The equilibrium potentials of iron‐chromium alloys in oxygen‐free 4% NaCl solution are active between 0% and 11.5% Cr showing a gradual trend in a noble direction with increasing chromium concentration. At 11.5% and higher chromium concentrations, the potentials are noble, characteristic of passive metals, with only slight potential change on increasing chromium to 20%. When cathodic hydrogen is diffused from a face of the alloy not in contact with the NaCl solution, the potentials all become more active by several hundredths to several tenths of a volt, the greater change being characteristic of the passive alloys. Relative passivity of the alloys containing interstitial hydrogen persists, the critical chromium concentration for passivity being at 15% instead of 11.5%. The data indicate that recently pickled iron and stainless steels are chemically active, probably because they contain dissolved hydrogen retained during pickling. The relatively long time to establish the iron or alloy equilibrium potential is presumably governed (1) by the rate at which a metal hydroxide film forms which establishes the ion concentrations and (2) by the rate at which interstitial hydrogen comes into equilibrium with the electrolyte. The data are explained on the viewpoint supported by additional facts that interstitial hydrogen directly increases the free energy for solution of lattice atoms to ions in aqueous solution. The potential change of the cell produced by interstitial hydrogen is shown to be delayed by some minutes or hours after starting the cathodic hydrogen discharge, the time depending on the chromium content of the alloy. This delay is a measure of the time for hydrogen diffusion in the alloy. For pickled specimens, the diffusion times are less than for unpickled specimens.