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The Effect of pH on Nickel Alloy SCC and Corrosion Performance

Description: Alloy X-750 condition HTH stress corrosion crack growth rate (SCCGR) tests have been conducted at 360 C (680 F) with 50 cc/kg hydrogen as a function of coolant pH. Results indicate no appreciable influence of pH on crack growth in the pH (at 360 C) range of {approx} 6.2 to 8.7, consistent with previous alloy 600 findings. These intermediate pH results suggest that pH is not a key variable which must be accounted for when modeling pressurized water reactor (PWR) primary water SCC. In this study, however, a nearly three fold reduction in X-750 crack growth rate was observed in reduced pH environments (pH 3.8 through HCl addition and pH 4-5.3 through H{sub 2}SO{sub 4} addition). Crack growth rates did not directly correlate with corrosion film thickness. In fact, 10x thicker corrosion films were observed in the reduced pH environments.
Date: October 10, 2002
Creator: Morton, D.S. & Hansen, M.
Partner: UNT Libraries Government Documents Department

Primary Water SCC Understanding and Characterization Through Fundamental Testing in the Vicinity of the Nickel/Nickel Oxide Phase Transition

Description: This paper quantifies the nickel alloy stress corrosion crack growth rate (SCCGR) dissolved hydrogen level functionality. SCCGR has been observed to exhibit a maximum in proximity to the nickel/nickel oxide phase transition. The dissolved hydrogen level SCCGR dependency has been quantified in a phenomenological model in terms of the stability of nickel oxide not the dissolved hydrogen level. The observed SCCGR dependency has been extended to lower temperatures through the developed model and Contact Electrical Resistance (CER) measurements of the nickel/nickel oxide phase transition. Understanding obtained from this hydrogen level SCC functionality and complementary SCC subprocesses test results is discussed. Specifically, the possible SCC fundamental subprocesses of corrosion kinetics, hydrogen permeation and pickup have also been measured for nickel alloys. Secondary Ion Mass Spectroscopy (SIMS) analysis has been performed on SCCGR specimens tested in heavy water (D{sub 2}O).
Date: May 8, 2001
Creator: Morton, D.S.; Attanasio, S.A. & Young, G.A.
Partner: UNT Libraries Government Documents Department

Stress Corrosion Crack Growth Rate Testing and Analytical Electron Microscopy of Alloy 600 as a Function of Pourbaix Space and Microstructure

Description: Stress corrosion crack (SCC) growth rate tests and analytical electron microscopy (AEM) studies were performed over a broad range of environments and heat treatments of Alloy 600. This effort was conducted to correlate bulk environmental conditions such as pH and electrochemical potential (EcP) with the morphology of the SCC crack. Development of a library of AEM morphologies formed by SCC in different environments is an important step in identifying the conditions that lead to SCC in components. Additionally, AEM examination of stress corrosion cracks formed in different environments and microstructures lends insight into the mechanism(s) of stress corrosion cracking. Testing was conducted on compact tension specimens in three environments: a mildly acidic oxidizing environment containing sulfate ions, a caustic environment containing 10% NaOH, and hydrogenated near-neutral buffered water. Additionally, stress corrosion cracking testing of a smooth specimen was conducted in hydrogenated steam. The following heat treatments of Alloy 600 were examined: mill annealed at 980 C (near-neutral water), mill annealed at 1010 C (steam), sensitized (acid and caustic), and mill annealed + healed to homogenize the grain boundary Cr concentration (caustic). Crack growth rate (CGR) testing showed that sensitized Alloy 600 tested in the mildly acidic, oxidizing environment containing sulfate ions produced the fastest cracking ({approx} 8.8 {micro}m/hr at 260 C), and AEM examination revealed evidence of sulfur segregation to the crack tip. The caustic environment produced slower cracking ({approx} 0.4 {micro}m/hr at 307 C) in the mill annealed + healed heat treatment but no observed cracking in the sensitized condition. In the caustic environment, fully oxidized carbides were present in the crack wake but not ahead of the crack tip. In near-neutral buffered water at 338 C, the CGR was a function of dissolved hydrogen in the water and exhibited a maximum (0.17 {micro}m/hr) near the transition between Ni ...
Date: October 4, 2000
Creator: Lewis, N.; Attanasio, S.A.; Morton, D.S. & Young, G.A.
Partner: UNT Libraries Government Documents Department

The Influence of Dissolved hydrogen on Nickel Alloy SCC: A Window to Fundamental Insight

Description: Prior stress corrosion crack growth rate (SCCGR) testing of nickel alloys as a function of the aqueous hydrogen concentration (i.e., the concentration of hydrogen dissolved in the water) has identified different functionalities at 338 and 360 C. These SCCGR dependencies have been uniquely explained in terms of the stability of nickel oxide. The present work evaluates whether the influence of aqueous hydrogen concentration on SCCGR is fundamentally due to effects on hydrogen absorption and/or corrosion kinetics. Hydrogen permeation tests were conducted to measure hydrogen pickup in and transport through the metal. Repassivation tests were performed in an attempt to quantify the corrosion kinetics. The aqueous hydrogen concentration dependency of these fundamental parameters (hydrogen permeation, repassivation) has been used to qualitatively evaluate the film-rupture/oxidation (FRO) and hydrogen assisted cracking (HAC) SCC mechanisms. This paper discusses the conditions that must be imposed upon these mechanisms to describe the known nickel alloy SCCGR aqueous hydrogen concentration functionality. Specifically, the buildup of hydrogen within Alloy 600 (measured through permeability) does not exhibit the same functionality as SCC with respect to the aqueous hydrogen concentration. This result implies that if HAC is the dominant SCC mechanism, then corrosion at isolated active path regions (i.e., surface initiation sites or cracks) must be the source of localized elevated detrimental hydrogen. Repassivation tests showed little temperature sensitivity over the range of 204 to 360 C. This result implies that for either the FRO or the HAC mechanism, corrosion processes (e.g., at a crack tip, in the crack wake, or on surfaces external to the crack) cannot by themselves explain the strong temperature dependence of nickel alloy SCC.
Date: October 12, 2000
Creator: Morton, D.S.; Attanasio, S.A.; Young, G.A.; Andresen, P.L. & Angeliu, T.M.
Partner: UNT Libraries Government Documents Department

