2. Corrosion is defined as the
deterioration of a material,
usually a metal, because of a
reaction with its environment.
3. CORROSION IS
A natural phenomenon that occurs over time.
An electrochemical reaction (on metals) Happens
at different rates with different metals and in
different environments
If we expose iron or steel to air and water we can
expect to see rust form in a short time, showing
the familiar color of red-brown iron oxide.
Depending on the environment the rust may
develop in minutes
4.
5.
6.
7. With other metals such as copper, brass, zinc,
aluminum, and stainless steel we can expect
corrosion to take place, but it might take longer to
develop.
One reason for the reduction of the corrosion rate
with these metals is the potential formation of
metallic oxides of copper, zinc, aluminum, and
chromium.
Unfortunately ordinary iron or steel does not form
this protective layer, so must be separated from the
environment by some other means.
Generally protective coatings are utilized for this
purpose.
8. Coating
A coating is a covering that is applied to the surface of an
object, usually referred to as the substrate.
The purpose of applying the coating may be decorative,
functional, or both.
The coating itself may be an all-over coating, completely
covering the substrate, or it may only cover parts of the
substrate.
An example of all of these types of coating is a product
label on many drinks bottles- one side has an all-over
functional coating (the adhesive) and the other side has
one or more decorative coatings in an appropriate pattern
(the printing) to form the words and images.
9. Functions of coatings
• Adhesive –
• adhesive tape, pressure-sensitive labels, iron-
on fabric
• Changing adhesion properties
• Non-stick PTFE coated- cooking pans
• Release coatings e.g. silicone-coated release
liners for many self-adhesive products
• primers encourage subsequent coatings to
adhere well (also sometimes have anti-corrosive
properties)
10. Optical coatings
• Reflective coatings for mirrors
• Anti-reflective coatings e.g. on spectacles
• UV- absorbent coatings for protection of eyes or
increasing the life of the substrate
• Tinted as used in some coloured lighting, tinted
glazing, or sunglasses
• Catalytic e.g. some self-cleaning glass
• Light-sensitive as previously used to
make photographic film
11. Protective
• Most paints are to some extent protecting the substrate
• Hard anti-scratch coating on plastics and other materials
e.g. of titanium nitride to reduce scratching, improve wear
resistance, etc.
• Anti-corrosion
• Underbody sealant for cars
• Many plating products
• Waterproof fabric and waterproof paper
• antimicrobial surface
• Magnetic properties such as for magnetic media
like cassette tapes, floppy disks, and some mass transit
tickets
13. CHEMICAL CONVERSATION COATINGS OR
SURFACE CONVERSATION COATINGS
These coatings are produced on the surface of a
metal or alloy by chemical or electrochemical
reaction.
The metal is immersed in a solution of suitable
chemical which reacts with the metal surface
producing and adherent coating.
These coatings protect the base metal from
corrosion. Moreover many of these coatings are
particularly useful to serve as excellent bases for
the application of paints, enamels and other
protective coatings.
The most commonly used surface conversion
coatings are chromate coatings, phosphate
coatings and chemical oxide coatings.
14. ANODIZING
The process uses the metal as an anode, by electrolytic process a layer of
hard metal oxide is formed at the anode i.e. on the surface of the part.
The coating provided on the metal surface may be of different colors
(usually black, red, blue)
Electrolytic treatment produces a stable oxide layer on the metallic
surface
Applications:Aluminium, Magnesium, zinc, titanium, and other
Metals
Dyes can be incorporated into anodizing process to create a wide
variety of colors
Especially common in aluminium anodizing
Functions: primarily decorative; also corrosion protection
15.
16. FLAME SPRAYING
This process is basically the spraying of molten
material onto a surface to provide a coating. Material
in powder form is melted in a flame (oxy-acetylene
or hydrogen most common) to form a fine spray.
When the spray contacts the prepared surface of a
substrate material, the fine molten droplets rapidly
solidify forming a coating.
This flame spray process carried out correctly is
called a "cold process" (relative to the substrate
material being coated) as the substrate temperature
can be kept low during processing avoiding damage,
metallurgical changes and distortion to the substrate
material.
