytic plating)
Process description
A limited number of metals can be deposited by chemical reduction rather than by electrical
reduction. The basic reaction is:
M2+ + 2e -→ M
The deposits produced by electroless plating are almost completely uniform in thickness
compared to electrodeposits, which vary in thickness. It is also possible to plate onto nonmetallic surfaces, for example plastics and ceramics. Chemicals need to be added to the bath
continually to replace materials as they are used up. This leads to a build up of breakdown
products in the bath, which reduces its efficiency.
Several metals can be deposited in this way, but in practice copper and nickel are the only two
deposited on a large commercial scale. Copper is used in printed circuits and electroless plating
is the major method used for depositing copper through the hole connections. Electroless
copper is also used for the decorative plating of plastics, but has being largely replaced in this
field by electroless nickel. The main use of electroless nickel is in engineering where it is applied
as a hard, corrosion-resistant coating. Electroless gold is being developed for use in the
electronics industry, though due to its high costs its use is likely to remain limited. Electroless
silver is used in the electroforming industry, as a means of metallizing non-metallic mandrels.
Electroless cobalt has a special application in computer memory discs, and possible
applications in rocket technology. The other electroless deposits have no serious commercial
applications at present.
Formulations
A common factor in all electroless formulations is the presence of complexing agents. These
range from very strong chelators such as EDTA to acids such as citric and tartaric acids. For
electroless nickel, carboxylic acids are used extensively. For instance the following compounds
are in regular use: acetic, propionic, lactic, glycollic, maleic, succinic, citric, and tartaric acid.
Addition agents are used sparingly in electroless formulations. Sulphur compounds such as
mercaptobenztriazole are used as stabilisers, and lead and cadmium salts can be used as
brighteners, though their use is declining.
Copper
Table 2.24 gives the typical formulation of a electroless copper bath. Copper sulphate is usually
used as the source of metal ions, though copper formate and copper nitrate may also be used.
Complexing agents used included the tartrates and EDTA. The stabilizers used are sulphurcontaining compounds such as thiourea, thiodiglycollic acid and mercaptobenzthiazole. Sodium
cyanide and vanadium oxide may also be used.
Releases
The losses due to dragout in normal use will be small and in many cases so small as to require
little or no treatment. Waste disposal problems may occur at the end of the working life of the
solution when the solution has to be discarded. The time interval between solution changes
varies depending upon the size of the user. For large scale users changes may be required at
2-3 day intervals while for smaller scale users changes at intervals of 1-2 weeks may be
required. The typical size of an electroless nickel tank is 200-1000 liters, though tanks up to
6000 liters are in use.
The solution for disposal contains a number of breakdown products. The typical content of a
spent electroless nickel solution is given in Table 2.30.
Table 2.30 TypicaL content of spent electroless nickel solution
Nickel
Approximately 5 g/l
Sodium hypophosphite
Approximately 10 g/l
Other phosphates and phosphites
30-50 g/l
Mixed carboxylic acids
50-80 g/l
Lead
2 PPM
Sulphur compounds
1 PPM
Unlike electroplating solutions, electroless plating solutions have a finite life. This is usually
expressed as the number of metal turnovers accomplished, and is commonly of the order of 3 to
8 metal turnovers with 6 metal turnovers being the mean.
At the point of disposal the solution will contain about 3-5 g/l of Nickel, together with a mixture of
phosphates and phosphites, a considerably quantity of sodium sulphate, and a quantity of the
complexing acids, typically acetic acid, lactic acid and glycolic acid. The actual concentrations of
these acids can vary considerably, but will normally be higher at this stage than the original
make up concentration, and could be as much as 50% higher.
Methods of disposal vary widely. Very small operators will bleed the spent solution into the main
effluent treatment system, where the nickel will be partially removed, but all other materials will
pass directly to the waste stream. Some operators treat the solution first to remove the nickel.
There are two main methods. Precipitation of the nickel as a fine powder by the addition of a
powerful reducing agent such as hydrazine or sodium borohydride is in some use. The other
method is to break the complex with sodium dithionite, then precipitate the nickel as hydroxide
at high pH. The resultant waste stream will still contain the various complexing acids and
phosphates.
None of the above methods is truly satisfactory and so there is a growing tendency for spent
solutions to be disposed of to landfill through licensed waste disposal contractors. There has
been some investigation into nickel recovery of bulk solutions by specific ion exchange, but this
has not proved financially viable. As a consequence when the material goes to landfill, it goes
as a total spent solution.
The nickel in the solution may be removed relatively easily by oxidation, precipitation, reduction,
electrowinning or ion exchange. The carboxylic acids can be removed by biological degradation,
though no viable system is in use at present. There is also no viable system for the removal of
phosphates at present.
Electroless copper solutions have a longer life. In this case the residual materials are copper
and formate. The life is often extended by a system of bleed and feed, which means that small
amounts are continually run to waste. As in the case of electroless nickel the final destination of
these materials is landfill.
In electroless copper solutions the copper metal is quite strongly complexed which has led to
considerable problems. The best method of removing copper is by treatment with complexing
ion exchange resins. This leaves a residue containing formaldehyde, formic acid, and assorted
complexing agents such as EDTA and tartrates.
Barrel Plating is used when the plating is done inside of a perforated barrel, and the barrel is
rotated to even the plating. It is mainly used in the plating small diverse objects, devoid of
sharp and long edges that tend to plate badly.
Functions of barrel plating
a. The primary function of barrel plating is to provide an economical means to electroplate
manufactured parts that also meets the customer’s specific finishing requirements.
b. The four most important requirements are:
- Engineering applications, such as building up the thickness of metal to change the
physical size of a part or to provide a good surface for some other treatment such as
painting or screening.
- Decorative coatings such as Bright Nickel, Brass, and Antiquing.
- Cosmetic uses such as Zinc plating to improve shelf life and selling ability.
- But by far, the most important use of barrel plating is to extend the corrosion
protection of the customers' parts.
Barrel plating fundamentals and the production process
a. Parts need only to be free-flowing enough to enter the mouth of the barrel.
b. Loads should not exceed half the volume of the barrel or improper tumbling will occur
and a loss of plating uniformity.
c. The surface area of the plated parts should generally be about 25 sq. feet for every foot
length of the barrel at a 14 inch diameter.
d. Parts must be able to tumble freely to insure a good plating distribution. Such interior
protrusions as breaker bars, dimples or ribbed sides should be used as necessary.
e. The rotation of the barrel while in the plating tank is also very important. Typically a
speed of 3 to 6 RPM is considered adequate but faster speeds facilitate a more uniform
deposit even though there may be some physical wear on the barrel itself. As long as the
parts themselves will not be harmed it is more desirable to maintain as fast a rotational rate
as is practicable.
f. Barrel sizes and hole perforations should be chosen with care depending on the size of
the parts to be plated. Too small a hole will trap solution by capillary action and drag the
chemicals all along the plating line. Too small a barrel and the parts will not tumble
properly.
Quality control
a. Proper and on-going training is extremely important for successfully barrel plating any
part.
b. Records should be kept regarding all of the important parameters involved in each step
along the plating cycle. These should include things such as part description, load size,
voltage, time, thickness readings, chemical additions and also any problems which may
have taken place during the cycle.
c. All relevant data and notes should be routinely reviewed to assure that the product will
remain at a consistent level of quality and that the process can be continuously improved.
d. There are numerous quality systems which the customer may require the barrel
electroplater to employ such as the ISO 9000 standard which is one of the more recent
attempts to help barrel electroplaters achieve the highest level of customer satisfaction
possible.
The single, most important, factor to be considered when purchasing barrel plating
equipment is to understand that the equipment you are buying is part of a system. Your
plating line is a kind of an industrial ecosystem. Every component barrels, tanks, rinsing
system, etc. affects the results generated by every other component. Any kind of slightest
change in one piece of equipment can result you to pay the penalty further down the line.
This principle applies to both new equipment purchases or the repair and retrofitting of
existing plating lines.
COMMON PROBLEMS ASSOCIATED WITH BARREL PLATING
How can one limit the amount of or recover the waste in the barrel plating process.
Barrel plating has existed—in one form or another—since the close of the Civil War. And
while the technology has seen some radical improvements in the last 140 years, modern day
barrel plating is not without its challenges.
Problem: Spikes in Cyanide Concentration
During a plant visit, along with a careful analysis of their operations it showed that
periodically, the conventional horizontal barrel line is over-loaded with work and the
oscillating barrel line is then used to plate the over-load. Unfortunately, the over-load
consists of cup-shaped parts that create a very high drag-out, as the oscillating barrel line
carries these cups into the rinse system without emptying them over the plating tank prior
to transfer. A normally rotating barrel would empty the cups over the tank (as is done on
your other plating line) and would do a better job of rinsing these parts. After measuring the
drag-out rate it was found to be about 1.5 gallons per barrel.
As a result of operating the oscillating line on these parts, a large amount of cyanide
entered the rinse system after plating. Even a well working waste treatment system can be
over-loaded by a spike in cyanide concentration. The systems were designed around 100500ppm of cyanide, while the spikes were around 2000-2500ppm.
The solution to the problem is to not use the oscillating barrel plating line on cup shaped
parts.
Problem: Damaged Parts
Since tin is a soft metal, it can easily be abraded in a barrel plating operation. The factors to
look at include the condition of the electrical contacts, barrel rotational speed and use of
ballast.
An examination of the parts under the microscope (see photo) indicated that some severe
scraping is going on in at least some of the barrels you are using. The following corrective
actions should be considered:
1. Change Method of Electrical Contact
The barrels used employ conventional danglers, which can build up in metal to the point of
being abrasion sources. Button contacts or rod contacts may be more gentle.
2. Maintenance of Electrical Contacts
One of the frequently neglected tasks in barrel plating is maintenance of the dangler. As
metal builds up on the electrical contacts within a barrel, they develop sharp edges that can
cause damage to a moving load. If the electrical contacts have any heavy build-up of metal,
this must be removed on a more frequent basis.
3. Change the Barrel Speed
The barrel speed may cause too much friction between the dangler and parts and between
the parts themselves. If possible change to rotational speed of the barrel. By lowering the
speed, you may also need to lower the barrel loading. The best combination of barrel
loading and speed will have to be determined by trial and error.
4. Use/Change Ballast
Ballast can be used to keep parts separated during plating, reducing damage from contact
with sharp features on the parts and also improving coverage.
If you are not using ballast, it is recommended to try this. Common ballast is copper beads
as which comes in a variety of sizes. But you need to experiment with both shape and
size to arrive at the optimum combination.
Problem: Plating Solution "Growth"
This is the most common reported problem. In most cases it is a case of more drag-in
than drag-out from the plating solution. The chloride zinc process typically contains a high
concentration of wetting agent (surfactant), which lowers the surface tension of the plating
solution and results in better drainage of the barrel as the barrel is removed from the
plating tank. Since the rinse before the plating tank does not contain any wetter, the barrel
does not drain as well before going into the plating tank. Over time, the difference in dragin volume vs. drag-out volume causes the plating solution to “grow.”
If there is a drag-out rinse, try going into this rinse before you bring the barrel into the
plating tank. Since the drag-out rinse will contain some of the wetter, this may solve the
problem. Some wetter may need to be added to the drag-out rinse to bring the surface
tension closer to that of the plating solution.
Solution: If there is no drag-out tank, some wetter may be bled into the last rinse prior to
plating. Heating the last rinse before plating may also help, as warm water drains better
than cold.
Brush Plating
Brush plating is an electrochemical process that uses systems to electroplate, anodize, and
electro polish localized areas on both OEM components and parts that need coatings for
repair and dimensional restoration.
Brush systems are portable. Unlike their tank counterparts, brush plating systems use very
small volumes of solution (usually only one or two gallons) and hand-held tools to apply the
deposits and coatings onto localized areas. These hand-held tools are covered with an
absorbent material that is saturated with a solution and then brushed or rubbed against the
part. Brush plating requires different hand-held tools for each different solution in the
operation.
A portable power pack (rectifier) provides the direct current required for all the processes.
The power pack has at least two leads. One is connected to the tool and the other is
connected to the part. The direct current supplied by the power pack is used in a circuit that
is completed when the tool is touching the work surface.
The work surface is prepared using the same types of tooling and equipment that are used
for the final finishing operation. As with a tank plating process, brush plating requires good
preparation of the work surface to produce an adherent deposit.
Brush plating has come a long way from the early days of tank plating when it was a
common practice to touch up bad spots on plated parts using solution saturated rags
wrapped around pieces of pipe.
Today, brush plating and anodizing systems are used to selectively apply engineered
deposits and coatings in very precise thicknesses for both OEM and repair applications.
Brush plating and anodizing are now completely divorced from their tank counterparts,
although some of the equipment and terms still resemble those used in tank processes.
Tools, equipment and solutions, however, cannot be used interchangeably between brush
and tank systems.
Since it is more difficult to control temperature and current density in portable finishing
processes than in tank processes, it was necessary to develop complete, integrated
portable finishing systems for commercial applications. These systems were developed for
operators who are not familiar with tank finishing techniques.
