Ministry of Higher Education
&
Scientific Research
University of Technology
Materials Engineering Department
Branch:
Class:
Lab: Corrosion and Protection
Prepared by
Assistant Prof. Dr. Rana Afif Majed
2014-2015
Materials Eng. Department
Lab.: Corrosion and Protection
Contents
Exp. No. Description
Page No.
1
Potential of Electrode
2
Electrochemical Measurements of Corrosion
3
Corrosion Measurement by Weight Loss
4
Pitting Corrosion
5
Study
of
High-temperature
Corrosion
(Oxidation)
6
7
Corrosion Protection by Inhibitors
Corrosion Protection by Metallic Coating
(electroplating)
8
Corrosion Protection by Metallic Coating
(flame spray)
2
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (1)
Potential of Electrode
The purpose of experiment:
Measurement the potential of metals and alloys.
Introduction:
If a metal is immersed in a solution of its own ions, such as zinc in
ZnSO4 solution, or copper in CuSO4 solution, the potential obtained is called
the reversible potential (Erev). If, on the other hand, the substances taking
part in the process are in their standard states, such that the activities of the
metallic ions are equal to unity or gases are at 1 bar pressure, the potentials
obtained are called ‘Standard Electrode Potentials’. A standard potential
refers to the potential of pure metal measured with reference to a hydrogen
reference electrode.
Standard potential differences are the actual cell potential differences
measured in reversible cells under standard conditions. Tables of standard
electrode potentials can be obtained if any one electrode, operated under
standard conditions, is designated as the standard electrode or standard
reference electrode with which all other electrodes will be compared. This
electrode is called the standard hydrogen electrode, abbreviated SHE. The
potential difference across a reversible cell made up of any electrode and a
SHE is called the reversible potential of that electrode, E. If this other
electrode is also being operated under standard conditions of pressure and
concentration, the reversible potential difference across the cell is the
standard electrode potential Eo of that electrode.
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Materials Eng. Department
Lab.: Corrosion and Protection
In many practical potential measurements, the standard hydrogen
electrode cannot be used because hydrogen reacts with other substances in
the cell or because other substances in the cell react with the catalytic
platinum electrode surface upon which the H+/H2 potential is established. It
is often much more convenient to use alternative electrodes whose potentials
are precisely known with respect to the SHE. The effect of changing the
reference electrode is to change the zero of a potential scale while leaving
the relative positions of all of the potentials unchanged.
Procedure:
Absolute single electrode potential is a characteristic property of a
metal. All metals have characteristic electrode potentials. Absolute single
electrode potentials cannot be measured directly. They can be measured with
respect to a standard electrode such as standard hydrogen electrode (SHE).
The standard hydrogen electrode is a widely used standard electrode. It is
arbitrarily assumed to have zero potential at all temperatures by definition.
The most acceptable method of obtaining standard electrode potentials
is by comparing the electrode potential of metals with the standard hydrogen
electrode. Since the SHE has zero electrode potential at all temperatures by
definition, the electrode potential of a metal is numerically equal to the emf
of the cell formed by SHE and the metal electrode. In other words, the emf
of the cell represents the electrode potential of the half cell formed by the
metal with respect to the standard hydrogen electrode. In such a cell,
reaction on the hydrogen electrode is oxidation and reaction on the other
electrode is reduction. Such a cell can be expressed as:
H2 → 2H+ + 2e
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Materials Eng. Department
Lab.: Corrosion and Protection
Also the potential of the metal electrode (working electrode) is
measured with respect to a standard Calomel electrode, which is nonpolarizable. The reference electrode is kept in a separate container and it is
connected electrically with the working electrode placed in a container in
contact with the electrolyte via a salt bridge. A high impedance voltmeter is
connected between the working electrode and the reference electrode. The
negative terminal of the voltmeter is connected to the working electrode and
the positive to the reference electrode.
Fig. (1) An experimental arrangement for making
corrosion potential measurement.
