Nature of the passive film on Fe–Cr alloys as studied by18O secondary ion mass spectrometry: reduction of the prior film and stability to ex situ surface analysis

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Abstract
Two fundamentally important aspects of the passivation of Fe–Cr alloys have been investigated using 18O SIMS and electrochemical techniques. First, the cathodic reducibility of the prior oxide film on Fe and Fe–Cr alloys was studied in neutral borate buffer and in sulphuric acid solution. Between 0.2 and 0.3 nm of the prior film was not removed in neutral solution, while the prior film could be completely removed in the more aggressive acid solution. Secondly, the stability of the passive film towards the ex situ exposure associated with surface analysis was investigated. The stability of the passive oxide film was found to decrease with increasing Cr content of the alloy. For Fe-26Cr, the film was not stable (with respect to oxygen exchange) to ex situ exposure after passivation at any potential in the passive region. This result has important implications for examining the composition and thickness of the passive film on high-Cr alloys using ex situ techniques. A model was proposed to account for the resistance of the passive film on Fe–Cr alloys towards breakdown.