Corrosion-Resistant Alloys UNS N09925 And N07725 For Oilfield Applications
Authors: Hibner E.L. , Moeller R.H. 
Published: October 2008

Alloy 925 (UNS N09925) and alloy 725 (UNS N07725) are highly corrosion resistant alloys capable of being age hardened to minimum O.2% yield strengths of 105 ksi (724 MPa) and 120 ksi (827 MPa), respectively. Alloy 925 and alloy 725 are austenitic nickel base alloys strengthened by precipitation of gamma prime (?') and gamma double-prime (?"), respectively. Large section sizes can achieve uniform hardening throughout the cross section, Both alloys are resistant to pitting and Stress Corrosion Cracking (SCC) in Deep Sour Gas Well (DSGW) environments containing NaCl, H2S, and S, and are resistant to Sulfide Stress Cracking (SSC) as illustrated by laboratory and field test data. The alloys also exhibit excellent resistance to other environments such as sea water and mineral acids. This paper is an in-depth summary of the mechanical properties and corrosion resistance of age-hardenable corrosion resistant alloys 925 and 725.

INTRODUCTION

The trend in oil and gas production has been toward deeper wells, enhanced recovery methods, and offshore field exploration. Deeper wells and enhanced recovery methods, such as steam injection, in combination with C02 result in hostile environments due to increased temperatures, pressures and presence of H2S, C02 and INCOLOY and INCONEL are registered trademarks of the Into family of companies.

Chlorides, For several decades, much research has centered around the need for Corrosion Resistant Alloys (CRA â??s) that can withstand the high temperature, high pressure and corrosive oil field production environments.1 Alloy 625 (UNS N06625) and alloy C-276 (UNS N10276) have the necessary corrosion resistance, but can only be strengthened through cold work, therefore, some large sizes cannot be produced with the exception of alloy C-276.2 Age-hardenable alloy X-750 (UNS N07750) and alloy 718 (UNS N07718) have been used in these applications but have insufficient corrosion resistance in many severe environments.3,4 lNCOLOY® alloy 925 (UNS N09925), strengthened by precipitation of gamma prime [Ni3(Ti,Al)], and lNCONEL® alloy 725 (UNS N07725), strengthened by precipitation of gamma double-prime [Ni3 (Nb,Ti,Al) ] are capable of being age hardened to minimum O.2% yield strengths of 105 ksi (724 MPa) and 120 ksi (827 MPa), respectively, Aging treatments given each alloy result in the optimum combination of strength, toughness, and corrosion resistance. Alloy 925 was developed for use in oil production tubular products, tool joints, and equipment for surface and down hole hardware for intermediate sour gas wells. Alloy 725 was developed for tubing hangers, SubSurface Safety Valves (SSSV'S), wellheads, Christmas trees, valve trim, packers, Polished Bore Receptacles (PBR's), and other down hole equipment for severe sour service. Alloy 725 has been used as pressure-containing, flow-wetted components in subsurface safety valves and as valve components and other down hole components in sour service5 at temperatures exceeding 175Â°C (350°F). Alloy 925 is resistant to pitting and Stress Corrosion.