Evaluation of Hydrogen Embrittlement Susceptibility of High-Molybdenum Nickel Superalloys Using SSRT

RIP2025-00056: Nickel-based superalloys are extensively used in the oil and gas industry due to their outstanding oxidation resistance and mechanical properties at elevated temperatures. However, hydrogen embrittlement (HE) poses a significant challenge, particularly in hydrogen-rich environments typical of subsea applications. In these environments, hydrogen is generated on the surface of materials through cathodic protection systems, which are used to prevent corrosion. This hydrogen can diffuse into the material, resulting in HE and, under combined stress and material susceptibility, may lead to hydrogen-induced stress cracking (HISC). This study focuses on evaluating the HE susceptibility of three nickel-based superalloys with high molybdenum content, previously cold-rolled, using slow strain rate testing (SSRT) in accordance with the NACE TM 0198 standard. The alloys were subjected to SSRT with in-situ cathodic polarization to simulate exposure to hydrogen-rich conditions. The tests were conducted at a strain rate of 10-6 s-1, a controlled temperature of 40 °C and a cathodic current density of 5 mA/cm². The SSRT specimens followed the subsize specifications of the NACE TM 0198 standard, with an exposed area of 5 cm². The electrochemical setup includes an Ag/AgCl reference electrode in 3M KCl, a platinum wire counter electrode with 0.5 mm diameter and 2 m length, and a counter-to-working electrode ratio of 1:10. The test environment was deaerated by continuously bubbling N2 5.0 gas at a controlled flow rate, as outlined in the NACE TM 0198 standard. After the tests, fracture surfaces were analyzed using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) to determine failure mechanisms. Additionally, the other half of each specimen was cleaned and immediately analyzed using a Bruker G4 PHOENIX DH device. The measurement of diffusible hydrogen was performed using the hot extraction method, with a carrier gas in a tubular furnace coupled to a thermal conductivity detector (TCD). This ongoing research aims to evaluate the susceptibility of these superalloys to HE, providing valuable insights for their application in the oil and gas industry. The experiments are currently in progress, and we anticipate obtaining the initial results from these tests in the near future.