Effects of alloying elements on hydrogen entry to low alloy steels under a cyclic corrosion condition

Abstract

Hydrogen entry in low alloy steels and effects of alloying elements were investigated using a hydrogen permeation technique under a simulated atmospheric corrosion condition and an acidic solution. In the base steel, the hydrogen permeation coefficient sensitively varied depending upon the wetting and drying processes in the cyclic corrosion condition. Additions of Mo, Cu and Ni to the base steel were effective for suppressing hydrogen entry under the cyclic corrosion condition. These elements were also effective in the acidic solution with an initial pH of 3.5. The mechanism of the beneficial effects of these elements was considered to be a change in hydrogen overpotential, which leads to a decrease in hydrogen surface coverage. S was also effective for suppressing hydrogen entry under the cyclic corrosion condition, although it drastically promoted hydrogen entry in the acidic solution. The detrimental effect in the acidic solution was considered to relate the production of H2S, a catalytic promoter of hydrogen entry, accompanying with the chemical dissolution of soluble sulfide inclusions such as MnS. On the contrary, under the cyclic corrosion condition, dissolution of MnS inclusions would lead to an increase in pH because of the consumption of H+ ions in the thin water layer on the steel surface, resulting in the suppression of hydrogen entry.