What is corrosion? How corrosion occurs?Corrosion is the gradualdestruction of material, In the presence of moisture, an oxidation reaction takes place on usually metals, by chemical the energized area of the metal surfacereaction with its environment. to elute metal as an ion (anode). A reduction takes place on low energy area (cathode).
WHAT IS CORROSIONCONTROL?• Protecting the surface of the metal sothat corrosion reaction dont take place atsurface is known as corrosion control.• There are several methods to controlcorrosion which are described below.
Corrosion control Metallic Organic Electrochemical coating coating Using alloys protection Corrosion Using pure By modifying Proper inhibitors metal environment designing •Paint •Enamel Inorganic •Sacrificial•Electroplating •Varnish coating Anodic•Metal cladding •lacquers protection•Hot dipping •Impressed•Cementation current cathodic•Metal Spraying 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 fromcorrosion 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 alloysare more corrosion resistant then pure metals.6. Inorganic coating The in organic coatings are produced by chemical or electrochemical reactions at a surface of metal. This coating providesprotection to the base metal from corrosion. Some inorganic coatingare 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 orsilica gel leads to decrease in corrosion.• The acidic corroding environment can be neutralized by addition ofammonia, lime or sodium hydroxide.8. Proper designing• Proper designing of the equipment plays a significant role in controlling thecorrosion.• Proper designing can slow down or even avoid corrosion .• Avoid the contact of dissimilar metals in the presence of corrodingenvironment
9. Electrochemical protection The Electrochemical protection or cathodic protectionevolves the supply of electrons to metal there by forcing the metalbehave 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 isconnected to a more anodic metal so the metallic equipment acts as cathode.As a result corrosion take place at anodic metal and cathodic metallicequipment is saved from corrosion• Impressed current cathodic protection This process consist of connecting the material to be protected tonegative terminal of DC source. The positive terminal of DC source isconnected to a insoluble anode like graphite, carbon, scrap iron . The currentsupplied is in opposite direction to the corrosion current. Thus the metal to beprotected act as cathode and get protected.