A new 13Cr martensitic stainless steel (0.025C-13Cr-Ni-Mo) with excellent resistance to C02 corrosion and good resistance to SSC is developed and its application limits in oil and gas environments is clarified. The C02 corrosion rate of the 13Cr steels with Ni and Mo is less than 0.3 mm/yr at 180ºC (356°F ) in 20% NaC1. It is less than that of the conventional 13Cr steel (0.2C-13Cr). Tire corrosion rate of the steel slightly decreases with the increase in Mo and Ni content. The SSC resistance improves with the increase in Mo content. The critical partial pressure of H2S for the 2% Mo steel is greater than 0.005 MPa at the pH value of 3.5. The effects of Ni and Cu on SSC are not distinctive for this kind of steel. These results depends on the hydrogen permeability. The critical H2S partial pressure for the 110 grade steel is the same as that of the 95 grade steel at the pH values of 4.5 and 3.0, and is slightly lower at the pH values between 3.0 and 4.5. The new 13Cr steel proves to have excellent properties in the sweet and slightlyt sour environment.
The field application of API 13% Cr steel is steadily increasing for Oil Country Tubular Goods (OCTG) owing to its good corrosion resistance in the high carbon-dioxide (C02) and chloride Environment(1, 2). However, the 13Cr steel is susceptible to sulfide stress cracking (SSC) in the sour environment containing hydrogen sulfide (H2S)(3,4) and less corrosion resistant at elevated temperatures. Furthermore high strength OCTG has been required for the development of deep wells. The duplex stainless steels have been used in these conditions (5),because of their high corrosion resistance and high strength. However, they are very expensive and push up production costs. With these backgrounds, a number of modified 13Cr steels have been proposed (6-9).They have been used in the field applications. The application limits of these steels have been also studied as well (10-13). However, their application limits in sour environment have been required to be clarified.
This paper describes the effect of environmental factors, alloying elements and strength on SSC resistance and discusses the mechanism of SSC for the 13Cr steel.
EXPERIMENTAL PROCEDURE
Six types of steel plate were used. They were rolled from small ingots to the thickness of 12mm in a laboratory scale rolling mill and air-quenched, followed by tempering to obtain the yield strength of the grade of 655 MPa(95ksi) and 758 MPa(l10 ksi). Their chemical composition is listed in Table 1. The specimens for the C02 corrosion test were cut from the mid-wall position. Their dimensions are 3x25x50mm3. Each specimen was placed in an autoclave filled with 20% sodium-chloride (NaCl) solution saturated with 3.0 MPa CO2 gas at room temperature, and heated to 180ºC for 7 days. The resistance to C02 corrosion was evaluated by the weight loss rate.
The resistance to SSC was evaluated by the constant load tensile test specified by NACE TM0177-90 method A.
