In Japan, main unit of oil refineries have been operating for 20 years or more. Many places of business receive authorization of the self-imposed maintenance engineering organization for extension of the open inspection of plant for production. However, accidents due to corrosion etc. have still been occurring frequently. In the Japanese chemical equipment field, safety evaluation method is not fixed yet. At present, the four law of Safety Act is applied to the defected equipment. The method requires that the defects are to be removed and to be recovered to the minimum thickness by repair welding etc. In this paper, the accident occurrence situation of chemical equipment and tanks are shown, and describes relation between the deposit repair welding and four law of Safety Act.

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Copper tubes were immersed in formic acid solutions ranging in concentration from 1 to 10000 mg/L for up to 84 days to investigate the effect of concentration of formic acid on copper corrosion. Concentrations of copper ion in test solutions were measured to determine the dissolution rate of copper. Concentrations of copper ion in formic acid solutions from 1 to 1000 mg/L were saturated in a certain value, while concentrations of copper ion in 5000 and 10000 mg/L formic acid solutions increase with an increase in immersion period. Corrosion form of copper tubes soaked in formic acid solutions from 1 to 1000 mg/L was an ant's nest corrosion, while the uniform attack occurred on copper tubes soaked in 5000 and 10000 mg/L formic acid solutions. Weight losses of specimens have a tendency to increase with increasing concentration of formic acid.

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A film formed on copper plates immersed in the synthetic freshwater containing silica and chloride ion has been assessed using FT-IR, SEM, EPMA and cathodic reduction. The scale formed on copper plates immersed in 20 ppm Cl⁻ solution without silica composed mainly cuprous oxide (Cu₂O) and its morphology was angular shape with facet faces. Granular scales aggregated and deposited on copper plates immersed in the solution containing silica and increased in size and number with immersion period. The composition of the scale was found to be a silicon oxide containing siloxane bonds by FT-IR analysis.

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To control stress corrosion cracking (SCC) of stainless steel used in marine and coastal environments, it is important to clarify its spatial progress and time dependence on its initiation and propagation. In order to analyze initiation and propagation behavior of SCC in three dimensional (3D) view, SCC of stainless steel in the presence of chloride was observed for the first time in the world by using micro focus X-ray computed tomography (CT) of highly general use. The two and three dimensional information about initiation and propagation of SCC in stainless steel was obtained with this technique more easily than with a conventional way.

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Cracking in steel induced by hydrogen in alkaline conditions was observed to investigate the initiation of hydrogen embrittlement. Fractures of steel due to pre-absorbed hydrogen propagated mainly inward from the surface of the steel. In contrast, fractures of steel due to non-pre-absorbed hydrogen extended along the specimen surface. The latter case is likely to correspond to the atmospherics corrosion conditions where hydrogen entry into the steel is periodic.

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Stress corrosion cracking may occur when the welding structure made from stainless steel is used in oceanic moist environment. The initiation and the propagation process of atmospheric SCC (ASCC) were modeled in this report. The initiation lifetime of ASCC is estimated to be 1.1 years under the following conditions of repair-welded joints: surface residual stress in the axial direction of the welded joints of 362 MPa, residual stress in the thickness of the pipe of 200 MPa, degree of sensitization of 20%, and salt deposition of 0.1 g/m²[NaCl]. According to the relationship between the crack propagation rate and stress intensity factor, the initiated crack would penetrate the thickness of 20 mm within 7.6 years.

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The iodine-sulfur thermochemical cycle for hydrogen production takes place in very harsh environments. Structural metallic materials for the hydrogen-iodide decomposition are exposed in the high-temperature halogen corrosion and the hydrogen embrittlement environment. To evaluate adaptability of the materials, corrosion rates and mechanical properties (the yield strength, the tensile strength, and the elongation) were measured. Prepared test specimens were exposed to ambient gas consisting of HI, I₂, H₂O and H₂ (molar fraction, 1：1：6：0.16) at 450℃ for 1000 hours at the atmospheric pressure. After the exposure, the corrosion rates were obtained by the weight loss of each specimen. Nickel-based alloys (Hastelly C-276, MAT21, Inconel 625) exhibited appropriate corrosion resistance (＜0.03 g m⁻²h⁻¹). In addition, no degradations of the mechanical properties for the MAT21 and the Inconel 625 were observed. The specimens of tantalum and titanium showed the hydrogen embrittlement ; the specimens of zirconium and niobium exhibited poor corrosion resistance. The specimens of molybdenum (Mo) exhibited good corrosion resistance, however strength degradation of Mo is causing concern. As the results, the nickelbased alloys are well suited for the structural materials within this environment from the viewpoint of the corrosion resistance. MAT21 among them is the outstanding material with an eye to its corrosion resistance and mechanical properties.

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