Calculations:
Mn+ + ne → M
Ecell =Ecathodic – Eanodic
= 𝐸𝑀𝑛+/𝑀 − 𝐸𝐻2 /𝐻 +
=𝐸𝑀𝑛+ /𝑀 − 0
=𝐸𝑀𝑛+ /𝑀
5
Materials Eng. Department
Lab.: Corrosion and Protection
Discussion:
1- Define the standard electrode potential?
2- What are the reference electrodes?
3- How measure the potential of electrode?
4- Calculate the reversible potential for a zinc electrode in contact with
𝑜
ZnCl2 when the activity of zinc is a Zn2+ = 10-3 and 𝐸𝑍𝑛
= −0.76 𝑉𝑜𝑙𝑡
6
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (2)
Electrochemical Measurements of Corrosion
The purpose of experiment:
Identify the behavior of materials in certain environment?
Introduction:
Potentiodynamic polarization (intrusive): This method is best known
for its fundamental role in electrochemistry in the measurement of Evans
diagrams. A three-electrode corrosion probe is used to polarize the electrode
of interest. The current response is measured as the potential is shifted away
from the free corrosion potential. The basic difference from the LPR
technique is that the applied potentials for polarization are normally stepped
up to levels of several hundred millivolts.
Instruments:
The techniques and instruments used may be classified as follows:
1. Potential measurements-a reference electrode and a potentiometer or
electrometer which require only a small current to give a measurement of
e.m.f. and thus minimise polarisation of the electrodes.
2. Current measurements - milliammeters or the measurements of the ZR
drop across a conductor of known resistance.
3. Galvanostatic polarisation -constant direct current power units, or banks
of accumulators or dry cells used in conjunction with a variable resistance.
4. Potentiostatic polarisation - potentiostats with varying output currents.
5. Determination of impedance- a.c. bridges of various frequencies.
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Materials Eng. Department
Lab.: Corrosion and Protection
Electrochemical Cells:
Since the single potential of a metal cannot be measured it is
necessary to use a suitable reference electrode such as the Hg/Hg,Cl,/KCI
electrode or the Ag/AgCl/KCl electrode, and although potentials are
frequently expressed with reference to the standard hydrogen electrode
(S.H.E.) the use of this electrode in practice is confined to fundamental
studies rather than testing.
Fig. (1) Electrochemical cell.
Calculations:
Can be calculate the corrosion parameters by using two methods. The
first Tafel extrapolation and the second linear polarization resistance as
shown below:
8
Materials Eng. Department
𝑅𝑝 =
Lab.: Corrosion and Protection
𝑏𝑐 𝑥 𝑏𝑎
2.303 𝑥 𝑖𝑐𝑜𝑟𝑟 𝑥 (𝑏𝑐 + 𝑏𝑎 )
Discussion:
1- Explain the electrochemical methods to measure the corrosion?
2- What is the potentiostat?
3- Compare between reference and auxiliary electrodes?
4- A corrosion rate of 1 mm/y for iron corresponds to what weight loss in
g/m2.d? Density of iron=7.86 g/cm3.
9
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (3)
Corrosion Measurement by Weight Loss
The purpose the experiment:
Measuring corrosion by simplest method.
Introduction:
The Weight Loss technique is the best known and simplest of all
corrosion monitoring techniques. The method involves exposing a specimen
of material (the coupon) to a process environment for a given duration, then
removing the specimen for analysis. The basic measurement which is
determined from corrosion coupons is weight loss; the weight loss taking
place over the period of exposure being expressed as corrosion rate.
The simplicity of the measurement offered by the corrosion coupon is
such that the coupon technique forms the baseline method of measurement
in many corrosion monitoring programs. The technique is extremely
versatile, since weight loss coupons can be fabricated from any
commercially available alloy. Also, using appropriate geometric designs, a
wide variety of corrosion phenomena may be studied which includes, but is
not limited to:
• Stress-assisted corrosion
• Bimetallic (galvanic) attack
• Differential aeration
• Heat-affected zones
Advantages of weight loss coupons are that:
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Materials Eng. Department
Lab.: Corrosion and Protection
• The technique is applicable to all environments - gases, liquids,
solids/particulate flow.
