An Integrated Theory of Stress Corrosion

Abstract

THE electrochemical-mechanical theory now favoured states that the dislocation substructure is the controlling factor in the process of stress-corrosion cracking of austenitic stainless alloys^1–4. If the dislocations become piled up on slip planes due to a low stacking fault energy, a susceptible alloy results. On the other hand, if a cellular arrangement is obtained as a result of a high stacking fault energy, the alloy is not susceptible to failure. The emergence of the dislocation pile-ups at the surface produces the chemically reactive sites for crack initiation. To explain the apparently anomalous stress-corrosion resistance of ‘Incoloy 800’ (high stacking fault energy and a planar dislocation arrangement) and nichrome (planar arrangement) the concept of short-range order was introduced^2,3. It was postulated that in the higher nickel alloys short-range order is present which on deformation leads to a planar structure in spite of a high stacking fault energy. The disorder on the slip planes created by the passage of dislocations results in chemically reactive sites on the surface. Furthermore, it was postulated that if re-ordering can occur at a rapid enough rate the chemically reactive sites are destroyed and the resistance to stress-corrosion failure is high^2,4.