17. The main advantage of this flame spray process over the similar Combustion
wire spray process is that a much wider range of materials can be easily
processed into powder form giving a larger choice of coatings. The flame
spray process is only limited by materials with higher melting temperatures
than the flame can provide or if the material decomposes on heating.
18. ELECTROPLATING
Electroplating is the process of depositing one metal onto another
metal.
Electrons travel from the negative end of the battery through the
cathode, through the solution, up through the anode, and into the
positive end of the battery.
The positively charged ion form the solution are attracted to the
negatively charged cathode.
These ions attached themselves to the cathode.
Electroplating can enhances-
Chemical properties- increase corrosion
resistance
Physical properties- increase thickness
of part
Mechanical properties- increase tensile
strength & hardness
19. ELECTROLESS PLATING
• Part is submerged into an aqueous bath filled with metal salts ,
reducing agents and catalysts.
• Catalysts reduce metal to ions to form the coating.
• This process can be used to plate non conducting parts with a layer
of metal.
• Excellent for complex geometries as deposition is uniform across
surface regardless of geometry (except very sharp corners ( 0.4 mm
radii)
20. Vapor Deposition-
They are of two types-
1) Physical vapor deposition(PVD)
2) Chemical vapor deposition(CVD)
Physical vapor deposition(PVD)
Family of processes in which a material is converted to its
vapor phase in a vacuum chamber and condensed onto substrate
surface as a very thin film.
The most common methods of PVD are Sputtering and Thermal
Evaporation.
Because in the PVD process, the material is transported and
accumulated atom-by-atom or molecule by molecule in a vacuum
to the substrate surface, the deposited films have high purity and
efficiency that for many applications compared to other deposition
methods are preferred.
22. Applications of PVD-
For example, in the construction of the most
important part of any microchip and semiconductor
device, durable protective layers, optical
lenses, solar panels and many parts and medical
devices, the PVD thin film provides basic functional
characteristics for the final product.
In general, wherever there is a need for coating with
very thin, pure, durable and clean thin films, PVD is
the key to solving the problem.
23. Its advantages include the following:
PVD-coated thin films are more
resistant to corrosion than coatings
made using other coating processes such
as plating. Most PVD coatings are
impact resistant and also very abrasion
resistant. These coatings are often able
to withstand high temperatures
High purity of the deposited thin films
and the possibility of controlling the
structure of the layers
Thin films are cheaper bulk of
material(for example, a thin layer of
gold is much cheaper than a piece of
gold)
Using the PVD method, it is possible to
deposit almost any type of inorganic
material and some organic materials on
a diverse and wide group of surfaces
and substrates
High environmental compatibility
Provide a variety of techniques for
deposition of a specific material
Disadvantages of PVD:
Limitations on the coverage of
parts with complex geometric
shapes. Of course, this limitation
has been largely overcome by
various methods of moving the
substrate
Some PVD methods and
techniques require high attention
and accuracy of the user due to the
high vacuum and high temperature
of the coating environment.
Cooling water circulation systems
are needed to dissipate the
generated heat load and prevent
damage to the components of
the coating systems
24. Chemical vapor deposition (CVD) is the formation
of a non-volatile solid film on a substrate due to the
reaction of vapor-phase chemical reactants. CVD is
an atmosphere-controlled process conducted at
elevated temperatures of around 1925°F (1051°C) in
a CVD reactor.
CVD is used to create coatings for a variety of
applications such as wear resistance, corrosion
resistance, high temperature protection, erosion
protection and combinations thereof.
CHEMICAL VAPOR DEPOSITION (CVD)
25. A basic CVD process consists of the following steps:
1.A mixture of reactant gases and diluent inert gases are introduced into
the reaction chamber.
2.The gas moves to the substrate.
3.The reactants are adsorbed on the surface of the substrate.
4.The reactants undergo chemical reactions with the substrate to form the
film.
5.The gaseous byproducts of the reactions are desorbed and evacuated
from the reaction chamber.
Chemical vapor deposition coatings are fine grained, have high purity
and are harder than similar materials produced using conventional
ceramic fabrication processes.
This process is a very versatile one that can be used on products that
may be difficult to coat with other methods. It is commonly used to
protect electronic components, such as integrated circuits, against
corrosion.
