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An electron-microscope study of the corrosion fracture surfaces of sensitized austenitic stainless steel specimens exposed to Strauss (CuSO4-H2SO4) solutions and Huey solution (HNO3), respectively, revealed marked differences in the corrosion attack. In the Strauss test the dissolution of the matrix occurs along a narrow region in the vicinity of the carbide particles which remain essentially unattacked, while in the Huey test both the carbide particles as well as a wider region of the matrix are attacked. The extent of the preferentially dissolved region around the carbide particles after the Strauss test suggests that the metal dissolution cannot be explained on the basis of a local chromium impoverishment, but that an electrochemical process between a more noble carbide and the less noble matrix is operative. This is supported by results of electrochemical potential measurements on carbide and steel samples in the various corrosive environments which show that in the Strauss solution the carbide has a more noble electrochemical potential than the steel matrix, while in the Huey solution the carbide is less noble than the steel matrix.

The dislocation arrangements in 304 stainless steel and Incoloy 800 have been determined as a function of temperature and strain. Dislocation pile-ups were observed in both alloys in spite of the fact that the stacking fault energies were considerably different, viz. 30 erg/cm2 for the Incoloy 800. The stress-corrosion behaviour of the two alloys in magnesium chloride boiling at 142°C was determined. The Incoloy 800 was found to be considerably more resistant than the 304 stainless steel. After critically reviewing the current theories of stress corrosion and structural and stress-corrosion data for a series of austenitic alloys, it was concluded that the data could best be correlated with the theory proposed by Swann. According to this theory, the presence of short range order is expected to markedly influence dislocation arrangement and stress-corrosion behaviour. A review of the literature revealed that there is some indirect evidence to indicate the presence of short range order in certain of the austenitic alloys, but the evidence is not conclusive.

The mechanism of stress-corrosion cracking was investigated in purified 16%Cr-20%Ni stainless steels, with and without single-element additions of manganese, molybdenum and titanium. Potential/time behaviour and potentiostatic current/time curves were studied in 42% MgCl2 at 146°C. The purified alloy did not crack under severe testing conditions, manganese addition left it apparently immune, but molybdenum or titanium additions induced cracking. The effect of deformation on dislocation mobility was examined by transmission electron microscopy. All of the results are in accord with a periodic electrochemical-mechanical mechanism of crack propagation, wherein susceptibility to cracking is associated with highly restricted slip, alloys which readily cross slip being immune. It is shown that cracking is associated also with low stacking fault energy.