The need to improve stress corrosion cracking resistance in H2S/CO2/CI-, at reasonably low prices and good mechanical properties, has led in the last decade to develop a plethora of superduplex stainless steels for sour oilfield environments application. The outstanding performance of superduplex stainless steels to SSC corrosion is diminished with the increase of chloride concentration and H2S partial pressure. In this work the performance of superduplex steel in very harsh oilfield simulated environments (H2S up to 20 psi (155 kPa), NaCl 25% and 80°C) were verified. Experimental work was carried out on two grades of superduplex steel seamless tubes, UNS S32760 (80ksi, 560 MPa), as the most commonly used, and UNS S39277, characterised by high strength in the annealed state (90 ksi, 630 MPa). The resistance to sulphide stress cracking (SSC) of duplex and superduplex stainless steels was evaluated in H2S/CO2/Cl- bottom hole simulated environments. SSC threshold was determined by means of C-ring (constant strain) testing method. The results were utilised to draw engineering diagrams of superduplex UNS S39277 and UNS S32760. Duplex 22%Cr steel was considered as reference material. SSC resistance of duplex steel was strictly correlated to the acidity of the solution. Critical H2S partial pressure in high chloride environments with addition of a buffering agent such as NaHCO3 is shifted towards higher values. Superduplex steel represents a suitable candidate for the use in the deep high pressure wells with H2S partial pressure up to 5 psi (35kPa) and 20% NaCl.
Keywords: Duplex stainless steel, Superduplex steel, Sulphide stress cracking, Sour environment, Chlorides, CO2, H2S, C-ring test