Microelectrochemical surface and product investigations during electrochemical machining (ECM) in NaNO3

The structure of the sample surface during electrochemical machining (ECM) of iron in neutral NaNO₃ solutions was investigated. From former experiments, we expected a duplex structure: a solid oxide film of some nm and, above, a meta-stable, highly soluble supersaturated iron nitrate film for ECM in NaNO₃ (current densities up to 100 A/cm², electrolyte flow). The total current (and charge) and the dissolution products during anodic pulses was measured. The formation of Fe³⁺ and Fe²⁺ was monitored by UV-VIS spectroscopy and enabled (for the first time) a quantitative product determination.A transition from predominant oxygen evolution to predominant iron dissolution was found in the range from 5 to 30 A/cm². At current densities >35 A/cm² the oxygen evolution is reduced to some percent of the total current. The Fe³⁺/Fe²⁺ formation ratio increases to 2, corresponding to a dissolution of Fe₃O₄.