One of the authors (T. M.) has investigated the effect of pH on the corrosion fatigue behaviour of a low carbon steel, and reported the relationship between the corrosion fatigue life and pH. The present work has been carried out to make clear the effect of pH on the fatigue crack propagation rate of the same material, and the results are compared with the previous data. Experiments have been made of fatigue crack propagation of 0.15% carbon steel in aerated HCl-NaCl-NaOH solutions with different pH values of pH 0.5, 5.3 and 11.2, at 25°C. In the acidic solution of pH 0.5, the crack propagation rate was an order of magnitude greater than that in the solution of pH 5.3 or pH 11.2. In the slightly acidic or in alkaline solution (pH 5.3 and pH 11.2), the crack propagation rate was influenced by the stress intensity factor, K. Some wedging effect by solid corrosion products formed on fatigue surface was also observed. At low K values, crack propagation rate was greater in the alkaline solution of pH 11.2 than that in the solution of pH 5.3. Increasing the K value tended to show a reversed relationship in the propagation rate between pH 5.3 and pH 11.2. The pH dependence of crack propagation rate was in good accord with previous results of pH dependence on the fatigue life as a function of cyclic stress amplitude.

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The electrode impedance of Al-Cu, Al-Mn and Al-Mg alloys have been measured at 10³Hz under potentiostatic-cathodic polarization conditions to investigate the relation between the cathodic polarization and the pitting corrosion of aluminum alloys. The cathodic current of Al-Mg and Al-Mn alloys decreased with increasing setting time but that of Al-Cu alloys remained nearly constant. Since the pitting corrosion depends on the cathodic reaction, the pitting corrosion resistance of Al-Mg and Al-Mn alloys in chloride solution may be estimated to be larger than that of Al-Cu alloy. From the time variation of AC impedance at metal-solution interface, the relationship between the local current and the measured polarization curve is discussed. It is likely that the film property influences significantly the pitting corrosion of aluminum alloys.

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Stress-corrosion tests of Cu-30% Zn alloy under a constant strain rate were conducted in Mattsson's type solution of both non-tarnishing (pH 4.3) and tarnishing (pH 7.0) conditions, in order to examine the relation between the mode of cracking in tarnishing and non-tarnishing solutions. Fractographic examination showed that intergranular cracking predominated in a non-tarnishing solution of pH 4.3, where it has been reported that U-bend specimen failured indicates the nature of transgranular cracks. It is thought that a single distinction of the cracking form by tarnishing or non-tarnishing conditions is not justified, due to the observed evidence of intergranular stress-corrosion cracking occured in both conditions.

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Corrosion problems of metals and alloys in caustic soda containing chloride and/or chlorate ions have been described in connection with the process conversion of Japanese chlor-alkali industry. Iron and carbon steels are useful in alkaline solutions even at high concentrations. Stainless steels are desirable in alkali free of chloride and chlorate, but are susceptible to localized corrosion of various forms such as pitting, crevice corrosion, and stress corrosion cracking including caustic embrittlement, when the solution contains aggressive impurities, i. e., either Cl^- or ClO₃^-.

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