Cathodic and anodic polarization curves and hydrogen permeation have been determined for Fe, Fe-0.5Cu and Fe-3.4Cu in sulfuric acid. During cathodic polarization at potentials more noble than -400 mV (NHE) a black layer consisting of metallic Cu and Fe-oxides or -hydroxides is formed on the surface of the alloys by preferential dissolution of Fe. The black layer acts as a barrier for hydrogen diffusion and a catalyst for H 2 evolution, the layer reduces the permeation rate by a factor of two compared to pure Fe, in the presence of H 2S the reduction is by a factor of six. Addition of H 2S and/or an increase in pH can shift the cathodic polarization curve to more negative potentials where the scale is not stable. Under such conditions the permeation rate is faster on the alloys than on Fe.