The development of HPHT (high pressure/high temperature) and XHPHT (extreme high pressure and high temperature) wells require high strength and corrosion resistant fasteners for subsea equipment and components. Precipitation hardened (PH) nickel alloys meet these requirements. Most PH alloys can offer high yield strength from 105 - 140 ksi (723 - 965 MPa) and some can offer a yield strength up to 150 ksi (1034 MPa). PH alloys contain a significant amount of Cr and Mo and hence are very resistant to seawater corrosion. However, their susceptibility to hydrogen embrittlement under cathodic protection is not well understood. Given that cathodic protection is a proven and widely applied technology to protect carbon steel subsea equipment and structures from corrosion, PH alloys must be resilient to exposure to typical subsea potentials. PH alloy fasteners, as part of subsea equipment and structure, are exposed to -1100 mV Ag/AgCl.

In this investigation, six PH and one cold worked nickel alloy were tested using SSRT (slow strain rate test) per NACE TM 0198. The two test solutions utilized were synthetic sea water and de-aerated synthetic seawater with 0.15 psi (0.01 bar) H2S partial pressure. The de-aerated synthetic seawater with 0.15 psi (0.01 bar) H2S partial pressure simulates an anoxic seawater condition. The degree of hydrogen embrittlement susceptibility for each alloy was evaluated based on time to failure ratio, elongation, and area reduction ratio as compared to a baseline sample tested in air. The fracture surfaces were examined as a secondary means to further confirm the degree of embrittlement. The microstructure of the examined alloys was evaluated for the amount of precipitation along the grain boundaries. Finally, all the alloys were ranked based on their susceptibility to hydrogen embrittlement under cathodic protection condition.

Subject
Hydrogen embrittlement, Materials, Microstructure, Nickel based alloys, Synthetic seawater, Mechanical failure, Elongation, Risk reduction, Grain boundary, Alloys, Yield strength, Ductility, Hydrogen
Keywords:
cathodic protection, hydrogen embrittlement, slow strain rate test, anoxic seawater, precipitation hardened nickel alloy
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2014
Association for Materials Protection and Performance (AMPP)
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