SURFACE COATING

Mallappa Komar
Dept. of Mechanical Engineering
Corrosion is defined as the
deterioration of a material, usually
a metal, because of a reaction
with its environment.
 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.
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.
 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.
 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)
 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
 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
 Coating processes
Coatings
 Conversion Coatings (oxidation, anodizing)
 Thermal Coatings (carburizing – flame spraying)
 Metal Coatings (electrochemical, electroless)
 Deposition
 Physical Vapor Deposition
 Chemical Vapor Deposition
 Organic
 C H E M I C A L CONVERSATION COATIN G S O R
S U R FA C E CONV ERSATION COATIN G S
 These coatings are produced on the surface
of a
metal or alloy by chemical or
electrochem ical reaction.
 The metal is immersed in a solution of
suitable
chem ical which reacts with the m etal
surface producing and adherent coating.
 These coatings protect the base metal
from
corrosion. Moreover many of these coatings
are
 particularly
The mostuseful to serve
commonly as excellent
used surface bases for
coatings
the are chromate
application
conversion coatings,
of paints, enamelsphosphate
and
coatings
other and chemical
protective oxide coatings.
coatings.
 Conversion Coatings
• Oxidation
• Phosphate Coatings
• Chrome Coatings
 Conversion Coatings - Oxidation
• Gun-bluing
– Heat steel to 700 deg F in steam
• Oxidation or oil
– Blue coating offers some
– Not all oxides are corrosion resistance, but little
wear benefit
detrimental – many are • Chemical Baths – similar in nature
tightly adhering leading to to gun-bluing
passivation and • Black Oxide – chemical
application
hardening of surface – Typically applied to steel, copper
• Al2O3 and stainless steel
• Chromium in Stainless steel • Anodizing – electrochemical
conversion
rapidly corrodes to – Usually done to Aluminum
passivate the surface – 2-25 m thick typically
– Multiple colors possible
– Improved Corrosion and Wear
Resistance
 Chemical Oxide Coatings
 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.
 A -Anodisation or Anodised Coatings
• Anodised coatings are generally produced on
non –
ferrous metals like Al, Z n , 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 H 2 S O 4 ,
chromic
acid, boric acid, phosphoric acid, oxalic acid etc
• Th e base metal to be anodized is susp ended
from the anode.

• Th e process is carried out by passing a

moderate direct current through the electrolytic
bath.
• As the anodized coatings are somewhat thicker than
• 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.
The O 2 evolved at the anode oxides the outer layer of Al to the oxide
film, Al 2 O 3 .

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

(Al 2 O 3 .H 2 O) which occupies more, volume, thereby the pores are
sealed.
4 Al + 3 O2 Al 2 O 3
Al 2 O 3 + H 2 O Al 2 O 3 .H 2 O
Anodized coatings may be coloured with organic dyes and inorganic
pigments to give decorative effects.
 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
 Phosphate coating
• These are produced by the chemical reaction of base
metal with aqueous solution of phosphoric acid and a
phosphate of Fe, Mn or Z n .

• 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, C d , Al and S n .

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
 Chromate Coatings

• 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 Z u , C d , Mg and Al
 Applications

• Aircraft parts, refrigerators, reflectors,

machine parts etc are anodized by
this method Al articles used as doors,
windows, showcase panels
household u tensils are & anodized
this method. by
 Thermal Treatments
• Surface Heat Treatment
• Diffusion Coating
• Hot-Dip Coatings
• Weld Overlay Coatings
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
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
 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
 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.
 Metal Coatings
• Electroplating
• Electroless Coatings
• Metallizing of Plastics and Ceramics
 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
 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))
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,
 Electroless Nickel vs. Chrome
Plating
ELECTROLESS NICKEL HARD CHROME

METAL DISTRIBUTION VERY GOOD POOR

1,000 HOURS 400 HOURS
CORROSION RESISTANCE
ASTM B117 ASTM B117
HARDNESS:
AS DEPOSITED 48-52 Rc 64-69 Rc
HEAT TREAT 70 Rc 48-52 Rc

