When the exhaust gas in marine engine is passing the supercharger, it is necessary to cool the water way of the supercharger by sea water or fresh water to prevent deterioration of it. A lot of corrosion product which is strongly affected by DO and pH in the water is formed in its water way. In this paper, the contents of crystalline hematite, goethite and magnetite in corrosion products of cast iron were examined by X-ray diffractions and the effect of cooling water rate of sea and fresh water on corrosion rates was also investigated. The amount of hematite in fresh water was less than that in sea water, and magnetite in fresh water was about twice than that in sea water. There was no goethite in both water. The corrosion rate increased with an increase in cooling water rate, but the corrosion rate in sea water was larger than that in fresh water.

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Effects of manufacturing processes and of solution treatment on hot corrosion behavior were investigated for a hot-corrosion-vulnerable Ni-base superalloy. The tested processes were single-crystallization (SC), directional solidification (DS), HIP-consolidation (HIP) and superplastic-forging of the HIP-consolidated material (SFG). Before solution treatment, specimens from DS were the most vulnerable to the corrosion. Their vulnerabilities differed depending on whether the corroding surface laid across or along the growing direction. The vulnerabilities of the specimens from HIP and SFG were 50 to 80% of those from DS. Effects of solution treatment differed depending on the manufacturing processes. It caused a considerable decrease in vulnerability for specimens from DS. The trend was reversed for those from HIP and SFG. As a result vulnerability for the specimens after solution treatment decreased in the order of SFG, HIP, SC and DS. Part of these effects could be understood by assuming that compositional and/or structural inhomogeneity within and around dendrites and crystal grains would make the alloy more vulnerable to the corrosion.

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Hydrogen embrittlement susceptibility of a 13Cr martensitic stainless steel of 495 in Vickers hardness is investigated in chloride solutions of 20-20000ppm Cl^- with pH of 3-7 at 20-80°C by means of slow strain rate test (SSRT) and constant load SCC test. HE susceptibility is found to increase with aggressiveness of the environment and no crack initiates in mild environment unless localized corrosion occurs. In the mild environment HE susceptibility depends on temperature, pH and chloride concentration. On the other hand HE susceptibility in the severe environment is determinedsolely by temperature. This may suggest that the composition of the solution inside the localized corrosion pit which serves as the crack initiation site is almost independent of the composition of the bulk solution and its aggressiveness is governed mainly by temperature. A simple electrochemical modeling indicates that HE susceptibility is determined by hydrogen reduction rate in the steel surface. Furthermore a good correlation between HE susceptibility evaluated under SSRT and that under constant load test may lead to a conclusion that SSRT test is valid for evaluation of HE susceptibility of the steel and environment examined here. Consequently HE preventive measures are suggested.

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Corrosion fatigue crack growth rate (da/dN) of (α+γ) dual-phase stainless steel SUS 329 J1 has been measured in synthetic sea water under free corrosion and the various cathodic potentials at a frequency of 0.17Hz. The effect of cathodic potential on da/dN as a function of ΔK was discussed in the range of ΔK≥20MPa·m^1/2. The da/dN under the cathodic potential of -0.40--0.80V (Ag/AgCl) became much larger than that in open circuit. Cathodically polarized potential gave rise to the large acceleration of crack growth rate due to hydrogen embrittlement.

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The Honshu-Shikoku bridges are planned to connect Honshu with Shikoku by three routes. Among these, the Kojima-Sakaide route was entirely finished on April 10., 1988, and the other two routes are partially opened to traffic. Therefore the feature of the Honshu-Shikoku bridges is to construct many large bridges over straits, many technical developments are required; the development of anticorrosion coatings is one of these. This anticorrosion coatings are so developped as durable against sea mists and of long recoating cycle, because the total area of coating would be 6, 000, 000m² after three whole routes finished. As for the bridge inspection which is a basis of maintenance, a specification is regulated, and by this, disposed full-time inspectors do the inspection work for rust condition and an extent of corrosion daily or regularly, then the coatings are treated at sooner period before the corrosion advances to be harmful. Moreover, many inspection ways and inspection vehicles are attached to the bridges for usual inspection and care. This report shows an outline of the coatings and inspection of the Honshu-Shikoku bridges.

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