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The Effect Of Interactions Between Cathodic Protection Potential And Stress Concentration On Hydrogen Embrittlement Of Precipitation-Hardened Nickel Alloys

Picture for The Effect Of Interactions Between Cathodic Protection Potential And Stress Concentration On Hydrogen Embrittlement Of Precipitation-Hardened Nickel Alloys
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Product Number: 51321-16663-SG
Author: Imran Bhamji; Kasra Sotoudeh; Menno Hoekstra; Herman Amaya; Bryan Fahimi
Publication Date: 2021
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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.

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.

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Hydrogen Embrittlement of High Strength Precipitation Hardenable Nickel Alloys

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Author: Michael Dodge
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The high strength and corrosion resistance of nickel-chromium alloys such as Alloy 718 and nickel-iron-chromium alloys such as Alloys 945 and 945X make them particularly good candidates for use in demanding environments in the upstream oil and gas industry. These materials generally perform well where resistance to sulphide stress cracking and chloride stress corrosion cracking is required. However whilst these alloys are considered ‘NACE compliant' environmentally-assisted failures can still occur.It is generally accepted that for hydrogen cracks to initiate there must be a critical combination of stress susceptible microstructure and hydrogen concentration. In this project the effect of microstructure is explored by heat treating Alloy 718 945 and 945X to standard and non-standard conditions. Tensile specimens were slow-strain-rate-tested in air and under CP to explore sensitivity to hydrogen embrittlement. Finally the effect of a severe stress concentration in the form of a sharp notch was used to determine whether there is an enhanced susceptibility to hydrogen embrittlement due to the presence of local stress raisers. The results are compared with tests undertaken by other authors under various hydrogen-charging conditions.
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