Evaluation of the K_IEC of UNS N07718 (Alloy 718) with and without Delta Phase in Simulated Seawater with Cathodic Protection using Double Cantilever Beam Tests

Most of the recent work done to characterize the fracture toughness of age-hardened nickel-based alloys in simulated seawater with cathodic protection (SW+CP) environments has used rising displacement techniques and compact tension specimens. The results of these tests are sometimes used in fracture mechanics calculations for design verification, though there is some uncertainty about the appropriate method for determining environmental fracture toughness from the results. There has not to date been much work performed to characterize threshold stress intensities in SW+CP environments using double cantilever beam (DCB) tests.

In this work, NACE TM0177 Method D (DCB) tests were performed on UNS N07718 (Alloy 718) in a simulated seawater environment with cathodic polarization. The tests were performed in a deaerated 3.5% NaCl solution at 4 °C (40 °F) with pH adjusted to 8.2 and an applied potential of -1050 mV versus a saturated calomel electrode (SCE). For comparison purposes, tests were performed on Alloy 718 with and without delta (δ) phase in the microstructure. Material and specimen examinations included microstructural characterization and scanning electron microscopy. The test data was compared to published data from several sources. The results support the current understanding that δ phase can be detrimental to the hydrogen embrittlement resistance of Alloy 718.

UNS N07718 (Alloy 718) is an attractive material for use in subsea and downhole environments due to its combination of corrosion resistance and high strength relative to carbon and low-alloy steels. However, use of improperly solution-annealed Alloy 718 and aerospace grades (i.e. ASTM⁽¹⁾ B637¹) in subsea and downhole environments have resulted in several high-profile failures.^2,3,4,5 Early work convincingly showed a relationship between grain boundary delta (δ) phase precipitates and the susceptibility of the material to hydrogen stress cracking / hydrogen embrittlement.⁶ These findings and others led the industry to draft API⁽²⁾ Standard 6A718⁷, later superseded by API Standard 6ACRA⁸, which specifies grades of Alloy 718 that are largely free of grain boundary δ phase and other deleterious microstructural constituents. The prevention of ? phase formation is achieved by specifying a solution annealing temperature above the ? phase solvus temperature.