Today, brush plating systems are available for electroplating, anodizing, hard coating and
electro-polishing. These systems vary in their sophistication and coating capabilities.
Small pen-type systems apply only flash deposits on small areas. Larger, more
sophisticated systems use power packs with outputs up to 500 amps and are capable of
producing excellent quality finishes and high thicknesses on large surface areas.
Chapter 9
<<Avoiding Contamination, Corrosion and Surface Preparation>>
Avoiding Contamination & Corrosion
Before anything is plated, the parts to be coated must be CLEANED. Electroplaters use
CLEANERS for this. They are alkaline materials that remove oils, dirt and rust. In a typical
plating line, the part is first immersed in a cleaning tank, then in an electro-cleaning tank
(uses power from a rectifier to aid in cleaning), and then into the plating tank.
A typical plating tank has three copper bars suspended over its top:
One connected to the negative lead from the rectifier and two connected to the positive lead.
The racks of parts to be plated hang from the bar that is connected to the negative lead, the
anodes (metal to be plated) from the positive bars. The solution in the tank may have to be
heated or cooled. For this, electroplaters use Immersion Heaters or Heat Exchangers.
The solution becomes contaminated with dirt and other particles, which would cause rough
plates. To prevent this, electroplaters use filters.
In some cases the plated part is chromated. Zinc plated parts; for example, will become bluish
or yellowish if they are chromated. You can see such appearances on nuts and bolts you buy in
a hardware store. The chromate coating is applied by dipping the zinc plated part in a tank
containing chromic acid and other chemicals. The acid reacts with the zinc plating to form a zinc
chromate. This is called a conversion coating, because the chromic acid solution converts the
surface to zinc chromate. This coating further improves corrosion resistance. There are also
black and olive drab conversion coatings.
Larger parts are usually plated on racks. But if you have a million nuts and bolts to plate, you
don't want to hang each of them individually on a plating rack. For this reason a plating barrel is
used. The parts are dumped into a plastic barrel with holes drilled into the plastic sides. Then
the barrel load of parts is immersed into the plating solution. Inside the barrel is a dangler, a
piece of flexible metal that reaches down into the load of nuts and bolts to carry current to them.
The current is conducted from part to part by their electrical conductivity and the whole load
begins to be plated. The barrel is rotated while current is applied. The nuts and bolts become
plated with zinc or cadmium or whatever is desired. This is barrel plating.
If you have lots of racks or lots of barrels and you don't want to hand carry them from tank to
tank you can attach them to a conveyor that moves the racks or barrels from tank to tank,
immersing them in each solution for a preset time. This is conveyor based plating, which may be
done from an automatic line or from a hoist line.
Understanding and Avoiding Corrosion
There are 3 types of corrosion:
•
•
•
Auto Corrosion
Contact Corrosion
External Corrosion
The most commonly occurring types of corrosion are Auto and Contact corrosion.
Auto corrosion occurs when a metal is in contact with an electrolyte but is not at the same
time in contact with any other electrical conductor, neither metallic nor non-metallic.
Simple case of Iron and Rust creation – In chemically pure iron, corrosion would proceed
simply by the exertion of the solution pressure of the metal, in conjunction with the presence
of hydrogen ions and the oxygen dissolved in the electrolyte, which depolarize the metal
surface, oxidize and precipitate the primary products of solution as ferric hydrate or rust.
In practical cases, auto corrosion proceeds by the galvanic action which is set up as a result
of the heterogeneous structure of the metal or alloy.
No commercial metal exists in which there is perfect homogeneity, there is always some
characteristic of structure, some slight degree of segregation or the presence of embedded
impurities which is sufficient to impart varying potentials or solution pressures to adjacent
areas of the metal surface.
For this reason auto electrolysis is set up by which the more electro-positive areas dissolve
and, in the case of ferrous material, are eventually precipitated as rust. The pronounced
heterogeneity of some alloys, such, for instance, as brass, is no ' doubt largely responsible
for the rapidity with which they frequently corrode, and in the case of iron and iron alloys
there is a large volume of evidence to show that heterogeneity, whether induced by
structure or segregation, etc., is conducive to accelerated corrosion.
Contact corrosion occurs when the metal is in contact with some other conducting
material, which is also immersed either wholly or partially in the electrolyte:
•
•
If this other conductor is a metal, then the corrosion of the first metal will be either
accelerated or retarded, according to the electro-chemical relationship between the
two metals.
If the second metal is electro-positive to the first, then it will protect the latter at its
own expense by itself corroding or dissolving preferentially, but if it be electronegative to the first metal then the corrosion of this will be accelerated (or the second
metal will receive protection at the expense of the first).
The practical recognition and application of this may be found in the practice of protecting
boilers from corrosion by inserting slabs of the more electro-positive metal zinc and in the
protective coatings of zinc which are applied to iron products by various processes. Other
conditions being the same, the rate of the contact corrosion of a metal is usually greater
than the rate of its auto corrosion. If the second conductor is nonmetallic in character, it may
generally be assumed to be electro-negative to the metal, and contact between them will
therefore result in an accelerated corrosion of the metal.
External Corrosion is the result of the passage of a current, generated from some external
source, through the metal whilst the latter is in contact with an electrolyte:
•
If the current flows in that direction which necessitates the metal acting as anode,
then corrosion results.
•
If the current flows in the opposite direction, i.e., from the electrolyte into the metal,
the latter receives protection from corrosion which may be complete provided the
E.M.F. of the current is sufficiently high.
Figure Demonstrating the chemical reaction of corrosion on Iron/Steel surface
Surface preparation
It is commonly accepted and often quoted by electroplaters that one can make a poor coating
perform with excellent pretreatment, but one cannot make an excellent coating perform with
poor pretreatment. Surface pre-treatment by chemical and/or mechanical means is important in
the preparation for electroplating. Surface treatment and plating operations have three basic
steps:
1. Surface cleaning or preparation. Usually this includes employing of solvents, alkaline
cleaners, acid cleaners, abrasive materials and/or water.
2. Surface modification. That includes change in surface attributes, such as application of
(metal) layer(s) and/or hardening.
3. Rinsing or other work-piece finishing operations to produce/obtain the final product.
Success of electroplating or surface conversion depends on removing contaminants and films
from the substrate. Organic and nonmetallic films interfere with bonding by causing poor
adhesion and even preventing deposition. The surface contamination can be extrinsic,
comprised of organic debris and mineral dust from the environment or preceding processes. It
can also be intrinsic, one example being a native oxide layer. Cleaning methods are designed to
minimize substrate damage while removing the film or debris.
If the chemistry and processing history of a metal surface is known, one can anticipate cleaning
needs and methods. In practice, extrinsic organic and inorganic soils originate with processing
of the substrate before plating, as well as from the environment. Specific residues include
lubricants, phosphate coating, quenching oils, rust proofing oils, drawing compounds, and
stamping lubricants.
In short, the mixture of potential contaminants to which a part is exposed is typically complex.
Again in case of a metal substrate it must be remembered that all metals form oxide and
inorganic films to a degree with environmental gases and chemicals. Some of these are
protective against continuing attack such as the aluminum oxide formed on aluminum alloys
That phenomenon is the reason of the usefulness of aluminum siding on some homes. On the
other hand, some are non-protective, such as iron oxide on steel. Some of these films can even
be plated directly with nickel over aluminum oxide over aluminum being an example. The
cleaning and activation steps must account for the fact that surface oxide re-forms at different
rates on different metals.
Specifically, in case of iron or nickel the oxide re-forms slowly enough that the part can be
transferred from a cleaning solution to a plating bath at a normal rate. In case of aluminum or
magnesium the oxide re-forms very fast such that special processing steps are required to
preserve the metal surface while it is being transferred to electroplating.
Cleaning processes are based on two approaches – Physical Cleaning and Chemical
Cleaning.
In Physical Cleaning, mechanical energy is introduced to release both extrinsic and intrinsic
contaminants from the (metal) surface. Examples are ultrasonic agitation and brush abrasion.
In Chemical Cleaning contaminant films are removed by active materials, dissolved or
emulsified in the cleaning solution. Extrinsic contaminants are removed with surface-active
chemicals while the chemical energies involved are modest. Intrinsic films are removed with
aggressive chemicals that dissolve the contaminant and often react with the surface (metal)
itself. The energy involved in surface preparation is substantial.
Pie-treatment
Pre-treatment is a sequence of processes necessary to ensure that the product for subsequent
coating or surface modification is in a suitable condition. For all metal finishing technologies
some form of pre-treatment is an essential requirement. The three main pre-treatment methods
are:
• Cleaning
Aqueous, solvent and mechanical (blasting)
•
Activation
•
Brightening
Acid rinsing
Bright dipping, chemical polishing and pickling
Cleaning
Cleaning may be defined as the removal of soils from metal surfaces by employing chemical
solutions or mechanical methods. Chemical cleaning can cope with a wide variety of soils
including those from heavy oils and greases, light cutting oils and polishing compositions.
Cleaning may be considered the most important process in metal finishing because the final
appearance and acceptance depends upon the presentation of a clean and active substrate,
irrespective of the final coating process.
Aqueous cleaning
•
Where products are to be treated by subsequent aqueous-based metal finishing
technologies, such as electroplating, electroless plating, and electrocoating, the cleaners
used are normally of an aqueous and alkaline nature. The type of cleaner used is
dependent upon the nature of the soil for removal and the material of the base substrate.
Where ferrous materials are to be cleaned then a highly alkaline solution may be
employed, but for copper and copper alloys, zinc based alloys and aluminium, only
mildly alkaline solutions are suitable.
•
Chemical cleaners act through solubilization, emulsification and saponification of the
contamination.
•
Most cleaners are supplied as proprietary product in powder form, the ingredients being
selected from sodium carbonate, sodium hydroxide, sodium metasilicates, trisodium
phosphate and sodium borates, with complexing agents (EDTA, gluconates, heptonates,
and polyphosphates) and organic surfactants to reduce the surface tension of water and
to promote oil emulsification. Complex phosphates are included to chelate calcium and
magnesium ions present in hard water, and to prevent their precipitation as insoluble
salts. The traditional use of phosphates has been reduced in recent years due to
environmental concerns.
•
The type of cleaner and strength used is dependent upon the metal being cleaned and
the soil to be removed. Heavy-duty cleaners may have an alkalinity of 20-30%
expressed as sodium hydroxide whilst light duty cleaners may have only 5-10%
alkalinity. Complexing agents in the cleaner are of the order of 1-2% by volume.
•
Many cleaners are used as soaks in which the oils and greases are softened and
released from the component surface. Alternatively electrolytic means (anodic or
cathodic) are used in which the gas generated assists contamination removal by its
scrubbing action. The efficiency of cleaners may be increased by air or mechanical
agitation. In certain applications spray cleaning is preferable. To make more efficient use
of aqueous cleaning solutions the use of ultrasonic vibration is sometimes
advantageous.
•
Where possible none or low foaming surfactants are used, being an essential
requirement for electrolytic cleaners and spray applications.
•
Thorough water rinsing after cleaning is essential. This is true particularly where high
sodium hydroxide concentrations are in use. A dip in dilute acid is required after rinsing,
due to the difficulty in rinsing caustic solutions from substrate.
•
Where a sequence of cleaners is in use e.g. soak, cathodic and anodic, cleaners may be
selected which are compatible with each other thus eliminating the need for interstage
rinsing and the consequent drag-out losses.
•
Some cleaners are of the emulsion type with the use of an organic hydrocarbon solvent
in alkaline solution Suitable emulsifiers are used to form an oil-in-water or water-in-oil
emulsion. Cleaners of this type were commonplace but have been superseded by more
sophisticated conventional alkaline materials. Further, these types of cleaners cannot be
rinsed 100% free of solvent and produce subsequent process problems.
•
The selection of the cleaning formulation is in some cases specific to the product
substrate although there are some, which have a more universal application.
•
Cyanide containing formulations are still available where cold electrolytic cleaning
applications are required, but their use has largely been curtailed due to the need to
treat the discharge for cyanide destruction.
•
Discharges are determined by the type of articles being processed and drag out into
water rinse systems (see Section 3). The outflow to effluent usually only requires pH
adjustment, unless it contains cyanide.
•
Periodic replacement of the total cleaning solution is required, the frequency of which is
dependent on the soil contamination removed from the articles processed, the plant
throughput and the volume of cleaning media contained in the tank. As a guide solutions
are disposed of after 4-8 weeks. The sludge produced is removed and disposed of to
landfill. It is normal to discharge cleaning materials at the same time as the acids from
the process line so that pH neutralization is nearly automatic. Following settlement of
solids the solution can then be discharged to sewer.
•
Typically the concentration of a commercially available cleaner used is in the range of
25-75 g/l at 50-80°C. Periodically additions would be made to the solution after simple
alkalinity analysis, to compensate for drag-out losses.
