Susceptibility of hydrogen induced stress cracking of Ni-alloys exposed to cathodic polarisation

Abstract

Testing of the susceptibility to hydrogen induced stress cracking (HISC) in three different cold deformed nickel based alloys have been conducted and compared. These included Datalloy-HP at two different strength levels, an alloy without UNS number but similar to Datalloy-HP called Datalloy-HP-X and Alloy 625. Smooth pre-charged tensile rods of each alloy were stepwise loaded in Cortest Proof rings exposed to cathodic polarisation until fracture. The fractured surfaces were examined and characterised in scanning electron microscope (SEM). Finally, the hydrogen concentration was measured in one sample of each alloy.

Each of the alloys were found to be susceptible to HISC determined by a ductile to brittle transition due to hydrogen degradation. The centre region indicated ductile behaviour while the surface regions indicated brittle behaviour. This indicated that the influence of hydrogen was primarily in the vicinity of the surface areas of the pre-charged samples.

The results from the HISC testing showed that every pre-charged specimen suffered from ductility loss when compared with not pre-charged specimens, reflected by reduction in reduction of area (RRA) values. There was not documented any reduction in strength for any of the alloys. The average RRA values for the highest and the lowest strength levels of Datalloy-HP were measured to 11.4 ± 11.1 % and 40.0 ± 6.9 %, respectively. The average RRA values for Alloy 625 and Datalloy- HP-X were 62.5 ± 17.0 % and 42.1 ± 10.7 %, respectively. Based on the reduction in ductility, the strongest Datalloy-HP was concluded to be least susceptible to HISC while Alloy 625 was most susceptible to HISC.