The precipitation of the carbides depletes the
matrix of chromium adjacent to the grain boundary.
The diffusion rate of chromium in austenite
is slow at the precipitation temperatures; therefore,
the depleted zone persists, and the alloy is
sensitized to intergranular corrosion. This sensitization
occurs because the depleted zones have
higher corrosion rates than the matrix in many
environments. Figure 1 illustrates how the chromium
content influences the corrosion rate of
iron-chromium alloys in boiling 50% H2SO4 containing
ferric sulfate, Fe2(SO4)3. In all cases, the
alloys are in the passive state. The wide differences
in the corrosion rate are the result of the differences
in the chromium content.With the lower chromium-
bearing stainless steels, the passive
film is more soluble in the acid, and therefore,
more metal must dissolve to repair the film.