26. Conversion Coatings - Oxidation
• Oxidation
– Not all oxides are
detrimental – many are
tightly adhering leading to
passivation and hardening
of surface
• Al2O3
• Chromium in Stainless steel
rapidly corrodes to
passivate the surface
• Gun-bluing
– Heat steel to 700 deg F in steam
or oil
– Blue coating offers some
corrosion resistance, but little
wear benefit
• Chemical Baths – similar in nature
to gun-bluing
• Black Oxide – chemical
application
– Typically applied to steel, copper
and stainless steel
• Anodizing – electrochemical
conversion
– Usually done to Aluminum
– 2-25 m thick typically
– Multiple colors possible
– Improved Corrosion and Wear
Resistance
27. These types of coatings are formed on the surface of
metals like Fe, Al, Mg etc by treating the base metal
with alkaline oxidizing agents like potassium
permanganate.
This treatment increases the thickness of the
original oxide film on the metal, there by increasing
the corrosion resistance.
Oxide coatings form a good base for paints.
These oxide coatings have got only poor corrosion
resistance. However, for better protection the
thickness of the oxide film can be increased 100 to
1000 times by electrolytic oxidation or anodisation.
Chemical Oxide Coatings
28. • Anodised coatings are generally produced on non –
ferrous metals like Al, Zn, Mg and their alloys by
anodic oxidation process.
• In this process, the base metal is made as anode
and the cathode is an inert electrode like graphite.
• The electrolytic bath is usually of H2 SO4 , chromic
acid, boric acid, phosphoric acid, oxalic acid etc
• The base metal to be anodized is suspended from
the anode.
• The process is carried out by passing a moderate
direct current through the electrolytic bath.
• As the anodized coatings are somewhat thicker than
the natural oxide film and they posses improved
resistance to corrosion.
A -Anodisation or Anodised Coatings
29. • Anodizing on Al has gained considerable
commercial importance.
• Al coated surface require oxidation to convert the
metal to its inert oxide.
• Anodising on Al is carried out by an electrolytic
process.
30.
31. The O2 evolved at the anode oxides the outer layer of Al to the oxide
film, Al2O3.
The oxide film initially very thin, grows from the metal surface
outwards and increases in thickness as oxidation continues at Al
anode.
The outer part of the oxide film formed is porous and to reduce
porosity, the article after electrolysis is kept immersed in a boiling
water bath.
This treatment changes porous alumina into its monohydrate
(Al2O3.H2O) which occupies more, volume, thereby the pores are
sealed.
4 Al + 3 O2 Al2O3
Al2O3 + H2 O Al2O3.H2O
Anodized coatings may be coloured with organic dyes and inorganic
pigments to give decorative effects.
32. Conversion Coatings – Phosphate
Coating
• Immersion in a Zn-P bath with Phosphoric acid
causes growth of a crystalline zinc phosphate layer
– Iron, Zinc or Manganese Phosphate layer formed
• Typically applied to C-steel, low alloy steel and cast
irons
– Sometimes applied to Zinc, Cadmium, Aluminum and Tin
• Typically very thin ~ 2.5 m
33. • These are produced by the chemical reaction of base
metal with aqueous solution of phosphoric acid and a
phosphate of Fe, Mn or Zn.
• The reaction results in the formation of a surface film
consisting of phosphate of a surface film consisting of
phosphates of the metal.
• These coatings are usually applied by immersing or
spraying or brushing. These coating do not give
complete corrosion resistance but can serve as base for
painting.
• These are applied on metals like Fe, Zn, Cd, Al and Sn.
Phosphate coating
34. Conversion Coatings – Chrome Coating
• Food cans
• Immersion in a chromic acid bath (pH ~ 1.8) with
other chemicals to coat surface
• Known carcinogen chemicals used, so alternatives
are currently under research
– Molybdate chemicals currently best subsititute for
aluminum coatings
• Very good to minimize atmospheric corrosion
– Many household goods – screws, hinges (yellow brown
appearance)
• Typically very thin < 2.5 m
35. • There are produced by the immersion of the
article in a bath of acidic potassium chromate
followed by immersion in a bath of neutral
chromate solution.