• Visual inspection can be undertaken.
• Corrosion deposits can be observed and analyzed.
• Weight loss can be readily determined and corrosion rate easily calculated.
• Localized corrosion can be identified and measured.
• Inhibitor performance can be easily assessed.
In a typical monitoring program, coupons are exposed for a 90-day
duration before being removed for a laboratory analysis. This gives basic
corrosion rate measurements at a frequency of four times per year. The
weight loss resulting from any single coupon exposure yields the “average”
value of corrosion occurring during that exposure.
The disadvantage of the coupon technique is that, if a corrosion upset
occurs during the period of exposure, the coupon alone will not be able to
identify the time of occurrence of the upset, and depending upon the peak
value of the upset and its duration, may not even register a statistically
significant increased weight loss.
Therefore, coupon monitoring is most useful in environments where
corrosion rates do not significantly change over long time periods.
Procedure:
1- weight the metallic plate of any metal or alloy (wo)
2- immerse the plate in corrosive environment for t1 and then weighted after
washing and drying with acetone (w1).
3- repeated the above process for t2, t3, ….and tn.
4- listed the data certain table to calculate the rate of corrosion.
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Materials Eng. Department
Lab.: Corrosion and Protection
Fig. (1) Immersed sample.
Calculations:
At any time can be calculating the rate in the term of weight loss:
𝑅𝑚𝑑𝑑 =
∆𝑊(𝑚𝑔)
𝑎𝑟𝑒𝑎(𝑑𝑚2 ) 𝑥 𝑡𝑖𝑚𝑒(𝑑𝑎𝑦)
𝑅𝑚𝑝𝑦 =
1.44
𝑅
= 39.4 𝑅𝑚𝑚/𝑦
𝑆. 𝐺 𝑚𝑑𝑑
Where
Rmdd: rate of corrosion in mg/dm2.day
∆W: W2-W1 =Wt-Wo
S.G.: specific gravity
Rmpy: rate of corrosion in mil per year.
Rmm/y: rate of corrosion in mm per year.
Discussion:
1- Define the corrosion and mention its types?
2- What is the effect of aeration on the rate of corrosion?
3- What is the effect of Time of immersion on corrosion rate?
4- Discuss the relation between weight loss and time?
5- A copper pipe corrodes at 6 g/m2.d in a certain environment. What would
this rate be if expressed as mm/y? density of Cu=8.9 g/cm3.
12
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (4)
Pitting Potential
The purpose of experiment:
Measurement of pitting potential.
Introduction:
Pitting corrosion is a form of localized corrosion of a metal surface
where small areas corrode preferentially leading to the formation of cavities
or pits, and the bulk of the surface remains unattacked. Metals which form
passive films, such as aluminum and steels, are more susceptible to this form
of corrosion. It is the most insidious form of corrosion. It causes failure by
penetration with only a small percent weight-loss of the entire structure. It is
a major type of failure in chemical processing industry. The destructive
nature of pitting is illustrated by the fact that usually the entire system must
be replaced. Generally, the most conducive environment for pitting is the
marine environment. Ions, such as Cl‾ Br‾ and I‾, in appreciable
concentrations tend to cause pitting of steel. Thiosulfate ions also induce
pitting of steels. Aluminum also pits in an environment that cause the pitting
of steel. Oxidizing metal ions with chloride, such as cupric, ferric and
mercuric, cause severe pitting. Presence of dust or dirt particles in water may
also lead to pitting corrosion in copper pipes transporting seawater. With
soft water, pitting in copper occurs in the hottest part of the system, whereas
with hard waters, pitting occurs in the coldest part of the system.
Studies on stainless steels have shown that sulfide inclusions are the
most probable sites for pit nucleation. Pits may grow in several forms, such
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Materials Eng. Department
Lab.: Corrosion and Protection
as circular, square, pyramidical and hexagonal. However, it is the depth of
the pit which matters more than the shape of the pit. Crystallographic pits
may be observed in certain alloys, such as Al 5052 associated with very low
dissolution rates. Pits generally grow downwards from horizontal surface.
