Today’s global exploration of oil and gas leads to a range of highly corrosive environments that, in turn, require corrosion resistant high nickel content alloys for subsurface and wellhead equipment. Materials have to meet criteria for corrosion resistance and mechanical properties in severe environments for the required service life. This paper presents the latest development of a higher strength grade of alloy 725 (UNS N07725), alloy 725HS. This new age-hardenable nickel base alloy 725HS offers many advantages such as higher-strength, toughness and excellent corrosion resistance. The standard alloy 725 grade is typically specified with 120 ksi (827 MPa ) minimum yield strength compared to alloy 725HS with 140 ksi (965 MPa ) minimum yield strength. The grades are approved to NACE MR0175 up to hardness levels of 40 HRC and 43 HRC maximum for the alloys 725 and alloy 725HS, respectively. The higher strength level of alloy 725HS offers higher strength for sour well service compared with the traditionally used alloy 718 (UNS N07718). Corrosion data is also presented showing the performance of the age-hardenable nickel alloys in seawater, resisting chloride stress corrosion cracking, and in sour well environments using the slow strain rate and C-ring test methods. Alloys 725 and 725HS are also shown to be suitable candidate materials in severely corrosive high temperature oil patch environments. Sour environments in which alloys 725 and 725HS have met the criteria for corrosion resistance are defined with and without free sulfur.
Keywords: nickel-base alloys, sour environments, corrosion resistance, age-hardened, high-strength, bar, oil field