Metal Surface Preparation and Cleaning
NATURE OF THE SURFACE
Metal surface under normal circumstances are not atomically smooth. Crystal effects
such as dislocation, twins and grain boundaries, emerging at the surface can give rise to
steps and ledges that can be many atoms high. Surface treatments prior to
electroplating can lead to further enhancement of this surface roughness. Atoms in
ledges and steps are even more energetic than those in smooth and are sites of strong
adsorption. Different crystal phases within the surface of alloys as well as impurities and
non-metallic inclusions such as entrapped slag, create additional surface in
homogeneities.
DEPOSIT GROWTH MODE
In the presence of large quantities of growth inhibitors it becomes impossible for even
the initially depositing atoms to follow the basis metal crystal structure. Under these
conditions epitaxial growth does not occur, growth being determined solely by plating
conditions and bath composition, often resulting in a fine-grained, randomly oriented
deposit structure.
ADHESION
Certain basis metals may contain non-conducting or poorly conducting phases. Where
such phases are exposed on the surface, plating will not occur, although coverage may
be produced by bridging of the deposit from neighboring areas that are conducting. The
same effect is produced is produced by abrasive particles such as silicon carbide that
become embedded in the surface during the polishing or grinding operation and are not
subsequently removed by electro polishing or etching.
POROSITY
Many of the factors which cause poor adhesion also produce porosity in the electro
deposit. Areas of the basis surface that have soils remaining or contain non conducting
phases such as slag particles are not plated and pores are formed as the deposit
bridges over them. Like-wise low hydrogen over-voltage phases produce hydrogen
bubbles which may be occluded by the growing deposit or create channels through the
deposit as the bubbles evolve from the surface. Aggressive mechanical treatment of the
surface may produce fine surface cracks that are not plated, again leading to porosity.
Some of these causes of porosity may be removed by treatments prior to plating such as
adequate degreasing, chemical or electro polishing. However there is little that can be
done about low conducting and low hydrogen over voltage phases except to avoid them
where possible.
BRIGHTNESS
The basis metal effect on brightness is essentially that due to surface topography. This
topography is very much dependent on surface treatments. Surface roughness greater
than the wavelength of visible light-i.e., about 0.15 micrometers (6 micro inches) causes
diffuse scattering of light and a dull appearance.
STRESS
It was earlier mentioned that stresses could result from the atomic mismatch between an
epitaxial deposit and the basis metal surface. If the interatomic spacing of the deposit is smaller
than that of the basis metal, the crystal structure of the initially depositing atoms will be
stretched and consequently be in the state of tensile stress. Compressive stress arises when
the deposit has a larger spacing than the basis metal. These stresses can be significant in thin
deposits, often leading to the formation of dislocations, which influence the mechanical
properties, and corrosion resistance of the deposit.
Apart from co-deposition during plating, atomic hydrogen may be introduced into the basis metal
during pre-plating treatment processes such as acid pickling or cathodic cleaning. This
hydrogen can collect as molecules as voids and produce considerable internal pressures,
leading to brittle cracking of the basis metal under relatively low stresses. Heat treatment is the
only way to remove the entrapped hydrogen, but this cure may itself cause problems if it
reduces desired properties such as hardness.
SOLVENT CLEANING
Cleaning is the removal of undesirable material from the base material and is normally
limited to the surface. The unwanted material may be rust or oxide films, metal fines,
shop dirt such as dust or paper, rust preventatives, buffing or polishing compounds, wax
oil, fingerprints, grease, asphalt, and even water in some cases. Earlier chapters of this
section have described buffing, polishing, barrel finishing and electropolishing. These
processes are valuable in removing rust or oxide films or metal oxide imperfections.
Salts and other water-soluble soils, soaps and some buffing and polishing suspending
agents can be cleaned from parts by alkali cleaning as described in section F. the
presence of organic compounds such as oil or grease can foul these cleaning processes
and add to their cost, maintenance and waste. These organic compounds are easily
solubilized by solvent and removed from the work parts.
In some cases solvent cleaning before other surface preparations can extend the life of
those cleaning operations; reduce their costs. In other cases solvent cleaning provides
work parts in a condition ready for the next operation such as assembly, painting,
inspection, further machining or packaging. Prior to plating solvent cleaning is usually
followed by an alkaline wash or other similar processes, which provide a hydrophilic
surface. Further solvent cleaning can be employed to remove water from electroplated
parts, as is common in jewelry industry.
Solvent cleaning can be accomplished in room temperature baths or by employing
vapor-degreasing techniques. Room temperature solvent cleaning is commonly referred
to as cold cleaning. Vapor degreasing is the process of solvent cleaning parts by
condensing solvent vapors of a non-flammable solvent on a work part(s).
COLD CLEANING
The prime properties of a solvent for simple cold cleaning are good solvency,
minimum flammability, low toxicity and a moderately fast evaporation rate. Carbon
tetrachloride was regarded as nearly ideal cold cleaning solvent before its highly toxic
properties were recognized. Cold cleaning solvent properties are summarized in table 1.
DIPHASE COLD CLEANING
Frequently, solvent cleaning is chosen to avoid exposing the processed work to
water. In some instances, such as before plating, aqueous cleaning after solvent
cleaning is desirable. In such situations, diphase cleaning can offer distinct advantages.
In this cleaning operation, a water layer, which may contain surfactants, is placed on top
of one of the chlorinated solvents or fluorocarbon 113. The chlorinated and fluorocarbon
solvents are heavier than water and remain below the water surface. Generally, the
petroleum solvents are not used in diphase cleaning because they are lighter than water
and float.
Control System for Cold Cleaning
Control System A
Control Equipment:
1.
2.
3.
Cover
Facility for draining cleaned parts
Permanent, conspicuous label, summarizing the operating requirement.
Operating Requirements:
1.
2.
3.
Do not dispose of waste solvent or transfer it to another party, such that greater than
20% of the waste (by weight) can evaporate into the atmosphere. *store waste
solvent only in covered containers.
Close degreaser cover whenever not handling parts in the cleaner.
Drain cleaned parts for at least 15 sec or until dripping ceases.
Control System B
Control Equipment:
1.
2.
3.
4.
5.
Cover: Same as in system A, except if (a) solvent volatility is greater than 2 kPa (15
mm Hg or 0.3 psi) measured at 38°C (100°F), ** (b) solvent is agitated, or (c) solvent
is heated, then the cover must be designed so that it can be easily operated with one
hand. (Covers for larger degreaser may require mechanical assistance, by spring
loading, counter weighting or powered systems.)
Drainage facility: Same as in system A, except that if solvent volatility is greater than
about 4.3 kPa (32 mm Hg or 0.6 psi) measured at 38°C (100°F), then the drainage
facility must be internal, so that parts are enclosed under the cover while draining.
The drainage facility may be external for an application where an internal type cannot
fit into the cleaning system.
Label: Same as in system A.
If used, the solvent spray must be a solid, fluid stream (not a fine, atomized or
shower type spray) and at a pressure which does not cause excessive splashing.
Major control device for highly volatile solvents: If the solvent volatility is > 4.3 kPa
(33 mm Hg or 0.6 psi) measured at 38°C (100°F), or if solvent is heated above 50°C
(120°F), than one of the following control devices must be used:
a.
Freeboard that gives a freeboard ratio*** = 0.7
b.
c.
Water cover (solvent must be insoluble in and heavier than water)
Other systems of equivalent control, such as refrigerated chiller or carbon
adsorption.
Operating Requirements:
Same as in system A
CONTROL SYSTEMS FOR CONVEYORIZED DEGREASERS
Control System A
Control Equipment: None
Operating Environment:
1. Exhaust ventilation should not exceed 20 m3/min/m2 (65 cfm/ft2) of degreaser opening,
unless necessary to meet OSHA requirements. Workplace fans should not be used near
the degreaser opening.
2. Minimize carry-out emissions by:
a. Racking parts for best drainage.
b. Maintaining conveyor speed at < 3.3 m/min (11ft/min).
3. Do not dispose of waste solvent or transfer it to another party, such that greater than
20% of the waste (by weight) can evaporate into the atmosphere. Store waste solvent
only in covered containers.
4. Repair solvent leaks immediately, or shutdown the degreaser.
5. Water should not be visibly detectable in the solvent exiting the water separator.
Control System B
Control Equipment:
1. Major control devices; the degreaser must be controlled by either:
a.
Refrigerated chiller,
b.
Carbon adsorption system, with ventilation = 15 m2/min/m2 (15 cfm/ft2) of air
/vapor area (when down time covers are open), and exhausting <25 ppm of
solvent by volume averaged over a complete adsorption cycle, or
c.
System demonstrated to have control efficiency equivalent to or better than
either of the above.
2. Either a drying tunnel, or another means such as rotating (tumbling) basket, sufficient to
prevent cleaned parts from carrying out solvent liquid or vapor.
3. Safety switches.
a.
Condenser flow switch and thermostat (shuts off sump heat if coolant is either
not circulating or too warm).
b.
Spray safety switch (shuts off spray pump or conveyor if the vapor level drops
excessively, e.g. > 10 cm (4 in).
c.
Vapor level control thermostat (shuts off sump heat when vapor level rises
too high.)
4. Minimize openings: Entrances and exits should silhouette work loads so that the
average clearance (between parts and the edge of the degreaser opening )is either <
10 cm (4 in) or < 10 % of the width of the opening.
5. Down Time Covers: Covers should be provided for closing of the entrance and exit
during shutdown hours.
Operating Requirements:
1 to 5. Same as for system A.
6. Down time cover must be placed over entrances and exits of conveyorized degreasers
immediately after the conveyor and exhaust are shut down and removed just before they are
started up.
ALKALINE CLEANING
The object of this pre-plating cycle is to remove those surface films which can be characterized
as soils, and replace them with films which will be compatible with the solutions being used to
apply the final finish. When the sequence is properly selected and operated, the parts will enter
the final processing solution with a surface in an activated or receptive state for the finish to be
applied. To accomplish this preparation, four basic steps are required:
1. Gross cleaning – the removal of heavy soil.
2. Fine cleaning- the removal of residues from gross cleaning, along with
fine particulate matter.
3. Oxide removal – the removal of the thin layer of oxide, which covers
every metallic surface.
4. pH adjustment- to bring the residual surface film close to the same pH
as the processing solution.
BUFFING COMPOUNDS
Buffing Compounds are mixture of lubricating materials (usually fatty acids), abrasives
(complex silicates, carbides or metal oxides) and materials to control the melting points
(often high melt parafinnic compounds or waxes). Since the buffing process is a friction
related process, very high temperatures may be generated at the point of contact, and
all the ingredients can react with each other and the metal surface. These temperatures
can vary widely with buffing conditions; the reaction can vary as well.
RUST PROOFING COMPOUNDS
Rust proofing compounds can roughly be placed in three categories:
•
•
Inorganic, water-soluble compounds for protection between operations or
short term protected storage. These normally do not present any cleaning
problems.
Emulsifiable organic mixtures cut back with water to form the required
emulsions. When the emulsion “breaks” due to a change of temperature
or the evaporation of water the organic portion is left on the surface as a
protective film. The formulation usually contains one or more volatile
constituents, which evaporates with the water during drying so the
protected film is no longer emulsifiable. Protection is adequate for long
term protected storage, or interplant transfer. Cleaning problems are
similar to the next category.
•
Solvent cut back organic mixtures provide a wide degree of protection of,
depending on composition and degree of cut back. Protection may be
adequate to permit out door storage for reasonably extended periods.
They may be formulated with water displacing characteristics so parts to
be protected may be immersed wet. The organic protective materials
generally contain an oil base, a highly protective material such as a fatty
acid, a metallic soap, or a polar material with an affinity for the substrate.
If they are not fully dry to touch they become magnet for shop dirt.
Dryness or lack of tack is usually imparted by incorporating a wax, a
drying oil or a film forming resin. Since these materials are designed to
protect by preventing the penetration of moisture to the metal surface,
they are often difficult to clean in aqueous system. Solvent or vapor
degreasing before aqueous cleaning is often helpful. A solvent dip to
penetrate the film and reduce its viscosity also helps. If waxes are used
for dryness, the temperature of the cleaning solution must be higher then
the melting point of the wax.
Age of the film can be an important factor. Some of the polar materials may react with the metal
surface. Unsaturated compounds may polymerize to form varnish like material. Evaporation of
the solvent use for cutback will alter the viscosity. Coiled Stock is particularly is susceptible to
these effect. Depending on the tightness of the coiling, these variations may occur at different
rates in various areas of the coil. Hence differences in cleaning requirement from point to point
on the coil are not unusual.
MACHINING AND FORMING OILS.
Increasingly often, these oils are being fortified with additives providing extreme
pressure lubrications. Since these adhere strongly to the substrate, aggressive, high
alkalinity cleaners may be required.
SMUTS.