Several months or sometimes even years may be needed before the pits
become visible. The classical techniques involving exposure to a corrosive
medium and metallographic examination by optical microscopy have been
supplemented by time-saving electrochemical techniques in recent years.
One such electrochemical technique is the polarization hysteresis technique
used for observing the anodic polarization behavior of steels, aluminum and
their alloys. The anodic polarization scan is commenced from the open
circuit potential of aluminum and continued in the noble direction at a slow
scan rate. The scan is reversed on reaching a predetermined current value
and continued in a reverse direction until the loop is closed at a more active
potential and the current drops to a minimum value. The plot obtained is
called a cyclic polarization curve, as shown in following figure. The diagram
is conveniently divided in three regions as below:
(1) Region I: This region represents immunity to pitting and at a potential
more negative than Epp (protection potential) pitting is not expected to
propagate.
(2) Region II: In this region, new pits do not initiate, but the pits already
initiated in Region III continue to propagate. This region is, therefore, the
propagation region for pits.
(3) Region III: In this region, pits initiate and propagation take place on and
above a certain potential, called breakdown or critical pitting potential (Ep).
The critical pitting potential signifies the onset of pitting.
14
Materials Eng. Department
Lab.: Corrosion and Protection
Fig. (1) Behavior of SS in acid.
Procedure:
As shown in polarization test for corrosion rate measurement, we can
calculate the pitting potentials from anodic polarization and cyclic
polarization.
SS 316L in 1N H2SO4 solution is the experimental condition to pitting
test. From polarization curve, can be calculate the Epp and Epit.
Discussion:
1- State the factors affecting pitting?
2- Explain types of pits?
3- How can prevent pitting corrosion?
15
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (5)
Study of High-temperature Corrosion (Oxidation)
The purpose of experiment:
Estimation of corrosion at high temperature.
Introduction:
High-temperature corrosion is a form of corrosion that does not
require the presence of a liquid electrolyte. Sometimes, this type of damage
is called dry corrosion or scaling. The term oxidation is ambivalent because
it can either refer to the formation of oxides or to the mechanism of
oxidation of a metal (i.e., its change to a higher valence than the metallic
state). Strictly speaking, high-temperature oxidation is only one type of
high-temperature corrosion, but it is the most important high-temperature
corrosion reaction. In most industrial environments, oxidation often
participates in the high-temperature corrosion reactions; regardless of the
predominant mode of corrosion. Alloys often rely upon the oxidation
reaction to develop a protective scale to resist corrosion attack such as
sulfidation, carburization, and other forms of high temperature attack. In
general, the names of the corrosion mechanisms are determined by the most
abundant dominant corrosion products. For example, oxidation implies
oxides, sulfidation implies sulfides, sulfidation/ oxidation implies sulfides
plus oxides, and carburization implies carbides.
16
Materials Eng. Department
Lab.: Corrosion and Protection
High-temperature corrosion is a widespread problem in various
industries such as:
_ Power generation (nuclear and fossil fuel)
_ Aerospace and gas turbine
_ Heat treating
_ Mineral and metallurgical processing
_ Chemical processing
_ Refining and petrochemical
_ Automotive
_ Pulp and paper
_ Waste incineration
The volume of the oxide formed, relative to the volume of the metal
consumed, is an important parameter in predicting the degree of protection
provided by the oxide scale. If the oxide volume is relatively low, tensile
stresses can crack the oxide layers. Oxides, essentially representing brittle
ceramics, are particularly susceptible to fracture and cracking under such
tensile stresses. If the oxide volume is very high, stresses will be set up that
can lead to a break in the adhesion between the metal and oxide. For a high
degree of protection, it can thus be argued that the volume of the oxide
formed should be similar to that of the metal consumed. This argument is the
basis for the Pilling-Bedworth ratio:
P.B.R 
Wd
Dw
where W is the molecular weight of the oxide, w is the atomic weight of the
metal, D and d are the specific densities of the oxide and metal respectively.
