ABSTRACT

The dissatisfactory corrosion rate of high-Mn steel in an aqueous acid environment extremely restricted its new application such as expandable casing and oil well pipe, although it possessed excellent mechanical property. In our study, high-Mn steel alloyed with Ni and Cr could depress its uniform corrosion. However, pitting corrosion occurred. The maximum pitting rate was 18.08 mm/a. In this research, the pitting corrosion of these steel in high temperature, high CO2, low H2S, and high Cl- environment was investigated by scanning electron microscope (SEM), energy dispersive detector (EDS) and X-ray diffraction (XRD). The results showed that in some local places the corrosion scale was bulged like hillocks which were the representation of pitting. The surface morphology and the cross section morphology were observed by SEM which showed that the corrosion scale had a bedded structure. According to the EDS and XRD results, the corrosion products of the uniform surface and the unbroken part of the hillock were sulfide which contained Fe7S8 and (Fe,Ni)9S8. The other part of the hillock was only Fe7S8 and the core was (Fe, Mn)CO3. The element Cl appeared only on the bottom of the pit. The accumulation of Cl was the incentive of pitting.

INTRODUCTION

In pace with the expansion demanding of fossil energy and the growing shortage of conventional petroleum and gas resource, plenty of resources in HTHP H2S/CO2 environment are exploiting. Especially in some super deep well, oil tubes and drive tubes suffer enormous loading and corrosion of aggressive acid medium. 1-3 Hence, pipes with high strength and good anti-corrosion property are badly in need. Moreover, the expandable casing technology which is a significant revolution in oil drilling and extraction industry also requires analogous materials. 4 The mechanical property and corrosion resisting ability must be ensured after expanding. Also, expansion characters such as ductility and low resilience shall be outstanding. Consequently, it is necessary to develop materials with high performance to satisfy the loading in deep well and deformability as well as corrosion resistance in the acidic environment.

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