Localized corrosion continues to be one of the most difficult forms of corrosion to predict and monitor, making implementing mitigation strategies limited. This reality leads corrosion engineers to use the blunt instrument of alloy selection as the only means of (hopefully) ensuring that such an attack does not occur on an engineering asset. In the late 1960s and early 1970s, the advent of electrochemical techniques in corrosion science was heralded as a way to address the problem by determining the critical conditions needed for pits to nucleate and grow and also those required for pit growth to stop (i.e., the pits to repassivate).The conceptual framework first suggested by Pourbaix, et al.,1 that a wetted surface needed to exceed some potential (usually referred to as the pitting potential, Epit) for pits to nucleate, and those pits would grow until the potential was below a lower potential (usually referred to as the repassivation potential, Erp). The position of these potentials relative to the corrosion potential (Ecorr) was also critical. If Ecorr was above Erp but below Epit, then the surface would be passive unless there was a disruption in the passive film (e.g., a mechanical breach). Slow cyclic potentiodynamic polarization (CPP) curves were heralded as a means of determining both the pitting potential (Epit) and Erp in a single measurement. The increasing availability of commercial electrochemical workstations promised a revolution in material selection against localized corrosion.The excitement came to a crashing halt when Brian Wilde, then at the Ohio State University, published papers2-3 that seemed to show that the measured values of the repassivation potential were highly sensitive to experimental variables. The work showed that the value of Erp was a strong function of scan rate and vertex current density, with results decreasing by hundreds of mV with increasing amount of corrosion damage. It was concluded that Erp was not a property of a material/solution combination and thus not of use for engineering purposes. For almost two decades, the interest in the engineering application of Erp waned. Rosenfeld, et al.,4 provided additional supporting data. Although it was still considered a valid concept, there seemed to be no way to operationalize it.In the early 1990s, work on a proposed repository for high-level nuclear waste (HLNW) was proceeding apace. Yucca Mountain (YM), Nevada had been selected as the location for the repository. The containment concept was defense in depth: the HLNW would be contained in stainless steel containers each of which would be placed inside an additional corrosion-resistant outer container. These would be buried under YM which is in the desert west of Las Vegas. The dry climate would limit the amount of water available; the mountain would provide a barrier to that water reaching the canisters, and then the corrosion-resistant outer canister would protect the stainless steel container which would protect the HLNW. At early stages after emplacement (several hundred years), the canister surfaces would be hot enough that any seepage water would evaporate upon contact, leaving behind the dissolved species it had collected during its transport through the mountain. Eventually, the canister surfaces would cool sufficiently that the salt on the surface would deliquesce, forming an electrolyte that would initially be saturated, but then dilute as the temperature continued to fall. Aqueous corrosion of any kind would not start until the deliquescence of the salts on the surface. That said, under conditions of elevated temperature and concentrated chloride solutions, there was concern that the canisters would eventually be breached by corrosion, allowing radionuclides to be carried to groundwater several hundred feet below the emplacement zone. EPA regulations required DOE to provide a technical basis for the safety of any design such that the biosphere near YM would be exposed to only acceptable levels of radiation over the design life. This design life increased from initial values of approximately 103 y to 106 y. Prediction of corrosion performance for those time scales was unprecedented and daunting.Projects aimed at developing such a technical basis were created and executed under the separate auspices of the U.S. DOE and the U.S. NRC. The latter work was being performed at the Center for Nuclear Waste Regulatory Analyses (CNWRA) at the Southwest Research Institute (SwRI) as part of their development of a Total System Performance Assessment. The U.S. DOE work focused on establishing the rates of both general and localized corrosion that would develop in repository environments, primarily through exposure testing of a wide range of alloys in solutions whose compositions aimed to replicate those expected to develop on the canisters due to the seepage of water through the fractures in YM.The team as CNWRA took a different tact, particularly regarding localized corrosion. They made the compelling argument that the uncertainty associated with the application of laboratory-measured corrosion rates to field environments rendered a corrosion-allowance approach of limited utility with such long service lives. The propagation of uncertainty in localized corrosion rates would render any prediction highly suspect in terms of reliability. Rather than using a rate-based