-------
_TECHMETALS, INC.
P.O. Box 665 • Dayton, Ohio 45401
-~~._--------
• 513/253-1250
ELECTROLESS NICKEL WITH AHARD CHROME OVERLAY
Electroless nickel and hard chrome are two competing deposits
used extensively for engineering purposes. Each deposit has its
own distinct properties and characteristics. When they are combined, they form a highly functional coating.
In many areas of application, hard chrome can be replaced by
electroless nickel. In some specific applications, it has even
proven to be an improvement over hard chrome. Due to the fact
that both electro less nickel and hard chrome are normally used
for functional (rather than decorative) applications, one is able
to gain many advantages. This can be seen by the comparisons in
Table 1.
STRUCTURE
Electroless nickel is an alloy of nickel and phosphorus which as
deposited forms a metallic glass coating. Unlike hard chrome and
other electroplates, electroless nickel coatings are completely
amphorous, and have no crystalline structure containing internal
segregation or separate phases. It is the lack of this structure
which produces the unusual properties and makes it well-suited
for protection against corrosion, erosion, and wear.
The internal stress of high phosphorous electroless nickel
deposits is very low on most substrates due to their high phosphorous content, purity, and homogeneity. Low phosphorous
electro less nickel and those which contain large amounts of codeposited contaminants (cadmium, lead, sulfur) will have high
stress and tend to be very porous which leads to a poor base coating underneath the hard chrome.
Hard chrome deposits are formed by electrodepositing a chromic
acid solution onto a base material. The coating itself may
exhibit different characteristics due to the complex forms of
acids used in the baths. The electrodeposition of chromium
differs from other plating processes not only with respect to the
bath, but also in the nature of the coating produced. In the
I~A famll, 01 Precision Metal finishers
I
thickness used in this type of deposit, the coating is normally
0.5 mil or less, forming a highly porous coating. This is the
reason why the electroless nickel base is very important to
corrosion resistance. The hard chrome deposits structure tends
to give it greater wear resistance and galling resistance due to
its crystalline structure.
Crack structure of
(above x 500) chromium
as-deposited and (left
x 500) chromi urn
electropolished to high
fi n l sh ,
The internal stress in the form of a compression stress will form
in the base material with this type of coating originating from
hydrogen absorption from the electroless nickel deposit. As
mentioned before, this stress will be very minimal due to the
electroless nickel's quality. These stresses are transferred to
the chromium coating where they overcompensate the tensile stress.
The compressed stresses which occur are determined by the effect
of the basis metal; they gradually fall over time, at the thickness of this type of application 0.5 mil thick or under. The
compression stress in this deposit is easily reduced by a heat
treatment after coating at 375°F for 2 to 3 hours.
UNIFORMITY
Electroless nickel coatings are applied without an electrical
current, but instead by a autocatalytic chemical reduction. By
this process, one is able to uniformly coat all surfaces it wets.
With this coating used as the initial coating of the basis metal,
it will evenly coat grooves, slots, blend holes, and even thread
areas. Because of this ability of uniformity, the base coat is
-------------
used as the one to increase or decrease a part's dimensions the
most in this type of coating. The thickness of the deposit may
be controlled up to 0.1 mil. This unique quality normally will
decrease the cost of plating if a complex part requires the
coating.
Hard chrome coatings, on the other hand, are applied using
electrolysis. This requires the use of a cathode (part being
plated) and an anode. It is this relationship between the part
and the anode which causes the variance in coating thickness.
These coatings tend to build up on edges, corners, and any other
sharp protrusions. Thus, by limiting this thickness in this
part of the coating to 0.5 mil thick, we are able to control
size with the coating fairly evenly.
HARDNESS AND WEAR RESISTANCE
Normally when dealing with chromium deposits the hardness and wear
resistance is directly related to the thickness of the chromium
deposit. But due to the fact that the chromium deposit in this
coating is very thin, 0.5 mil or less, the base coating of
electroless nickel is very critical to the hardness and wear
characteristics.
Electroless nickel as plated has a 48 Rockwell C hardness, and
is a very good support material for a hard chrome coating. When
parts are subject to heavy loads or wear, the outer coating may
break down due to a failure of either the base material or if the
coating between the top coating and bottom is of a softer hardness. As such, plating of this coating on soft base materials has
been found to be very advantageous on aluminum, beryllium, and
other soft alloys.
This special structure of the hard chrome deposit is responsible
for its hardness and resistance to wear. In this application the
hardness of deposit is further added by the very thin, yet very
hard and dense network of cracks in the deposit. Comparisons of
hard chrome and electroless nickel can be seen in the figure
below and further related in Table #2.
... ...
Tnt r.mperltur" "F
zoo
,zoo
Effect of temperature
on the elevated
temperature hardness
of a 10 percent
phosphorus electroless
nickel.
1000
--
400
,~
....
......
%
J
-,
As d'POsitMi
--....., .....
0-
•00
200
100
200
'400
,,400 °c (750 oF!
'00
600
1200
:<:,, . .,
>
t
1000
300
'"
""
4110
TlSt tlmperalure,
-<Chromium
......
~
500
"c
......
......
'.
'"--=:
'00
700
'00
SUPERIOR CORROSION RESISTANCE
One of the most beneficial differences between the electroless
nickel coating and the hard chrome is that they both are able to
offer some corrosion resistance. Both coatings are capable of
protecting the base material by sealing it off from the
environment.
Because of the limited amount of chrome being deposited, its
corrosion resistance is very limited due to the cracks in the
deposit and the thin amount of plating. 0.5 mil or less.
