Effects of CO2 and H2S on corrosion of martensitic steels in NaCl(aq) at low temperature

Abstract

Corrosion studies were conducted for martensitic carbon steels in brine solutions at 4 °C and 10 MPa (1450 psi), which simulated the subsurface environments encountered in Arctic drilling. Three environments with a 5 % wt. NaCl brine were used: (1) 0.312 mole of CO2 per mole of H2O in brine, (2) 3.12 x 10-4 mole of H2S per mole of H2O in brine, and (3) a mixture of 3.12 x 10-4 mole H2S and 0.312 mole CO2 per mole of H2O in brine. Two martensitic carbon steels were selected for the investigations: a high strength low alloy (HSLA) carbon steel commonly used for drill pipe (G41000) and a newly designed ultra-high strength low alloy (UHSLA) steel (G41300). Electrochemical and mass loss measurements found corrosion rates on the order of 0.05 mm y-1 in the systems containing H2S and CO2+H2S, while the CO2 systems saw corrosion rates between 0.5 and 2 mm y-1. Surface analyses of the tested samples were performed using scanning electron microscopy and energy dispersive X-ray spectroscopy to identify corrosion products. The corrosion products were unstable and oxidized quickly after taking out of the system. All samples had a high oxygen content across the surface, though the samples exposed to H2S and CO2+H2S had relatively higher levels of sulfur present in an inner region of the film. Experimental corrosion rates were compared to model predictions, and a good agreement was found for the CO2 and H2S cases. However, the model predicted that corrosion rates in the mixed case would match those of the CO2-only system. The experimental results showed that CO2:H2S ratios as high as 1000 can result in predominantly sour corrosion at the low temperature, high pressure conditions tested.