Measurement of the Nickel/Nickel Oxide Phase Transition in High Temperature Hydrogenated Water Using the Contact Electric Resistance (CER) Technique

Abstract

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.

The Effect of Temperature on the Preferential Intergranular Oxidation Susceptibility of Alloy 600

“…Furthermore, while the value of EcP expressed vs the standard hydrogen electrode (SHE) would require the knowledge of the dissolved H 2 and pH, the potential EcP expressed vs the Ni/NiO transition (EcP vs.Ni/NiO ) is not dependent on the pH but solely on the dissolved H 2 in the system. [38,39] Further thermodynamic considerations, as well as the value of dissolved H 2 and H 2 fugacity required to be at the Ni/NiO transition are discussed in the publications by Morton et al [38,39] and by Moss and Was. [40] Several Alloy 600SA coupons were exposed to simulated PWR primary water at 320°C for 1000 hours with 40 cc/kg (3.64 ppm) of dissolved H 2 , which corresponds to an equilibrium potential more reducing than the Ni/NiO equilibrium potential, [41,42] and with addition of H 3 BO 3 (1200 ppm) and LiOH (2 ppm).…”

“…At 320°C the measured H 2 fugacity was 42.76 kPa and the electrochemical corrosion potential of Alloy 600 was on the reducing side of the Ni/NiO transition. [39] The total pressure in the autoclave at 320°C was 11.3 MPa.…”

Crack growth rates and metallographic examinations of Alloy 600 and Alloy 82/182 from field components and laboratory materials tested in PWR environments.

“…While the mechanistic origin is unclear, there are consistent data showing that CGRs of Ni-base alloys peak near the Ni/NiO phase boundary. [35][36][37] Thus, to isolate the effect of temperature, the tests are conducted so that the difference between the corrosion potential of the specimen and that of the Ni/NiO phase transition was maintained approximately constant. The Ni/NiO phase transition is very close to the H 2 /H 2 O phase transition, and the latter controls the corrosion potential of the simulated primary water environment.…”

“…where: E test = potential at test temperature T E Ni/NiO = potential of the Ni/NiO transition at test temperature T R = universal gas constant = 8.314 x 10 -3 kJ/mol K ( The hydrogen concentration at the Ni/NiO transition as a function of temperature was determined by fitting the data by Attanasio et al 36 with a fourth-order polynomial (Fig. 11).…”

“…However, the corrosion potential for the Bechtel Bettis tests conducted at 360°C (680°F) with 150 cc/kg hydrogen was farther removed from the Ni/NiO phase transition than in the case of the 338°C (640°F) tests with 40-60 cc/kg hydrogen. As such, since the CGRs are highest near the Ni/NiO phase transition and decrease as the potential deviates from this transition, 36,37 the CGRs in the 360°C tests may have been reduced because of the high dissolved hydrogen levels. Thus, the actual activation energy for the Bechtel Bettis data may be somewhat higher than that determined in Fig.…”