A comparison of the pitting susceptibility and semiconducting properties of the passive films on carbon steel in chromate and bicarbonate solutions

The semiconducting properties and pitting susceptibility of the passive fil ms formed on A516-70 carbon steel in chromate and bicarbonate solutions wer e studied by polarization measurements, electrochemical noise (EN) and Mott -Schottky analysis. A stable passivity is established in 0.01 M CrO42- solu tion with a potential range of more than 1000 mV and the pitting potential of 900 mV (Ag/AgCl), Similar passive properties are obtained in bicarbonate solution only when the concentration of HCO3- reaches 0.5 M. Upon the addi tion of Cl-, metastable pitting is initiated and indicated by the typical c urrent transients, which have the shape of a quick current rise followed by a slow recovery. There is a higher pit-initiation rate and a smaller noise resistance in the bicarbonate solution than in the chromate solution. The strong frequency dependence of the capacitance behavior shows that the pass ive films formed in both solutions are Ii-type semiconductors with a highly disordered nature. There is a thicker space-charge layer, a lower donor de nsity and a more positive flat-band potential for the passive film formed i n the chromate solution. Electrochemical results show that the passive film that formed in the chromate solution is more stable than that formed in th e bicarbonate solution. (C) 2000 Elsevier Science B.V. All rights reserved.