Pitting corrosion and its protection of stainless steel were studied by using an electrochemical technique. In order to prevent pitting corrosion, it is important to know the inversion potential at which the repassivation of pit begins to undergo. This potential was a function of the bulk concentration of chloride ions and the time of polarization. The alkylamines which had the alkyl groups of large carbon numbers, i. e., di-n-amylamine and di-n-hexylamine were beneficial as the inhibitors of pitting corrosion of stainless steel. Pitting corrosion played an important role in stress corrosion cracking. Inversion potential was also important as a characteristic value to determine the susceptibility to such a cracking. And such a cracking will be prevented in the region between the inversion and the protection potentials.

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In order to find out good inhibitors to retard hydrogen entry and there by hydrogen embrittlement of a high strength steel, the hydrogen content in the steels immersed in 3% NaCl solutions was measured with an addition of different inorganic additives. As the addition of such additives changed the pH value of solution, the hydrogen content was measured as a function of pH and was found to decrease with an increase of pH in acidic region and to become almost constant in basic region. Sodium nitrite was found to reduce hydrogen content significantly which could not be explained with the change of pH.

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Decomposition behavior of sodium nitrite, which has been added to the circulating cooling water system of marine diesel engines as a corrosion inhibitor for steel, was investigated in 50ppm SO₄^2- ion solutions. In the absence of steel, no loss of sodium nitrite was observed in any case. When the sodium nitrite over the critical concentration required for corrosion inhibition was added, it was stable without decomposition and high inhibitive effect was observed. On the other hand, when the sodium nitrite was added below the critical concentration, the rate of decomposition was accelerated with decrease of ratio of the solution volume to the specimen area, and simultaneously pH of solution became high markedly and NH₄⁺ ion was detected. The decomposition of sodium nitrite, which forms NH₃ as the reduction product, may proceed as follows:
NO₂^-+5H₂O+6e^-→NH₃+7OH^-

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The effect of pre-strain on Intergranular Stress Corrosion Cracking (IGSCC) susceptibility of Type 304 stainless steel has been studied using a constant load tensile specimen in 288°C water containing 26ppm dissolved oxygen. The results obtained are summarized as follows: 1) The time to failure of specimens which are pre-strained at room temperature or 288°C are longer than those of non pre-strained specimens. It is indicated that IGSCC initiation is controlled by pre-straining. 2) The stress-strain behavior of material changes by pre-straining. The effect of pre-strain on IGSCC initiation can be explained in terms of a relation between rupture of surface oxide film and stress-strain behavior of material.

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The effects of alloying elements on the atmospheric corrosion of stainless steel, the results of exposure tests in different environments in Japan and the evaluation methods of atmospheric corrosion resistance were discussed.

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