Kinetics of oxide film growth and destruction on iron surface in carbonate solutions

The variation of the open circuit potential of the Fe electrode in strongly oxygenated solutions of Na2CO3 of different concentrations is followed until steady state values are established. In all solution concentrations, the steady state potentials E-st. are approached from negative values, denoting oxide film repair and thickening. The rate of oxide film thickening was determined from the liner relationship between the open circuit potential of the Fe and the logarithm of immersion time t as evident from the relation: E=a+b log t, where a and b are constants. This indicates that the oxide film growth is by ion conduction under a high field. The rate of oxide film thickening increases as the concentration of Na2CO3 solution increases. The presence of the aggressive salts Na2S, NaCl, NaBr and NaI decreases the rate of oxide film growth and finally causes breakdown of passivity and initiation of pitting corrosion. The aggressiveness of these anions decreases in the order: S2- > Cl- > Br- > I-. The concentration of the carbonate ions that can withstand a certain aggressive ions concentration varies according to the relation: log C-CO3(2-) = k+n log C-agg., where k and n are constants.