H2S and CO2 Corrosion of Some 9Cr-1Mo Alloys for Downhole Applications

Abstract In the 80’s 9Cr-1Mo alloys were extensively used as tubing or casing in the completion of gas wells with no H2S or in environments where the concentration of this corrosive compound was a few ppm. In many others applications 9Cr-1Mo steels are used in downhole and wellhead equipment including packers, tubing hangers and safety valves. Due to the significantly lower cost relative to duplex and Corrosion Resistant Alloys (CRA’s) there is considerable interest in extending the current field of application of 9Cr-1Mo alloy to higher H2S contents. This paper describes the corrosion behaviour of some 9Cr-1Mo alloys for jewelry applications with particular emphasis to Sulfide Stress Cracking (SSC) resistance. In this context, engineering diagrams were drawn individuating the applicability limits in H2S containing environments. The influence of some metallurgical parameters such as austenitic grain size, yield strength and hardness on the SSC resistance was verified. The susceptibility to localised corrosion, pitting and crevice, as well as the resistance to uniform corrosion of 9Cr-1Mo alloys was also investigated in conditions simulating actual downhole environments. Long term exposure tests were performed in the range 100-150°C, in high salinity concentration solution (100 g/1 NaCl) and at elevated CO2 partial pressure (up to 1.4 MPa).