The electroless nickel, on the other had, is very similar to high
alloy nickels such as stainless steel. Electroless nickel coatings are resistant to alkalies, salt solutions, many acids, and
all types or organic media. Refer to Table #3 at the end of this
section for further comparisons.
APPLICATIONS KNOWN OF TODAY
Aircraft
(pistons, hatch locks, brakes and lubrication
components)
Increased life, added corrosion resistance.
Good wear at high temperatures.
Food Industry
(flatware, gears, bacon presses, shafts, and
packaging equipment)
Corrosion resistance on complex shapes and
increased wear.
Hardware
(marine use)
Increased life and wear over conventiOnal
hard chrome and electrodeposited nickel.
Hydraulics
(rams and shafts used in steel mills, around
salt and other chemicals)
Gained in corrosion resistance over
conventional hard chrome.
Oil Field Equipment
(ball valves, shafts)
Added wear and corrosion. Protection of
nickel from weld splatter.
Molds
(plastic and rubber)
Added corrosion resistance, infinite throw
into baffles, ports, and molding surfaces.
In electroless nickel plating of rubber
molds it is particularly important to hard
chrome the molding surfaces as many types
of rubbers stick to electroless nickel.
Stainless Steel
(replacement with steel)
Electroless nickel applied in place of
stainless requiring hard chrome on specific
areas.
CONCLUSION
Electroless nickel with a hard chrome overlay has many unique
properties which make it a superior engineering coating. The
coating offers high strength, superior abrasion and wear resistance, superior corrosion resistance, and a complete and uniform
coating. This coating has proven to be useful in improving life
of parts and reducing machining costs as well as material costs.
Its list of applications is ever expanding and its potential has
only began to be realized.
IABLE 1
COMPARISON OF ELECTROLESS NICKEL AND COMMERCIAL HARD CHROMIUM COATINGS
PROPERTY
MATERIAL
STRUCTURE
ELECTROLESS NICKEL
COMMERCIAL HARD CHROMS,
Alloy of 10 to 11 percent
dissolved in nickel.
Chromium plus trace amounts
of oxides and hydrogen.
Amorphous; no phase structure,
Crystalline; fine grained
lamination or segregation.
with numerous cracks.
INTERNAL STRESS
ON STEEL, MPa
<7
200-300
DENSITY, g/cm 3
7.75
6.90-7.18
890
1610
ELECTRICAL
RESISTIVITY,
I1rl-cm
90
14-66
THERMAL
CONDUCTIVITY,
W/cm-oK
0.08
0.67
MAGNETIC COERCITY
Non-magnetic
Non-magnetic
TENSILE STRENGTH,
MPa
>700
<200
DUCTILITY, %
ELONGATION
1 to
MODULAS OF
ELASTICITY, GPa
200
100-200
COEFFICIENT OF
THERMAL
EXPANSION,
o
l1/m/ C
12
6
ADHESION STRENGTH,
MPa
300-400
Good
HARDNESS, VHNIOO
480 to 500, as deposited;
heat treatable to 1100
800 to 1000
COEFFICIENT OF
FRICTION VS STEEL
(LUBRICATED)
0.13
0.16
TABER WEAR,
RESISTANCE,
mg/lOOO cycles
15 to 20, as deposited;
2 to 9 after heat treatment
2 to 3
CORROSION RESISTANCE
Excellent resistance to attack
by all but the most severely
Poor due to cracks;
resists oxidizing
environments; attacked
MELTING POINT,
°c
1~
oxidizing environments.
«0.1
by halogens and reducing
solutions.
-------------------_._--------
IDLE 2
COMPARISION OF THE TABER ABRASER RESISTANCE
OF DIFFERENT ENGINEERING COATINGS
COATING
HEAT TREATMENT
TWI,
MG/1000
Watts Nickel
None
25
E1ectro1ess Nickel
None
17
E1ectro1ess Nickel
300
E1ectro1ess Nickel
500
E1ectro1ess Nickel
650
Hard Chromium
(1)
0C/l
0C/l
0C/l
hr
10
hr
6
hr
4
None
CYCLES (1)
2
Taber Wear Index, CS-I0 abraser wheels, 1000 gram load, determined as average
weight loss per 1000 cycles for total test of 6000 cycles.
TABLE 3
COMPARISON OF THE CORROSION BEHAVIOR
OF CHROMIUM AND ELECTROLESS NICKEL
IN DIFFERENT ENVIRONMENTS
ENVIRONMENT
CORROSION RATE,
llm/y
ELECTROLESS
NICK~~
TEMPERATURE
10% Acetic acid
nil
10% Citric acid
nil
660
19
100,000
Conc. Hydrochloric acid
25
46
25,000
30
nil
19
51
17
10% Nitric acid
nil
44
Conc. Nitric acid
nil
>25,000
10% Hydrofluoric acid
10% Lactic acid
10% Malic acid
10% Phosphoric acid
25
5
16
10% Sulfuric acid
280
12
Conc. Sulfuric acid
760
25
10% Sodium hydroxide
nil
nil
10% Ammonium chloride
nil
nil
10% Cupric chloride
380
25
10% Cupric nitrate
51
12
10% Ferric chloride
nil
780
10% Sodium chloride
nil
0.5
(1)
Commercial hard chromium deposit; corrosion rates less than
25 um/y reported as nil.
(2)
Cast chromium metal.
(3)
Electroless nickel containing
0.05% other elements.
10~%
phosphorus and less than