Write short notes on Pitting corrosion b) stress corrosion c) Conc.cell d) Intergranular e) Galvanic cell.

a) Pitting corrosion

Pits are holes or cavities that are developed on the metal surface when the protective oxide film breaks.  Around such pits, small anodic and cathodic areas are formed. Due to this corrosion is accelerated in localized area known as pitting corrosion. Pits may also be formed due to surface roughness, scratches, sliding under the load, action of some chemicals etc. 

b) stress corrosion

Stress corrosion or stress cracking is due to the presence of stress caused by heavy working like rolling, drawing or insufficient annealing. This type of corrosion is the combined effect of static tensile stress and the corrosive environment like, Strong caustic alkalis or nitrates (which affected mild steel) Traces of ammonia (Affecting brass) Marine water or H2S gas or solution of acid halides (chlorides particularly) affecting stainless steel etc.  Stress corrosion is probably due to localized electrochemical reaction, forming local anodic & cathodic areas on the metal surface

c) Conc.cell

This type of corrosion occurs when one single metal is exposed to different concentration of electrolyte or air. It is the most important type of concentration cell corrosion which occurs when one part of a metal is exposed to a different air concentration from the other part of the metal. As a result, difference in the potential takes place between differently aerated areas. It has been found experimentally that poor oxygenated (poorly aerated) parts of metals are anodic and highly oxygenated (highly aerated) parts of metal are cathodic.

d) Intergranular

Smaller subunit of an alloy or metal is called as grains. Each grain consists of all the constituents of the alloy. All the grains of a given metal/alloy have identical arrangement of atoms.  This type of corrosion occurs along the grain boundaries where the material is sensitive to corrosion attack (anodic) as compare to that at the centre in particular corrosion medium. .

e) Galvanic cell.

When two metallic materials (dissimilar metals) are electrically connected and exposed to an electrolyte, the metal occupying higher position in electrochemical/galvanic series undergoes corrosion, is called ‘galvanic corrosion’.