Corrosion Science - Definition and Theory

Definition:

Corrosion is defined as destruction and consequent loss of metals through chemical or electrochemical attack by the environment.

Main Types of Corrosion:
1. Dry Corrosion
2. Wet Corrosion

Theory of Corrosion:

Corrosion takes place in a metal due to the formation of anodic and cathodic regions in the same metal surface, or when two metals are kept in contact with each other.

Oxidation and Reduction reactions takes place at Anode and Cathode Respectively. Since Oxidation involves liberation of electrons, metal undergoes corrosion at anodic region. The liberated electrons move towards cathodic region constituting corrosion current. The corrosion product is formed due to the diffusion of metal ions liberated at anodic and cathodic regions.

Corrosion Reactions:

At anode oxidation takes place liberating electrons. Metal atoms are converted into metal ions.

M --> M^n^++ne^-

Cathodic Reactions are of several types. The most common types are explained here. They are:
1. Hydrogen Evolution ( Hydrogen type Corrosion )
2. Oxygen Absorption ( Oxygen type Corrosion )

1. Hydrogen Evolution Type:

It takes place in the Absence of oxygen

(i) Acidic Medium:

2H^++2e^--->H~^2

(ii) Basic or Neutral Medium:
2H~^2O+2e^--->H~^2+2OH^-

2. Oxygen absorption type

This type of reaction takes place in the presence of oxygen.

(i) Acidic Medium:
O~^2+4e^-+4H^+-->2H~^2O

(ii) Basic or Neutral Medium:
H~^2O+O~^2+e^---> 4OH^-