CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
solutions containing metallic ions (Cr 6+ , V 5+ , Ce 4+ , Fe 3+ ) and is an unusual form of intergranular
corrosion.
According to accepted mechanism, in sensitized alloys intergranular corrosion occurs due
to the depletion of chromium at the grain boundaries, and formation chromium rich carbide
precipitates (Cr 23 C 6 ), if the carbon concentration is 0.03 % or higher. The degree of sensitization
depends on the chromium and carbon content of the alloy and in generally increases with increase
in carbon content and decrease in chromium content. Chromium from the solid solution is utilized
for Cr-rich Cr 23 C 6 formation resulting in lower chromium content adjacent to such carbides along
the grain boundaries. Such chromium depleted regions are venerable to corrosive attack, because it
does not contain sufficient chromium to form passive film. The intergranular corrosion of
austenitic stainless steel due to depletion of chromium is shown in Fig. 2.7 [2].
Fig. 2.7: Intergranular corrosion of austenitic stainless steel in nitric acid [2].
Nonsensitized austenitic stainless steels also undergo intergranular corrosion in highly
corrosive solutions containing metallic ions in their higher valance state. This happens when
austenitic stainless steels are used in the transpassive region. The effect of minor alloying elements
such as sulphur, silicon, and phosphorus have deleterious effect on such type of corrosion due to