ABSTRACT This paper describes a series of tests on two grades of stainless steels UNS S31600 (AISI 316) and UNS S66286 (ASTM A453 Gr. 660? Alloy A286) in simulated H2S-contining oil and gas service environments. The purpose of this study was to evaluate these materials for stress corrosion cracking. Triplicate, stressed C-ring specimens of each material were exposed to a 5% NaCl and 0.4% NaCH3COO solution equilibrated with a 1.5 psia (10.3 kPa) H2S at 200 F (93 C), 250 F (121 C) and 300 F (149 C) for 30 days. After exposure, none of the C-rings showed signs of cracking. The UNS S31600 specimens did exhibit pitting after testing at all three temperatures. None of the UNS S66286 (A286) C-rings exhibited pitting after exposure. Following this initial effort, additional tests at 300 F (149 C) were performed consisting of triplicate specimens immersed in three different solutions saturated with 15 psia (103 kPa) H2S: (a) 5% NaCl solution with a pH of 5.5, (b) 16% NaCl solution with a pH of 5.5, and (c) 5% NaCl solution with a pH of 4.5. After these exposures, none of the C-rings immersed in the 5% NaCl solution with a 5.5 pH showed signs of cracking. Two UNS S31600 and one UNS S66286 specimens failed after exposure to the 16% NaCl solution with a pH of 5.5. In addition, one UNS S31600 all three UNS S66286 specimens failed after testing in the 16% NaCl solution with a pH of 4.5. Several of the UNS S31600 specimens exhibited pitting after testing. The maximum pit depth was 10.2 mils (0.26 mm) on a UNS S31600 specimen exposed to the 16% NaCl / pH 5.5 environment. None of the UNS S66286 exhibited pitting after exposure.
INTRODUCTION With the development of new and deeper water offshore locations, and the increasing H2S levels of both newer and older reservoirs, special considerations have arisen for material selection relative to selective forms of environment cracking associated with exposure to oil and gas environments containing H2S and chlorides in produced water. Under these conditions, environmental cracking can be manifested in the form of either:
? Sulfide Stress Cracking (SSC) caused by atomic hydrogen absorbed through the process of sulfide corrosion on the metal surface or galvanic couple to a more active metal, and
? Stress Corrosion Cracking (SCC) that initiates on the metal surface and propagates through localized anodic attack (dissolution) driven by the presence of chloride and sulfide (or oxygen) in the environment.
Deep water completions are operating under conditions of generally increasing H2S partial pressure encountered in both new and existing (aging) reservoirs. Consequently, there are unresolved questions regarding serviceability limits for environmental cracking of many commonly used materials. In some cases, industry guidelines as found in NACE MR0175/ISO 15156 (2003) have been developed based on laboratory testing or documented field experience. However, in several cases, the serviceability limits have been reached by consensus without verification by testing or evaluation. Materials in this latter category include some martensitic, austenitic and precipitation hardened stainless steels. These materials are widely used in sweet service, but some materials have not been evaluated for cracking limits to the same degree as some of the newer alloys included in this NACE standard. In these latter situations, the MR0175/ISO 15156 standard provides very conservative serviceability limits resulting from the lack of corrosion data or documented field experience.
For example, austenitic stainless steel UNS S31600 is addressed in Section A.2 Table A.2 of MR0175/ISO15156 (2003) and has maximum serviceability limits of:
? HRC 22 (without cold work desig