Intergranular corrosion passivation required

The Cr-depletion theory has been accepted as the mechanism by which a sensitized stainless steel becomes susceptible to intergranular corrosion [60]. Recall that a minimum of 10 to 13wt.% Gr in solid solution is required to form protective passive films on Fe-Ni-Gr alloys... 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

Sensitized steels were maintained at potentials between 0 and -1-350 mV in 2N H2SO4 and the products of dissolution were analyzed. On analysis of the products, it was found that the chromium content decreased from 18.6% at 0 to 13.7% at -1-350 mV. It has been shown by different tests on steels that a decrease in chromium content is accompanied by an increase in passive current density (the density required to achieve passivation). Intergranular corrosion was... 

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