protection of metals from corrosion

M. AWAIS YAQOOB

Protection from Corrosion

Contents

What is corrosion?Cause of corrosionChemistry of Corrosion……Protection

What is corrosion?

Corrosion is the deterioration of materials by chemical interaction with their environment.

Cause of corrosion

Oxygen

Humidity Chemical Salts

Chemistry of Corrosion……

When a metal is exposed to atmospheric oxygen in the presence of water, the well known rusting process takes place.

e.g. “Iron”, when exposed to atmosphere, the metal is degraded to form ferric rust, a red-brown compound is formed.

4Fe + 3O2 + 2H2O → 4FeO.OH

Cont……

Protection……….

There are many methods of protecting metals against corrosion.

They are

1) Design

2) Barrier protection

3) Sacrificial protection

4) Cathodic protection.

1) Design

Generally speaking we want a design that:

• Avoids entrapment of water.

• Shape should be round.

• Allows access for surface preparation and coating operations.

1) Barrier protection

Here, a thin barrier is developed between the surface of iron and atmosphere

by one of the following methods:

a) Painting of the surface

b) Coating the surface with a thin film of oil or grease

c) Developing a thin layer of some non –corrosive metal like nickel,

chromium copper etc., by electroplating.

a) Painting of the surface

To do paint on a metal surface is the most common and useful method of prevention from corrosion.

Paint, an organic compound , actually prevents the metal surface from direct interact with the moisture or any other chemical.

b) Coating the surface with a thin film of oil or grease

This type of method is just as to paint on the surface of metal but the main difference is that here in oily method we just cover the surface with an oil or grease.

The basic purpose of oiling the surface is is to creat hindrence for moisture to get interact with metal surface.

Grease better than oil as it is more thick than oil and covers the surface.

c) Developing a thin layer of some non –corrosive metal by electroplating.

This mehtod is known as some how electrochemivcal metod to avoid from corrosion.

In this method we use the technique of electroplating to cover up the surface of objective material

2) Sacrificial protection

In this method, the surface of metal is covered with a more electropositive metal.

Sacrificial metal can be

3) Cathodic protection.

This method is especially used for underground iron pipes

. Here, the iron pipe or tank is connected to a more electropositive metal like magnesium or aluminum.

. The more electropositive metal acts like anode (supplies electrons) and iron acts like cathode (receives electrons).