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Materials Eng. Department
Lab.: Corrosion and Protection
Procedure:
1- Prepare the sample by grinding and polishing.
2- Weight the sample W0.
3- Put the sample in the furnace at certain temperature T1 for time t1.
4- Cool the sample and then weight it W1.
5- Repeat the steps (1 – 4) at different time t2, t3, ….
6- Repeat the steps (1 – 4) at different temperature T2, T3,….
Calculations:
Make the table of results.
No. of
sample
1
2
3
Weight before
oxidation
gm
W0
W0
W0
Weight after
oxidation
gm
W1
W2
W3
Weight gain
gm
Weight gain
per area
gm/cm2
Discussion:
1- Explain the mechanism of oxidation?
2- What is Oxide Defect Structure?
3- Mention oxidation rate laws?
4- Plot the relation between weight gain and time?
5- Calculate the activation energy of oxidation?
18
Time
hrs.
Temp.
o
C
t1
t2
t3
T1
T2
T3
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (6)
Corrosion Protection by Inhibitors
The purpose of experiment:
Using inhibitors to corrosion control.
Introduction:
The practice of corrosion prevention by adding substances which can
significantly retard corrosion when added in small amounts is called
inhibition. Corrosion inhibitors are selected on the basis of solubility or
dispersibility in the fluids which are to be inhibited. For instance, in a
hydrocarbon system, a corrosion inhibitor soluble in hydrocarbon is used.
Two phase systems composed of both hydrocarbons and water; utilize
oil soluble water-dispersible inhibitors. Corrosion inhibitors are used in oil
and gas exploration and production, petroleum refineries, chemical
manufacturing, heavy manufacturing, water treatment and product additive
industries. Corrosion control by use of inhibitors is extremely useful in many
environments; however, there are certain exceptions, such as:
(a) equipment and components subjected to turbulent flow.
(b) systems operating above the stability limits of inhibitor.
(c) equipment subjected to high velocity, beyond 4 m/s.
There are two major classes: inorganic and organic. The anodic type
of inorganic inhibitors includes chromates, nitrites, molybdates and
phosphates, and the cathodic type includes zinc and polyphosphate
inhibitors. The film forming class is the major class of organic inhibitor as it
includes amines, amine salts and imidazolines - sodium benzoate
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Materials Eng. Department
Lab.: Corrosion and Protection
mercaptans, esters, amines and ammonia derivatives. The following figure
shows the classification of inhibitors.
Fig. (1) Classification of inhibitors.
Procedure:
Electrochemical method can be used to study the corrosion protection
by inhibitors as shown in the following steps:
1- corrosion test for any metal or alloy can be tested by potentistat in
experimental solution before added the inhibitor.
2- from polarization behavior can be calculate the corrosion parameters such
as corrosion potential (Ecorr) and corrosion current density (icorr), the later
called corrosion current density without addition of inhibitor (i1).
3- corrosion test for same metal or alloy can be achieved after added the
certain concentration of inhibitor.
4- calculate the corrosion parameters (Ecorr and icorr), the corrosion current
density called the current density after added the inhibitor (i2)
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Materials Eng. Department
Lab.: Corrosion and Protection
5- the following figure shows the effect of addition of inhibitors on potential,
discuss this case?
Fig. (2) Tafel plot before and after adding inhibitor.
Calculation:
To know the effect of inhibitor which added to environment through
calculate the protection efficiency (P%):
𝑖2
𝑃% = [1 − ] 𝑥100
𝑖1
Where :
i1: corrosion current density without inhibitor
i2: corrosion current density with inhibitor
1- Classify the inhibitors?
2- What is the effect of inhibitor on the corrosion parameters?
3- From the below data, indicate the best inhibitor?
Type of inhibitor
Current density without inhibitor
(A.cm-2)
Current density with inhibitor (A.cm-2)
A
B
C
2x10-6
2x10-6
2x10-6
1x10-10
1x10-8
1x10-4
21
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (7)
Corrosion Protection by Metallic Coating (Electroplating)
The purpose of experiment:
Using coating method to reducing corrosion.
