Pitting corrosion and its protection of carbon steel are studied by using an electrochemical technique. In order to prevent pitting corrosion, it is important to know whether the pre-existing pits continue to propagate or undergo repassivation. A critical potential at which the repassivation of pit begins to undergo is designated as the inversion potential and proposed as a characteristic value that determines susceptibility to pitting corrosion. 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 benificial as the inhibitors for pitting corrosion of carbon steel.

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A new method of titanium coating on mild steel was developed by one of us, at recent year. The method was as follows, the suspended aqueous solution of titanium hydride powder was coated on steel by spray gun, then it was heat-treated in vacuum at a temperature over eutectic temperature (1080°C) of titanium and iron. The formed coating on steel, therefore, became alloy of these metals. If this coated steel is considered about employment as the base of anode for electrolysis, we have to know that how large anticorrosivity does it have under high potential. Accordingly, test specimens varied conditions of heat treatment were prepared, and those corrosivity were investigated in hydrochloric acid under high potential, as compared with the constitution and the composition. As a result of the examination, a good anticorrosive coating was obtained, when the thickness of coated titanium hydride was thicker than 0.4mm, heat treatment temperature was 1090°C near upon the eutectic temperature, and the retention time at the above temperature was 1hr. In order that the coating resists to the corrosion for long term, however, it is necessary that the coating is uniform in composition not only of the surface but of the inside.

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A brief review of the technical literature shows that numerous investigations have been made on alternating stray current electrolytic corrosion of lead, iron, copper, alminum and other metals.
Alternating current corrosion on lead, iron and copper seems to be caused mainey during the anodic half cycle, but alternating current corrosion on alminum may be caused during both anodic and cathodic cycles.
Alternating current corrosion may be also caused by the rectification effects of corrosion product film and or polarization.
Weight loss effect of alternating current on lead, iron and copper is less than 2 percents compared with the effect of equal direct current, but its effect on alminum is very high and more than 40 percents of equal direct current.
The effective remedial measures of alternating current corrosion are selective drainage method and cathodic protection by galvanic anodes.

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In this article, as a part of guide series for introduction to metallic corrosion, various cases of natural corrosion were explained by superposing of the polarization curves for anodic reaction of corroding metals and for catholic ruction of reducible agents dissolved in corrosive medium. Similar electrochemical explanation were also carried out on the heterogeneous corrosion such as galvanic and differential aeration corrosion.

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