A smut is defined as finely divided particulate matter strongly adherent to the metal
surface. It may be conductive or non-conductive. The nonconductive smuts consists of
inorganic residues including carbon from acid treatment of high carbon steels or from
heat treating operations such as oil quenching or controlled atmosphere heat treatment;
pigments from the use of pigmented drawing compounds; insoluble constituents of an
alloy brought to the surface by previous chemical treatment; i.e. silicon in aluminum
alloys beryllium in beryllium copper etc.;
BASE METAL EFFECTS
The nature of the base metal has a critical bearing on the type of cleaning system
selected. Materials must be selected to provide the required cleaning action without
undue or selective attack on the base metal. Since metals vary greatly in reactivity,
allowable limits of pH, temperature and concentration and the type and concentration of
inhibiting agents are dictated by the base metal. Cleaners for aluminum or zinc will
generally be quite different from those for brass or steel.
BUFFING COMPOUND REMOVERS
These are essentially highly specialized form of soak cleaners, designed for the effective
removal of buffing compound residues. They fall into three basic categories:
1. Neutral detergent-usually liquids; mixtures of surfactants; pH close to
neutral with buffering provided by the surfactants used.
Concentrations in the range of 1 to 10% by volume.
2. Enhanced detergent-similar to neutral detergent but fortified with
organic alkalis, which can react with the fatty acid in the buffing
compound to form organic soaps. Concentrations 2 to 10% by
volume
3. Modified soak cleaners-similar to soak cleaners (q.v.) but modified to
be especially effective on buffing compounds. Concentration 45 to
120 g/1 (6 oz/gal)
Type 1 and 2 often show poor performance on oily soils other than buffing compounds.
Temperature of operation should be above the melting point of the buffing compound 60
to 80 C (140 to 180 F) Use of ultrasound. Or vigorous agitation will often permit
operation at lower temperatures.
ALKALINE CLEANERS
Alkaline cleaners are blends of various inorganic alkaline salts with deflocculants,
Inhibitors and surfactants as required providing the various cleaning mechanisms and
functions discussed below.
Saponification:
The chemicals action by which a fatty acid, oil or other reactible soil is converted to a
water-soluble compound such as soap. Elevated temperature, Concentration and pH
promote the speed and completion of the reaction. The main advantage is that cleaning
will proceed in the absence of surfactants, and that the reaction products may function
as additional cleaning agents to improve the performance of the cleaner.
Disadvantages include the fact that at least initially only reactible soils will be affected;
the reaction products may build up tolevels that cause rinsing and drying on problems;
incomplete rinsing may result in re-deposition of the soils in a subsequent acid treatment
the solubilized soils unless separated will contribute heavily to hexane solubles in the
effluent, and such separation is not always easy to attain.
Emulsification:
The chemical process by which surfactants penetrate oily soils and break them down
into globules sufficiently small to allow dispersion and suspension in the solution.
Advantages include the fact that the reaction is often independent of pH; temperatures
and concentrations required can be somewhat lower than with saponification; all types of
oily soil will be removed: and rinsing will generally be somewhat better than for
saponified soils.
Disadvantages are similar to those for saponification except as noted, and with the
added possibility that the surfactant concentration may be depleted at a rate different
from the alkali depletion. The cleaner may therefore drift out of balance and fail to
perform even when concentrations appear to be within limits.
Deflocculation:
The process by which special chemical compounds surround particles of solid soil,
removing them from the surface and dispersing them in solution. The process is
generally improved by mechanical action and/or the development of gas by electrolysis.
Elevated temperatures may also be helpful. Different deflocculants may be specific to
certain solids, so complex soils may require mixtures of several agents for effective
action.
Displacement
The process by which surfactants lifts oily soils from the surface of the parts to be
cleaned. A film of surfactant and solution is left on the part surface. The oily soil floats to
the surface of the cleaning bath. Advantages include longer solution life and the
possibility of operating at lower concentrations and temperatures.
The main
disadvantage is the need to continually skim the solution surface to remove the
displaced oil.
Failure to keep the solution surface properly cleaned my result in their deposition of the
oily soil as the parts are removed from the solution. When properly operated. Hexanes
soluble in the effluent are reduced, since the oil soil is constantly separated from the
cleaning solution.
SPRAY CLEANERS
Cleaning solutions, which are sprayed on the parts, sometimes under considerable
pressure. Any of the mechanisms previously discussed, including emulsified solvents
may be used. Careful attention must be give to choosing materials with low foaming
characteristics. The combination of chemical action and the mechanical action of the
spray produces effective cleaning.
Spray patterns must be designed to provide complete coverage of the parts, and the
units given periodic maintenance to insure that nozzles are not plugged. Except for the
foaming requirement, alkaline spray cleaners are similar to soak cleaners.
Concentrations and temperatures, however, are generally much lower, in the range of 15
to 30 g/1 (2 to 4 oz/gal) and 35 to 60 C (100 to 140 F). The newer low temperature
spray cleaners often operate at 4 to 15g/1(1/2 to 2 oz/gal) and 20 to 30 C (70 to 90 F)
Liquid forms of the materials are sometimes available and operate at ½ to 2% by
volume.
ULTRASONIC CLEANING
•
•
•
•
•
Cavitation
Electrical effects
Transmission effects
Cleaning materials
Equipment considerations
OXIDE REMOVAL
Oxide removal operations represent the greatest single use of chemicals in
metalworking. During 1952 such operations consumed 5% of all sulfuric acid produced,
25% of the hydrochloric acid and most of the hydrofluoric acid. In addition large
quantities of nitric acid, phosphoric acid and ferric sulfate are used. Oxide removals in
the metal finishing trade are far over-shadowed by the large scale pickling operations
necessary during the metal production. In general heavy oxides scales are produced
and must be removed during metal production operations.
Oxide removal prior to plating is usually necessary because heat-treating, welding or
other similar operations have oxidized surfaces, which are to be plated. Occasionally
work is allowed to rust and corrode between fabrication steps, necessitating oxide
removal treatments. Reclaiming old machinery and fabricated metal parts require special
oxide removal operation. Bright dipping and removal of superficial oxide films are an
important part of metal finishing procedures.
CHARACTER OF OXIDES AND SCALES ON METALS
For instance heat-treated scales on steel consist of largely Fe2O3 on the outside Fe3O4
as an intermediate phase and approximately FeO next to the metal. The same general
condition is sometimes noted for copper alloys with CuO on the outside and more or less
Cu2O on the inside
OXIDE REMOVAL FROM LOW AND MEDIUM ALLOY STEEL
Oxide removal, pickling or de-scaling operations are practiced to a far greater extent on
low and medium alloy steels than on all other classes of metallic materials combined.
This is true for total tonnages as well as for the number of de-scaling installations. Most
steel in fabricated articles has undergone at least one de-scaling operations and in many
cases three or four at various stages of manufacture. Because of this commercial
interest there is more detailed technical information available on oxide removal from
steel then for most other metals
¾ PICKLING WITH SULFURIC ACID
• Pickling process
• Effect of acid concentration and temperature
• Effect of dissolved iron
• Effect of scale breaking
• The Effect of inhibitors
• Electrolytic Pickling
¾ PICKLING WITH HYDROCHLORIC OR PHOSPHORIC ACID
• Hydrochloric Acid
• Phosphoric Acid
¾ GAS-PHASE PICKLING
¾ BRIGHT DIPPING
Bright Dipping solutions are used for Ferrous and MonoFerrous alloys and usually
involve mixtures of two or more of the acids, sulfuric, phosphoric, chromic, nitric and
hydrochloric
An Example of a typical bath is:
•
•
•
•
20% nitric acid
25% acitic Acid
55% phosphoric acid
0.5% hydrochloric Acid
The bath is operated at 190F, and the dipping time ranges up to 5 min. another example
is:
• 40% nitric acid
• 30% phosphoric acid
• 30% acitic Acid
• 1.0% sodium chloride
A typical bright dipping sequence for racked work is as follows:
1)
2)
3)
4)
5)
6)
•
•
•
•
•
Dip in a scaling bath containing three parts concentrated nitric acid and
one part concentrated sulfuric acid (by volume)
Cold water rinse
Dip in a bath containing one part concentrated nitric acid and three parts
concentrated sulfuric acid (by volume)
Double cold water rinse or spray rinse
Rinse in cold 5% sodium cyanide solution
Final cold and hot water rinses
3 part sulfuric acid
1 part nitric acid
1/20 part hydrochloric acid
6 parts water
½ lb chromic acid per gallon of solution
Chapter 10
<<Controlling Thickness and Most Used Tools>>
Thickness of coating is the most important parameter, which decides the quality of coating
and therefore it needs to be controlled.
Controlling the thickness of the electroplated object is generally achieved by altering the
time the object spends in the salt solution. The longer it remains inside the bath, the thicker
the electroplated shell becomes but there must also be an adequate amount of metallic
ions in the bath to continue coating the object. The shape of the object will also have an
effect on the thickness of the coating. Sharp corners will be plated thicker than recessed
areas. This is due to the electric current present in the bath since it flows more densely
around corners.
Before electroplating an object, it must be cleaned thoroughly and all blemishes and
scratches should be polished. As mentioned, recessed areas will plate less than sharp
corners, so a scratch will become more prominent, rather than being smoothed over by the
plated material.
Determining Coating Thickness
There are five basic, non-destructive methods of determining coating thickness. Each
method is devised to achieve cost-effective, accurate, and repeatable results. These
methods are:
•
X-Ray fluorescence
•
Eddy-current
•
Magnetic induction
•
Beta backscatter
•
Micro-resistance
Each method is particularly suited to a specific coating(s)/substrate combination. We'll
discuss below each system and applications for each.
X-Ray Fluorescence
When a material is subjected to x-ray bombardment, some of its electrons will gain
energy and leave the atom, creating a void in the vacated shell, thereby releasing a
photon of x-ray energy known as x-ray fluorescence.
The energy level or wavelength of fluorescent x-rays is proportional to the atomic
number of the atom and is characteristic for a particular material. The quantity of
energy released will be dependent upon the thickness of the material being measured.
Basically, the x-ray fluorescence unit consists of an x-ray tube and a proportional
counter. Emitted photons ionize the gas in the counter tube proportional to their
energy, permitting spectrum analysis for determination of the material and thickness.
X-ray fluorescence is the most precise measurement method, especially for smalldiameter parts, or dual coatings such as gold and nickel over copper.
Eddy-Current
The eddy-current technique is used for measuring both non-magnetic, metallic
coatings (zinc, cadmium, copper, etc.) over steel as well as non-conductive coatings
over non-ferrous metals such as anodize or paint over aluminum.
When a conductive material is subjected to an AC magnetic field from a probe, eddycurrents occur in the material in proportion to the frequency and resistance. The
induced eddy currents generate an opposing magnetic field which alters the circuit
reactance and the output voltage of the probe. The change in output voltage is used to
calculate the coating thickness. Electrical conductivity between the coating and
substrate should differ by a ratio of 2:1 for optimum accuracy.
Non-conductive coatings introduce a gap (lift-off) between the probe and non-ferrous
base material. This gap produces a loss in eddy current penetration which is
compared to a measurement directly on the base material to determine coating
thickness.
With conductive coatings over steel, eddy currents are generated in both the coating
and ferrous base material. Eddy current loss in both materials is proportional to the
coating and substrate material thickness and will range somewhere between readings
taken directly on pure samples of each material. The eddy current loss differential is
used to calculate coating thickness.
Magnetic Induction
This principle is used for measuring the thickness of a non-magnetic coating (zinc,
cadmium, paint, powder coating, etc.) over a steel substrate.
The probe system is essentially the secondary of a transformer circuit that reacts to
the presence of a magnetic material. The circuit efficiency and output voltage increase
when the probe is brought near a magnetic surface, providing parameters which may
be used to measure the distance (coating thickness) from the magnetic surface.
Beta Backscatter
Beta rays are electrons emitted from unstable radio isotopes. If a highly collimated
beta source is directed at a plated sample (gold over nickel on a printed circuit board,
for instance), the electrons will penetrate the plating material and be reflected back
(back scattered) toward the source. They can be collected and counted with a GeigerMueller tube for subsequent conversion to coating thickness. The atomic number of
the coating material must be sufficiently different (at least four atomic numbers) from
the atomic number of the base material to achieve accurate readings of coating
thickness.
Micro-resistance
The new micro-resistance method of determining plating thickness is ideally suited for
printed circuit board plated-through-holes and for surface copper measurements.
This technique requires precise measurement of the resistance of the copper cylinder
that forms the plated through hole. Once this parameter is known, it is combined with
data on the board and hole aspect ratio to calculate the average copper plating
thickness. Calculations are performed automatically by software associated with the
measurement device.
Specially designed, pyramidal, electrically isolated probe tips simultaneously inject
current and take voltage-drop measurements. The resistance is then calculated by
Ohm's Law.
For electro coatings, powder coating and paint over a non-ferrous substrate, use the eddycurrent method. Use the magnetic induction method on a ferrous substrate.
Portable Tools to Measure Thickness
HELMUT FISCHER offers a family of hand-held instruments with features to meet the
individual requirements of customers.
