Cracking of Sensitized Austenitic Alloys in Polythionic Acids

6 Cracking of Sensitized Austenitic Alloys in Polythionic Acids 

One function of polythionic acids is to act as cathodic depolarizers, thereby stimulating dissolution of chromium-depleted grain-boundary material. Another possible function is that their cathodic reduction products (H2S or analogous compounds) stimulate absorption to interstitial hydrogen by chromium-depleted 

Martensitic, Precipitation-Hardening, or Ferritic Stainless Steeb 

Since stainless steels are passive and exhibit a noble potential, they can be coupled successfully to metals that are either passive or inherently noble. This includes metals and alloys like silver silver solder copper nickel 70% Ni-Cu alloy 76% Ni, 16% Cr, 7% Fe alloy and, usually, aluminum in environments in which it remains passive. 

As discussed in Section 19.2.4, stainless steels are best employed under fully aerated or oxidizing conditions, which favor the passive state. Whether used in handling chemicals or exposed to the atmosphere, the alloy surface should always be kept clean and free of surface contamination. Otherwise, crevice corrosion may cause pitting and localized rusting. Austenitic stainless steels cooled too slowly through the sensitizing temperature zones tend to rust in the atmosphere. 

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