Effects of Mo, Cu and C on the corrosion rate and the type of corrosion of 18 Cr-8 Ni stainless steels which were deformed into a U shape have been investigated in 5N-H₂SO₄ solutions containing NaCl up to 0.4mol/l. The results are summarized as follows:
(1) The general corrosion, the intergranular corrosion and/or the stress corrosion cracking occur according to the contents of Mo, Cu and C in the steels and the concentration of NaCl in the solution.
(2) The intergranular corrosion is mainly observed when the steels with a larger amount of Mo are corroded in a solution containing 0.2mol/l NaCl. The stress corrosion cracking occurs in solutions containing 0.1-0.3mol/l NaCl if the steels contain both Cu and Mo amounting repectively to more than one percent, and in a solution containing 0.4mol/l NaCl it occurs in the steels with lesser amounts of Cu and Mo.
(3) Cl^- ion in the solutions acts as an activator or as an inhibitor of corrosion reactions according to its concentration and the chemical compositions of steels.

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Ellipsometric study combined with electrochemical measurements has been made on anodic oxide films on nickel in neutral sodium borate-boric acid solution in the potential regions of passivity and oxygen evolution. It is found that the oxide film once formed in anodic oxidation is not easily removed by the cathodic reduction, and that the surface treated with the potentiostatic reduction immediately after electropolishing gives good reproducibility in subsequent measurements. The reduced surface thus obtained is used as the optical reference. Films formed in the passive region are assumed to be NiO with optical constant of 2.7-0.25i, and the thickness estimated from ellipsometry increases linearly with rise of the potential from 4 to 13Å. The thickness is about 1.3 times thinner than that calculated from the electrochemically measured anodic film charge assuming the film as NiO.
In the oxygen evolution potential region, the optical surface state is unsteady even after 1hour anodic oxidation. It is shown, however, that the film in oxygen evolution not only undergoes the change of its optical constant but also thickens rapidly with rise of the potential, and that at a certain potential the optical change begins to deviate from the expected direction of film growth. It is concluded that in the oxygen evolution region the film initially converts to a higher nickel oxide, probably Ni₂O₃, with continuation of growth up to a limiting thickness and finally takes a constant thickness independent of the potential.

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