Electrochemical impedance of thin rust film of low-alloy steels

The quality of the rust film formed on the steel is a significant factor in atmospheric corrosion. In the present paper, ion-exchange membranes were adopted to simulate the anion and cation selective permeability of the rust film. The i-E curve and the electrochemical impedance of a carbon steel covered with the ion-exchange membrane were measured to discuss the effect of the ion transfer inside the rust film on the anodic dissolution. The electrochemical impedance of the rust film membrane was measured. The electrochemical impedance of the rust film membrane shows the apparent capacitive behavior. It was found that the film had huge capacitance of order 10 mF cm^-2. The resistance of the rust films of Fe-3%Ni, Fe-1%W and Fe-0.6%Mo was large value, indicating that the low ion permeability rate inside the rust film contributed to decreasing the corrosion rate of low-alloy steel.