1. Corrosion Control methods

2. Methods of corrosion control
Surface Coatings
Proper designing
Use of pure metal
Using metal alloys
Modifying the environment
Use of Inhibitors
Cathodic Protection

3. SURFACE COATINGS
The common and oldest method of protection of a metal from its surroundings is by surface coatings
These may be
i) Metallic coatings
ii) Organic coatings
The coated metal isolates the metal from its corroding environment

4. Requirements for protection
Coating applied must be chemically inert to the environment
Coatings must prevent the penetration of the environment
The metals used for coatings may be placed under two categories
i) metals which are anodic to the metal i.e., metals above it in galvanic series.
ii) metals which are cathodic to the metal i.e., metals below it in galvanic series.

5. Functions of metal protective coatings
To protect the metal from corrosion
To impart aesthetic sense on metal surface
To impart specific mechanical, physical and electrical properties
To give oxidation resistance and thermal insulating properties

6. Cleaning of metal surface
Solvent cleaning : - Removes oils, grease and fatty substances by the organic solvents like naphtha, CCl4, toluene, acetone.
Then the metal surface is cleaned with steam and hot water containing wetting reagents

7. Alkali Cleaning :- Removes old paint – alkali cleansing agents like trisodium phosphate along with soaps and wetting agents

8. Mechanical Cleaning :- Removes loose rust, scales, dirt – by using bristle brush/sand paper/detergents like soap.

9. Flame Cleaning:- Removes loosely adhering scales – heating the metal surface with hot flame followed by wire brush

10. Sand Blasting:- Removes oxide scales – sand of different grain size were blasted on the metal surface with pressure of 25 to 100 atm

11. Pickling and etching:- Removes scales which are adherent - metals except aluminium are immersed in acid pickling solution

12. Metallic Coatings
Hot dipping: Hot dipping is used for producing a coating for low melting metals such as Zn, Sn, pb ,Al.
on Iron, steel or copper
The process involves immersion of a metal in a bath of its molten coating & covered by a molten layer.
The flux cleans the base metal & prevent oxidation of metal coating with molten solution.
Hot dipping is widely applied either by (1)GALVANIZING (2)TINNING

13. Galvanizing
It is the process in which iron or steel is protected from corrosion by coating with a thin layer of zinc.

14. Steps involved in the process of Galvanisation
The base metal iron or steel is cleaned by acid pickling with dil. H2SO4 for min
At a temperature of C
Sheet is then washed and dried
It is dipped in a bath of molten zinc maintained at 425°C – 430°C
The surface of the bath is covered with ammonium chloride flux to prevent oxide formation
Sheet is taken out and excess zinc is removed by passing it between a pair of hot rollers
Sheet is subjected to annealing process at 650°C and cooled slowly
An alloy of zinc and iron are formed at the junction of the base metal and coating metal

15. Uses of Galvanization: It is used to protect Iron used for roofing sheets, wires, pipes, nails, bolts, screws, buckets, and tubes.
Defects : Galvanised utensils are not used for cooking because of solubility of Zn.

16. Tinning
Tinning is the process of coating of tin over the Iron sheet or steel articles
A cleaned iron sheet is passed through a bath of molten flux, then passes through a tank of molten tin
and finally through a layer of palm oil which protect hot tin coated surface against oxidation.

18. Uses: It is widely used for coating of steel, copper, brass & bronze etc .
It is used to store food stuff, ghee oils, kerosene & pickles and used for refrigerators equipment.