Intergranular Precipitation on the Precipitation-Hardened Nickel Alloy UNS N07725: Double-loop Electrochemical Potentiokinetic Reactivation Test

Precipitation hardened (PH) nickel alloy UNS N07725 used in oil and gas subsea service have experienced several brittle fractures linked to hydrogen embrittlement (HE). The alloy’s susceptibility to cracking was associated with the precipitation of a Cr-and Mo-rich phase at grain boundaries (GBs). Since the failed components were compliant with the current standards, developing a new quality control test capable of distinguish susceptible microstructures became a priority for the oil and gas industry. Herein, the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test was optimized for UNS N07725 as an alternative approach to detect microstructures with abundant intergranular precipitation. These precipitates lead to a Cr- and Mo-depletion along the decorated GBs (i.e., sensitization). Thus, the study of the intergranular corrosion susceptibility by the DL-EPR test served as an indirect quantification method of the degree of GB precipitation. The DL-EPR test solution, temperature, deaeration, scan rate, and surface finish were evaluated to distinguish between UNS N07725 batches with different degrees of sensitization. The optimized DL-EPR test conditions were 2 M HCl + 1 M H₂SO₄ + 0.0001 M KSCN aqueous solution at 30°C with a 1.667 mV/s scan rate and a vertex potential of E_corr + 700 mV. Results were reproducible and consistent with the metallographically measured degree of GB decoration.