In Situ Determination of the Composition of Surface Films Formed on Fe‐Cr Alloys

Abstract

A method for using in situ X‐ray absorption near edge spectroscopy to determine the composition of passive, transpassive, and nonreducible thin oxide films is described. The method is demonstrated by determining the composition of the passive films formed in pH 4.5, 0.1 M acetate buffer, on 100 Å thick Fe‐y/Cr alloys (y = 8.5, 15, 19, 23 atom %) at potentials: (i) low in the passive region (−0.3 V vs. mercurous sulfate reference electrode, MSE); (ii) high in the passive region (+0.2 V MSE); (iii) in the transpassive region (+0.8 V MSE); and (iv) in the cathodic region where the air‐formed film is not fully reduced (e.g., −1.2 V MSE). The nonreducible film at −1.2 V is entirely a oxide/hydroxide, due to reductive dissolution at −1.2 V of all the Fe that was present in the air‐formed film. This nonreducible film persists at anodic potentials. At −0.3 V, the passive film on each alloy is significantly enriched in Cr, present as . Fe, if present, exists as both and . The film at −0.3 V consists primarily of the nonreducible oxide/hydroxide plus a very small amount of additional oxidized alloy; at this potential most Fe dissolves as . At +0.2 V, Fe is present in the passive film only as , Cr as , and the passive film is less enriched in Cr than at −0.3 V. The composition of the transpassive film at +0.8 Vis nearly the same as the composition of the film at +0.2 V, with Fe present at , but Cr is present as both and a small amount of (10–25% of the total Cr in the oxide). At +0.2 and +0.8 V, the passive film consists of the nonreducible oxide/hydroxide plus significant amounts of additional oxidized alloy. Formation of the +0.2 and +0.8 V films proceeds with little dissolution of Fe or Cr.