DU- B.Sc Hons or program - Contemporary methods of Surface Coating( Galvanization, Carburising, sherardising)

1. Institute – Rajdhani College of DU
Name – Sunny Gupta
Course – B.Sc (H) Chemistry
Roll no. – 5872
Section - B
Semester – 5th
Submitted To – Dr. Harish Sir

2. Contemporary Surface Coating Methods
 Galvanization , Sherardizing
 Carburizing
Galvanization
The process of applying a protective Zn, Cd, Al or Sn coating to iron or steel, to
prevent rusting
Coating material should have : rust resistance property, Low melting point than iron
and steel
Galvanization is Classified into :
1 Hot dip Galvanization
2 Cold dip Galvanization ( Electroplating Galvanizing )
3 Sherardizing ( Vapour galvanizing / dry Galvanization)
Steps of Galvanization :
Surface preparation  Galvanization
(Rinsing  Pickling  Fluxing ) (HDG/ CDG/ Sherardizing)

3. Hot dip Galvanization
 Rinsing - washing iron or steel with
water
 Pickling – heating iron or steel in dilute
sulphuric acid at 60-70°C
 Fluxing agent – Zinc ammonium Chloride
 Zinc bath - At 450°C
 Annealing - Heating above critical
temperature and then slow cooling.
It relives the strain, makes material
. more ductile and reduces the hardness.
Cold dip Galvanization
Anode (+) – Pure Zinc
Cathode (-) – Iron or Steel
Electrolyte – Zinc Sulphate/ Zinc Hydroxide
 Rolling - To get uniform Coating

4. Comparison between HDG and CDG
Hot dip Galvanization
 It is done In molten metal
 Coated layer is strongly adhered
 Coated layer is thick and uniform
 Long life time
 Comparatively less shiny
 Expensive
Cold dip Galvanization
 It Is done in wet environment
 Comparatively weakly adhered
 Comparatively thin and non uniform
 Short life time
 Coating is shiny
 Cheaper

5. Sherardizing
 Sherardizing is the formation of a corrosion-resistant, zinc-iron alloy coating, on the
surface of steel or iron. The method involves the heating of the object in an airtight
container that has zinc powder. A thermal diffusion process occurs and the zinc
diffuses into the object’s metal surface, forming the zinc-iron alloy.
 The external layer of the zinc-iron alloy formed is a pure zinc adherent coating with
a smooth surface and uniform thickness. It provides a very strong and long-term
protection against corrosion and abrasion.
 This process, which is ideal for small objects is also a slow process.
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6. Carburizing ( Case Hardening)
 Carburising is a thermochemical process in which carbon is diffused into the
surface of low carbon steels to increase the carbon content to sufficient levels
so that the surface gets harden while the bulk remains unaffected (i.e. soft).
 Objective of Carburizing :
1. To get lighter and harder steel
2. To make water 💧 resistant
 Carburizing is classified into :
 Solid Carburizing (pack Carburizing) :- A low carbon steel is heated in closed
vessel containing mixture of Coal and Barium Carbonate in this process carbon
monoxide derived from a solid compound decomposes at the metal surface
into nascent carbon and carbon dioxide. Then diffusion of nascent Carbon into
the steel takes place.
C + BaCO3  BaO + 2 CO
CO + CO  C + CO2

7.  Liquid carburization :- Steel is heated in the salt bath So that the carbon to
be diffused in the workpiece comes from a molten salt that is a mixture of
sodium cyanide (NaCN) and barium chloride (BaCl2).
Concentration of cyanide in solution is constantly monitored throughout the
process So that the process remain stable.
 Gas Carburization :- The steel is heated in contact with carbon monoxide
and/or a hydrocarbon which is readily decomposed at the carburizing
temperature.
 Post Carburization process
When the iron or steel is cooled rapidly by quenching, the higher carbon
content on the outer surface becomes hard due to the transformation from
austenite to martensite, while the core remains relatively soft and tough.

8.  Conditions affecting depth of diffusion:
1 Heating Temperature
2 Carburizing Time
3 Available Carbon
Depending on the amount of time , temperature and available carbon, the affected
area can vary in carbon content. Longer carburizing times ,higher temperatures
and higher availability of Carbon typically increase the depth of carbon diffusion.
 Among all three type of Carburization, gas Carburization is the best