The Mechanism of Hydrogen Evolution on Iron in Acid Solutions by Determination of Permeation Rates

The permeation of electrolytic hydrogen in Armco iron membranes has been investigated by the electrochemical technique described elsewhere. In purified solutions it has been found that the permeation rate follows the equations previously derived. This rate is inversely proportional to the thickness and to the surface coverage with atomic hydrogen θ. The intercept of the plot of reciprocal of the permeation rate against the thickness varies inversely as the coverage as also predicted by the equation. Anomalies in the thickness dependence often reported in the literature have been traced to the adsorption of impurities and to redeposited iron, which then limit the over‐all permeation rate. The variation of permeation with cathodic overpotential has been determined. It is found that at moderate overpotentials indicating a slow discharge rate‐determining step followed by Tafel recombination. At higher overpotentials , indicating a constant coverage brought about by a transition to electrochemical desorption. In confirmation of this mechanism attempts have been made to determine the pseudocapacitance of the electrode system by galvanostatic cathodic transients. The results show the complete absence of pseudocapacity in acid solutions, thus proving that discharge of hydrogen ions is the rate‐determining step.