This paper presents environmental performance data on welded nickel alloy under high temperature, high pressure, ‘ultra-sour’ conditions. Welded coupons of nickel alloy UNS N06625 were subjected to three levels of nominal stress at 200 °C (50%, 75%, 100% AYS) and tested for 30 days in an environment consisting of 3.5 wt% NaCl solution with a partial pressure of 8.8 MPa (88 bar) H2S, 2.65 MPa (26.5) bar N2 and 1.55 MPa (15.5 bar) H2O. Coupons were visually assessed and showed no signs of environmentally assisted cracking (EAC). However, a thin, tenacious scale comprising various sulfides was formed with no significant metal loss observed over the test period. More detailed characterization, including scanning electron microscopy and X-ray diffraction studies, revealed the microstructure and phases in the scales formed on the surface of the coupons. Examination showed regions with two distinct scale morphologies: a fine adherent scale of mixed nickel sulfides and larger, faceted features in regions adjacent to the weld, which were determined to be FeS2 (pyrite).
One may conclude that nickel alloy UNS N06625 performs well in an ‘ultra-sour’ (>5 MPa pH2S) environment at high temperature (200 °C) and high pressure (13 MPa) and shows no evidence of environmentally assisted cracking. However, it must be noted that UNS N06625 is not immune to corrosion in such environments.
Key words: ultra sour, sour corrosion, sour service, high pressure and high temperature.