approach, Dunn, Cragnolino, and Sridhar considered what conditions could exist in a repository that would preclude localized corrosion from propagating, even if initiated. Thus, the concept of Erp was particularly attractive in that once established, a go-no-go decision could be made on any combination of alloy and environment. In addition, one could in principle monitor the Ecorr of the material, and as long as it was below Erp, localized corrosion could not propagate, even if the passive film was breached.The challenge involved how to measure Erp. In a series of papers, the CNWRA team explored several methods, all of which involved initiating and growing localized attack (i.e., pitting/crevice corrosion) at a highly positive potential for some time, and then lowering the potential until the current density fell to low levels, indicative of repassivation. Others had used a similar approach to establish Erp for different alloy/environment systems. The key differences are that Dunn, Cragnolino, and Sridhar decided that (a) the anodic charge density passed (Q) would be the critical independent variable, and (b) they would extend the range of Q to values much, much higher than in most previous studies in an attempt to establish a lower limit to Erp. They supplemented their work by extracting Erp vs. Q data from the literature to demonstrate the existence of a plateau in Erp for sufficient levels of Q for a range of alloy/environment combinations, as shown in their Figure 3.The most powerful part of the work of Dunn, Cragnolino, and Sridhar was their demonstration of the validity of the concept via a large matrix of long-term (nearly 3 y) potentiostatic tests that demonstrated that when the potential was above the properly measured Erp, localized corrosion eventually occurred, but when the potential was at or below Erp no form of localized corrosion (either pitting or crevice corrosion) was observed. The higher the potential above Erp, the shorter the time until localized corrosion is initiated.The importance of the work of Dunn, Cragnolino, and Sridhar is its provision of a basis for engineering implementation of the concept of repassivation potential. The approach has been the basis for further experimental work, modeling, and monitoring. Experimental work has applied the concept to a wide range of alloys.5-9 Other work has sought to connect this work to other aspects of localized corrosion.10-14 Characterization of the effects of experimental variables on Erp has also been the focus of work.15-17 The use of Erp in corrosion modeling has allowed predictive frameworks to be developed18-21 and applied for corrosion monitoring.22The impact of the Dunn, Cragnolino, and Sridhar papers in the engineering application of corrosion science is on par, in my view, with the work of Stern and Geary23 in establishing polarization resistance as an effective means of evaluating corrosion rate as well as Pourbaix's development of E-pH diagrams.24 In all three cases, the publication provided a tool of great use to corrosion engineering that was based on a combination of deep insight and strong fundamental science.Lifetime predictions of containers used for the disposal of high-level radioactive waste (HLW) are necessary to determine the overall performance of a HLW repository. Failure of the containers by localized corrosion and release of radionuclides into the accessible environment may occur if chloride-containing groundwater infiltrates the repository and interacts with the containers. To assess the repository performance, it is necessary to develop a methodology for predicting the long-term susceptibility of container materials to localized corrosion. Laboratory tests have demonstrated that pitting and crevice corrosion are initiated when the corrosion potential of the material exceeds the repassivation potential for localized corrosion. Once corrosion is initiated, the propagation of the localized attack will result in rapid penetration of the waste packages. However, no localized corrosion can be initiated, and all active corrosion sites repassivate when the corrosion potential of the material is lower than the repassivation potential. Results of this investigation demonstrated that the corrosion potential and repassivation potential are useful parameters to predict the long-term localized corrosion susceptibility of corrosion- resistant candidate container materials such as type 316L (UNS S31603) stainless steel and alloy 825 (UNS N08825).Editor's Note: Originally published in Corrosion 56, 1 (2000): p. 90-104. When citing "An Electrochemical Approach to Predicting Long-Term Localized Corrosion of Corrosion-Resistant High-Level Waste Container Materials," please cite the original version.KEY WORDS: alloy 825, chloride, high-level nuclear waste containers, localized corrosion, nickel-based alloys, radionuclides, repassivation potential, stainless steelThe primary goals of the U.S. Department of Energy (DOE) updated waste containment and isolation strategy for the proposed repository at the Yucca Mountain (YM) site are the near-complete containment of radionuclides within waste packages (WP) for several thousand years and acceptably low annual doses to a member of the public living near the site.1 The WP consists of containers, fillers, basket materials, and waste forms (spent fuel [SF] or vitrified waste). One of the most important