The effects of applied potential and chloride ion on the repassivation kinetics of pure iron

The kinetics of repassivation on the bare surface of pure iron wire electrode in 0.5 M Na2SO4 solutions have been investigated as a function of applied anodic potential and chloride ion concentration by means of potentiostatic current transients obtained from the abrading electrode. At an applied anodic potential of -0.6 V (SCE) near the Tafel region, the current transients showed the formation of an adsorbed hydroxide film on the bare surface of Fe near the open circuit potential. Below the active-passive transition potential of -0.17 V (SCE) the anodic current density increased with increasing applied anodic potential. In contrast, above the active-passive transition potential the anodic current density necessary for the establishment of passivity decreased with increasing applied anodic potential, and the pure iron repassivates according to a linear relationship of log i vs log t. The dissolution of adsorbed hydroxide species is a predominant anodic reaction below the transition potential. Above the transition potential the growth of passivating oxide film is a dominant anodic reaction. There are consecutive electrochemical reaction steps of the competitive adsorption of chloride ions and hydroxyl ions, dissolution of adsorbed species [FeOH(ad), FeCl(ad)] and passivating oxide film growth. There exists a critical concentration of chloride ions below which chloride ions may help the formation of the oxide film by inhibiting the dissolution of FeOH(ad) or facilitating the place exchange reaction between metal and hydroxyl ions. Copyright (C) 1996 Elsevier Science Ltd.