CORROSION

 CORROSION

Corrosion is the destruction of metals and alloys by the chemical or electrochemical reaction with the environment. 

During corrosion, metals are converted to metallic compounds at the surface and these compounds wear away as corrosion products. The corrosion of iron is known as rusting. Examples of corrosion are:

1. Rusting of iron in the moist atmosphere form Fe2O3.2H2O
2. Formation of basic CuCO3 (green film) on the surface of copper.
3. The reaction of chlorine gas with Mg or Sn.

Rate of corrosion

It is expressed in Milligrams per year (mpy). mpy= 534W/DAT 

Where W=weight loss in mg, D= the density of the material in g/cm2, A= the area of the material in sq. inch, T= exposure time in an hour.

Mechanism of Corrosion

They are of two types

• Direct chemical corrosion or dry corrosion (oxidation, corrosion due to other gases, liquid metal corrosion) and
• Electrochemical corrosion or wet corrosion.0

Electrochemical or WET corrosion

This type of corrosion takes place when 

• Metal is in contact with a conducting liquid
• Two dissimilar metals or alloys are immersed partially in a conducting solution. According to electrochemical theory,
• Separate anodic and cathodic areas are developed between which current flows through the conducting medium. 
• At the anodic area oxidation and generation of electrons takes place.
• These electrons formed in the anodic area move through the conducting medium and it is called corrosion current.
• The environment is the cathodic area where the electrons are received( reduction ) non-metallic ions like OH- or  O2- are formed. 
• Diffusion towards the metallic ions formed in the anodic area and nonmetallic ions in the cathodic area takes place and the corrosion product is formed in between the anodic and cathodic area.