The variation in corrosion behavior of steel in neutral solutions caused by the presence of copper, nickel or chromium has been investigated by means of electrochemical methods. Sodium sulfate and sodium chloride aqueous solutions have been chosen as corrosion environments, in order to reveal the anion effect on the corrosion of steels studied.
Results obtained are as follows:
1) The corrosion resistance of steels in neutral solution may be evaluated by the corrosion potential and the anodic polarization curve.
2) In sodium sulfate aqueous solution, the corrosion resistance of steel was not affected by the addition of nickel, chromium or copper in steel, whereas in sodium chloride solution, the addition of nickel or chromium tends to increase the corrosion resistance of steel. This phenomenon seems to be correlated with the formation of compact corrosion products on the surface of the steels embracing these elements.
3) In the early stage of corrosion in sodium chloride solution, corrosion behavior of steel was affected by the auto-reduction of corrosion product.

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On two examples of submarine pipelines, agreement of the measured potential values with the calculated ones at the end point of the pipelines were confirmed as the extension of the previous report, and the performance of various coatings was investigated based on the potential values measured on the submarine pipelines under cathodic protection as follows.
1) It was confirmed that the potential value at the end point of submarine line calculated by the potential distribution formula shown in the previous report was in good agreement with the measured potential value.
2) By the potential distribution formula, changes of resistance of various pipe coatings were estimated.
3) Mortar lining applied to submarine lines with various coatings improves the coating performance under cathodic protection.
4) Combination systems of impressed current cathodic protection and various pipe coatings were discussed from a economical point of view.

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This study has been made of the effectiveness of corrosion inhibitors and coatings relating to protecting from the sulfide corrosion cracking of high strength steel used in H₂S-containing LP gas systems.
In order to select the effective inhibitors, at first, the corrosion rates by means of weight loss measurements and by polarization resistance measurements were determined on specimens of a high strength steel exposed to aqueous solutions and 0.5 percent acetic acid solutions containing several inorganic and organic inhibitors under the conditions that the solutions were saturated with nitrogen and with hydrogen sulfide, and it was found that oleyl amine-type and commercial organic inhibitors could effectively protect the steel from the sulfide corrosion. Then, the influence of inhibitor concentration upon the protective effect and durability of these selected inhibitors was examined in H₂S-saturated environments.
The specimens treated with these inhibitors at a concentration of 0.5 percent were exposed to liquid propane containing hydrogen sulfide with and without water for 168 hours, and also the specimens coated with zinc-inorganic and polyurethane paints were exposed to the similar environments for periods up to 1, 000 hours, and the hydrogen absorption which is responsible for the sulfide corrosion was examined for these specimens. It was found that the volume of hydrogen absorbed in the specimens treated with amine-type inhibitor and coated with zinc-inorganic paint decreased to considerably small amount.
On the basis of these observations, it is assumed that the sulfide corrosion cracking of high strength steel used in LP gas containing hydrogen sulfide can be prevented by the use of the effective corrosion inhibitors or coatings.

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The anodic dissolution current of iron at constant potential in acid solution decreases markedly by addition of a small amount of iodide ions into the solution. The transient fall in the dissolution current was recorded to follow the inhibition process of iron dissolution by iodide ions, and the following results were obtained.
The rate of the inhibition process by iodide ions is represented by
dφ/dt=Aexp(-Bφ)
where φ is the relative amount of inhibition in percentage, and A and B are constants independent of the potential. Parameter A is proportional to the concentration of iodide ions, while B appears to be almost independent of the inhibitor concentration. Parameter A, which represents a rate constant of the inhibition process, is calculated to be much larger in the case of perchloric acid than that in sulphuric acid. It is suggested that the adsorbability of matrix anions of the solution on the surface of iron is closely related to the rate of the inhibition process by iodide ions; the stronger the adsorbability of matrix ions, the smaller the rate constant A.

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