The DUALSCOPE® MP0R/ MP0RH are designed for quick and easy measurements. This
economic instrument accurately and conveniently displays the coating thickness readings on
two LCD displays: on a large front panel display and a top panel display. It features an
ergonomic design with an integrated constant pressure probe allowing easy one hand
operation.
The possibility of a statistical evaluation of the measurement series exists, with an integrated
radio transmitter for wireless online or offline transmission of the measurements directly to a
computer,
up
to
10
–
20
m
(33
–
66
feet)
away.
The DUALSCOPE® MP0R/ MP0RH utilize both the eddy current test method according to
DIN EN ISO 2360, ASTM B244 and magnetic induction test method according to DIN EN ISO
2178, ASTM B499. It is used to measure non-conductive coatings on non-ferrous metals as
well as non-ferrous metal coatings and non-conductive coatings on iron and steel. It will
automatically recognize the material to be measured and utilize the appropriate test method.
The DUALSCOPE® MP0RH has a considerably larger measurement range. The
DUALSCOPE® MP0R/ MP0RH is ideal for measuring:
• Non-ferrous metal coatings (e.g. chromium, copper, zinc etc.) on steel and iron.
• Paint, lacquer and synthetic coatings on steel and iron.
•
Electrically non-conductive coatings on non-ferrous metals such as paint, lacquer,
synthetics on aluminum, copper, brass, zinc and stainless steel.
•
Anodized coatings on aluminum.
Table 1: Suggested Norms @ Thickness, hours needed and type of objects to be plated
Degree
Exposure
of
Min.Thick
(microm
eter) (1)
Chromate
Finish
MILD-Exposure
too
indoor
atmospheres
with
rare
condensation &
subjected
to
minimum wear
or abrasion
MODERATEExposure mostly
to dry indoor
atmospheres but
subjected
to
occasional
condensation,
wear or abrasion
SEVEREExposure
to
condensation
perspiration,
infrequent
wetting by rain,
cleaners
5
None
VERY SEVERE
(2) Exposure to
bold
atmospheric
conditions, and
subject
to
frequent
exposure
to
moisture,
cleaners
and
saline solutions
plus
likely
damage
by
denting,
scratching
or
abrasive wear
25
Clear
Salt Spray
Hrs to
White
Corr.
12-24
24-72
iridescent
yellow
olive
drab
8
None
12-24
24-72
iridescent
yellow
olive
drab
None
iridescent
yellow
olive
drab
25.4 Micrometers = 1.0 mil.
None
Tools,
zipper,
pulls,
shelves,
machine parts
72-100
Clear
Screws,
nuts,
and
bolts,
buttons,
wire
goods, fasteners
72-100
Clear
13
Typical
Applications
12-24
24-72
72-100
Tubular furniture,
insect screens,
window fittings,
builders’
hardware,
military
hardware,
washing
machine parts,
bicycle parts
Plumbing
fixtures, pole line
hardware
(1)
(2)
Thickness of the coating after chromate treatment.
Although there are some applications for heavy electrodeposited coatings for very
severe service they are most usually satisfied by hot dipping or sprayed coatings.
THICKNESS TESTS
In as much as corrosion resistance has often been shown to be intimately related to the
thickness of the deposit, the stipulation of minimum thickness in the product specification is
obvious. However, corrosion resistance is not the only criterion that makes thickness
specification necessary; at least of equal importance is the functional requirements of the
deposit itself. Many products are plated to achieve definite physical and chemical properties
such as conductivity on printing wiring boards and other electronic devices, wear resistance in
industrial chromium plating and, in some instances, electroless nickel plating, and silver plating
on bearing retainers to impart lubricity at relatively high temperatures. Plating specific coatings
greatly enhances the functions of a particular item, and there are minimums and maximums in
plating thickness specifications, which must be adhered to in order for the item to perform as
designed.
•
•
•
•
•
•
•
•
•
•
Copper
Nickel
Chro
miu
m
Auto
catalytic
Nickel
Zinc
Cadmium
Gold
Palla
dium
Rhodium
Silve
r
Tin
Lead
TinLead
Alloy
NonMeta
l
CM
CMA
CM
CBMA
CM
BCM
BM
BM
BM
BC
M
BCM
BCM
BCCC
M
BM
CED
CMA
C
CB
C
BC
B
B
B
BCE
BC
BC
BCCC
BE
C only
on
CMA
C
CB
C
BC
B
B
B
BC
BC
BC
BCCC
BE
Coatings
Substrat
es
Magnetic
Steel
(includin
gcorrosion
resisting
steel)
Non
magnetic
Stainless
steel
Copper
and
MICROSCOPIC-OPTICAL METHODS
DOUBLE BEAM INTERFERENCE MICROSCOPE, INTERFEROMETRY
CHEMICAL METHODS
MAGNETIC METHODS
EDDY CURRENT
MASS PER UNIT AREA
COULOMETRIC
Factor = Standard calibration Unit
Machine Reading
X-RAY METHODS
BETA BACKSCATTER (BBS)
MICRORESISTANCE TECHNIQUE
D
alloys
Brass
and CuBe
C
MA
…
…
…
B
B
B
B
B
B
B
BC
BE
BC
BCMA
BC
BCBEA,
BC
BC
B
B
B
BC
BC
BC
BCCC
E
B
BMA
B
B
B
B
B
B
B
B
B
B
BC
E
C
…
C
…
C
BC
B
B
B
BC
BC
BC
BCCC
BE
Silver
B
BMA
B
B
B
…
B
…
…
…
…
BC
BC
BE
Glass
Sealing
NickelCobaltIron
alloys
UNS No.
K94610
Non
metals
Titanium
M
CMA
M
CBMA
M
BM
BM
BM
BM
BM
BM
BCM
BACC
M
BM
BCED
BCMA
BC
BCB
BC
BC
B
B
B
BC
BC
BC
BCCC
…
B
BMA
…
BEA, B
B
B
B
B
B
B
B
B
BC
BE
Zinc and
Alloys
Aluminu
m
and
alloys
Magnesi
um and
alloys
Nickel
A
B
Method is sensitive to permeability variation of the coating.
Method is sensitive to variation in the phosphorus content of the coating.
C
Method is sensitive to alloy composition.
D
Method is sensitive to conductivity variation of the coating.
B
Chapter 11
<<Metal Plating for Polymers>>
Polytetrafluoroethylene (PTFE) is better known by the trade name Teflon®. It's used to make
non-stick cooking pans and anything else that needs to be slippery or non-stick. PTFE is also
used to treat carpets and fabrics to make them stain resistant.
What's more, it's also very useful in medical applications. It can be used for making artificial
human body parts, because human bodies rarely reject it.
These materials can be used to make a variety of articles having a combination of
mechanical, electrical, chemical, temperature and friction-resisting properties unmatched by
articles made of any other material. Commercial use of these and other valuable properties
combined in one material has established PTFE resins as outstanding engineering materials
for use in many industrial and military applications.
Table 9a shows data that an EN/PTFE process should be able to fulfill to meet today's
environmental and economical needs. To achieve the "perfect coating" and the required
lifetime, it is necessary to modify the electroless nickel electrolyte and the manufacturing of
the dispersion to develop an improved proprietary process.
A nice side effect of this was that the lifetime of the plating solutions was extended to five
metal turnovers or more under normal job-shop conditions. Customers using EN/PTFE
coatings of the newest generation report generally the same deposition quality for five metal
turnovers or more. Also, the incorporation rate of the PTFE remains at about 25 to 30%
volume during the entire bath life.
To incorporate higher PTFE content does not appear practical because the plating rate is
reduced and the nickel matrix becomes more fragile, increasing the wear rate. Also,
agglomeration of PTFE particles is hardly a problem.
TABLE 9a- EN/PTFE Coating Characteristics:
PTFE incorporation - Variable in between 15 - 30 volume - %
Particle size - 0.3 - 0.5 mm
Bath lifetime - About 5 MTO
Deposition Rate - 5 mm/h at 28 volume % up to 5 MTO
Agitation during Plating - None
Wear properties - A widely held prejudice is that only hard coatings can solve wear problems.
That is correct for abrasive wear, but for nearly all other problems, a reduction of the
coefficient of friction can handle it and is sometimes even better. The main reason for the
success is that forces that could fatigue one or both partners are reduced, so less wear is
transferred into the material.
In the top view, the color is coordinated with height, and in the hardness map with hardness.
Even on the surface, the PTFE is uniformly distributed so phenomena such as fretting and
galling are minimized. The comparison of a heat-treated electroless nickel and hard chromium
layers shows why hardness is not the main factor for low wear. Both layers have an overall
hardness of about 1000 Hv (Vickers hardness) 0.1.
The EN coating is abraded more quickly than the chromium coatings, because EN has a
higher coefficient of friction. Even though the EN/PTFE coating is much softer (320-400 Hv
0.1), the coefficient of friction is lower than that of hard chromium (about 0.07 0 0.1 for
EN/PTFE and 0.12 to 0.25 for hard chromium). Therefore it wears more quickly.
Because of the fatigue behavior of the coating, PTFE and EN particles are broken out of
the surface. In the case of the very smooth surface, the small Ni-P particles can increase
the wear on the next layer of the coating, and the PTFE balls are simply wiped away.
Against that the PTFE particles remain longer in the area of wear and can be used like ball
bearings on the rougher surface. The Ni-P particles are too small to increase the wear on
the rough surface.
Comparison to PVD/CVD coatings (PVD: physical vapor deposition. CVD: chemical vapor
deposition)
In comparison to other wear-protective coatings such as PVD and CVD, there are some
other advantages of soft dispersion coatings:
1. Because the hardness of the deposited layer is similar to that of the substrate (in most
cases), there is no need for a hardening process before plating the protective layer. This
is necessary in a PVD or CVD system to achieve an appropriate adhesion. Similar to the
PVD/CVD processes, the surface has to be purely metallic (no oxides).
2. Because of PTFE's softness, the coating can store and dissipate more energy in small
volumes before it is plastically deformed.
3. The EN/PTFE coating can use the base material as a support structure because of the
similar hardness.
4. Because of the porosity-free deposition, it is possible to get another degree of freedom
for the deposit's lifetime (thickness).
5. The corrosion behavior is much better than that of PVD/CVD coatings (EN: 500 hrs at a
plating thickness of 12mm).
6. The "soft dispersion coatings" cannot increase the wear at the tribological partner in the
system.
In most PVD/CVD coatings, corrosion resistance is a problem. To cope with that, some
companies apply "sandwich coatings." Usually plating of 10mm of a high-phosphorus EN
layer (d=10mm) below the thin ceramic layer can help with the problem.
Galling and corrosion problems are covered best by EN/PTFE dispersion layers. This is
because of the anti-adhesive and hydrophobic properties of the surface and the thermal
conductivity of the coating. While ceramic coatings are electrical and thermal insulators,
EN/PTFE coatings have electrical and thermal conductivity.
Different applications for EN/PTFE coatings
Table 9b shows some of the most common applications for EN/PTFE coatings.
New improvements in the coatings have helped in many industries: drilling equipment for
paper; gardening tools; gas meters; and spiral pumps. In automotive industries there are
new parts tested to avoid noise and wear or reduce weight and costs (Table 9c).
Combining electroless nickel deposits with sub-micron PTFE particles has not yet reached its
peak. That is due not only to a lack of information, but also to the quality of the solid layers.
The consistency was not good enough, and the results differed too much, so that many
electroplaters did not dare apply this coating in high-tech applications. Since improving the
chemistry and the deposit, most applications can now be approached successfully. Also, the
life of the electrolyte convinces some of the electroplating industry customers to use this
trend-setting coating. Many problems can be solved where some years ago no one thought
this could be done with a soft coating. Only some physical properties have to be changed
and many applications could be managed easily.
Chapter 12
<<Plating on plastics>>
Plastics cannot be plated in the same way as metals because plastics are not electrically
conductive. Thus one cannot immerse a plastic part connected to the negative lead and
expect it to plate. Instead, electroless plating is applied first to get a conductive surface, and
then the electroless plated parts are electroplated. Many automotive parts, including grilles
and all manner of decorative trim have been plated plastic.
The Term plated plastics generally reefers to plastic parts finished with bright chrome
electroplate. Plated plastic parts are used in a variety of automotive, appliance, and
hardware applications. This section will review the general procedures for electroplating
plastics.
SELECTION AND MOLDING OF THE PLASTIC
At present, about 85% of plated plastic parts are injection molded of acrylonitrilebutadiene-styrene (ABS) terpolymer. Special plating grade abs molding compounds are
generally preferred for better-quality plating. Also, for successful plating, basic design
criteria should be observed avoiding blind holes, large flat surfaces, and sharp corners.
During the molding special; parameters recommended by the resin manufacturer should
be observed. Finally, silicone mold releases should be avoided in all cases.
PLATING
The typical plating cycle for ABS plastics is described in fig 1, other plastics based on
polyphenylene oxide, nylon, polysulfone, fluoropolymers, and alloys of ABS and
polycarbonate have been commercially plated. Special plating cycles are available for
these materials, either from the manufacturer of the resin or from the suppliers of the
plating chemicals. These cycles differ mainly by requiring special cleaning, solvent
treatment, or etching procedures.
