Stress corrosion cracking (SCC) behavior of two 13Cr steels was investigated in aqueous CO 2-H 2 S environments. U-bend tests, constant load tensile tests, and corrosion tests were performed in CO2 environments containing different amounts of H 2 S. In addition, the slow strain rate tensile (SSRT) tests and hydrogen absorption measurements were done under cathodic hydrogen charging conditions to determine why 13Cr steels are more susceptible to SCC in H 2 S environments than low alloy steels are.The 13Cr steels were less resistant to SCC in the CO 2 -H 2S environments than low alloy steels were, but some 13Cr steels were not subject to SCC even at a hydrogen sulfide partial pressure of 0.3 atm. Furthermore, it was found that SCC in a CO 2 -H2S environment was caused by hydrogen embrittlement and that the SCC susceptibility of 13Cr steels was affected by their intergranular cracking behavior. Thus, their microstructures and carbide dispersions are important factors in defining SCC susceptibility of 13Cr steels.* Presented during CORROSION/84 (Paper No. 212),
Experimental
Ma ter/alsTwenty kinds of 13Cr steel were used in this investigation. They included two types: (1) a high carbon 13Cr type (C type) and (2) a low carbon Ni-13Cr type (Ni type). In addition, three low alloy steels of different strengths were used to compare them with the 13Cr steels. The chemical compositions of the above are listed in Table 1. Various alloying elements such as carbon, nitrogen, niobium, vanadium, nickel, and molybdenum were investigated to determine their influence on SCC for each type of 13Cr steel. The specimens of Heat Nos. 2 and 3 were cut from milt-finished seamless pipes produced by the Kawasaki prag milt and heat treated to their respective strengths. The austenitizing temperature was 950 C. All of the other 13Cr steels were laboratory heats. They were melted in a vacuum induction furnace and hot rolled into 5 mm thick plates. They were heated at 1000 C for 1 h and air cooled or water quenched, and then they were tempered at varying temperatures from 450 to 780 C. The specimens of low alloy steels were cut from mill-finished seamless pipes having strengths of API L80, C90, and C95.
ProcedureThe U-bend tests, constant load tensile tests, and corrosion tests were performed in CO2 environments containing various amounts of H 2 S at a pressure of 1 atm. The gas compositions, NaCI concentrations, testing temperatures, and testing periods are shown in Table 2. The Condition 1 test was conducted in an aerated NaCI solution without CO 2 or H2S. Condition VII is the "NACE condition," which is used to observe the SCC susceptibility of steels in H 2S environments.In the U-bend test, 2 mm' x 20 mmw 14 mm specimens were immersed in a 0.1 m3 vessel. The gas was continuously bubbled through the vessel after deaerating with nitrogen gas.In the constant load SCC test, the solution saturated with a given gas was circulated through the cel] shown in Figure 1.The specimen was the NACE subsize (2.5 mm 0 x 25.4 mm ,).
