Intergranular corrosion principles

These steels can also be liable to intergranular corrosion. In addition to Cr carbides, Cr nitrides may be precipitated and thus contribute to the depletion of Cr at the grain boundaries. The mechanism is in principle the same as for austenitic steels, but in the ferritic steels the attacks can occur for a wider spectrum of environments and lower contents of C + N. The critical range of temperatures that makes the metal sensitive is higher, namely above 925°C, and the attack occurs therefore close to the weld or in the weld metal itself. The material becomes resistant again after annealing for 10-60 minutes at 650-815°C. It is noted that the effects of the different temperature ranges are opposite to what is the case for the austenitic steels. 

Intergranular corrosion is best known to occur with certain types of stainless steels and aluminum alloys, but in principle, many other alloys can be susceptible to this kind of corrosion damage. 

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. 

The method is based on the principle that if an aluminum alloy is susceptible to intergranular corrosion, this susceptibility will show as... 

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