Defining the Limits of Application of Duplex Stainless Steel Coupled to Carbon Steel in Oilfield Environments

Slow strain rate testing of solution-annealed (σ_0.2 = 552 MPa [80 ksi]) and cold-worked (σ_0.2 = 1,068 MPa [155 ksi]) 22%Cr duplex stainless steels (DSS) was conducted at different hydrogen charging currents in acid-brine solutions at 80°C (176°F). The charging currents and environments were designed to simulate coupling of DSS downhole tubulars to carbon steel casing, assuming a leak of production fluid into the annulus. In addition, a limited number of tests were conducted using less severe loading conditions, such as constant load and fluctuating load. To provide a framework for evaluating the results of the stress corrosion tests, hydrogen uptake was measured under equivalent conditions using primarily the electrochemical hydrogen permeation technique. Complementary tests were carried out using hot extraction techniques. In environments without hydrogen sulfide (H₂S) at 80°C, there was no reduction in the plastic strain-to-failure of cold-worked DSS at charging currents typical of coupling to carbon steel (e.g., 100 μA/cm² [645 μA/in.²]). Addition of 1% H₂S caused a reduction in the strain-to-failure at charging currents as small as 20 μA/cm² (129 μA/in.²). There was evidence of cracking on the surface of specimens in proof ring and ripple loading tests with 1% H₂S and a charging current of 20 μA/cm². In H₂S-saturated environments, the plastic strain-to-failure in slow strain rate tests was low at all charging currents and at open circuit. The cold-worked material was more susceptible to cracking than the solution-annealed material at all charging currents. For the same charging current density, the plastic strain-to-failure relative to oil was lower at 20°C (68°F) than at 80°C. The resistance to cracking did not correlate simply with bulk hydrogen uptake, as measured in the permeation tests. This suggests that the dominant factor is enhanced local hydrogen uptake at sites where the passive film, which acts as a barrier to hydrogen uptake, is ruptured.