Effects of Surface Film on Hydrogen Entry

Abstract

Effects of several types of surface films on hydrogen entry in low alloy steels were investigated using a hydrogen permeation technique. Effects of Zn plating under cathodic hydrogen charge, additions of alloying elements such as Ni, Cu and Mo in an acidic solution, and sulfide film formed in wet H₂S environments were evaluated, and classified into the following two types.

Zn plating drastically suppressed hydrogen entry into low alloy steel. The mechanism was considered to be a resistance against hydrogen permeation, due to small hydrogen diffusivity in Zn or small hydrogen evolution rate on Zn surface. Formation of sulfide film in a wet H₂S environment was also effective for suppressing hydrogen entry according to the similar mechanism. Zn plating and sulfide film were classified into the “surface resistance type”

Additions of Ni, Cu and Mo were also effective for suppressing hydrogen entry in the acidic solution. The mechanism was considered to be a change in hydrogen overpotential, not a resistance to hydrogen permeation. The decrease in hydrogen overpotential would lead to a decrease in hydrogen surface coverage. These alloying elements were considered to work as the “small hydrogen overpotential type”.

In the case of the “resistance” type surface films such as Zn or sulfide, thickness of the steel plate affects the hydrogen concentration just beneath the surface film because it was determined by the balance of the diffusion of hydrogen through the surface film and the base steel.