13Cr martensitic stainless steel is widely used in the oil and gas industry. It is well known that the addition of molybdenum (Mo) enhances the passivity of steel. However, the role of Mo in the passive film has not been clarified completely. The subjects of the present research were to reveall the role of Mo in terms of semiconductivity. The Mott-Schottky plot, obtained by impedance spectroscopy, revealed the effect of Mo addition on the semiconducting property of a passive film under an H2S environment in a pH 4.0 solution at 25°C. The results revealed that Cr-O of the inner film and sulfide of the outer film exhibited semiconductor p-type and n-type characteristics, respectively. The number of defects in the film on 2.5 mass% Mo-added steel was approximately half of that in the film on Mo-free steel. The role of Mo was discussed through investigation using x-ray photoelectron spectrometric analysis based on the state of the passive films.

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