• The surface film consisting of a mixture of
trivals and hexavalent Cr is formed.
• Chromate coatings possess more corrosion
resistance and can also be used as a base for
paints. These are applied on Zu, Cd, Mg and Al
Chromate Coatings
36. • Aircraft parts, refrigerators, reflectors,
machine parts etc are anodized by
this method Al articles used as doors,
windows, showcase
household utensils are
panels &
anodized by
this method.
Applications
38. Thermal Treatments – Surface Heat
Treatment
• Basic concept is to heat the surface to austenitic
range, then quench it to form surface martensite -
workpiece is steel
• Heating Methods
– Flame Treatment
– Induction Heating
• Copper coil wraps around part to heat by induction
– Electron Beam or Laser Beam Hardening
• Typically heat small area and allow the bulk solid heat capacity to
quench the small heated area
39. Thermal Treatments – Diffusion
Coating
• With low carbon steel, the surface can be enriched by
diffusion of C or N into surface
• Carburizing
– Heat steel to austenitic range (850-950 ºC) in a carbon rich
environment, then quench and temper
• Nitriding
– Nitrogen diffusion into steels occurs around 500-560 ºC to form a
thin hard surface
– Good for Cr, V,W,and Mo steels. Will embrittle surface of
Aluminum.
• Metal Diffusion
– Chromizing – Chromium diffuses into surface to form corrosion
resistant layer.
• Take care with carbon steels as surface will decarburize
– Aluminizing – Used to increase the high temperature corrosion
resistance of steels and superalloys
40. Thermal Treatments –
Hot-Dip Coatings
• These coatings are used for corrosion protection
• Galvanizing
– Parts are dipped into a molten zinc bath
• Galv-annealing
– Galvanized parts are then heat treated to ~500 ºC to form Fe-Zn inter-
metallic
• Used for metals that need spot welded to protect copper electrode from alloying
with zinc and reducing its life
• Zn-Al Coatings
– Gives a different corrosion protect and a more lustrous appearance (can
greatly reduce spangles easily observed on galvanized parts)
• Aluminum Coatings
– Alloyed with Si
– Coatings used on steel for high temperature applications that need a
lustrous appearance
• Example – Automobile exhaust
41. Thermal Treatments –
Weld Overlay coatings
• Typically used to improve wear resistance by creating a hard
surface over a tough bulk body
• Hard Facing
– Weld buildup of parts – alloy composition controls final properties
– Examples – cutting tools, rock drills, cutting blades
– Cladding of material for corrosion resistance
• Thermal spraying
– Molten particle deposition – a stream of molten metal particles are
deposited on the substrate surface
– Major difference from hard facing is that the surface of the substrate is
not subjected to welding. Instead it just undergoes a bonding process
with the molten particles.
43. Metal Coatings - Electroplating
• Used to increase wear and corrosion resistance
• Electrochemical process used to create a thin coating
bonding to substrate
• Process is slow so coating thickness can be closely
controlled (10-500 m)
• Applications
– Tin and Zinc are deposited on steel for further working
– Zinc and Cadmium are deposited on parts for corrosion resistance
(Cadmium is toxic and can not be used for food applications)
– Copper is deposited for electrical contacts
– Nickel for corrosion resistance
– Chromium can be used to impart wear resistance to dies and reduce
adhesion to workpieces such as aluminum or zinc
– Precious metals for decoration or electronic devices
44. Metal Coatings – Electroless
Coatings
• Part is submerged into an aqueous bath filled
with metal salts, reducing agents and catalysts
– Catalysts reduce metal to ions to form the coating
• Excellent for complex geometries as
deposition is uniform across surface regardless
of geometry (except very sharp corners (0.4
mm radii))
45. Metal Coatings -Electroless Nickel
Plating
• Has the appearance of
stainless steel
• Autocatalytic immersion
process
• Key characteristics:
– Heat treatable coating (to 68
Rc) very hard
– Non-porous
– Corrosion resistant
– .001” thick typical
– Withstand load to 45 ksi
• Can be applied to:
– steel and stainless steel,
iron, aluminum, titanium,
magnesium, copper, brass,
bronze, and nickel
46. Electroless Nickel vs. Chrome
Plating
ELECTROLESS NICKEL HARD CHROME
METAL DISTRIBUTION VERY GOOD POOR
CORROSION RESISTANCE
1,000 HOURS
ASTM B117
400 HOURS
ASTM B117
HARDNESS:
AS DEPOSITED
HEAT TREAT
48-52 Rc
70 Rc
64-69 Rc
48-52 Rc
MELTING POINT 1800oF 2900oF
WEAR RESISTANCE GOOD VERY GOOD
CO-EFFICIENT OF FRICTION:
DYNAMIC
STATIC
0.19
0.20
0.16
0.17
DUCTILITY 1-2% Very Low Almost 0
EFFLUENT COST RELATIVELY LOW HIGH
DEPOSITION RATE
(PER HOUR PER HOUR)
.0002 - .0003 .001 - .002
EFFECTIVE OF HYDROGEN
EMBRITTLEMENT ON PLATED
COMPONENTS
FAIR/NOT SERIOUS USUALLY SERIOUS
47. Metal Coatings –
Metallizing of Plastics and Ceramics
• Poor adhesion is the major challenge (As in all
coating processes, however it is more
challenging in this case.)
