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Alloys 600 and X-750 have been shown to exhibit a maximum in primary water stress corrosion cracking (PWSCC) susceptibility, when testing is conducted over a range of aqueous hydrogen (H{sub 2}) levels. Contact electric resistance (CER) and corrosion coupon testing using nickel specimens has shown that the maximum in SCC susceptibility occurs in proximity to the nickel-nickel oxide (Ni/NiO) phase transition. The measured location of the Ni/NiO transition has been shown to vary with temperature, from 25 scc/kg H{sub 2} at 360 C to 4 scc/kg H{sub 2} at 288 C. New CER measurements show that the Ni/NiO transition is located at 2 scc/kg H{sub 2} at 260 C. An updated correlation of the phase transition is provided. The present work also reports CER testing conducted using an Alloy 600 specimen at 316 C. A large change in resistance occurred between 5 and 10 scc/kg H{sub 2}, similar to the results obtained at 316 C using a nickel specimen. This result adds confidence in applying the Ni/NiO transition measurements to Ni-Cr-Fe alloys. The understanding of the importance of the Ni/NiO transition to PWSCC has been used previously to quantify H{sub 2} effects on SCC growth rate (SCCGR). Specifically, the difference in the electrochemical potential (EcP) of the specimen or component from the Ni/NiO transition (i.e., EcP{sub Ni/NiO}-EcP) has been used as a correlating parameter. In the present work, these SCCGR-H{sub 2} correlations, which were based on SCCGR data obtained at relatively high test temperatures (338 and 360 C), are evaluated via SCCGR tests at a reduced temperature (316 C). The 316 C data are in good agreement with the predictions, implying that the SCCGR-H{sub 2} correlations extrapolate well to reduced temperatures. The SCCGR-H{sub 2} correlations have been revised to reflect the updated Ni/NiO phase transition correlation. New data are presented for EN82H weld metal (also known as Alloy 82) at 338 C. Similar to other nickel alloys, SCC of EN82H is a function of the aqueous H{sub 2} level, with the SCCGR exhibiting a maximum near the Ni/NiO transition. For example, the SCCGR at 8 scc/kg H{sub 2} is {approx} 81 x higher than at 60 scc/kg H{sub 2}. The 8 scc/kg H{sub 2} condition is near the Ni/NiO transition (located at {approx} 14 scc/kg H{sub 2} at 338 C), while 60 scc/kg H{sub 2} is well into the nickel metal regime. A hydrogen-SCCGR correlation is provided for EN82H. The data and understanding obtained from the present work show that SCC can be mitigated by adjusting the aqueous H{sub 2} level. For example, SCCGR is typically minimized at relatively high aqueous H{sub 2} levels, that are well into the nickel metal regime (i.e., far from the Ni/NiO transition)

Measurement of the Nickel/Nickel Oxide Transition in Ni-Cr-Fe Alloys and Updated Data and Correlations to Quantify the Effect of Aqueous Hydrogen on Primary Water SCC