Abstract
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 -1050mVAg/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.
