Duplex stainless steels (DSS) possess a ferritic (α) phase matrix dispersed with austenitic (γ) phase precipitates. Due to the chemical compositional arrangement and the characteristic α/γ dual phase structure, DSS possess superior mechanical properties and corrosion resistance without sacrificing economic performance. In any case, even DSS suffer from crevice corrosion. Accordingly, the measure of crevice corrosion is of pragmatic importance. The modes of crevice corrosion on DSS were complex because of its preferential dissolution and distinguishable as some types from the outside to the center of the crevice: the passivity retention region, the region with preferential dissolutions of γ phase precipitates, and the region with preferential dissolution of the α phase matrix. It is described the study to elucidate the preferential dissolution mechanism of crevice corrosion on DSS, based on the in-situ observation of crevice corrosion dissolution behavior, and the analysis of dissolution behavior of DSS, α and γ phases in a simulated crevice solution.
A copper part (C1100), used in an EGR-equipped engine, got corroded. This copper part was directly contacting an iron part (S45C), for which the iron part should have been corroded before the copper one began being corroded, because iron is less noble than copper. However, it was found that the corrosion had taken place only on the copper part. In order to clarify the reason, we investigated the phenomenon for substances related to the corrosion and tried to identify how the environments became corrosive and what mechanisms caused the corrosion, using a chemical method. As the result, we came up with the conclusion that a solution consisting of sulfuric acid and nitric acid might probably have worked on the corrosion, by verifying that copper becomes more corrosive than iron in a place where nitric acid is present.
An ammonium thiocyanate (NH4SCN) solution is widely used in hydrogen embrittlement evaluations of high-strength steel materials. It is known that an increase in the specific solution volume to the specimen surface area results in a severe evaluation in hydrogen embrittlement testing. The reason for that is explained in this paper based on the change in the solution pH induced by a cathodic reaction, which accompanies thiocyanate ion decomposition and governs hydrogen absorption. In addition, the pH dependence of the cathodic reaction governing hydrogen absorption is made clear by using a sodium thiocyanate solution containing a buffer solution to control the solution pH. It is shown that immersing steel specimens in the pH-controlled sodium thiocyanate solution achieves a higher hydrogen content compared with the level attained with the NH4SCN solution.