system attributes recognized in this strategy is container lifetime. To predict the container lifetime confidently, the adequacy of methodologies for extrapolating short-term laboratory data to long-term performance needs to be demonstrated.Several WP designs have been evaluated by the DOE over the history of the YM repository program. The canistered fuel design is considered most likely to be used for high-level radioactive waste (HLW) disposal.2 In this design, either 21 pressurized water reactors (PWR) or 40 boiling water reactors (BWR) assemblies are contained in a type 316L (UNS S31603)(1) stainless steel (SS) canister (3.5 cm wall thickness) surrounded by an inner overpack made of a corrosion-resistant alloy (2.0 cm wall thickness), which is contained in an outer overpack made of a corrosion allowance material (10 cm wall thickness). The total surface area of such a WP is ∼ 37 m2 and the loaded weight is ∼ 65,000 kg. Alternate designs include WP for uncanistered fuel, small-canistered fuel (12 PWR or 24 BWR SF assemblies), and vitrified waste. Nickel-based alloys, such as alloys 825, 625, and C-22 (UNS N08825, N06625, N06022, respectively) presently are being evaluated for the inner corrosion-resistant overpack, and a carbon steel (such as A516 grade 55) is proposed for the outer corrosion allowance overpack.3 If groundwater contacts the container, it is expected by the DOE that uniform corrosion of the carbon steel overpack will proceed at a slow and predictable rate. Evaporative effects and interactions with concrete in the repository may result in an alkaline groundwater that, in the presence of chloride, can lead to localized corrosion of the carbon steel surface.4 If the carbon steel overpack is perforated, then the inner overpack will be exposed to the repository environment. Corrosion of the inner overpack may be mitigated by the use of a corrosion-resistant alloy and galvanic coupling of the inner overpack to the carbon steel overpack.5-6The most common approach to long-term corrosion prediction involves obtaining the corrosion rates of candidate materials exposed to environmental conditions that are similar to, or preferably more severe than, the anticipated environmental conditions of the particular application. These rates then are integrated over time to predict service life. The corrosion rates are obtained through the use of controlled laboratory tests, coupon exposures in the field, or performance evaluation of similar materials in analogous applications. An example of this approach is found in the DOE Total System Performance Assessment (TSPA) conducted in 1995.7 This analysis uses atmospheric and natural water corrosion data to estimate the corrosion rate of outer overpacks in the repository. For the corrosion-resistant inner overpack, degradation was assumed to occur only by localized corrosion that was described by a pit growth rate that varies with temperature. The general corrosion of the corrosion-resistant container is determined by the passive dissolution rate of the material. This approach, referred to here as the empirical corrosion rate approach, has many drawbacks when applied to corrosion estimates for the repository time scales (eg., 103 years to 104 years). These limitations include:First, the relationship between the laboratory or field service environments in which corrosion rates are obtained and the repository environment seldom can be established clearly. Inferring analogous behavior between short-term experiments and the repository setting is especially problematic in the case of the hydrologically unsaturated zone at the YM site. In the proposed repository, the fluid in the environment surrounding the container (typically referred to as the near-field or very near-field environment) may evolve in time and may vary in ionic strength from that of a dilute groundwater to that of a saturated electrolyte because of a complex combination of processes including evaporation, interaction with minerals and man-made materials, and radiolysis.Second, several corrosion phenomena, especially localized corrosion and stress corrosion cracking (SCC), have relatively long initiation times that may be longer than the time elapsed in short-term tests, which are typically < 7 days.8 Once corrosion initiates, however, the rates can be quite high. The initiation time may be a function of the material and test environment. Thus, short-term corrosion rate data may be more useful in eliminating materials that are not suitable for a given environment than for predicting the long-term performance of materials.Third, it is difficult to evaluate effects of design features in corrosion processes such as galvanic coupling. Hence, a large number of long-term tests (i.e., several weeks to years), using various possible design configurations, have to be conducted.Finally, proof of the performance of a material in a repository cannot be obtained easily because of the lack of long-term exposure data for the candidate corrosion-resistant container materials. If feasible, models developed to simulate the long-term corrosion behavior of a material should be constructed from a mechanistic understanding of uniform and localized corrosion processes. The predictive capability of the models can be verified using a diverse set of corrosion data. However, the empirical corrosion rate approach is not based on a mechanistic