surface coating

1. Surface Coating
Dr. Jawahar
Dept. of Applied Chemistry

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

4. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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)

13. 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

14. 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

15. Coating processes
Coatings
 Conversion Coatings (oxidation, anodizing)
 Thermal Coatings (carburizing – flame spraying)
 Metal Coatings (electrochemical, electroless)
 Deposition
 Physical Vapor Deposition
 Chemical Vapor Deposition
 Organic

16. 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.

17. Conversion Coatings
• Oxidation
• Phosphate Coatings
• Chrome Coatings

18. 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 mm thick typically
– Multiple colors possible
– Improved Corrosion and Wear
Resistance

19.  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

20. • 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 H2SO4, 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

21. • 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.

23. 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 + H2O Al2O3.H2O
Anodized coatings may be coloured with organic dyes and inorganic
pigments to give decorative effects.

24. 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 mm

25. • 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

26. 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 mm

27. • 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

28. • Aircraft parts, refrigerators, reflectors,
machine parts etc are anodized by
this method Al articles used as doors,
windows, showcase panels &
household utensils are anodized by
this method.
Applications

29. Thermal Treatments
• Surface Heat Treatment
• Diffusion Coating
• Hot-Dip Coatings
• Weld Overlay Coatings

30. 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

31. 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

32. 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

33. 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.

34. Metal Coatings
• Electroplating
• Electroless Coatings
• Metallizing of Plastics and Ceramics

35. 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 mm)
• 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

36. 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))

37. 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

38. 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

39. 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

40. Vapor Deposition
• Physical Vapor Deposition (PVD)
– Thermal PVD
– Sputter Deposition
– Ion plating
• Chemical Vapor Deposition (CVD)

41. 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 mm to as thick as 1 mm.

42. 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)

43. 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

44. 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

45. 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

46. 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

47. 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

48. Contents:
Protective coatings:
Surface preparation for metallic Coatings
Solvent cleaning
Acid pickling
Alkali cleaning
Sand blasting
Electro plating One example with explanation

49. 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.

50. 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.

51. 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

52. 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.

53. • Metallic coatings are mostly
applied on Iron and steel because
these are cheap and commonly
used construction materials.
There are two types of metallic
coatings.
Metallic Coatings

54. • The base metal which is to be
protected is coated with a more anodic
metal for eg. Coatings of Zn, Al and
Cd steel are anodic because their
electrode potentials are lower than
that of the base metal ie. Fe.
i. Anodic coatings

55. • 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

56. METHODS OF APPLICATION
OF METALLIC COATING

57. • 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

58. • 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

59. a. Galvanization
 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.
• The article is then washed with water in a
washing bath & dried in a drying chamber.

60.  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

61. 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.

62. • 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

63. • 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

64. 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

65. 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.
2. Metal Cladding

66.  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

67. • 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

68. • 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

69.  it is probably the most important and most
frequently applied industrial method 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

70. ©2010 John Wiley & Sons, Inc.
M P Groover, Fundamentals of
Modern Manufacturing 4/e
Electroplating

71. • 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.

72. Contents:
Chemical conversion coatings: anodizing,
phosphating and chromate coating.

73. • 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
b. VITREOUS COATINGS OR CEREMIC
PROTECTIVE COATINGS

74. 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.

75. 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 MgSO4, MgCO3, Na2Co3
etc.