Selection of pressure vessel materials under hydrogen sulfide stress corrosion

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I. Definition of a wet hydrogen sulfide corrosion environment: The standard MR 0103—2005 \"Materials Resistance to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments\" issued by the National Association of Corrosion Engineers (NAcE) defines it as follows: A wet H2S corrosion environment exists when the medium in contact with the container contains free water in the liquid phase, and one of the following conditions is met: (1) The concentration of H2S dissolved in the free water is greater than 50 mg/L； (2) The pH value of free water is less than 4.0, and it contains H2S ； (3) The pH of free water is >7.6, and the water contains 20 ppm of dissolved hydrogen cyanide (HCN) as well as some dissolved H2S. (4) The partial pressure of H2S in the gas phase (absolute pressure) is greater than 0.3 kPa. Classification of wet H2S corrosion environments: Based on different corrosion mechanisms, wet H2S corrosion environments can be divided into Category I and Category II. Class II wet H2S corrosion (HB, HIC, and SOHIC) environment: An environment in which the container operates at temperatures ranging from room temperature to 150°C and meets any one of the following conditions is considered a Class II wet H2S corrosion environment: (1) High hydrogen activity resulting from aqueous corrosion ； (2) The concentration of H2S in water is greater than 2000 mg/L, and the pH value is greater than 7.6 ； (3) The concentration of H2S in water is greater than 50 mg/L, and the pH value is less than 4.0 ； (4) The pH of free water is greater than 7.6, the HCN or cyanide content in the water is greater than 20 mg/L, and H2S is present in the water. Class I wet H2S corrosion (SSC) environment: An wet H2S corrosion environment that does not meet the criteria of Class II is referred to as a Class I wet H2S corrosion environment. II. General principles for material selection: For the selection and evaluation of materials used in equipment designed and manufactured in accordance with elastic design criteria, materials that have been approved according to ISO 15156-2 (GB/T20972.2) and ISO 15156-3 (GB/T20972.3) or NACE MR0175-2003 standards can be chosen. These standards specify the environmental conditions that must be met in order to prevent stress corrosion; only when these conditions are satisfied can materials be selected in accordance with the requirements of relevant specifications ； If these requirements are not met, HIC and SSC tests must be conducted to meet the corresponding standard specifications. III. Conditions under which carbon steel, low-alloy steel, and cast iron materials are exempt from SSC and HIC tests 1. The materials shall be delivered in one of the following conditions: hot-rolled (only for carbon steel), annealed, normalized, normalized + tempered, normalized, austenitized, quenched + tempered, or austenitized, quenched + tempered. 2. The hardness of the materials shall not exceed 22HRC, and the nickel content shall be less than 1.0% ； S≤0.003%, P≤0.010%. 3. The yield strength of the material must be less than 355 MPa. 4. The tensile strength of the material must be less than 630 MPa. 5. Carbon equivalent limits: For low-carbon steel and carbon-manganese steel: CE≤0.43, where CE = C + Mn/6. For low-alloy steel: CE≤0.45, where CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. 6. When the thickness is greater than 12 mm, ultrasonic testing must be performed on each piece individually. 7. The hardness of welds and heat-affected zones must not exceed 22 HRC. 8. Stamping or any other permanent marking is not allowed. 9. Stress-relief heat treatment is required after welding. IV. Conditions under which austenitic stainless steel materials are exempt from SSC and HIC tests: 1. Chemical composition limits (see image). 2. Solution annealing followed by quenching, or annealing at a suitable temperature to achieve a stable microstructure, is required. 3. Mechanical properties cannot be improved through cold working. 4. The hardness of the raw material, welds, and heat-affected zones must not exceed 22 HRC. 5. Environmental constraints (see image). For HIC testing, NACE ETM0284–2016 is generally used; Solution A is typically employed, with the requirement that CLR≤5% ； CTR≤1.5% ； CSR≤0.5%, where: CLR is the crack length ratio ； CTR crack thickness ratio ； The sensitivity to cracking in the CSR test is higher than that in the SSC test, which is a test for the resistance of base materials to sulfide stress cracking. The standard method used is NACE TM0177-2016; the specific requirements are as follows: A set of specimens is taken from the forging with the highest σb or HB value within each batch of forgings, and another set of specimens is taken from each batch of steel plates for the sulfide stress cracking test. A pass result is achieved when the threshold stress σth is ≥ 247 MPa. For the sulfide stress cracking resistance tests of welding joints of types A, B, and D, a set of specimens is taken from the welding procedure qualification test plates to conduct these tests; a pass criterion is met when the threshold stress σth is ≥ 247 MPa