The corrosion behavior of Type 316 stainless steel in boiling formic and acetic acid solutions was investigated by scanning electromicroscope (SEM) and surface analysis of X-ray photoelectron spectorscopy (XPS). In 50% formic acid, intergranular corrosion occurred, and MoO₃, Cu₂O and CuO existed in the film produced on the steel surface. But it was considered that the film is insufficient to inhibit the corrosion. In 100% formic acid, since the film consisting of MoO₃, MoO₂, Ni₂O₃ and NiO has somewhat corrosion resistance, the corrosion form did not show the welldefined intergranular corrosion, but the preliminary step of its intergranular corrosion. In any acetic acid concentration, intergranular corrosion was observed. In 90% acetic acid, unevenness on the steel surface was particularly formed by difference of the corrosion rate of each grain. But in acetic acid, stable compounds of Cr were produced at underlayer from the steel surface, and their compounds were supposed to be contributed to the improvement of corrosion resistance.

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Corrosion fatigue crack growth rate (da/dN) of 80kgf/mm² grade high strength steel HT80 has been investigated in synthetic sea water under the open circuit, anodic and the various cathodic potentials at a frequency of 0.17Hz. The effect of cathodic potential on da/dN as a function of ΔK or ΔK_eff was examined and the crack closure effect caused by corrosion reaction products within crack was discussed. The arrangement by the effective stress intensity factor range ΔK_eff, which was taken from the crack closure effect of white cathodic reaction deposits (a mixture of CaCO₃, MgCO₃ and Mg(OH)₂), was useful to evaluate the influence of cathodic potential on the crack propagation. The da/dN under the cathodic protection potential of -0.7--0.9V (Ag/AgCl) became smaller than that in open circuit, while cathodically polarized potential more negative than -1.0V gave rise to the large acceleration of crack growth rate due to hydrogen embrittlement.

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The effect of various factors produced by welding on the corrosion fatigue crack growth rate was investigated by measuring crack length, the corrosion potential and fractography. For simulate test of welded joint, hardness and grain size were varied with heat treatments. The tensile residual stress and the compressive residual stress were caused by electron beam welding. In actual test, the welding joint was used. The da/dN of specimens with hardness and residual stresses was less than that of the base metal. The da/dN of the specimen with large grain size was as same as that of the base metal. When da/dN is smaller than that of the base metal, the corrosion potential is lower than that of the base metal. Increasing the stress ratio from 0.1 to 0.4, da/dN is decreased. Removeing the coating of the specimen makes da/dN small. The crack growth rate of the corrosion fatigue crack in the welded joint is smaller than the base metal.

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Two kinds of SCC tests were conducted with Type 310S steel potentiostatically in 35% MgCl₂ solution a t80°C. In the SCC test I, both a tensile stress and an electrode potential of -350mV. SCE were simultaneously applied immediately after passivating at -400mV for 1h. The SCC test II had an additional polarization procedure at -330mV before loading to introduce relatively large pits on the specimens. In the SCC test I, “Type A” cracks issued from small pits of which depths were less than 10μm under tensile stresses at and above 19.5kgf/mm² nearly equal to the yield stress of 21.3kgf/mm². In the SCC test II, “Type B” cracks issued from large pits deeper than critical depths which depended on applied stresses at and above 14kgf/mm², below which no cracks initiated. Threshold stressing condition for the initiation of Type B crack could be expressed in terms of threshold stress intensity, K_ISCC≅8kgf/mm^3/2, calculated from the applied stress and the depth of large pit as an initiation site of the crack. This value of K_ISCC is equal to that for the cracks from a sharp notch previously reported.

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Mechanism of pit initiation in passive metals is considered on the basis of the assumption that pits nucleate through two-step processes. First, perturbations of the ionic concentration and of the electric field occur in the solution, resulting in the local accumulation of halide ions at the anodic sites on the passive film. The local accumulation of halide ions then brings about the second process on the passive film where hemispherical halide nuclei are formed and the nuclei grow inwards in the passive film. The growth of the nuclei causes finally the film breakdown and pit initiation. The possibility of each process to proceed continuously is discussed by use of the dynamic and the irreversible thermodynamic stability theories. It is shown that the present model sufficiently explains such kinetic features of pit initiation processes as the pit initiation and the repassivation potentials and the induction time, as well as the noise phenomena.

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The morphologies and facets of crystallographic pit formed as faceting dissolution process of SUS 304 steels single crystal were potentio-statically examined on (100), (110) and (111) specimen surfaces in 0.5kmol/m³-H₂SO₄+0.5kmol/m³-NaCl and 2.5kmol/m³-H₂SO₄+0.5kmol/m³-NaCl solutions. All types of crystallographic pit with {100}, {110} and {111} facets, and with their mixed facets such as {100}+{110}, {100}+{111}, {110}+{111} and {100}+{110}+{111} appeared on three specimen surfaces. It seems that these results give an important indication for the mechanisms of stress corrosion cracking and anodic dissolution.

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The problem concerning electrolysis corrosion of pipe line caused by stray current from DC electrified railroad was fairly solved by appling drainage method. In this report next item is explained. (1) Leakage current from railroad and electrolysis corrosion. (2) Construction of drainage method. (3) Recent problem of them.

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