Abstract
The anodic polarization behavior of mild steel and pure iron under high current densities was investigated by using a flow-channel cell in 2mol solutions of NaCl, NaNO3, NaClO3 and NaH2PO4 at 25°C. The overpotential transients during galvanostatic electrolysis and the potential decay transients after switch-off activation were measured. It was proved that iron was dissolved in active state in NaCl solution; but the dissolution was inhibited by the formation of a passive film in NaH2PO4 solution. In NaNO3 and NaClO3 solutions, the formation of the anodic film in the early stages of electrolysis, followed by the breakdown of the film, was derived from the potential decay transients. The anodic dissolution mechanism of a transpassive iron electrode in NaNO3 and NaClO3 solutions was proposed from the fact that the quantity of electricity necessary for the breakdown of the film depended upon the polarization current density. This mechanism proposed could successfully explain the dependence of the current efficiency for dissolution of iron on the current density, and it was applied to the interpretation of anodic dissolution behavior obtained in the electrochemical machining experiments reported in the preceding paper.
