Electrochemical passivation of iron in NO3-, SO42-, and ClO4- solutions

Electrochemical passivation of iron was studied in NO3-, SO42-, and ClO4- solutions of different pH values. Passivation potential of iron in NO3- and ClO4- was similar for any pH examined, while in SO42- it was more anodic. Passive current was lowest in NO3- (10 +/- 2 mu A/cm(2)), in SO42- was in the range of 40-60 mu A/cm(2) while in ClO4- passive current of 30-50 mu A/cm(2) was reached only at 600-800 mV. After passivation, the properties of passivated iron are similar in NO3- and SO42- but differ in ClO4-. In the latter, the passive region is shorter by 200-300 mV. The transpassive potential in ClO4- is 1.2 V vs SCE and independent of pH, but shifts anodically with decrease in ClO4- concentration. The transpassive potential in NO3- and SO42- is higher, O-2 evolution starts, and the potential shifts cathodically with increase of pH. At the transpassive potential in ClO4-, a black substance forms which dissipates in acidic solutions and is stable in strong alkaline solutions. Microphotographs of anodically polarized iron in these solutions show no pits, only uniform corrosion over the whole electrode surface. We assume that the rapid increase of anodic current at transpassive potential in ClO4- is due to oxidation of iron and iron oxides from the passive film to hexavalent iron.