Introduction:
The use of metallic coatings is only justified by longer life as they are
substantially more expensive than the organic coatings. Metallic coatings
protect the substrate either by acting as a barrier between the environment
and the substrate or by corroding preferentially with reference to the
substrate. The following are the major methods used for applying metallic
coatings:
(a) Electroplating or electrodeposition
(b) Diffusion
(c) Spraying
(d) Hot dipping
(e) Chemical vapor deposition
(f) Physical vapor deposition
(g) Welding, cladding, bonding
(h) Miscellaneous methods.
Procedure:
Metallic coatings are obtained in a conducting substrate by electrodeposition. The metal which is coated is exposed to a solution container
containing a salt of the coating metal in a specially designed tank. The metal
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Materials Eng. Department
Lab.: Corrosion and Protection
which is to be electroplated (work piece) is made the cathode whereas the
anode consists of a rod or sheet of the coating metal. The cathode is
connected to the negative terminal of a DC power source and anode to the
positive terminal. A specified voltage is given to the system depending on
what metal is to be electro-deposited. The anode contains the rod of the
metal which is to be electro-deposited, for instance, a copper rod is made the
anode if copper is to be electro-deposited. If an anode of inert metal is to be
used, a suitable metal salt must be added to the electrolyte.
Pure metals, alloys and mixed metals can be electro-deposited as
shown in the following figure. Three methods are used - vat, selective
plating and electroless plating.
Fig. (1) Arrangement of electrodes in electroplating.
Calculation:
1- calculate the thickness of coating film by weight the work piece before
and after coating or by ultrasonic inspection.
2- calculate the cathodic current efficiency:
23
Materials Eng. Department
𝐶. 𝐶. 𝐸. =
Lab.: Corrosion and Protection
𝐴𝑐𝑡𝑢𝑎𝑙 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓𝑐𝑜𝑎𝑡𝑖𝑛𝑔 𝑓𝑖𝑙𝑚
𝑥100
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓𝑐𝑜𝑎𝑡𝑖𝑛𝑔 𝑓𝑖𝑙𝑚
Where the theoretical thickness can be calculate as follow:
𝑊=
𝐼𝑡𝑀
𝑛𝐹
3- microstructure can be tested before and after coating to show smoothness
of coating film.
Discussion:
1- What are the major methods used for applying metallic coatings?
2- How can achieved coating by electroplating?
3- What are the factors affecting electroplating technique?
24
Materials Eng. Department
Lab.: Corrosion and Protection
Experiment (8)
Corrosion Protection by Metallic Coating
(Flame spray)
The purpose of experiment:
Coating by flame spraying to reducing corrosion.
Introduction:
The general procedure is to melt the coating material and blow it on to
the surface to be coated. The coating material is in the form of very small
molten particles or droplets.
Procedure:
Four methods based on the form of the coating material are generally used:
(1) The coating material is in the form of rod which is melted by an
oxyacetylene flame and blown onto the surface to be coated.
(2) The coating material is in the form of a powder which is heated by an
oxyacetylene frame, atomized and blown onto the surface.
A cross section of a typical powder flame gun is shown in following Figure.
(3) The coating is in the form of a wire. The wire is fed into the central
orifice of a nozzle. It passes through an oxyacetylene flame and sprayed to
the metal surface.
(4) The coating material in the form of a wire is heated by passing it through
the plasma of an electric arc.
The resulting molten metal is blown out of the arc by an auxiliary gas
stream, as droplets. The process is versatile with low capital investment. In
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Materials Eng. Department
Lab.: Corrosion and Protection
the electrostatic spraying process, the particles released from the spray gun
are electrostatically charged and propelled at low velocity by air or revolving
spray head. Too much air pressure is to be avoided. This procedure produces
a good wrap around without the need to positioning the work piece. The
process is suitable for tubular articles.
Discussion:
1- What are the metals which coated by flame spraying?
2- Mention the parts of instrument?
3- What are the parts that coated with this technique?
26