Intergranular corrosion is caused by Microsegregation of impurities and alloying elements on the grains boundaries.
The driving force of intergranular corrosion is the difference between the Electrode potentials of the grain boundary and the grain itself, which form a galvanic cell in presence of an electrolyte.
If the phases segregated at the grain boundaries have lower value of electrode potential (higher position in the table of Electrochemical series) they will oxidize (anodic reaction) and the grain metal having higher value of electrode potential will provide cathodic reaction (reduction).
Dissolution of anodic grain boundaries starts from the surface and advances along the grains interfaces. The process results in deterioration of the bonding between the grains and drop of mechanical properties.
If the precipitates at the grain boundaries have higher electrode potential the grains will dissolve (anodic reaction). In this case the grain boundaries will not be attacked.
Intergranular corrosion is a typical defect of austenitic and Nickel alloys. At the temperatures 900-1400ºF (482-760ºC) chromium carbides Cr23C6 form along the austenite grains. This causes depletion of chromium from the austenitic grains resulting in decreasing the corrosion protective passive film. The grain boundaries are anodically attacked in presence of an electrolyte.
This effect is called sensitization. It is also called weld decay since it usually happens during welding process when the zone around the weld is heated.
The chromium carbides form mainly at the grain boundaries and not within the grains. This effect is caused by different diffusion rate of atoms of chromium through the grains volume and along the boundaries saturated with crystal lattice imperfections.
Means of preventing sensitization:
Aluminum alloys containing magnesium (Mg), zinc (Zn), copper (Cu), iron (Fe) are susceptible to intergranular corrosion: