Hydrogen permeation through steel electroplated with Zn or Zn–Cr coatings

Abstract Hydrogen permeation characteristics with steel samples, already electroplated with Zn or Zn–Cr are obtained. Alloy coatings with a thickness of 2.5 and 5.0 μm and various Cr content (3, 5 and 10 mass%) were prepared at conditions, close to industrial line trials. The investigations are performed in a conventional Devanathan–Stachurski cell in 0.5 M Na 2 SO 4 solution, pH 6 at two current densities (30 and 300 mA cm −2 ). The Zn and Zn–Cr coatings are impermeable during hydrogenation at a low charging current density and they permeate small amounts of hydrogen when a high charging current density is applied. The amount of hydrogen permeating into the steel substrate is in orders of magnitude lower in comparison to a bare steel substrate. Therefore, Zn and Zn–Cr coatings protect steel effectively under conditions which cause hydrogen ingress. The influence of the thickness of the Zn and Zn–Cr coatings as well as the influence of the Cr content in the Zn–Cr alloy on the hydrogenation of the steel substrate is expressed by negligible differences in the hydrogen permeation rates. The hydrogen permeation through Zn and Zn–Cr coatings is not a continuous process. Fluctuations of the permeation current which could be associated with the specific dynamics of hydrogen bubbles at the cathode surface and with the catalytic and barrier properties of the alloy coatings, regarding hydrogen evolution and permeation are registered. More expressed fluctuations of the permeation current are recorded in the case of the Zn–Cr alloy. During the charging of the Zn or Zn–Cr coatings at a high current density permanent structural changes occur. Disintegration of a subsurface metal layer and oxidation of the Zn coatings is observed, accompanied by crack formation in the case of Zn–Cr alloys.