The steps indicated in fig 1 can be carried out as one single plating line or divided into
two lines, the first comprising all steps needed to render the plastic conductive and the
second containing the electroplating operations. In either approach, automatic plating
machines, in general programmed hoist units, are used for better economy and
consistent quality. Adequate water rinses should be included after each step.
The main characteristics of steps of fig 1 are as follows:
Cleaning
Cleaning of the plastics may require a separate treatment, particularly when molding and
plating are being done at different locations. Extra care should be taken in the handling,
packaging, and transportation of molded parts to eliminate such dirts as airborne dust,
oils, and fingerprints. Some of these materials could absorb into the plastic and effect
subsequent processing. The purpose of cleaning is to remove materials that might
interfere with uniform chemical attack in the next steps. Alkaline detergent solutions are
commonly used.
Etching
The etching solutions are specifically designed to render the plastic surface hydrophilic,
and to produce a micro-etch of the surface by selectively attacking components in the
plastic. It is this selective micro-etch that supplies the required bond between the plastic
and the conductive first coating. The etchants usually are strongly oxidizing solutions,
often containing high concentrations of chromic acids and sulfuric acid. These etching
solutions are used for ABS at 60 to 70 C (140 to 160 F), for 5 to 10 minutes.
Neutralizing
Neutralizers normally are used following the chromium containing etchants so as to
eliminate as completely as possible the carryover of hexavalent chromium compounds
which are detrimental to operation of all other treatments in the cycle. The elimination of
chromium salts from the plastic surface also improves the absorption of the catalyst or
activator in the next step.
Alkaline Cleaning
Etching
Neutralizing
Catalyzing
Accelerating
Electroless Nickel
Electroless copper
Electrolytic Strike
Bright acid copper
Bright
Chromium
Duplex Nickel
Micro cracked Chromium
Fig 1: Typical electroplating process for ABS plastics. Rinses have been omitted
Catalyzing
In this step a catalyst is adsorbed on the plastic to initiate the electroless deposition
process. In general, catalysts or activators are strongly acidic mixtures of tin and
palladium salts designed to be adsorbed onto the plastic surface in limited and
controllable amounts. The tin compound is strongly attracted to the organic surface and
bonds the palladium to produce a catalytic surface. These catalysts are generally
supplied as proprietary concentrates which are suitably diluted (1 to 10% by volume) at
15 to 50C(60 to 120 F) for immersion times of 2 to 10 minutes.
Accelerating
An accelerator or post-activator is used in the next step to remove excess tin
compounds to expose the palladium. Dilute acid or alkaline solutions are used at 20 to
50 C (70 to 120 F) for 1 to 2 minutes immersing times.
Electroless Plating
Electroless Nickel or electroless copper deposits without electric current on the
catalyzed surface to produce the conductive coating required for electroplating. Most
present operations utilize electroless copper because of specific outdoor performance
requirements. Electroless nickel baths are more stable and are acceptable for many less
severe applications.
In general proprietary electroless solutions are used. These baths will deposit sufficient
metal at room temperature in 5 to 10 minutes to render the parts conductive. Present
proprietary baths are highly stabilized. Baths lives of several months for electroless
copper and over a year for nickel can be expected. Since the usual thickness of these
electroless deposits will only be 0.12 to 0.75 mm (0.005 to 0.030 mil), it is very important
that the parts now be handled with extreme care.
Electroplating
The previous steps can be carried out having the parts racked or in bulk using barrels or
baskets. Most of the electroplating is done on racks, and only small parts plated in
specially designed barrels, since plastic parts float in the plating bath. For same reason,
the racks should make strong positive contact with the parts to avoid floating and give
adequate electric conductivity. Although the conductivity of the electroless deposit may
be sufficient to carry the required electroplating current on small and medium sized
parts, it often is insufficient for large parts such as automotive grilles when electric rack
contacts may be quite far apart. In general, it is advisable to use a nickel or copper strike
deposit from a high efficiency solution at low current density in order to avoid burn-off of
the thin electroless layer. Thickness of the strike deposit usually is less than 2.5 mm
(0.1mil).
The next electrodeposit is bright, leveling ductile copper obtained from acid copper
sulfate baths. This layer of copper serves both to improve surface appearance prior to
other metal deposits and to act as a stress-absorbing layer both for stresses in the
following nickel deposits and the stresses which may be set up because of the difference
in the thermal expansion of plastic and the plated metals. Thickness of this deposit will
vary with the size and design of the plastic part and with its intended use. For example,
15 mm (0.6 mil) usually will be sufficient for decorative knobs a frame on radios and
television sets and decorative parts of small appliances. A minimum 20 mm (0.8 mil)
may be necessary on larger appliance parts subject to some temperature variation and
on most interior automotive parts. Exterior automotive parts, which usually are fairly
large and which must withstand extreme temperature changes, require thicker deposit
with the average usually being 20 to 25 mm (0.8 to 1 mil). In all cases, greater thickness
may be applied in order to improve final surface finish.
Over the bright copper, any of the other plated finishes may be applied. However, the
most common finish is bright nickel and chromium, usually to meet a specific
specification. Most appliance and interior automotive specifications will call for only
bright nickel and chromium with exterior automotive specifications require a deposit of
sulfur-free, semi bright nickel, a full bright nickel, and a special micro cracked or micro
porous chromium. Table 1 shows the minimum plate thickness normally considered
advisable for various service conditions. Major manufacturers will specify thickness and
specific performance tests. These may include thermo cycling, adhesion tests, and
corrosion tests.
Table 1. ELECTRODEPOSIT THICKNESSES FOR PLATING OF PLASTICS
Minimum Thickness
Type of service
Copper
µm
Mild
indoor 15
exposure; warm dry
minimum wear
Indoor
exposure 15
with
moderate
temperature
and
moisture changes,
medium wear
Indoor or outdoor 15
exposure, periodic
wetting,
possible
major temperature
changes;
some
abrasive wear
Very severe outdoor 15
exposure such as
exterior automotive
Bright Nickel
mils
0.6
Semi-Bright
Nickel
µm
mils
0
0
µm
7.5
mils
0.3
Special*
Chromium
Nickel
µm
mils µm
mils
0
0
0.125 0.005
0.6
0
0
15
0.6
0
0
0.25
0.01
0.6
15
0.6
5
0.2
2.5
0.1
0.25
0.01
0.6
19
0.75
12.5
0.5
2.5
0.1
0.25
0.01
* Special deposit of non-metallic particles in nickel matrix. Used to produce micro porous
chromium structure. Special micro cracked chrome may be an acceptable substitute for some
applications.
Making Printed Circuits Boards
The simplest printed circuit begins with a plastic board that has copper foil glued onto its
surface. The circuit paths are made by printing onto the copper foil coatings that resist the
etchants used. These coating or resists are applied in the shape of the circuits that must
remain. Then the exposed copper is etched away and the circuit paths remain.
The resist is removed and you have a rudimentary printed circuit. Some circuits or portions of
them are plated with tin or tin lead to provide solder ability or with gold to provide contact
reliability. So this is another market for plating chemicals and equipment. Many small
electronic parts are also plated with tin or tin lead or gold or other metals, to provide various
electrical properties.
A printed circuit board consists of "etched conductors" attached to a sheet of insulator. The
conductive "etched conductors" are called "traces" or "tracks". The insulator is called the
“substrate”.
The vast majority of 'printed circuit boards' are made by adhering a layer of copper over the
entire substrate, sometimes on both sides, (creating a "blank PCB") then removing
unwanted copper by etching in an acid or ferric chloride solution, leaving only the desired
copper traces. A few PCBs are made by adding traces to the bare substrate usually by a
complex process of multiple electroplating. Some PCBs have trace layers inside the PCB
and are called multi layer PCBs. These are formed by bonding together separately etched
thin boards. After the circuit board has been manufactured, components are connected to
the traces by soldering (usually by passing their leads through holes pre-drilled in the
board)
There are three common methods used for the production of printed circuit boards:
1. Silk screen printing, using etch-resistant inks to protect the copper foil. Subsequent etching
removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank
(non-conductive) board. The latter technique is also used in the manufacture of hybrid
circuits.
2. Photoengraving, the use of a photo-mask and chemical etching to remove the copper foil
from the substrate. The photo-mask is usually prepared with a photo-plotter from data
produced by a technician using computer-aided PCB design software. Laser-printed
transparencies are sometimes employed for low-resolution photo-plots.
3. PCB Milling, the use of a 2 or 3 axis mechanical milling system to mill away the copper foil
from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates in a
similar way to a plotter, receiving commands from the host software that control the
position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper
is extracted from files generated in PCB design software and stored in HPGL or Gerber file
format.
PCBs are rugged, inexpensive, and can be highly reliable. They require much more layout
effort than either wire-wrapped or point-to-point constructed equipment.
Originally, every electronic component had wire leads, and the PCB had holes drilled for each
wire of each component. The components' leads were then passed through the holes and
soldered to the PCB trace.
This method of assembly is called through-hole construction. Soldering could be done
automatically by passing the board over a ripple, or wave, of molten solder in a wavesoldering machine. Through-hole mounting is still used.
However, the wires and holes are wasteful. It costs money to drill the holes, and the
protruding wires are merely cut off.
Most PCBs have alignment marks and holes (called fiducials) to align layers and permit the
PCB to be mounted in equipment that automatically places and solders components. Some
designs place alignment and etch test-patterns on break-off tabs that can be removed before
installation.
Layers may be connected together through drilled holes called vias. Either the holes are
electroplated or small rivets are inserted. High-density PCBs may have blind vias, which are
visible only on one surface, or buried vias, which are visible on neither, but these are
expensive to build and difficult or impossible to inspect after manufacture.
Chapter 13
<<Plating in Semiconductor Industry – Lead Free Plating>>
Tin-lead (SnPb) solder has been widely utilized for electrical connections because of its
convenience, economy, and electrical and mechanical characteristics. As a result of recent
environmental concerns regarding lead (Pb), the requirement for Pb-Free semiconductors has
been receiving increasing attention within the semiconductor industry. Countries around the
world continue to enact stricter bans on the content of hazardous materials in semiconductors.
Such legislation has prompted the semiconductor industry to develop environmentally friendly
products.
The semiconductor industry is under pressure to develop environment friendly interconnection
and packaging technologies.
Lead and lead alloys have had a long history of being components in solder and in connector
leads for a good reason: the lead/tin compound known as solder forms good electrical
connections with other materials such as copper and silver at the relatively low temperature of
183°C. The resulting joints are reliable and the process is cost effective.
So finding Pb-free replacements involves a combination of materials science and clever
process control. Process control is, in fact, the key. "It is getting tougher to make a reliable
joint," says Philips' van de Water. "The window for success (in the soldering process) is
getting smaller."
Semiconductor companies have four primary problems to solve when designing Pb-free
packages: solderability, reliability, whiskers, and moisture sensitivity. Not surprisingly, the
attributes are interrelated.
Solderability relates largely to the ability to melt the Pb-free solder at temperatures close to
those of lead-based solder and lead coatings. Compatibility with the equipment used in most
of today's wave-soldering assembly lines is an imperative if it to be kept in service with a
minimum of retrofitting. Reliability addresses the strength of the joint; and, moisture sensitivity
determines how long the component may be kept in storage before it is attached to a board.
Solderability
Conventional Pb-based soldering takes place in a range of 215° and 240°C for lead frame
devices. Due to higher melting temperatures, matte tin requires a range of 235° to 260°C and
this potentially has an impact on reliability. Tests have shown that tightly managing the
temperature profile during the soldering process provides an acceptable solution, says van
der Water. Other package types have similar results.
Reliability
The reliability of a solder joint can be compromised by changes in temperature while the chip
is in service and is known as thermo-mechanical solder fatigue. Joint failure follows a well
known process that begins with diffusion and re-crystallization. Crack initiation and growth
follow until the fracture can actually be observed.
Testing the reliability of solder joints has been conducted by cycling the product through a
range of temperatures from -40° to 125°C for 10,000 cycles. Solder fatigue failure is
visualized and analyzed using a technique called Weibull statistics. Many package types have
been tested. Reliability has been comparable to conventional connectors and solder pastes.
Whiskers
As previously mentioned, when pure tin is used for plating a lead frame, the growth of tin
filaments has been identified as a potential reliability problem. If these "whiskers" grow long
enough they can conceivably short circuit two pins. Irregular intermetallic growth at the
copper-tin interface causes stresses that extrude the tin whisker from the surface. Whisker
growth is not immediately apparent. It occurs during storage at ambient temperatures—not
during the soldering process—so countermeasures must be taken.
One approach is to make the tin layer thicker. This dramatically reduces the length of
whiskers because a thicker tin layer can absorb more stress. In test results reported by ST
Microelectronics, maximum whisker length decreased 160 microns to less than 10 by
increasing the thickness of the tin layer from 1.82 microns to 10 microns.
