The nature of surface films formed on Fe-18Cr-8Ni and Fe-8Cr-10Ni alloys, which simulate the grain matrix and the Cr depleted zone of sensitized Type 304 stainless steel, respectively, has been examined by multiple-angle-of-incidence ellipsometry and scanning electron microscopy after the potentioststic oxidation of the alloys in pH 6.5, 0.5kmol·m^-3 Na₂SO₄ at 523K. A steep increase in the thickness of surface film was found on the Fe-18Cr-8Ni alloy in the potential region above -0.2V (SHE), which corresponds to the transpassive region on the steady state anodic polarization curve of the alloy. No such an increase in the thickness of surface film was recognized on the Fe-8Cr-10Ni alloy. The films formed on the Fe-18Cr-10Ni alloy in the transpassive region was rough and porous, whereas the films on the Fe-8Cr-10Ni alloy was relatively smooth and compact. The susceptibility to IGSCC of the sensitized Type 304 stainless steel occurred at the potential where transpassive dissolution started and it was enhanced with increasing potential. The occurrence of susceptibility to IGSCC was found to have good correspondence to the occurrence of a steep increase in the thickness of the surface film on the alloy simulating the grain matrix. An IGSCC model, which is based on the formation of thick porous surface film on the grain matrix, cracking of the film, a decrease in pH in the cracks and the accelerated dissolution of the Cr-depleted zone, was proposed.

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Hot corrosion behavior of a nickel-base superalloy, MM 247, was examined at 800-1, 050°C in combustion gases flowing at Mach 0.35-0.6 which were prepared from 1.2-24ppm Na₂SO₄-containing air and 0.5% sulfur-containing fuel. The results showed that the corrosion morphologies were classified into two categories. The first was characterized by a thin corroded zone. The second was characterized by a thick corroded zone with severe internal oxidation and severe sulfidation. In the case of the specimen corroded at Mach 0.5 and 900°C with 6ppm Na₂SO₄, the first was observed at the central area of the leading edge and the second was observed at the marginal area of the leading edge and the down-stream region. The corrosion morphology of the central area changed from the first to the second with increasing temperature and gas velocity, and with decreasing Na₂SO₄ content. It is proposed that these changes in the corrosion morphology are attributed to variation in the thickness and replacement frequency of the salt deposit layer, which depend on test conditions.

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A duplex stainless steel, manufactured from the mixture of SUS 316L stainless steel powder and SUS 444 one, was investigated on its localized corrosion resistance in aqueous chloride environments. The duplex stainless steel had almost the same resistance to pitting and crevice corrosion as SUS 316L and SUS 444. The pitting corrosion was initiated on its γ-phase for its less content in Cr and Mo. The crevice corrosion propagated through the dissolution of the α-phase because the corrosion potential of SUS 444 was lower than that of SUS 316L in its active state. The duplex stainless steel exhibited excellent resistance to stress corrosion cracking (SCC) at 200°C, 10³ppm Cl^- condition while SUS 316L and conventional 18% Cr duplex stainless steel suffered SCC. The galvanic behavior between the α-phase and the γ-phase was found to suppress SCC on the γ-phase.

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A parameter was proposed which predicts the occurance of pitting corrosion of aluminum alloy 5052 which was galvanic-coupled with tin free steel in NaCl solution. The parameter is identified as a kind of electrochemical anodic polarization resistance. It was shown that the occurance of pitting of 5052 related to decrease of the value.

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The photochemical machining of metals using a ferric chloride etchant is useful for producing micro machining parts and electronic parts such as printed circuit boards, shadow masks for CRT devices, IC lead frames etc. This paper presents the outlines of their examples of products, photo etching processes, photoresist materials for etch mask, machining accuracy, etchant composition, etching mechanism and characteristic phenomena.

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