Arsenic added as arsenite (AsO2-) to acid solutions promotes the entry of hydrogen into steel structures during cathodic polarization. The extent of the hydrogenation depends on electrochemical potential. At the corrosion potential of the steel, arsenite inhibits the entry of hydrogen; the reduction of arsenite to elemental arsenic suppresses the hydrogen evolution reaction. Only when the specimen is polarized to sufficiently cathodic potentials is there noticeable entry and permeation of hydrogen in steel. The greater the concentration of arsenite is in the solution, the greater is the extent of cathodic polarization needed to bring about measurable permeation. The entry of hydrogen does not depend on arsine formation; the greatest relative permeation occurs in the potential range where arsenic is the stable phase.