Another approach is to post-bake the component at 150°C for an hour. It was found that the
higher temperature created bulk diffusion in the material and regular intermetallics. No
whisker growth has been observed under these circumstances.
Still another approach involves chemically pre-treating the tin surface of the lead frame to
create a matte—as opposed to shiny—finish. A matte finish has proven to be less susceptible
to whisker formation that a shiny tin finish. There is no reason for choosing just one of these
solutions because they are, in fact, compatible.
Moisture Sensitivity
If any component is stored outside a dry pack for a significant amount of time moisture can
accumulate which will change its solderability and reliability characteristics. Pb-free
components are more inclined to be susceptible to moisture but using a different soldering
profile (245°C for packages greater than 350 mm² and 250°C for packages smaller than 350
mm²) helps alleviate the problem, says Freescale's Mike Thomas.
Every Pb-free SMD package will have to be re-qualified according to JEDEC standards,
however, and the Moisture Sensitivity (MSL) classification of some will drop, which means
more care will have to be taken when they are stored and used in board manufacturing
facilities.
For lead-free products, there are different types of solder pastes available and would work in
board reflow at 260° C and below:
Sn/Ag4.0/Cu.5
Sn/Ag2.5/Cu.8/Sb.5
Sn/Ag3.5
Sn/Cu.75
Sn/Bi3.0/Ag3.0
217° C
216° C
221° C
227° C
213° C
(240-255° C)
(225-240° C)
(245-255°C)
(250-260° C)
(225-244° C)
Chapter 14
<<Safety Hazards>>
CONTROLLING HAZARDS IN THE ELECTROPATING INDUSTRY
The Occupational Safety and Health Regulations 1996 set down specific requirements for
workplaces that use hazardous substances. These cover such things as:
•
•
•
•
•
•
Labeling of containers
Material Safety Data Sheets (MSDS)
Induction and safety training
Record keeping
Risk reduction; and
Health surveillance
The Regulations say employers, main contractors and self employed persons must:
•
•
•
Identify hazardous substances;
Assess the risk of injury or harm; and
Reduce the risk by:
1. Preventing exposure to the hazardous substance
2. Means other than personal protective equipment; and
3. Where 1 and 2 are not practicable, by the use of personal protective equipment.
The Act says employees must take reasonable care of their own safety and health and avoid
adversely affecting the safety and health of others. They must comply within reason with safety
instructions, use personal protective equipment provided and report hazards or injuries.
•
•
•
Manufacturers of hazardous substances must prepare a material safety data sheet
Suppliers of hazardous substances must ensure containers are adequately labeled.
They must provide a current MSDS to the workplace when first supplying a hazardous
substance, and thereafter when requested.
Designers, manufacturers, importers and suppliers must ensure, as far as practicable,
that people installing, maintaining or using their plant are not exposed to hazards.
RISKS:
Workers at electroplating workplaces may be exposed to hazardous substances.
These substances are mainly in the form of:
•
•
•
Fumes
Vapors or mists
Metal dusts
Other hazards in electroplating involve the use of:
•
•
Electricity
Mechanical plant
•
Manual handling
What are the health risks?
Workers exposed to electroplating chemicals can develop:
•
•
•
Short term throat, lung, sinus, skin and eye irritation and burns
Long term health problems such as asthma, heart, lung and nerve disorders and
Cancer
The risk of developing health effects depends on how much chemical is absorbed into the body.
In addition, electrolysis releases hydrogen bubbles which, unless safely contained or ventilated,
can:
•
•
Become explosive
Carry other chemicals in a toxic mist
What are the hazardous substances?
Hazardous substances in electroplating include:
•
•
•
•
•
•
•
Solvents such as methylene chloride, phenol, cresylic acid (a chemical similar to
phenol)
Gases such as hydrogen cyanide
Acids such as chromic acid, sulphuric acid and hydrochloric acid
Alkalis such as sodium hydroxide ( also known as caustic soda)
Cyanides such as sodium and potassium cyanide
Heavy metals such as nickel, chromium, cadmium and lead
Toxic wastes
These substances are commonly used or produced in the:
•
•
•
Preparation
Coating
Polishing of metal items.
CONTROLLING HAZARDS IN THE ELECTROPATING INDUSTRY
When can chemical exposure occur?
People working in electroplating industry can be harmed when:
•
•
•
•
•
•
Containers leak or spill during transport, storage, decanting or disposal
Explosive or toxic gas or fumes build up during storage in confined areas
Operators are splashed by items entering or leaving plating tanks
Excessive bubbling or fuming occurs in acids, caustic or other chemicals
Dust is breathed in during buffing or grinding of plated items
Excessive hydrogen or oxygen is emitted during electrolysis or anodizing, causing an
explosive or flammable atmosphere
•
•
•
•
•
•
Local exhaust ventilation fails, or is inadequate to handle escaping gases, fumes and
mists
Overhead gantry cranes, hooks or slings fail when lowering or lifting items from dip tanks
Residue liquid and sludge is removed from dip tanks
Maintenance and repair work is done to tanks
Chemical wastes are disposed of in sewers before being properly neutralized
Chemical wastes are disposed of at tipping sites without proper authority approvals
How can hazardous substances enter the body?
Hazardous substance can enter the body through:
•
•
•
The skin or eyes, following contact with liquids or droplets
The lungs and nasal passages, when fumes, droplets, gases or dusts are inhaled
The mouth, when eating or smoking with contaminated hands
How can hazards be identified?
Workplace hazards can be identified through:
•
•
•
•
•
Checking packaging or container labels and material safety data sheets
Regular communication between workers, supervisors and employers about likely
hazards
Regular inspection of workplaces, plant and equipment
Regular review of tasks and procedures
Checking of previous incident and injury records for recurring situations.
How can risk be assessed?
General hazards:
The risk of injury or harm from general workplace hazards can be assessed by:
•
•
•
•
Assessing the likelihood of the hazard causing injury or harm, e.g.. very likely or
remotely possible
Assessing the likely severity of injury or harm, e.g.. serious or minor injury
Checking records of previous incidents and injuries where hazards have caused injury or
harm
Checking plant and equipment to make sure hazards are properly controlled
Hazardous substances:
In addition, the risk of injury or harm from hazardous substances can be assessed by:
•
•
Obtaining information about the hazards
Checking work processes to make sure hazards are adequately controlled;
•
•
Conducting atmospheric monitoring to determine levels of exposure to chemicals such
as chromic acid
Conducting health surveillance to detect any adverse health effects from chemicals at an
early stage.
How can risk be reduced?
Risk can be reduced by using control methods, in the following order of priority:
•
•
•
•
•
•
Eliminate or remove the hazard – e.g. do not use a chemical or item in the plant if it is
not required.
Substitute or replace it with safer plant, equipment or substance.
Isolate it from workers – e.g. enclosed systems for chemicals, relocation of employees or
physical barriers.
Introduce engineering controls – e.g. guarding or exhaust ventilation.
Administrative controls – e.g. limiting workers’ time spent near the hazard.
Personal protective equipment – e.g. safety goggles and respirators. While essential for
some work procedures, these should be last in the list of priorities.
What information and training is required?
•
•
•
•
•
•
All workers must be informed of hazards from exposure to harmful substances.
They must be given information, instruction, training and supervision in safe procedures,
including personal protective equipment.
Workers should know how to identify hazards, and to report them to a supervisor.
Training on hazardous substances must include potential health effects of the
substances used, control measures, correct use of protective equipment and the need
for and details of health surveillance.
Workers from non-English speaking backgrounds may have special needs and should
be provided with information in their first language.
Training should be ongoing, with regular revision of safe procedures.
Controlling plating tank hazards
•
•
•
•
•
•
•
Substitute hazardous substances with less hazardous ones.
Where possible, pump chemicals into plating tanks rather than pouring manually from
containers.
Pumps need to be cleaned before use with a different chemical.
Use local exhaust ventilation along one or more sides of the tank to remove mists and
vapors.
Use a suppressant to minimize the amount of mist generated during electro plating.
Minimize risk of items accidentally dropping into tanks, splashing operators.
Ensure overhead cranes, hooks and slings are regularly maintained.
Controlling cyanide hazards
•
Acids and cyanides are an explosive combination, and should be clearly labeled and
stored in locked, dry places, well away from each other.
•
•
Articles treated in acid baths should be thoroughly rinsed with water before being placed
in plating tanks.
Drainage should be designed so there is separation of acid spillage from cyanide
spillage or effluent.
Buffing, grinding and polishing
•
•
•
•
•
Newly electroplated surfaces on heavy machinery parts are usually finished with
portable or fixed grinding machines.
Finer finishes on personal, hobby or household items are achieved with buffing and
polishing wheels, containing various polishes and waxes.
These processes generate large amounts of metal dusts, some of which are hazardous
if inhaled.
Local exhaust ventilation should be fitted to grinding and buffing machines to remove
dust as it is generated.
Where substances that are known to be carcinogenic are used, exposure levels should
be kept as low as possible
Chapter 15
<<Miscellaneous Topics>>
Treating Metal Plating Effluents
The field of electroplating or metal finishing today is moving away from large manufacturing
operations and into smaller job shops. With increasingly strict regulations governing metals
in wastewater and increasing costs for disposal of metal-contaminated waste, many metal
finishers find that implementing pollution prevention measures such as filtering or treating
process water to reduce or eliminate metal contamination and allow water reuse is the best
option, both environmentally and economically. Plating processes in this sector include
chrome, bright and electroless nickel, zinc, copper, tin, conversion coatings, and more.
Because metal plating has many significant environmental aspects and is a highly polluting
industry, there are many resources available to help the metal plating industry improve its
environmental performance. Trade associations have websites with technical guidance,
suppliers, and discussion forums where questions can be asked and answered, all for free.
There are several major programs focused specifically on helping the sector with pollution
prevention. Researchers looking for cost-effective ideas for improvement should examine
the sites dedicated to environmental improvement, and the trade association sites.
The tables below list low-cost or no-cost solutions to reduce waste and pollution in any
metal plating company, including ones in developing countries. All of these ideas have
been proven to help small companies, anywhere in the world, save money while protecting
the environment.
Top Low-Cost Solutions to Increase Efficiency and Reduce Waste in Metal Plating Operations
Install spill
containment
Spills can be contained and managed to prevent losses of valuable resources.
Allows more chemical to drip back to process tank, so reduces the amount of
chemical introduced in rinse water.
Post dragout times on signs at tanks to remind employees. A drain time of at
Establish dragout timing least 10 seconds has been demonstrated to reduce dragout by 40+%,
compared to the three-second industry average.
Install drain boards or Boards and guards minimize spillage between tanks and are sloped away from
drip guards
rinse tanks so dragout fluids drain back to plating tanks.
These sensors indicate the cleanliness of the rinsed water. They cost only a
Use conductivity
few hundred dollars to order, and can greatly improve plating quality and
sensors
prevent unnecessary dumping of rinsed water.
Agitation promotes better rinsing. Agitate water or part manually or with
Agitate rinse bath
mechanical means (stirring or air bubbling).
Reuse spent
Spent acid can be used to neutralize an alkaline waste stream. Spent alkali
acid/alkaline
can be used to neutralize an acid waste stream.
Concentrate rinsed water and captured dragout liquids for reuse; the water
Evaporation
condensate can also be reused. Mechanical evaporators or simple boilers can
be used.
Evaporates bath water so relatively clean waste rinsed water can be reused as
Increase bath
bath makeup water. Reduces solution viscosity so more chemical drains back
temperature
to process tank during dragout. Do Not Use On Cyanide or Hexavalent
Chromium Baths.
Run tests with successively lower bath concentrations to find the minimum
level needed to achieve quality. This saves money by reducing overuse of
Optimize bath
chemicals and reduces contamination in wastewater. Remember that
concentrations
concentrations recommended by vendors are usually higher than the minimum
needed for quality.
Lengthen dragout time
How to restore old chrome plated parts
When chrome plated finishes become scratched or marred there really isn't much you can do
with them. Beyond cleaning it with metal polish and keeping it waxed, there isn't much you
can do yourself. The items can certainly be re-chromed by a plating shop. Some people use
chrome polish to help shine the surface, but if the surface is scratched, it will eventually
corrode.
Your best bet is to take them to a custom plating shop in your area. They will strip the chrome
and hopefully any underlying nickel or copper. Conversely, some shops will only strip the
chrome and polish the nickel and reapply another layer of nickel, perhaps acid copper,
perhaps copper buff and more nickel and chrome. As you can see, the process can be quite
extensive and therefore, rather expensive.
Most people assume we just "dip it in a vat" and the part miraculously emerges with the shiny,
reflective surface we all associate with nickel/chrome. Please be aware that there is a
tremendous amount of manual labor involved in stripping the parts, buffing them, and replating them, so you should expect that you will pay considerably more than you probably
expected.
What are organic additives
Organic additives (carriers, brighteners, levelers) work to increase the current density or plating
rate that can be maintained with satisfactory throwing power.
