Nickel alloys are used for subsea applications in components exposed to both seawater and well fluid. These alloys are normally selected due to acceptable corrosion resistance and high strength (precipitation hardened alloys). However, the high strength Ni-alloys can suffer from hydrogen embrittlement when exposed to a hydrogen source. A lot of work has been done to establish "safe operation" windows for different Ni-alloys exposed to well fluid containing H2S (reference to ISO(1) 15156-3). 1
When exposed subsea a Ni-alloy component will be connected to a cathodic protection system. As part of the cathodic reaction, hydrogen will develop on the surface to be protected. Parts of this hydrogen will absorb on the surface and diffuse as atomic hydrogen into the metal. Since nickel has an austenitic structure, the overall hydrogen diffusion will be slow while the solubility will be high.
UNS(2) N07718 and UNS N07725 are used for subsea applications. Some un-expected failures have occurred and hydrogen has been explained as one possible reason for the failures. A research project to examine the susceptibility of Hydrogen Induced Stress Cracking (HISC) has been executed. In this project hydrogen charged samples of UNS N07718 and UNS N07725 were exposed to cathodic polarization and subjected to a tensile stress. The results were used to establish a critical stress level for initiation of HISC. The critical stress level for hydrogen charged samples was compared to fracture stress for un-charged samples. Examination in SEM was executed to document fracture morphology of fractured samples. In addition the hydrogen content of test samples was documented with melt extraction technique.