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

Description: Prior studies of Alloy 600 and Alloy X-750 have shown the existence of a maximum in stress corrosion 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 (H{sub 2}) concentrations. It has also been shown that this maximum in SCC susceptibility tends to occur in proximity to the nickel/nickel oxide (Ni/NiO) 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 H{sub 2}O. 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-free at a given condition. The transition aqueous hydrogen (H{sub 2}) 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 (i.e., 7 to 58 scc H{sub 2}/kg H{sub 2}O) between the measured Ni/NiO transition and the theoretical Ni/NiO transition previously calculated using free energy data from the Journal of Solution Chemistry. The CER-measured position of the Ni/NiO 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{sub 2} level at which the maximum in SCC susceptibility is likely to be observed at temperatures lower than 338 to 360 C, at which SCC tests are time-consuming to perform. Limited SCC data are presented which are consistent with the observation that SCC susceptibility is maximized near the Ni/NiO transition at 288 C.
Date: May 8, 2001
Creator: Attanasio, S.A.; Morton, D.S.; Ando, M.A.; Panayotou, N.F. & Thompson, C.D.
Partner: UNT Libraries Government Documents Department

Influence of dissolved hydrogen on nickel alloy SCC in high temperature water

Description: Stress corrosion crack growth rate (SCCGR) tests of nickel alloys were conducted at 338 C and 360 C as a function of the hydrogen concentration in high purity water. Test results identified up to a 7 x effect of hydrogen levels in the water on crack growth rate, where the lowest growth rates were associated with the highest hydrogen levels. At 338 C, the crack growth rate decreased as the hydrogen levels were increased. However, different results were observed for the test conducted at 360 C. As the hydrogen level was increased in the 360 C tests, the crack growth rate initially increased, a maximum was exhibited at a hydrogen level of {approximately} 20 scc/kg, and thereafter the crack growth rate decreased. Based on this testing and a review of the commercial literature, the thermodynamic stability of nickel oxide, not the dissolved hydrogen concentration, was identified as a fundamental parameter influencing the susceptibility of nickel alloys to SCC. These test results are discussed in relation to the accuracy of extrapolating high temperature SCC results to lower temperatures.
Date: March 1, 1999
Creator: Morton, D.S.; Attanasio, S.A.; Fish, J.S. & Schurman, M.K.
Partner: UNT Libraries Government Documents Department

Effect of soluble zinc additions on the SCC performance of nickel alloys in deaerated hydrogenated water

Description: Stress corrosion crack growth rates (SCCGR) of alloy 600, EN82H and X-750 were measured in deaerated hydrogenated water to determine if soluble zinc mitigates SCCGR. Constant load compact tension specimen tests were conducted. Two test strategies were used to discern a possible zinc effect. The first strategy employed separate SCCGR tests in zinc and non-zinc environments and compared the resulting crack growth rates. The second strategy varied zinc levels at the midterm of single specimen SCCGR tests and characterized the resulting crack growth rate effect through an electrical potential drop in-situ crack monitor. Results from the direct comparison and midterm changing chemistry tests did not discern a zinc influence; any apparent zinc influence is within test to test variability ({approximately}1.5{times} change in crack growth rate). AEM, AUGER and ESCA crack tip fracture surface studies identified that zinc was not incorporated within crack tip oxides. These studies identified nickel rich crack tip oxides and spinel, with incorporated zinc, ({approximately}5 atom percent) bulk surface oxides.
Date: August 1, 1997
Creator: Morton, D.S.; Thompson, C.D.; Gladding, D. & Schurman, M.K.
Partner: UNT Libraries Government Documents Department

Measurement of the Fundamental Parameters for the Film-Rupture/Oxidation Mechanism

Description: Identification of the mechanism responsible for primary water stress corrosion cracking (PWSCC) in nickel-base alloys is a controversial topic. Numerous mechanisms, including the film-rupture/oxidation (i.e., slip-oxidation or slip-dissolution) mechanism, have been proposed to explain PWSCC. According to this mechanism, the observed sensitivity of PWSCC to material and environmental factors may be explained by the combined effects of repassivation kinetics, oxide rupture strain, and crack tip strain rate (which includes creep). Previous research has shown that increasing the Cr content of Ni-9%Fe-Cr from 16 to 30 wt% strongly decreases PWSCC susceptibility. Consequently, measurements of these three fundamental parameters (repassivation, oxide rupture, and creep) were performed as a function of Cr content, and SCC crack growth rates were predicted on the basis of the resulting data. This paper illustrates that considering these three parameters concurrently may contribute to the understanding of Cr effects on PWSCC of Ni-base alloys. However, it is not clear whether the film-rupture/oxidation mechanism can adequately predict the observed crack growth rates for Alloy 600 at 338 C.
Date: April 23, 1999
Creator: Attanasio, S.A.; Fish, J.S.; Morton, D.S.; Rosecrans, P.M.; Was, G.S.; Wilkening, W.W. et al.
Partner: UNT Libraries Government Documents Department