The dissolution rate of hydrous oxides of iron, chromium and nickel in acid solutions was expressed as follows; v=ka_H+^la_A^m, where a_H+ is the proton activity, a_A the anion activity, l the reaction order for proton and m the reaction order for anion. The value of l is close to 0.5 irrespective of the species of hydrous oxides but the value of m depends upon the anion species. The dissolution mechanism of hydrous oxide was discussed from electrochemical point of view. The dissolution reaction may be regarded as consisting of the following two coupling and coupled reactions.
OH^-_(H.O.)+H⁺_aq→H₂O_aq
[Me(OH)_n(A^r-)_p]^(z₊-n-rp)+_(H.O.)→[Me(OH)_n(A^r-)_p]^(z₊-n-rp)+_aq
where[Me(OH)_n(A^r-)_p]^(z₊-n-rp)+ represents the intermediate metal-anion complex transferring across the Helmholtz layer at the hydrous oxide-electrolyte interface. The rate of these two reactions is controlled by the potential difference, φ_HL, across the Helmholtz layer, and the net-dissolution rate of hydrous oxides can be determined by superimposing the polarization curves (potential difference, φ_HL, vs. reaction rate) of the two coupling and coupled reactions. The intermediate metal-anion complex has been estimated from the comparison of experimental results with theoretical derivation by assuming the Freundlich isotherm for the adsorption of anions on hydrous metal oxide.

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The passivation behavior of nickel in sodium sulphate solutions up to 290°C has been studied in an autoclave. An external reference electrode system was applied to the electrochemical measurements in the autoclave. The composition of the passive film formed on nickel was identified by the transmission electron diffraction analysis. The corrosion potential of nickel in the deaerated 0.1N Na₂SO₄ solution above 200°C was very close to the hydrogen reversible potential, indicating that nickel behaved as a hydrogen electrode. Under the presence of dissolved oxygen, the corrosion potential of nickel increased to higher potentials and the specimen showed a locallized attack. Anodic polarization experiments showed that the stability of the anodic passivity of nickel increased with increasing temperature. However, the typical pitting which will be caused by the aggressive nature of sulphate ions, was observed at relatively less noble potentials at 200°C. At temperatures above 250°C pitting occurred in the transpassive region. However, no indication of pitting was found in borate, phosphate, bicarbonate and nitrate solutions at 200°C. Transmission electron diffraction analysis showed that the passive film formed on nickel mainly consisted of NiO and unknown substances which are assumed to be hydroxides. With increasing temperature and anodic potentials, the number of diffraction rings decreased and those of NiO became predominant.

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Effects of prestrain and prestrain rate on stress corrosion cracking susceptibility of austenitic stainless steels have been investigated by transmission electron microscopy and constant load tests. Specimens prestrained at low strain rates showed planar distribution of dislocations. On the other hand, specimens prestrained at high strain rates showed cellular distribution of dislocations having specific orientations and intersecting each other at certain angles. Surface morphology of the latter specimens indicated double slips. Constant load SCC tests by using specimens prestrained at various strain rates in air exhibited that the lower the strain rate, the higher was the susceptibility to stress corrosion cracking. This was interpreted in terms of the dislocation distribution formed by prestraining.

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