• Applications
– Decorative (plumbing fixtures, automotive parts),
reflectivity (headlights), electrical conduction
(electronic touchpads), and EMF shielding
49. Physical Vapor Deposition –
Thermal PVD
• Thermal PVD – also called Vacuum Deposition
– Coating material (typically metal) is evaporated by melting
in a vacuum
– Substrate is usually heated for better bonding
– Deposition rate is increased though the use of a DC current
(substrate is the anode so it attracts the coating material)
– Thin ~0.5 m to as thick as 1 mm.
50. Physical Vapor Deposition – Sputter
Deposition
• Vacuum chamber is usually backfilled with Ar gas
• Chamber has high DC voltage (2,000-6,000 V)
• The Ar becomes a plasma and is used to target the
deposition material. The impact dislodges atoms from the
surface (sputtering), which are then deposited on the
substrate anode
• If the chamber is full of oxygen instead of Ar, then the
sputtered atoms will oxidize immediately and an oxide will
deposit (called reactive sputtering)
51. Physical Vapor Deposition – Ion
Plating
• Combination of thermal PVD and sputtering
• Higher rate of evaporation and deposition
• TiN coating is made this way (Ar-N2
atmosphere)
– The gold looking coating on many cutting tools to
decrease the friction, increase the hardness and
wear resistance
52. Chemical Vapor Deposition
• Deposition of a compound (or element) produced by a
vapor-phase reduction between a reactive element and gas
– Produces by-products that must be removed from the process as
well
• Process typically done at elevated temps (~900ºC)
– Coating will crack upon cooling if large difference in thermal
coefficients of expansion
– Plasma CVD done at 300-700ºC (reaction is activated by plasma)
• Typical for tool coatings
• Applications
– Diamond Coating, Carburizing, Nitriding, Chromizing, Aluminizing
and Siliconizing processes
– Semiconductor manufacturing
53. Organic Coatings - paint
• Enamels
– Form film primarily by solvent evaporation
– 30 % Volatile Organic Content (VOC)
• Lacquers – solvent evaporation
• Water-base paints – water evaporation,
therefore much better
• Powder Coating – superior – more detail to
follow
54. Powder
Coating
• Fully formulated paint
ground into a fine
powder
• Powder is sprayed onto
part, retained by static
electricity
• Heat cured onto part
• Can virtually eliminate
VOCs
55. Teflon and dry lubricant coatings
• Sprayed, dipped or
tumbled to coat,
followed by heating to
bond
• Key characteristics:
– Low friction coefficient
(0.02 – 0.08)
– Can sustain load of 250
ksi
57. Protective coatings
An important method for protecting a metal from corrosion is
to apply a protective coating.
The protective coatings may be of metal, inorganic or organic.
The coated surface isolates the metal from the corroding
medium.
The coating applied must be chemically inert towards the
environment.
58. Protective Coating
Surface preparation for Coating:
1. Cleaning:
To prepare for suitable condition
Removing contaminants to prevent detrimental reaction product
- E.g. de-greasing, sand blasting, vapour degreasing, pickling and
alkaline cleaning.
