Intergranular corrosion:

This type of corrosion occurs along the grain boundaries and only where the material is especially sensitive to corrosive attack moreover corrosive liquid posses a selective character of attacking only at the grain boundaries but leaving the interior untouched or slightly attacked.

This type of corrosion is believed to be due to fact that the grain boundaries contain material which shows electrode potential grain anodic than that of the grain centre in the particular corroding medium.

This may result precipitation of certain compounds at the grain boundaries there by leaving solid solution just beneath to the grown boundary.

This solid solution is anodic with respect to grain centre as well as to the precipitated compound so that it will be preferentially attacked by corrosive environment.

EXAMPLE:

The grain boundary corrosion is generally encountered in alloy. For example during the welding of stainless steel (fe, c, cr) chromium carbide is precipitated at grain boundaries.

Thus the region just adjacent to grain boundaries become depleted chromium in composition is more anodic with respect to solid solution with in the grain which is richer in chromium. For the same reason it is also anodic to the particle of chromium carbide is precipitated.

Usually corrosion in welded stainless steel is not serious problem which can be restored by heat treatment method which dissolves chromium carbide precipitated during the welding. However when stainless steel assembles are to large to be heat treated it is then necessary to prevent the depletion of chromium by special means use of law carbon 0.03 is one remedy.

Intergranular corrosion occur on microscopic scale with out any external sign of intensive attack on account of this sudden failure of the material occurs (without pre warning ) due to loss of cohesion between grains.

The remedy for such corrosion of alloy is rapid heat treatment followed by rapid quenching to avoid the heterogeneous precipitation that usually occurs due to slow cooling.