David S. Morton

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

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).

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

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.

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

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.

Stress Corrosion Crack Growth Rate Testing of Novel Composite Arrest Specimens

Stress corrosion crack (SCC) arrest tests have been conducted on composite material specimens to study the SCC susceptibility of highly SCC resistant materials. The “composite arrest” test method entails fabricating composite material specimens consisting of a highly SCC susceptible material welded to a highly SCC resistant material. Results from Alloy 690, Alloy 690 weld metal and stainless steel composite arrest specimen tests showed that these materials are extremely resistant to SCC in deaerated water environments. Under aggressive 360°C (680°F) test conditions, cracks readily grew within the SCC susceptible starter material but did not transition into growth within the SCC resistant materials. In contrast, SCC readily grew within Alloy 600 control specimens. Results from extensive specimen analytical (microprobe, AEM, FIB, EBSD) characterization efforts and SCC mechanistic insight will also be discussed.

SCC Initiation Testing of Alloy 600 in High Temperature Water

Stress corrosion cracking (SCC) initiation tests have been conducted on Alloy 600 at temperatures from 304 to 367°C. Tests were conducted with in-situ monitored smooth tensile specimens under a constant load in hydrogenated environments. A reversing direct current electric potential drop (EPD) system was used for all of the tests to detect SCC initiation. Tests were conducted to examine the effects of stress (and strain), coolant hydrogen, and temperature on SCC initiation time. The thermal activation energy of SCC initiation was measured as 103 ± 18 kJ/mol in hydrogenated water, which is similar to the thermal activation energy for SCC growth. Results suggest that the fundamental mechanical parameter which controls SCC initiation is plastic strain not stress. SCC initiation was shown to have a different sensitivity than SCC growth to dissolved hydrogen level. Specifically, SCC initiation time appears to be relatively insensitive to hydrogen level in the nickel stability region.

Oxide Film Characterization Along Crack Paths in Stainless Steel in Aerated and Deaerated Water Environments

The oxide films that formed along stress corrosion crack (SCC) paths in both high temperature aerated and deaerated water (AW and DW) were characterized through extensive analytical transmission electron microscopy (ATEM), energy dispersive spectroscopy (EDS), Auger, X-ray diffraction (XRD), and focused ion beam/scanning electron microscopy (FIB/SEM) analyses. SCC growth rate experiments were conducted in AW and DW environments at temperatures as high as 360°C, with anion additions during portions of testing. Rapid growth rates occurred in anion faulted AW and were as much as two orders of magnitude faster than those measured in the DW environments. The oxides generally consisted of a dual-layered structure with an inner layer of chromium enriched oxide (relative to the metal substrate) and an outer layer of iron rich oxide. Nickel enrichment was often observed at and ahead of crack tips and along crack flanks in the DW environment and was infrequently observed in the AW environment. Localized corrosion, here referred to as “oxide bulbs” were solely detected in the AW environment and occurred periodically along crack paths and at some crack tips. The development of a nickel enriched zone ahead of the crack tip in the DW environment is speculated to be a signature of a slow growing crack rather than an essential signature of the crack propagation mechanism.

The Effect of pH on Nickel Alloy SCC and Corrosion Performance

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.