Detecting Intergranular Phases in UNS N07725. Part II: Double-Loop Electrochemical Potentiokinetic Reactivation Test Validation

Abstract

The formation of interconnected grain boundary (GB) phases in precipitation hardening (PH) nickel Alloy 725 (UNS N07725) compromises its mechanical properties and has been linked to hydrogen embrittlement failures. However, normative industry standards such as API 6ACRA are inadequate for differentiating between Alloy 725 batches with a critical extent of GB precipitation, creating the need for a complementary qualification method.In Alloy 725, intergranular phases produce a Cr and Mo depletion adjacent to GBs (i.e., sensitization). In Part I of this work, this impoverishment in Cr and Mo was used as the basis to develop a double-loop electrochemical potentiokinetic reactivation (DL-EPR) test, optimized to quantify the extent of GB precipitation in Alloy 725. Part II explores the reproducibility and sensitivity of the new approach using three new independent commercial batches containing different extents of precipitation. Results were highly reproducible and confirmed the method’s sensitivity to detect small amounts of GB phases. Additionally, the DL-EPR procedure revealed clear microstructural inhomogeneities across the bars’ thickness, which increased with increasing bar diameter. This investigation strongly supports the validity of the new DL-EPR technique as a reliable and straightforward industry-friendly qualification method to quantify the extent of intergranular phases in Alloy 725.