The anodic polarization behavior and time dependence of anodic current of 99.99% aluminum in 0.5N NaCl solution (pH 4, 6, 8.5, 10 and 12) were examined potentiostatially. The surface structure and the composition of corrosion product films formed on the surface were also observed by a scanning electron microscope, X-ray microanalyzer and energy dispersion type X-ray spectrograph, and the pitting mechanism of pure aluminum was discussed. Pitting corrosion of pure aluminum in 0.5N NaCl at pH 8.5 seemed to be caused by Cl^- which enters through cracks at the interface between the surface film and a rhombic corrosion product formed at less noble potentials than the pitting potential. This rhombic substance was observed only at pH 8.5, and was not formed at pH 4, 6, and 10 in 0.5N NaCl solutions. In 0.5N NaCl solution at pH 12, the pitting corrosion was affected by chemical dissolution, thus the pitting potential became rather noble. The limiting current observed in the anodic polarization curves in the solutions of pH 8.5, 10 and 12 seems to be controled by the diffusion of OH^-. The potential at which pits are induced for pure aluminum in 0.5N NaCl was about -0.743V at pH 4, -0.757V at pH 6, -0.760V at pH 8.5, and -0.710V at pH 10. The main components of the rhombic corrosion product were ascertained as aluminum, iron, and silicon.

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Corrosion resistance of nitrified titanium was evaluated by measurement of corrosion potential and corrosion rate in 20% HCl and H₂SO₄. It was found that nitride layer remarkably inhibits active dissolution of titanium in these acid solutions. There is a correlation between the structure of nitride layer and potential decay curve. X-ray analysis showed the existence of only TiN and ε-TiN in the nitride layer, but the results of potential measurement and chemical analysis of dissolved ions suggested the existence of unknown higher nitride which composes the outermost thin layer.

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As is well known, there are three types of laboratory tests for Stress Corrosion Cracking (SCC), i. e, constant load, constant total strain and constant strain rate tests. The first one has advantages that the state of stress is well defined and the quantitative data for practical engineering design are available, while the latter two provide a rather qualitative measure of the susceptibility to SCC. A number of studies on SCC have been reported using these types of tests, but there seems to be no established theory which can explain the mechanism of stress corrosion cracking in many alloy-environment systems. Needless to say, SCC is a result of the conjoint effect of stress and corrosion under the specified combination of materials and environments. It is utmost important and necessary to take into consideration, simultaneously, the various conditions such as metallurgical, mechanical and environmental conditions in order to clarify the mechanism of SCC. Therefore, the authors developed a new SCC test apparatus employing constant load method with the measuring system for elongation, potential and acoustic emission which enable us to study the SCC mechanism from the more extensive viewpoint. Results obtained are summarized as follows: i) The new test apparatus developed in our laboratory have demonstrated an excellent performance in clarifying the crack propagation processes. ii) Quantitative data on SCC of SUS 304 stainless steel in boiling 42% MgCl₂ are presented. Interrelationships among the potential, elongation and AE are also discussed. The initiation stage of crack was detected in the order of gas evolution from test specimen, generation of AE, shift of the potential to more active direction, and small stepwise elongation. iii) Acoustic emission during tensile and SCC tests were measured. Frequency analysis revealed that four types of AE were generated during SCC tests. Discussion was also made on the relationship between the stepwise elongation and AE signals.

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