The Effect of Interactions between Cathodic Protection Potential and Stress Concentration on Hydrogen Embrittlement of Precipitation-Hardened Nickel Alloys

Precipitation-hardened nickel alloys (PHNAs) are widely used for demanding oil and gas subsea applications, because of their high strength and corrosion resistance. However, several failures were associated with hydrogen embrittlement, under cathodic protection (CP), as subsea CP systems can provide a source of atomic hydrogen, which can diffuse into the material and compromise its toughness. The risk of hydrogen embrittlement might be reduced if CP systems were appropriately designed, e.g. more positive CP potentials could locally be applied to CRA components. Whilst tailored CP profiles will remain a practical challenge, the relationship between the CP potential and localised loading, due to the presence of stress raisers, is yet to be established.

This paper aims to address whether there is a threshold potential, above which embrittlement will not occur, and whether this threshold will be a function of stress intensity/concentration factor. This was explored through conducting slow strain rate tensile (SSRT) and incremental step load (ISL) testing on UNS⁽¹⁾ N07716, which has previously been associated with failures, at potentials between -750 and -1050mV_Ag/AgCl, on plain-sided specimens. Additional fracture-toughness-based testing, using single edge notch bend (SENB) specimens, was undertaken to understand if the stress intensity (associated with a fatigue pre-crack), can increase susceptibility to embrittlement.

Precipitation hardened nickel alloys are commonly used in demanding and aggressive environments where a combination of high strength and corrosion resistance is required. They are therefore frequently selected for applications requiring high strength in downhole, wellhead, subsea and Christmas tree applications¹. The microstructure of all PHNAs consist of a primary austenitic (Y) phase, which is solid solution strengthened by alloying elements, such as Cr, Mo and Ti. UNS N07716, includes additions of Nb and Ti and is therefore also strengthened by Y' (Ni₃Al/Ti) and Y" (Ni₃Nb) precipitates, which are dispersed through the matrix². The microstructure of PHNAs can also include the non-strengthening and deleterious δ and laves phases, and various metal carbides.