Effect of the microstructure of nickel alloy 718 Oil Patch on its corrosion behavior

The resistance against stress corrosion cracking of a metallic material strongly depends on the quality of the present oxide layer as well as material resistance to hydrogen induced cracking. In case of nickel alloy UNS N07718 (alloy 718), the material susceptibility to stress corrosion cracking has been assumed to depend in particular on the material hardness. A systematic study of the corrosion properties of alloy 718 has been conducted with the main objective to investigate the nature of the correlation between aging temperature and the material corrosion performance as this correlation has been reported in the literature. Electrochemical experiments conducted on samples aged at different temperatures indicated a higher localized corrosion resistance for those samples which have been heat treated at lower temperature (8h at 760 °C) compared to those heat treated at higher temperature (8h at 870 °C). However, the observed differences in the corrosion behavior could not be attributed solely to the precipitation of the intermetallic phases, such as γ'+γ'' and δ. Complementary investigations of the oxide layers revealed differences in their composition as a result of the aging temperature. Further, the presence of atomic hydrogen in the material bulk was found to reduce the material ability to build a protective passive layer. The degree of the impairment increased with an increasing heat treatment temperature. No correlation between the hardness of the alloy 718 and its hydrogen embrittlement susceptibility could be demonstrated. In addition, besides the recognized detrimental effect of orthorhombic phase precipitation δ on the hydrogen embrittlement resistance of alloy 718, the hydrogen embrittlement susceptibility of the material was found to correlate with the size and fraction of γ'+γ'' precipitates. Among the tested material variants, the double aged material (8h at 720 °C followed by 8h at 620 °C) revealed the lowest degree of susceptibility. Thus, it is the morphology of the strengthening phase but not the hardness (as suggested in the API specification) that provides the significant contribution to the degradation mechanism of alloy 718 in hydrogen containing environments