Sour Service limits for Super Martensitic Stainless Steel bar-stock: influence of testing approaches on materials performance

摘要

Super Martensitic Stainless Steel (SMSS) bar-stock is widely used in the Oil & Gas industry, for their good mechanical properties and corrosion resistance in mildly sour production environments. Furthermore, SMSS represent a cost effective solution for downhole equipment when compared to the more expensive Nickel alloys: intermediate price material grades such as super duplex are often not acceptable due to their lower strength. Many studies have been aimed at defining the Sour Service (SS) limits for this class of materials with respect to parameters such as chloride concentration, pH, temperature and H_2S partial pressure. However, the testing methodologies adopted have not always been successful for creating reliable and repeatable outcomes. Indeed, SMSS grades are strongly susceptible to Sulphide Stress Cracking (SSC) and the risk of a "false negative" is very high when conventional testing approaches are adopted. There is also some doubt as to the legitimacy of some of the performance limits published in NACE MR0175/ISO 15156 that were established based on testing and acceptance criteria that fall behind today's standards. Commonly, H_2S performance limits are evaluated with regards to the associated reservoir gas partial pressure. At high pressure, however, the "effective" partial pressure (fugacity) will be less due to the system non-ideality (i.e. the fugacity of the H_2S will often be lower than the partial pressure of H_2S) and then the aggressiveness of sour gases will be reduced. A testing method comparison is made and a critical interpretation of the results is presented.