Prior studies of Alloy 600 and A!loy X-750 have shown the existence of a maximum in stress coqosion cracking (SCC) susceptibility in high temperature water (e.g., at 360°C), when testing is conducted over a range of dissolved (i.e., aqueous) hydrogen (Hz) concentrations. It has also been shown that this maximum in SCC susceptibility tends to occur in proximity to the nickehickel oxide (NiNiO) phase transition, suggesting that oxide phase stability may affect primary water SCC (PWSCC) resistance. Previous studies have estimated the Ni/NiO transition using thermodynamic calculations based on free energies of formation for NiO and H20. The present study reports experimental measurements of the Ni/NiO transition performed using a contact electric resistance (CER) instrument. The CER is capable of measuring the surface resistance of a metal to determine whether it is oxide-covered or oxide-f?ee at a given condition. The transition aqueous hydrogen (Hz) concentration corresponding to the Ni/NiO equilibrium was measured at 288, 316, 338 and 360°C using high purity Ni specimens. The results showed an appreciable deviation (;.e., 7 to 5 8 scc Hzkg H20) between the measured NiNiO transition and the theoretical Ni/NiO transition previously calculated using free energy data from the Journal of Solution Chemistry. The CERmeasured position of the N m i O transition is in good agreement with the maxima in PWSCC susceptibility at 338 and 360°C. The measured Ni/NiO transition provides a reasonable basis for estimating the aqueous H? level at which the maximum in SCC susceptibility is Iikely to be observed at temperatures lower than 338 to 360"C, at which SCC tes& are time-consuming to perform. Limited SCC data are presented which are consistent with the observation that SCC susceptibility is maximized near the NdNiO transition at 288°C.

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The importance of knowing the electrochemical corrosion potential (ECP. also referred to as E,,,,, 1 of nickel-base alloys in hydrogenated water is related to the need to understand the effects of dissolved (i.e., aqueous) hydrogen concentration ([H2]) on primary water stress corrosion cracking (PWSCC). Also, the use of a reference electrode (RE) can improve test quality by heightening the ability to detect instances of out-of-specification or unexpected chemistry. Three methods are used to measure and calculate the ECP of nickel-based alloys in hydrogenated water containing -1 to 150 scdkg H7 (0.1 to 13.6 ppm H?) at 260 to 360°C. The three methods are referred to as the specimen/component method, the platinum (Pt) method, and the yttria-stabilized zirconidiron-iron oxide ( YSZFe-FeJO,) RE method. The specimen/component method relies upon the assumption that the specimen or component behaves as a hydrogen electrode, and its E,,,, is calculated using the Nernst equation. The present work shows that this method is valid for aqueous H? levels 2 -5 to 10 scc/kg H?. The Pt method uses a voltage measurement between the specimen or component and L! Pt electrode. with the Pt assumed to behave as a hydrogen electrode; this method is valid as long as the aqueous H? level is known. The YSZFe-Fe304 method, which represents a relatively new approach for measuring E,,,,, in this environment, can be used even if the aqueous H2 level is unknown. The electrochemical performance of the YSZFe-Fe304 probe supports its viability as a RE for use in high temperature hydrogenated water. Recent design modifications incorporating a teflon sealant have improved the durability of this RE (however, some of the RES do still fail prematurely due to water in-leakage). The Pt method is judged to represent the best overall approach, though there are cases where the other methods are superior. For example, the specimen/component method provides the simplest approach for calculating the E,,,, of plant components, and the YSUFe-Fe30, RE method provides the best approach if the H? level is unknown, or in off-nominal chemistry conditions. The present paper describes the use of these methods to determine the ECP of a specimen 01-component versus the ECP of the nickelhickel oxide (NiNiO) phase transition, which is important since prior work has shown that this parameter (ECP -ECPNi/NiO) can be used to assess aqueous H? effects on PWSCC. BACKGROUND