To develop a surface-finishing technique, an Fe-Cr surface alloying of an iron substrate was investigated using alternating pulsed electrolysis in an aqueous solution containing only Cr(III) ions as a single metal component. In this electrochemical process, the iron substrate was slightly dissolved during the anodic pulse, providing iron ions into the solution in the vicinity of the substrate, while Fe and Cr were both electrodeposited on the substrate surface during the subsequent cathodic pulse. Crack-free or fracture-free Fe-Cr alloy layers with continuous composition gradients were formed on the iron surface under appropriate pulse conditions. Both the thickness and Cr content of the Fe-Cr alloy layer increased with increasing anodic pulse time, suggesting that the Fe(II) ions which dissolved and stayed near the substrate induced the electrodeposition of chromium atoms. The time-dependent concentration distribution of Fe(II) ions in the vicinity of the substrate was digitally simulated through a simple diffusion model using a set of experimental data on current density and current efficiency. The effect of the Fe(II) ion concentration near the substrate surface was discussed in terms of the layer properties, such as chromium content and its anticorrosion behaviors. (c) 2007 The Electrochemical Society.