Abstract

13Cr martensitic stainless steel is widely used in oil and gas industry. It is well known that addition of molybdenum (Mo) enhances the passivity of steel. However, the role of Mo in passive film has not been clarified completely. The aim of the present research was to reveal role of Mo in terms of semi-conductivity. 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 existent state of the passive films.

Introduction

Oil and gas wells are highly corrosive environments because they contain H2S and CO2. The 13Cr martensitic stainless steel is widely used in the oil and gas industry because of high good corrosion resistance in CO2 gas wells. Generally, the addition of Mo increases the passivity of steel. However, the role of Mo in passive films has not been completely clarified.

The role of Mo was investigated not only in H2S but also in N2, such as deaerated seawater. An inhibition effect of molybdate was proposed, since it is insoluble and can prevent metastable pit formation and growth.1-6 The effect of molybdate ions (MoO42-) as inhibitor was considered in understanding the role of alloyed Mo in H2S environments, and it was reported that it improved the resistance to corrosion pitting.7-8 Contrarily, Ueda and Kudo have proposed a role of alloyed Mo in H2S environments, which differed from that in non-sour environments; they have also proposed that stable Mo sulfide enhances the formation of a protective chromium oxide film differentiated between sour and non-sour environments, indicating that stable Mo sulfide formation is enhanced. 9 Furthermore, there is no unified view on the effect of Mo, and its role remains unclear, especially in H2S-CO2 environments. 10-12

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