This paper describes the results of a laboratory study evaluating cracking resistance of 4130 (UNS G41300), 13Cr-L80 (UNS S42000), and two modified “13-5-2” type 13Cr alloys (UNS S41426) with 110 ksi (758 MPa) specified minimum yield strength in three different oil and gas well environments. Sulfide stress cracking resistance was evaluated using tensile specimens stressed to 90% actual yield strength and double cantilever beam specimens. Three environments were evaluated that included: (a) NACE Solution A with 15 psia (0.1 MPa) H2S; (b) a simulated oil well with 100,000 mg/L Cl-, pH 4.5, 0.5 psia (3.5 kPa) H2S; and (c) a simulated gas well with 1,000 mg/L Cl-, pH 3.5, 0.5 psia (3.5 kPa) H2S. These three environments were used for tests at both room temperature and at 40 °F (5 °C). Also, the oil well and gas well simulated environments were tested using both an acetate and bicarbonate buffer solution for pH control. Cracking and corrosion observations are reviewed and implications of alloy sour service limits, effect of temperature, and effect of buffering agent in laboratory testing are discussed. Findings of the current study are compared with previously presented studies to confirm or challenge assumptions on the role of temperature and buffering systems.
Materials that are utilized in oil and gas production must be resistant to corrosion and cracking in increasingly severe environments. In particular, offshore operations require a high degree of confidence in material reliability. Offshore wells continue to produce in deeper water depths and from deeper formations where many wells exhibit high concentrations of H2S. In many deep-water locations, low seawater temperatures in and around 40 °F (5 °C) induce these conditions on materials and equipment. Deep offshore wells require high strength materials for well applications with specified minimum yield strength (SMYS) of materials commonly needed at or above 110 ksi (758 MPa). Martensitic 13Cr stainless steel materials are favorable candidates for these applications due to their high strength and demonstrated resistance to CO2 corrosion. While 13Cr materials possess beneficial qualities for well applications, sulfide stress cracking (SSC) in the presence of H2S poses a significant concern. Several studies have investigated the sour limits of modified 13Cr martensitic stainless steels.1-Modified refers to alteration of typical alloying element contents from the base AISI420 13Cr grade (UNS S42000); modified 13Cr martensitic stainless steels typically have lower C, higher Ni, and higher Mo compared to AISI420. Research has found that many factors affect the SSC resistance of 13Cr martensitic stainless steels, including alloy content, material strength, pH, H2S partial pressure, temperature, chloride content, and buffer chemistry. A summary of the most important findings from this research are given below: 1. Alloy content of 13Cr martensitic stainless steels can affect the susceptibility of the alloy to general corrosion, localized corrosion, and SSC. Hashizume et. al. demonstrated that the higher the content of Cr, Mo, and Ni alloying elements, the lower the pH of the environment that could be tolerated by the material while maintaining passivity.5
