Evaluation of the Performance of Inconel 718 Fasteners Subjected to Cathodic Protection Systems in Offshore and Subsea Applications

Abstract

Fasteners manufactured with Inconel 718 alloy are being widely used in offshore and subsea applications due to the material’s high strength, when compared to other nickel alloys, and its inherent corrosion resistance. However, concerns have been raised over its utilization in applications where cathodic protection or impressed current systems are in place. These concerns relate to the susceptibility to hydrogen embrittlement that Inconel 718 alloy may present depending on its processing, microstructure, hardness and actuating stresses. Over the last few years, much has been discussed on the suitability of the alloy for subsea applications. The development of special thermal cycles for the ageing of the alloy has been necessary to provide a consistent material with a maximum hardness of 35 HRc, and a microstructure free of detrimental phases without jeopardizing the overall mechanical properties of the alloy. Wellstream has developed a test programme focused on the assessment of Inconel 718 behavior when subjected to cathodic protection systems. Through this programme, it was possible to demonstrate the suitability of Inconel 718 alloy in subsea applications when the resulting microstructure and hardness are properly controlled, and bolt loading is within normal working limits.

Effects of alloy 718 microstructure on hydrogen embrittlement susceptibility for oil and gas environments

“…Slow strain rate (SSR) tensile testing is one of the most common methods for evaluating alloy susceptibility to hydrogen embrittlement due to the relatively short test times; however, the SSR test is a more severe test method compared with constant displacement/constant load tests because of the dynamic straining. 6 Many studies have been performed using SSR tensile testing to evaluate the susceptibility of nickel-base CRAs to embrittlement in hydrogen-rich environments, 1,3,[7][8][9][10][11][12][13][14][15][16] where susceptibility is often based on a comparison of tensile properties in the presence of hydrogen and in an inert or ambient environment. While increased hydrogen embrittlement susceptibility has been correlated with increased strength, 8,15 the influence of microstructure at high strength levels is not fully understood for many nickelbase CRAs.…”

“…One microstructural feature that is known to increase hydrogen embrittlement susceptibility is δ-phase precipitation on grain boundaries. 1,3,[10][11][12][13] However, even removing the presence of extensive δ-phase does not clarify the correlation between hydrogen embrittlement and strength level. Figure 2 is a plot of total elongation ratios versus yield strength for alloy 718 without extensive precipitation of δ-phase at the grain boundaries; the data were obtained from several different studies.…”

“…13 On the other hand, multiple studies have found that the presence of the grain boundary strengthening δ-phase can significantly increase susceptibility to intergranular cracking (IGC) due to hydrogen embrittlement. 1,3,[10][11][12][13] API standard 6ACRA provides acceptable ranges for annealing and ageing heat treatments for 718 in downhole environments, as well as examples of acceptable microstructures and acceptance criteria for room-temperature mechanical properties such as yield strength, ultimate tensile strength, percent elongation, reduction of area and hardness. 24 The heat treatments in the API standard 6ACRA are intended to achieve high strength while also avoiding growth of δ-phase at grain boundaries.…”

“…13 On the other hand, multiple studies have found that the presence of the grain boundary strengthening δ-phase can significantly increase susceptibility to intergranular cracking (IGC) due to hydrogen embrittlement. 1,3,10–13

3 Time–temperature–transformation diagram for alloy 718 showing the approximate ageing temperatures and times required to precipitate the δ-phase (Ni 3 Nb) and γ′-phase (Ni 3 (Ti and Al)). Adapted from Oradei-Basile and Radavich 17

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