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 μA/cm2), in SO42- was in the range of 40−60 μA/cm2 while in ClO4- passive current of 30−50 μA/cm2 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, O2 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 ir...