Abstract
A recently developed hyper duplex material, UNS S33207, characterized by high strength and excellent localized corrosion resistance, can be utilized as downhole tubing, for raw seawater injection, in umbilicals and for other applications where high strength combined with high chloride induced corrosion resistance is needed as the material has a Pitting Resistant Equivalent number (PRE) minimum of 50. For tubes with a wall thickness larger than 4 mm the minimum yield strength is 700 MPa (100 ksi). Currently the limit of H2S for super duplex material with PRE 40 to 45 in NACE MR0175 is 20 kPa (3 psi) H2S. Previously published data established that UNS S33207 passed C-ring testing at 900C with 3 psi H2S. However, the limit for UNS S33207 performances in a sour environment was not defined and the aim of this work is to investigate how UNS S33207 performs at higher H2S partial pressures than the present limit in the NACE MR0175 standard for super duplex.
Stress corrosion cracking experiments were performed on UNS S33207 in different H2S environments containing low pH levels. The tests were conducted based on NACE TM0177 using constant strain four point bend (4-PB) loaded at 100% of AYS as well as constant load at 90% of AYS, both at 900C.
No cracks were observed in the constant load specimens after testing at a partial pressure of 35 kPa (5 psi) H2S with 25% NaCl and at 52 kPa (7.5 psi) H2S, 15% NaCl. Slight localized corrosion was observed at 7.5 psi H2S. No cracks or corrosion was observed after testing using four point bend even at 55kPa (8 psi) partial pressure of H2S and 25% NaCl concentration. These results are set in relation to the limits in NACE MR0175.