2. Solvent Cleaning:
Must be non-inflammable and nontoxic.
Trichloro trifluoroethane which has low toxicity are costlier.
Vapour de-greasing is economical and advantageous because of
continuous cleaning with small quantities of solvent.
59. 3. Electrolyte Pickling:
Provides better and rapid cleaning by increasing hydrogen
evolution resulting in agitation and blasting action.
Sand blasting is mechanical cleaning.
4. Alkaline Cleaning:
Cheaper and less hazardous.
Used in conjunction with surface active (wetting) agent.
Ability depends on pH, rapidly decreases below 8.5.
Other abilities are rinsability, detergent properties,
sequestering, wetting etc
60. 5. Acid Cleaning
Acid such as HCl, H2SO4, H3PO4 is very effective.
5-10% H2SO4 and HCl used to remove inorganic
contaminants.
Pickling are performed at high temp. (60 ̊C).
It is effective for removal of grease, oil , dirt and rust.
61. • Metallic coatings are mostly
applied on Iron and steel because
these are cheap and
used construction
commonly
materials.
There are two types of metallic
coatings.
Metallic Coatings
62. • The base metal which is to be
protected is coated with a more anodic
metal for eg. Coatings of Zn, Al and
electrode potentials are lower
Cd steel are anodic because their
than
that of the base metal ie. Fe.
i. Anodic coatings
63. • It is obtained by coating a more inert metal
having higher electrode potential. Than the
base metal. Eg. Coating of Sn, Cr, Ni on Fe
surface.
• The coating should be continuous and free
from pores and cracks.
• These coating metals usually have higher
corrosion resistance than the base metal.
ii. Cathodic Coatings
65. • It is used for producing a coating of low melting metal
such as Zn, Sn, Ph, Al etc on relatively higher melting
metals such as iron, steel, copper etc.
• This is done by immersing the base metal covered by
a layer of molten flux.
• The flux is used to keep the base metal surface clean
and also to prevent oxidation of the molten metal.
• Most widely used hot dipping methods are : (i)
galvanization and (ii) tinning
1. Hot Dipping
66. • It is the process of coating Zn over iron
or steel sheet by immersing it in molten
Zn. The procedure involves the following
stages.
• The iron or steel article is first cleaned by
pickling with dil H2So4 for 15 – 20 min.
at 60 – 900C in an acid bath.
• This treatment also removes any oxide
layer present on the surface of the metal.
a. Galvanization
67. a. Galvanization
• The article is then washed with water in a
washing bath & dried in a drying chamber.
⚫It is then passed through a pair of hot rollers to
remove excess of Zn and to get uniform thickness
for coating.
⚫Then it is annealed at about 6500C & cooled
slowly.
⚫In the case of Zn coating even if the protecting
layer has cracks on it, iron being cathodic does not
get corroded.
68. ⚫It is then dipped in a bath of molten Zn
kept at 425 – 4350C.
⚫The Surface of the bath is covered with
NH4Cl flux to prevent oxide formation.
⚫The article gets coated with a thin layer of
Zn.
a. Galvanization
69. Applications
This method is widely used for protection of
Fe from atmospheric corrosion in the form
of articles like roofing sheets, wires, pipes,
nails, screws, tubes etc.
It is to be noted that galvanized utensils
should not come in contact with acids.
70. • It is an eg. For cathodic coatings. It is the process of coating of
Sn over Fe or steel articles by immersing it in molten Sn.
• The process consists in Ist treating the iron sheet with dil
H2So4 to remove any oxide film.
• After this it is passed through a bath of ZnCl2 flux which helps
the molten Sn to adhere to the metal sheet.
•
• Next the sheet passes through palm oil which prevents
through a pair of hot rollers to remove excess of Sn & produce
uniform thickness for Sn coating.
ii. Tinning
71. • Tinning is widely used for coating steel, Cu and brass
sheets which are used for making containers for
storing food studs, oils, kerosene & packing food
materials.
• Tinned Cu sheets are used for making cooking
utensils & refrigeration equipments.
