Investigation on the surface layer formed during electrochemical modification of pure iron

This paper presents an experimental and analytical investigation on electrochemical modification of pure iron, aiming to obtain a surface layer which has good machinability for further processing. The microstructures of the modified layer were compared from a relative low current density (10 A/cm²) to high current density (30 A/cm²) and characterized by various detection equipment. A detailed X-ray photoelectron spectroscopy (XPS) analysis was conducted to measure the composition profile of the black film on the modified layer. The results show that, during electrochemical modification, the subsurface formed a loose porous layer covered with a flimsy oxide. At low current density, uniform dissolution was obtained and the surface was not corroded off completely. However, at large current density, due to the strong electrochemical action and uneven flow field, the cross section of pure iron became waved and the surface shape deteriorated. The results of the water contact angle measurements show that the surface hydrophilicity was enhanced by electrochemical modification, which might improve the cooling effect of the machining region. The XPS analysis indicated a bi-layer structure of oxidation film on the modified layer and the outmost Fe³⁺ species layer was considered to be the oxidation of Fe²⁺ species from inner layer.