Anodic Films and ECM Dimensional Control: A Study of Steel Electrodes in Solutions Containing Na[sub 2]SO[sub 4] and NaClO[sub 4]

The relationship between the passive film and the dimensional control of electrochemical machining (ECM) has been demonstrated experimentally with the behavior of steel electrodes in neutral and solutions. Potentiostatic transient measurements were first made with a mild steel rotating spherical electrode to reveal the mechanism of passivity, and the results were then compared to those obtained with rotating rods under actual ECM conditions. It is found that passivation is initiated by the formation of a salt layer resulting from supersaturation of dissolution products in the immediate vicinity of the anode. The rate of film formation is diffusion controlled, and the ECM dimensional control achieved by passivity of this kind is convection dependent. The results also indicate that this anodic film of the dissolution‐precipitation type can serve only as partial protection for the metal surface during ECM.