Applications
72. In this process, a thick homogeneous layer of coating metal
is bonded firmly & permanently to the base metal on one
or both the sides.
This method enhances corrosion resistance.
The choice of cladding material depends on the corrosion
resistance required for any particular environment.
2. Metal Cladding
73. Nearly all existing corrosion resisting metals like Ni, Cu, Al,
Ag, Pt and alloys like stainless steel, Ni alloys, Cu alloys can
be used as cladding materials.
Cladding can be done by different means.
a. Fusing cladding material over the base
b. Welding
metal.
c. Rolling sheets of cladding material over base metal.
2. Metal Cladding
74. ⚫In this process, the coating metal in the
molten state is sprayed on the previously
cleaned base metal with the help of a sprayer.
⚫The sprayer coatings are continuous but
somewhat porous a sealer – oil is applied on
such a coating to provide a smooth surface.
⚫However, adhesion strength of metallic
spraying is usually lesser that obtained by hot
dipping or electroplating.
⚫It is therefore essential to have a cleaned
metal surface. Spraying can be applied by the
following two techniques.
3.Metal spraying
75. • In this method, the coating metal in the
form of thin wire is melted by an oxy –
acetylene flame and vaporized by a blast of
compressed air.
• The coating metal adheres to the base
metal. Al is coated on aircraft steel parts
using this techniques.
i. Wire – gun method
76. • In this method, the coating metal is supplied in the
form of tine powder which is converted in to a
cloud of molten globules by a blower and are
adsorbed on the base metal surface.
ii Powder – metal method
77. frequently applied industrial method
⚫it is probably the most important and most
of
producing metallic coatings.
⚫Electroplating is carried out by a process called
electrolysis.
⚫Thus in this process, the coating metal is
deposited on the base metal by passing direct
current through an electrolyte containing the
soluble salt of the coating metal.
⚫The base metal to be electroplated is made the
cathode of the electrolytic cell whereas the anode
is either made of the coating metal itself or an
inert material of good electrical conductivity like
graphic.
4. Electroplating or Electrodeposition
79. • For electroplating of Ni, NiSO4 and NiCl2 are
used as the electrolyte.
• For electroplating of Cr, chromic acid is used
as the electrolyte.
• For Au plating, AuCl3 solution is taken as the
electrolyte.
• For Cu plating CuSO4 solution is used as the
electrolyte.
• In silver plating, AgNO3 solution is used as
the electrolyte.
81. b. VITREOUS COATINGS OR CEREMIC
PROTECTIVE COATINGS
• Ceramic protective coatings can be
broadly divided into vitreous enamel
coatings and pure ceramic coatings.
These coatings have the following
advantages.
1.They posses high refractoriness and
inertness
2. They are wear resistant & easily be cleaned
3. They are glossy in appearance
4. They are good thermal & electrical
insulators
82. Vitreous enamels are defined as glossy
inorganic composition that can adhere to
metals by fusion and protect them from
corrosion, abrasion, oxidation and high
temperature.
Vitreous enamel coatings consists of a
ceramic mixture of refractories and large
proportion of fluxes. These coatings are
usually applied on steel and cast iron
equipments. The raw materials used for the
vitreous coatings are the following.
83. Vitreous coatings
1. Refractories like quartz (SiO2), clay etc.
2. Fluxes like borax (Sodium tetra borate
Na2B4O7), cryolite (Na3AlF6) (Sodium
alumino fluoride), Soda ash (anhydrous
sodium carbonate Na2CO3) etc.
3. Opacifiers like TiO2, SnO2 , Al2O3 etc
4. Pigments like metallic oxides organic
dyes etc
5. Floating agents like plastic, clay, gum etc
6. Electrolytes like MgSO 4, MgCO 3, Na2Co3
etc.
84. Anodizing:
•“Anodizing is a process of forming a thick oxide
layer of aluminum. Aluminum develops a thin oxide
layer when exposed to air.”
•This aluminum oxide coat makes it resistant to
further corrosion.
The procedure of anodizing:
•During anodizing, a clean aluminum article works
as an anode and is electrolyzed with dilute
sulphuric acidH2SO4.
•The oxygen O2gas evolved at the anode reacts
with aluminum to make a thicker protective oxide