The additives fall into three main categories:
• Carriers
• Brighteners
• Levelers
Carriers increase the polarization resistance and are current suppressors. The suppression is a
result of the carrier being adsorbed to the surface of the cathode; this results in increasing the
effective thickness of the diffusion layer. The result is better organization. This gives rise to a
deposit with a tighter grain structure. The carrier modified diffusion layer also improves plating
distribution without burning the deposit.
The brightener is a grain refiner. Its random adsorption may produce a film that will suppress
crystallographic differences. Alternatively, brighteners may be adsorbed preferentially on
particular active sites such as lattice kinks, growth steps, or tops of cones, or surface projections
in general; growth at these locations is then blocked.
Levelers or leveling agents are inhibitors present at low concentrations in the electrolyte as
compared to the depositing metal. In case of a micro profile, the diffusion layer does not follow
the profile contour, but is maximum at the valleys and minimum at the peaks. Consequently, in
absence of a leveling agent, depositing ions diffuse more rapidly to the peaks than to the
valleys, and deposits grow more rapidly on the peaks, resulting in an exaggerated profile.
With good solution agitation, the leveler will accumulate more rapidly and readily at the peaks
and it will inhibit growth or deposition. The valleys will allow faster deposit growth and allow the
valleys to catch up to the peak, thus creating leveling.
CHAPTER 16
<<PLATING BATH COMPOSITIONS AND OPERATING CONDITIONS>>
The purpose of this chapter is to present in brief information on plating baths, for the most part
commonly used in production, essential for intelligent handling of the engineering problems that
may arise in connection with their installation and operation.
The three variables of temperature, current density, and agitation are interrelated in their effect
on a plating operation. It is customary to control all three closely by standardization and
instrumentation. Increasing temperature and agitation usually enables one to obtain higher
current efficiencies and to use higher current densities. It is good practice to operate at the
higher stvalues of temperature and current density consistent with the limitation imposed by the
quality of deposit required, or by the equipment and materials of construction. One thus
obtains the maximum production from the available facilities.
The operating temperatures in the tables represent the best average, if a single value is given or
the usual temperature range, if two values are given. The operating temperature is an important
consideration in the selection of suitable linings and protective coating. It also determines what
provision must be made for heating or cooling.
Single values of current density in the tables are good average values for the conditions of
temperature and agitation shown. A practical operating range of current density is indicated by
a high and low value. It is well to allow for somewhat higher values when estimating current
requirements.
The various types of anodes to be used with the different plating baths are mentioned in the
tables. Soluble and insoluble anodes of various types and compositions are discussed in
Chapter 29. Most modern plating processes and specifications require the use of high purity
soluble anodes. The ration anode-to cathode area is also important in determining the anode
current density. Too high an anode current density may cause objectionable polarization when
using soluble anodes with resultant loss in anode efficiency, formation of solid particles, or
sludge rough deposits and adverse changes in the composition of the plating bath.
Key No.
1
Material
Steel, low carbon
2
3
4
5
6
7
8
Cast Iron
Stainless Steel
High silicon cast iron (‘Duriron’)
Lead usually 6% antimony alloy
Copper
Nickel
Carbon (“Karbate”)
9
10
Glass (“Pyrex” or tempered)
Chemical stoneware
11
12
13
14
“Haveg”
Hard rubberb
Rubber (approved compositions) b
Plastics (approved compositions) b
15
16
Acid resisting bricks
Wood
Uses
Tanks, filters, pumps, pipes, fittings,
heating coils
Pumps, filters, valves, fittings
Tanks, pumps, filters
Pumps, pipe, fittings, heat exchanges
Linings, pipe
Heating coils
Heating coils
Heaters and heat exchangers primarily,
pumps, air diffusers
Heat exchangers, tanks, pumps
Tanks, tower concentrators and tower
packing
Tanks, pipe
Pipe, fitting, pumps
Linings, hose
Linings, hose, pipe, fittings, barrels,
heating coils and exchangers
Linings
Tanks
EFFECT OF HYDROGEN
Embrittlement of metals by hydrogen has been recognized for many years but, as two authors
have pointed out124 some other defects in electrodeposits, such as blistering, cracking, gas pits,
peeling poor adhesion may also be related to hydrogen in ways not yet identified. Their
conclusions, quoted below, have added greatly to our understanding of these matters.
“1. Many electroplating processes favor the absorption of large quantities of hydrogen by
metals. Embrittlement of the metals may be due to hydride formation, to strain imposed by
occlusion of molecular hydrogen in submicroscopic rifts and to deposition of hydrated ionsphenomena that are already recognized by electroplaters.
“2. Less recognized, perhaps, are the functions of hydrogen occluded within the steel. The
extremely low solubility of steel for hydrogen at ordinary temperature, combined with the
extraordinary ability of this metal to absorb huge quantities of gas when presented atomically at
the surface during pickling and cathodic electrolysis, causes important effusion of this gas from
the steel when the atomic layer is removed by the presence of an applied coating or by
cessation of the hydrogen-producing process.
“3. When the effusion occurs beneath the coating of any material, including metals, whose
permeability is unsuited to the quantity of the effusion, the coating may be (1) lifted from the
steel or ruptured; or both, by pressure of the accumulating gas, or (2) blistered, depending upon
the qualities of the coating.
“4. If the plated ware is heated, the effusion of hydrogen is accelerated and the coating is
weakened, so that the occurrence of the above defects is favored.
“5. Effusion of the gas from the steel base during electroplating leads to the formation of
the gas pits in the coating.
“6. Cathodic cleaning in either acid or alkaline solutions provides large quantities of
hydrogen for absorption. “
A true bronze consisting of a plated copper tin alloys similarly used as a stop off for nitriding.
These coatings can be removed by one of the proprietary strippers listed above or by one of the
following methods.
EXAMPLE
1. Treat anodically in a solution of 12.2 oz/gal sodium cyanide and 2.7 oz/gal sodium
hydroxide at room temperature using 6 volts.
2. Immerse in solution of 13.4 oz/gal sodium hydroxide, 2.7 oz/gal sodium cyanide and 2.7
oz/gal sodium chlorite at a temperature of 80-90 C (175-195 F). Note that sodium
chlorite is a hazardous chemical and instructions should be followed implicitly.
EXAMPLE
1. Anodically treating at 20 to 40 asf and 82 C (180 F) in a solution consisting of sodium
hydroxide 13 oz/gal, sodium metasilicate 10 oz/gal and Rochelle salts 6.7 oz/gal.
2. Anodically treating at 20 to 200 asf and 20 to82 C (70-180 F) in a sodium nitrate
solution of 67 iz /gal at a pH of 6 to 10.
3. Immersion in a solution of acetic acid 10 to 85% by volume and hydrogen peroxide (100
vol) at 5% by volume.
EXAMPLE
1. Reverse current in a sodium cyanide solution (4 to 8 oz/gal) at room temperature and an
anode current density of 10 to 20asf.
2. Same as (1) but at high pH. I.e. with an addition of 4 to8 oz/gal of sodium hydroxide to
the stripping bath.
3. Immersion in an acid mixture consisting of 95 and 5% by volume of concentrated sulfuric
and nitric acids, respectively, and operated at about 80 C (175 F) This bath should be
covered when not in use to prevent dilution by moisture from the air parts should be dry
when immersed, otherwise the basis metal attack will be excessive.
Well, that’s about it! If you’ve read this far than you can now call yourself a metal plating expert. You
should now have a solid grasp of exactly what is entailed in the metal or electroplating process, across a huge
range of possible metal plating applications and situations.
We hope that you have enjoyed reading the “Metal Plating Bible”, and most of all, we hope you can now feel
confident doing your own metal plating, in your own specific plating situation. Metal plating of course, is a
technical and oftentimes advanced topic by nature.
This guide has been an extensive attempt to take what was normally an activity that only chemistry experts
could partake in, and give beginners and other not so knowledgeable people a step-by-step guide that they
could learn from to form their own metal plating action plan.
On that note, I would like to wish you (my treasured customer) a pleasant evening, and the best of success in
your future metal plating activities! And remember, I’m always an email a way if ever need assistance.
Warm Regards,
Craig Bellinger
Owner and Co-Author
GLOSSARY
Anion- A negatively charged ion or radical which is attracted to the anode because of the
negative charge.
Anode -The positively charged electrode at which oxidation or corrosion of some component
occurs (opposite of cathode). Electrons flow away from the anode in the external circuit.
Barrel plating (or cleaning) - Plating or cleaning in which the work is processed in bulk in a
rotating container.
Brass - An alloy consisting mainly of copper (over 50%) and zinc, to which smaller amounts
of other elements may be added.
Brightener - An agent or combination of agents added to an electroplating bath to produce a
smooth, lustrous deposit.
Cathode -The negatively charged electrode of an electrolytic cell at which reduction occurs.
Cation - A positively charged ion that migrates through the electrolyte toward the cathode under
the influence of a potential gradient. See also anion and ion.
Cell - Electrochemical system consisting of an anode and a cathode immersed in an electrolyte.
The anode and cathode may be separate metals or dissimilar areas on the same metal. The cell
includes the external circuit, which permits the flow of electrons from the anode toward the
cathode. See also electrochemical cell.
Chromate coating (chromating) - A corrosion protection technique; can be applied to steel,
aluminum, magnesium, and zinc. It results in the formation of metal oxide on the surface of the
work piece which reacts to form metallic chromates. Chromating of aluminum and magnesium
improves corrosion resistance considerably. With steel it is much less permanent.
Conversion coating - A coating produced by chemical or electro-chemical treatment of a
metallic surface that provides a superficial layer containing a compound of the metal; for
example, chromate coatings on zinc and cadmium or oxide coatings on steel.
Corrosion - The chemical or electrochemical reaction between a material, usually a metal, and
its environment that produces a deterioration of the material and its properties.
Electrochemical Cell - An electrochemical system consisting of an anode and a cathode in
metallic contact and immersed in an electrolyte. The anode and cathode may be different
metals or dissimilar areas on the same metal surface.
Electro-deposition - The deposition of a substance on an electrode by passing electric current
through an electrolyte.
Electroless plating - A process in which metal ions in a dilute aqueous solution are plated out
on a substrate by means of autocatalytic chemical reduction. Electroless plating uses a redox
reaction to deposit metal on an object without the passage of an electric current.
Electrolysis - Production of chemical changes of the electrolyte by the passage of current
through an electrochemical cell.
Electrolyte - (1) A chemical substance or mixture, usually liquid, containing ions that migrate in
an electric field. (2) A chemical compound or mixture of compounds which when molten or in
solution will conduct an electric current.
Electrolytic cells - An assembly, consisting of a vessel, electrodes, and an electrolyte, in which
electrolysis can be carried out.
Electro-cleaning - An electrochemical cleaning process by which a metal is first made the
cathode in an electrolytic cell. When current is applied, electrolysis of water occurs at the
surface of the metal. This results in generation of Hydrogen gas. This gas creates a highly
efficient scrubbing action. Following initial treatment as a cathode the circuit is reversed so that
the metal is the anode. Oxygen gas, which is generated at the surface, produces a final
cleaning action.
Electrolytic etch - A technique generally applied to steels which attack the surface to produce
a clean, oxide free material. It is often used prior to electroplating, especially chromium plating.
Since it preferentially attacks edges it will open us small cracks in the surface of the metal. Due
to this, this process can be used to inspect finishes for flaws.
Etching - Etching is sometimes used a surface preparation technique prior to electroplating or
for removal of metal such as in the printed circuit industry where material not required on the
finished product is removed by a chemical solution. It can also be used as an inspection
technique due to its ability to accentuate surface cracks and defects.
HCD - High Current Density - High amperes per surface area
Indicator (pH) - A substance that changes color when the pH of the medium is changed; in the
case of most useful indicators, the pH range within which the color changes is narrow.
Leveling - Electrodeposited materials tend to be concentrated at sharp corners, peaks, and
ridges, therefore, when a metal with a rough surface is electroplated, the rate of deposition will
be faster on convex irregularities resulting in an accentuation of the item's original roughness.
To counteract this effect, additives are added to the electrolyte solution to produce a polarization
effect concentrated at the peaks and ridges. This polarization effect lowers the current density at
the peaks and reduces deposition rates. The net result is to smooth or "level" the surface of the
metal.
Reducing agent - A compound that causes reduction, thereby itself becomes oxidized.
Sensitizing - A relatively non-specific term used to cover a range of metal finishing processes
that improve the treatment ability of a metal for subsequent processes. It often refers specifically
to a part of electroless plating procedure on plastics or non-metal surfaces. After the surface
has been etched it is reacted with solution that deposits a very thin film of a metal or metallic
compound. The surface is then referred to as sensitized.
Substrate - Surface material or electroplate upon which a subsequent electro-deposit or finish
is made.
Solder plating - The term covers deposition of an alloy of 60% tin and 40% lead that is widely
used in the electrical and electronics industries. It provides two valuable features, corrosion
resistance and "solderability".
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