Effect of ammonium thiocyanate on hydrogen entry behavior of low alloy steel under galvanostatic cathode charging

Abstract

The effect of ammonium thiocyanate (NH4SCN) on the behavior of hydrogen entry into low alloy steel under cathode charging was investigated using electrochemical hydrogen permeation technique. In this study, hydrogen entry sides were polarized galvanostatically to control the rate of hydrogen evolution reaction. The potential, hydrogen charging current density and hydrogen permeation current density were measured at pH of 3.0 in acetic buffer solution with and without 3 g·L−1 NH4SCN. From the Tafel slope of the cathode reaction and the dependence of hydrogen concentration on hydrogen charging current density, it was confirmed that the hydrogen evolution reaction proceeds under Volmer-Tafel mechanism in this study. NH4SCN drastically increased hydrogen entry into steel. To analyze the results of this study, the efficiency of hydrogen entry was calculated from the relationship among hydrogen charging current density, hydrogen permeation current density and hydrogen overpotential. It was found that the hydrogen entry efficiency was drastically higher in NH4SCN environment than that in NH4SCN free environment. However, the coverage of adsorbed hydrogen atoms on hydrogen entry side decreased in NH4SCN environment. To discuss the mechanism that NH4SCN increases hydrogen entry efficiency, the activation energies of hydrogen adsorption and hydrogen absorption were estimated by temperature dependence of the hydrogen charging current density and the hydrogen permeation current density. It is suggested that NH4SCN increased the activation energy of hydrogen adsorption although it decreased that of hydrogen absorption.