A study was made on the electrochemical characteristics of nickel-containing and nickel-free high strength steels in hydrogen sulfide aqueous solution. It was demonstrated that the overpotential of cathodic reaction of each of the steels examined is decreased by the addition of hydrogen sulfide and the overpotential gradually decreases with the lapse of immersion time. These phenomena are quite pronounced when nickel is embraced in the steel. It is indicated that the change in overpotential depends upon the corrosion products, described in the preceding report, formed on the surfaces of these steels. The degree of decrease in the cathodic overpotential appears to have a close relationship with the susceptibility of sulfide corrosion-cracking of high strength steel.

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The test results in the previous report indicated such tendencies that by applying some coatings to the steel surface in submerged zone macro-corrosion could be suppressed and also current density for cathodic protection could be reduced. So, successively, the protective effect of applying one or two times zinc-coating to the steel surface in submerged zone alone, has been investigated and the following results were obtained:
(1) When area ratio of steel surface in sea water to that in mud zone saturated with sea water was less than two or four, the rate of macrocorrosion between sea water and mud zone reduced to about one-fifth by applying one time coating to the steel surface in sea water alone and further reduced to about one-tenth by applying two times zinc-coating.
(2) As the galvanic action by zinc-coating ceased in a short time, the above protective effect of zinc-coating on macro-corrosion seemed to depend on the covering effect of vehicle rather than the galvanic action by zinc.
(3) Because movement of mud line due to wave is inevitable at harbors, one should apply zinc-coating not only to the steel surface in sea water but also to that in mud zone near mud line.

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