MELTING POINT 1800oF 2900oF

WEAR RESISTANCE GOOD VERY GOOD
CO-EFFICIENT OF FRICTION:
DYNAMIC 0.19 0.16
STATIC 0.20 0.17
DUCTILITY 1-2% Very Low Almost 0
EFFLUENT COST RELATIVELY LOW HIGH
DEPOSITION RATE
.0002 - .0003 .001 - .002
(PER HOUR PER HOUR)
EFFECTIVE OF HYDROGEN
EMBRITTLEMENT ON PLATED FAIR/NOT SERIOUS USUALLY SERIOUS
COMPONENTS
 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
 Vapor Deposition
• Physical Vapor Deposition (PVD)
– Thermal PVD
– Sputter Deposition
– Ion plating
• Chemical Vapor Deposition (CVD)
 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.
 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)
 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
 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
 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
 Powde
r
• 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
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
 Contents:
Protective coatings:
Surface preparation for metallic Coatings
 Solvent cleaning
 Acid pickling
 Alkali cleaning
 Sand blasting
 Electro plating One example with explanation
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.
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.
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
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.
 Metallic Coatings
• Metallic coatings are
mostly applied on Iron and
steel arebecause
these cheap and commonly
used construction
There are two types ofmaterials.
metallic
coatings.
 i. Anodic coatings

• The base metal which is to be

protected is coated with a more anodic
metal for eg. Coatings of Z n , Al and
C d steel are anodic because their
electrode potentials are lower than
that of the base metal ie. Fe.
 ii. Cathodic Coatings
• It is obtained by coating a more inert metal
having higher electrode potential. Than the
base metal. Eg. Coating of S n , C r, 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.
M E T H O D S O F APPLICATION
O F METALLIC COATING
 1. Hot Dipping
• 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
 a. Galvanization
• It is the process of coating Zn over iron
or steel sheet by immersing it in molten
Z n . The procedure involves the following
stages.

• The iron or steel article is first cleaned by

pickling with dil H 2 So 4 for 15 – 20 min.
at 60 – 90 0 C in an acid bath.

• This treatment also removes any oxide

layer present on the surface of the metal.
 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.
 a. Galvanization
⚫It is then dipped in a bath of m olten
Zn kept at 425 – 435 0 C.

⚫The S u rface of the bath is covered

with NH 4 Cl flux to prevent oxide
formation.

⚫The article gets coated with a thin layer of

Zn.
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.
 ii. Tinning

• 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.
Applications
• 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.
 2. Metal Cladding

 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

 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 metal.

b. Welding

c. Rolling sheets of cladding material over base metal.

 3.Metal spraying
⚫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 continuou s
but
somewhat porous
⚫However, adhesion a sealer – oil isofapplied
strength
on s u c h ais coating
spraying
metallic usually to provide
lesser thata obtained
smooth by hot
surface.
dipping or electroplating.
⚫It is therefore essential to have a
cleaned
metal surface. Spraying can be applied by
the following two techniques.
 i. Wire – gun method
• 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.
 ii Powder – metal method

• 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.
 4. Electroplating or Electrodeposition
⚫it is probably the most important and
frequently applied industrial method most
producing metallic coatings. of

⚫Electroplating is carried out by a process

called electrolysis.
⚫Th u s 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
 Electroplating

©2010 John Wiley & Sons, Inc.

M P Groover, Fundamentals of
Modern Manufacturing 4/e
• For electroplating of Ni, NiSO 4 and NiCl 2
used as the electrolyte.
are
• For electroplating of C r, chromic acid is
used as the electrolyte.

• For Au plating, AuCl 3 solution is taken as the

electrolyte.

• For C u plating C u S O 4 solution is used as

electrolyte.
the
• In silver plating, AgNO 3 solution is used
as the electrolyte.
Contents:

Chemical conversion coatings: anodizing,

phosphating and chromate coating.
 b. VIT REO U S C OAT I N G S O R C E R E M I C
PROTECTIVE C OAT IN G S

• 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
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.
 Vitreous coatings
1. Refractories like quartz (SiO 2 ), clay etc.
2. Fluxes like borax (Sodium tetra borate
Na 2 B 4 O 7 ), cryolite (Na 3 AlF 6 ) (Sodium
alumino fluoride), Soda ash (anhydrous
sodium carbonate Na 2 CO 3 ) etc.
3. Opacifiers like TiO 2 , S n O 2 , Al 2 O 3 etc
4. Pigments like metallic oxides
organic dyes etc
5. Floating agents like plastic, clay, gum etc
6. Electrolytes like MgS O 4 , MgC O 3 ,
Na 2 C o 3 etc.