Unit 2- Class 6- CORROSION (Corrosion Control)

In the classes to come, let us learn about corrosion control.



CORROSION CONTROL: Corrosion of a metal is a natural spontaneous process, by which metal is converted into a more stable compound so that corrosion control is more realistic than corrosion prevention. In general preventing the formation of galvanic cells can control corrosion.
The methods used to control corrosion are as follows:

1.    Protective coatings:
Corrosion of metal can be controlled by isolating them from the corrosive atmosphere. This can be done by covering the metal (base metal) with a layer of another metal. This process is known as metal coating.
The Principal type of coatings applied on the metal surface are :
1. Metal coating 2. Inorganic coating 3. Organic coating.     


Metal coating: This coating is the deposition of a protective metal over the surface of the base metal. The method can be applied by electrodeposition, flame spraying, cladding, hot dipping etc.
On the basis of coating there are two classes:

1.    Anodic coating: It is produced by coating a base metal with more active metal which are more anodic with to the base metal for egs: Iron is coated with Zn, Mg, Al etc.,
One of the important properties of this type of coating are that, even if the coating is ruptured, the base metal does not undergo corrosion. The exposed part of the base metal is cathodic with respect to the coating metal and coating metal only undergone corrosion there by protecting the base metal. The protection is there as long as coating is there. Galvanisation is one of the best egs in anodic coating.

Galvanisation: It is a process of coating a base metal (iron) with zinc(Zn) metal. This process usually carried out by hot dipping method.
Process: first the base metal surface is washed properly with organic solvents to remove any organic matter (like oil, grease etc) on the surface afterwards it washed with dil. H2SO4 to remove any inorganic matter (like rust). Finally the base metal is well washed with water and air-dried. The base metal then dipped in a bath of molten zinc maintained at 425-4300c and covered with a flux of NH4Cl to prevent the oxidation of molten zinc. Then excess zinc on the surface is removed by passing through a pair of hot rollers so that a proper thin coating is obtained.
Application: Galvanized articles are mainly used in roofing sheets, fencing wire, buckets, bolts nuts, pipes and tubes etc. but galvanized articles are not used for preparing and storing food stuffs. Since zinc dissolves in dil. Acids and become toxic.

2.    Cathodic coating: These are the coating produced by coating a base metal with more cathodic (noble metal) for egs: iron is coated with tin, nickel, Cr and Cu. But these coatings provides protection only when it is undamaged and absolutely free from gaps otherwise rapid corrosion of the base metal takes place as a result the formation of large cathodic and small anodic area.
Tinning is the best egs to explain cathodic coating.

Tinning: It is a process of coating base metal (iron) with tin (Sn). It can be carried out by hot dipping method. 
            The iron sheet (base metal) first washed thoroughly with organic solvents to remove any organic matters. Then treated with dil. H2SO4, to remove rust and seal deposits, finally it is washed well with water and air-dried. It is then passed through ZnCl2 and NH4Cl flux (molten) so that molten tin can adheres properly on the metal surface then base metal passed through tank that contains molten tin.
Finally passed through a series of rollers immersed in palm oil. So that uniform, undamaged, continuous deposit of tin takes place. Tinning will provide complete protection against corrosion if it covers the surface completely.
Application: tinned articles are largely used in the manufacturing of containers used for storing foodstuffs, copper utensils are coated with tin so that contamination of food with copper can be prevented.


We will discuss of the next control methods in the classes to come