Effect of Ammonium Thiocyanate on Hydrogen Absorption by Cathodic Charging in Weak Alkaline Solution

To clarify the mechanism underlying hydrogen absorption enhancement by adding ammonium thiocyanate (NH4SCN) during cathodic charging in a weak alkaline solution, we constructed a kinetic model of the hydrogen evolution reaction and the hydrogen absorption reaction and performed analysis by fitting the kinetic model to the cathodic current density and steady-state permeation current density obtained from hydrogen permeation tests. Hydrogen permeation tests were conducted on pure iron by constant potential polarization at the hydrogen entry side in a weak alkaline solution with or without NH4SCN. The results indicated that NH4SCN enhances hydrogen absorption into pure iron as a result of the increasing surface hydrogen coverage on iron due to the suppression of the chemical hydrogen recombination reaction (Tafel reaction) and the electrochemical hydrogen desorption reaction (Heyrovsky reaction) in this condition.