The anodic polarization behavior of nickel and nickel-chromium alloys in molten Na₂SO₄ at 1173K was investigated from the observation of corrosion morphologies for specimens polarized at various anodic potentials. The anodic polarization curve of nickel exhibited an increase in current density due to active reaction and a subsequent passive region with an increase in polarization potential. A number of fine particle consisting of Ni and O were observed in the salt adhered on nickel after polarization at potential in active region and the surface oxide was scarcely detected on the nickel, whereas the surface oxides of 40-100μm in thickness corresponding to NiO were observed for nickels polarized at potentials in passive region. On the other hand, the anodic polarization curves of nickel-chromium alloys revealed a self-passivation behavior without current peak corresponding to the active reaction. The formation of thin and dense surface oxide was detected for nickel-chromium alloys after both immersion and polarization at the passive potentials. Their surface oxides were found to consist mainly of Cr₂O₃ by X-ray diffraction and ESCA analysis. The corrosion loss of nickel after the immersion test in molten Na₂SO₄ was extremely large in comparison with those of nickel-chromium alloys. This result could be explained from the difference in anodic polarization behavior between nickel and nickel-chromium alloys.

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Utilizing a combined loading Stress Corrosion Cracking (SCC) test apparatus, SCC threshold stress criteria of two material-environment systems where environmentally assisted passive films are formed has been studied. Two systems studied in this report are SCC of carbon steel in boiling nitrate solution (60%Ca(NO₃)₂+3%NH₃NO₄) and SCC of 70/30 brass in Mattsson's solution (ammonical solution) at room temperature.
The test results obtained are summarized as follows:
1) The threshold stress of nitrate SCC of carbon steel could be well explained by the maximum principal stress theory although the elastic fiailure of substrate metal was ruled by the shear strain energy theory. The environmentally assisted passive film of this system is supposed to be thich magnetite, and so brittle that it could suffer brittle fracture at lower stresses than the yield stress of substrate metal. Intergranular cracks might inititate at the fractured passive film along the grainboundaries oriented in perpendicular to the maximum principal tensile stress.
2) SCC threshold stress of 70/30 bass in Mattsson's solution was in good agreement with the maximum principal stress theory. The SCC threshold stress is much lower than the elastic failure stress of substrate metal. The tarnished film of this system is supposed to have an elastic modulas higher than that of substrate metal. and fracture stress lower than the yield strength of film-free substrate metal.
3) Based on the test results of previous and this reports, it was concluded, that the SCC threshold stress of the systems which developed an environmentally assisted film could be ruled by the maximum principal stress theory, and that the SCC threshold stress of metals which had a selfhealing ability of passive film in atmosphere could be ruled by the strain energy theory which is similar to the elastic failure stress criteria of substrate metal. Whether the threshold stress of APC-type SCC is evaluated by the strain energy theory or maximum principal stress theory seems to depend on the mechanical characteristics of passive film in a given system.

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Effect of sensitization heat treatment at 973K on the probability distribution of failure times of stress corrosion cracking of Type 304 stainless steel in boiling magnesium chloride solution at 399K, 411K and 427K and mainly at the stress of 100MPa has been examined. In all conditions examined, total failure time T_f is found to be divided into crack initiation time T_i and propagation time T_p, their distribution obeying a three parameter Weibull distribution with the high correlation coefficient of 0.941-0.996. Effect of sensitization heat treatment is observed only at 399K, at which T_p determines T_f. The shape parameter m of the distribution shows a maximum value for the highest sensitization heat teratment condition at 2.00×10³s, at which intergranular cracking is observed in a part of fractured surface. From the comparison of fracture mode with the shape of the discribution, a mixed type distribution is also suggested to be fitted instead of a single three parameter Weibull distribution.

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In the cooling water system treated with zinc-polyphosphate inhibitor, the relationship between inhibitor performance and corroded conditions of heat exchangers was studied. When cooling water system was kept in wet lay-up state, inhibitor concentration in the water jucket of heat exchangers decreased 15-30 percent per week, and turbidity increased 30-150 percent per week. These results show that corrosion rate of shell-plate in stagnant cooling water is more rapid than in flowing cooling water. Applied trouble discrimination method based on SiO₂ ratio to the chemical composition of corrosion products, corrosion trouble was observed in shell-plates of heat exchangers.
When cooling water system is kept in wet lay-up state, cooling water in the water jucket of heat exchangers is isolated fro mouter system. In this perfectly closed system, zinc-polyphosphate inhibitor was not effective for protection of corrosion of carbon steel, and metal (carbon steel) dissolution occurred. However, in the perfectly closed system, since the dissolved oxygen content of the system was reduced with lapse of time, reduction process at cathodic region was stopped, so corrosion of metals seemed to be inhibited.

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On the basis of previous studies, the effects of environmental and metallurgical factors on the degradation of creep rupture and fatigue strength properties of metallic materials in high temperature corrosive environments were reviewed. The weakening and strengthening mechanisms due to high temperature corrosion were also discussed.

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