CORROSION CONTROL

What is corrosion?

Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment.

How corrosion occurs?

In the presence of moisture, an oxidation reaction takes place on the energized area of the metal surface to elute metal as an ion (anode). A reduction takes place on low energy area (cathode).

WHAT IS CORROSION CONTROL?

• Protecting the surface of the metal so that corrosion reaction don't take place at surface is known as corrosion control.

• There are several methods to control corrosion which are described below.

Corrosion control

Using alloysElectrochemical

protectionOrganic coating

Metallic coating

Corrosion inhibitors

Using pure metal

By modifying environment

Proper designing

•Electroplating•Metal cladding•Hot dipping•Cementation •Metal Spraying

•Paint •Enamel•Varnish•lacquers

Inorganic coating

•Sacrificial Anodic protection•Impressed current cathodic protection

1. Metallic coating

Metallic coatings provide a layer that changes the surface properties of the work piece to those of the metal being applied.

• Electroplating It is the process by whish a coating metal is deposited on the base metal by passing a direct current through an electrolyte solution.

• Metal cladding• The base metal to be protected and coating metal are sandwiched by pressing through rollers under the action of heat and pressure.

• Hot dipping The base metal to be coated is immersed in a bath of the molten coating metal

• Cementation In this process a uniform surface coating is obtained by heating the base metal in a power of coating metal.

• Metal Spraying The coating metal in molten state is sprayed on base metal by means of spraying gun.

2. Corrosion inhibitors It is the substances which when added in a small amount in an

environment reduces the rate of corrosion of a metal exposed to that environment. These are of two types anodic and cathodic.

3. Organic coating These are applied on metallic surface for protection from corrosion and also to impart decorative value. It includes :

• Paints These are mechanical dispersion mixtures of finely divided solid called pigments in a fluid medium.

• Enamel It is a dispersion of pigments in a varnish.

• Varnishes it is a colloidal solution of natural or synthetic resin in solvents like oil or sprit.

• Lacquers It is a colloidal dispersion of a solution of cellulose derivatives like cellulose nitrates, resins and plasticizers in a solvent.

4. Using pure metal Pure metals are homogenous and corrosion resistance . The greater the purity of metal more it will be corrosion resistant.

5. Using alloys It is a homogenous mixture of two or more metals. The homogenous alloys are more corrosion resistant then pure metals.

6. Inorganic coating The in organic coatings are produced by chemical or electro chemical reactions at a surface of metal. This coating provides protection to the base metal from corrosion. Some inorganic coating are chome,phospate,oxides etc.

7. By modifying environment • Lowering the temperature reduces the corrosion . • The removal of moisture by the addition of adsorbents like alumina or silica gel leads to decrease in corrosion.• The acidic corroding environment can be neutralized by addition of ammonia, lime or sodium hydroxide.

• Proper designing of the equipment plays a significant role in controlling the corrosion.• Proper designing can slow down or even avoid corrosion .• Avoid the contact of dissimilar metals in the presence of corroding environment

8. Proper designing

9. Electrochemical protection The Electrochemical protection or cathodic protection evolves the supply of electrons to metal there by forcing the metal behave as cathode and as the result metal is not corroded.

Cathodic protection can be achieved in two ways: • Sacrificial Anodic protection In this protection the metallic equipment to be protected is connected to a more anodic metal so the metallic equipment acts as cathode. As a result corrosion take place at anodic metal and cathodic metallic equipment is saved from corrosion

• Impressed current cathodic protection This process consist of connecting the material to be protected to negative terminal of DC source. The positive terminal of DC source is connected to a insoluble anode like graphite, carbon, scrap iron . The current supplied is in opposite direction to the corrosion current. Thus the metal to be protected act as cathode and get protected.

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