不働態および酸素発生電位域における鉄のアノード酸化皮膜量の測定

Abstract

Galvanostatic-cathodic reduction behaviour of anodic oxide films on iron has been investigated in neutral boric-borate solutions. The amount of iron cathodically dissolved from the film and the decay of potential during the cathodic reduction are much affected by both the solution pH and the cathodic current density. The current efficiency for the reductive dissolution of the film increases with lowering the solution pH and attains 100% at pH=6.35. In the solution of pH=6.35, however, dissolution of iron substrate begins to occur immediately after completion of reduction of the film. Addition of arsenic trioxide into the solution is proved to inhibit the iron corrosion completely, and the maximum amount of dissolved iron from the film is now equal to the total amount of iron in the film calculated from the anodic film charge assuming the film as Fe2O3. This result not only provides evidence for the single hydrous ferric oxide layer model of anodic oxide films on iron, but also gives an electrochemical method for estimating the amount of iron in anodic oxide films on iron.This method has been applied to determination of iron in the films formed in the oxygen evolution potential region. It is shown that the film in the oxygen evolution region thickens with rise of potential, but becomes thin at a critical potential where transpassive dissolution occurs. Further rise of potential results a constant film thickness independent of the potential.