The corrosion and hydrogen embrittlement behaviours of tantalum in concentrated sulfuric acid solutions ranging from 80 to 98% at the elevated temperatures from 205 to 300°C have been studied in order to determine feasibility of tantalum as a construction material of the acid digestion equipment for the pultonium-contaminated organic wastes. Moreover, the inhibiting effects of the addition of N-O compounds such as nitric acid, nitrates, nitrites and NO_x, gases on both the corrosion of tantalum and the hydrogen absorption in tantalum have been also investigated. The results obtained are summarized as follows: (1) The corrosion rate of tantalum and the amount of hydrogen absorbed by tantalum increase with increase of sulfuric acid concentration and solution temperature. (2) Tantalum is uniformly attacked and the amount of hydrogen absorbed by tantalum increases proportionally with corrosion rate of tantalum. (3) Tantalum becomes brittle when the hydrogen content in tantalum exceeds about 100ppm. (4) The corrosion rate of tantalum and the amount of hydrogen absorbed by tantalum are simultaneously reduced by the addition of N-O compounds to the sulfuric acid solutions. (5) These inhibiting effects can be attributed to that nitrogen dioxide formed from N-O compounds in the sulfuric acid solutions is adsorbed on tantalum.

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In order to investigate the effects of environmental factors on the atmospheric corrorion of carbon steel in Niigata, the rate of corrosion and the amount of chemical pollutants in atmosphere were determined by gauze method and the data of some meteorological elements were obtained for four years from 1974 to 1978. It looks the amount of sulfate, sulphur oxides and nitrogen oxides in industrial area were larger than the ones in the others. The effects of those factors on the rate of corrosion were investigated and the correlations of some couples among the factors were analyzed by the single correlation coefficients. Summary of the results are as follows: 1) The effects of chloride, sulfate and sulphur oxides on the rate of corrosion showed strong positive correlations and their influences were singificant. 2) The effect of wind velocity on the rate of corrosion showed strong positive correlation and it was described in the previous paper. 3) The effect of each element of air temperature, duration of sunshine and solar radiation on the rate of corrosion showed strong negative correlation. It seems to conceal the effects of the meteorological elements because the amount of chloride, sulfate and sulphur oxides in atmosphere become large values in cold winter season. 4) The effects of precipitations and relative humidity on the rate of corrosion were very weak, then the both elements were not important in Niigata. 5) The correlation coefficient between wind velocity and each factor of chloride, sulfate and sulphur oxides showed strong positive value and the one between air temperature (duration of sunshine and solar radiation) and the each factor above showed strong negative value. It seems to be understandable because chloride was carried by northwestern monsoon from seaside and sulphur oxides were increased by discharged gases from combustion in cold winter season.

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Classification of the case histories of 965 encountered for last eleven years made clear that more than 30% of failures were caused by corrosion and that 98% of those were owing to localized corrosion such as pitting and stress corrosion cracking. Localized corrosion failures often results in considerable economic loss and disaster. This paper describes a few cases of localized corrosion and preventive studies which were conducted in order to improve structure, material and environment.
The first case is pitting corrosion of a stainless steel strainer in the purification plant of waste water using active carbon. Active carbon promoted the occurrence of pitting corrosion. The design modification of structure and cleaning of the strainer was employed as a countermeasure.
The second case is stress corrosion cracking which originated from corrosion pits under small deposits in a reactor made of austenitic stainless steel. From the results of plant tests, material of the reactor was changed from austenitic stainless steels to titanium. The third case is stress corrosion cracking which occurred around weld zone of SUS 316L in a distillation plant. The same material was used for the new fabricated plant, while stress corrosion cracking was prevented by solution heat treatment using large furnace.

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Service experiences were summarized on the hydrogen absorption by titanium and factors affecting the absorption were analyzed and reviewed both on environmental side and metallurgical side. Some environmental conditions have been known to cause this absorption but further investigations are still needed in the future. Several preventive methods, such as surface oxidation treatment, have been applied with satisfactory results in some cases.

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Corrosion control for mobil marine metallic structures is described. Almost all parts of marine metallic structures are protected from corrosion by paint coating. The outer surfaces of marine metallic structures, which are immersed in the sea, are protected by paint coating together with cathodic protection. Interior surfaces of ballast sea water tanks are protected from corrosion by paint coating, cathodic protection, inhibitor and/or innert gas method.
Material selection for paint coating, cathodic protection, inhibitor, etc. and their application methods are summerized.

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