understanding of the processes involved. As a result, long-term estimations made using this approach are more uncertain. The usefulness of the empirical corrosion rate approach is constrained further when most of the field and laboratory measurements of corrosion rates do not include measurement of the corrosion potential of the metal in the test environment. As a result, data do not provide a suitable means to predict the performance of the material when the duration of the exposure is increased. Because the corrosion potential of the test specimens is not always measured, valuable information necessary to predict the onset of localized corrosion in long-term exposures often is not available.An electrochemical approach can overcome many of the limitations of the empirical corrosion rate approach by explicitly calculating corrosion rates in terms of constituent electrochemical reactions9-10 or by using critical electrochemical parameters for determining the onset of various corrosion processes.5,11 An electrochemical approach has several advantages.First, the occurrence of various corrosion modes such as pitting, crevice corrosion, and SCC can be accelerated in short-term tests using a range of environments and applied potentials. The critical potentials for the occurrence of various corrosion modes then can be determined. The extrapolation from these short-term data is made then by calculating the evolution of the natural potential of the metal (corrosion potential) in the repository environment and comparing these to calculations derived from the measured critical potentials. The calculations of corrosion potentials are supported by fundamental electrochemical models based on the kinetics of the corrosion reactions. Thus, a clearer and more defensible relationship can be established between the test environments and the anticipated repository environment.Second, the initiation time for localized corrosion and SCC explicitly can be considered in terms of its relationship to the corrosion potential. In some cases, kinetics of the corrosion modes can be measured independently and supported by electrochemical models.Finally, the corrosion performance of a given material in diverse applications can be assembled in a single map of critical electrochemical parameters. This approach, similar to the one used by Staehle,12 could provide greater confidence in the performance prediction.Electrochemical models do not dispense with the need to measure corrosion rates; rather, they assign different corrosion rates to different regimes of corrosion, with the delineation of the regimes of corrosion and the rates of corrosion within each regime supportable by fundamental, mechanistic reasoning. It also must be emphasized that electrochemical models, just as other models, are only as good as the quality of the underlying data. Thus, electrochemical models offer an additional advantage over empirical corrosion rate models because they can be used to evaluate critical data needs.A number of investigators have proposed the use of the repassivation potential (Erp) as a parameter to predict the long-term initiation of localized corro- sion13-16 and SCC.11,17 If the corrosion potential (Ecorr) of the metal exceeds Erp, then localized corrosion can be initiated. Prior to initiation of localized corrosion, the metal is expected to corrode at a slow rate corresponding to the passive current density. After the initiation of localized corrosion, the pit would propagate through the metal container at a rate that is orders of magnitude greater than would be expected during passive dissolution. If the Ecorr of the inner barrier decreases below Erp, active localized corrosion would cease and the metal will repassivate. The concept of repassivation is not new. Pourbaix, et al., proposed the use of the Erp (also called the protection potential) below which localized corrosion would not be initiated.18 However, later investigations by Wilde,19 and Rosenfeld, et al.,20 indicated that the Erp decreased with increasing extent of prior corrosion, leading to the conclusion that this parameter cannot be used as a conservative criterion. Hence, to be a conservative parameter for predicting long-term localized corrosion, it must be demonstrated that a lower bound value for Erp can be established.In the present study, data on type 316L SS and alloy 825 generated by the Center for Nuclear Waste Regulatory Analyses (CNWRA) program during the past few years was reviewed along with data from the literature to demonstrate the conservatism of Erp. Additionally, the applicability of Erp for prediction of the long-term occurrence of localized corrosion was examined using results of ongoing long-term tests. Finally, the current mechanistic understanding of localized corrosion initiation and repassivation processes was assessed.A variety of corrosion-resistant and corrosion allowance materials are being tested in the CNWRA program. However, this study focused mainly on alloy 825 and type 316L SS because its objective was to establish the predictive methodology for corrosion- resistant materials. The compositions of type 316L SS and alloy 825 are given in Table 1.Table 1.Composition of Alloy 825 and Type 316L Stainless Steel (wt%)Tests were conducted at 95°C±2°C in an ASTM G-3121-type electrochemical cell with ≈ 1,500 mL solution containing 1,000 ppm Cl−, 85 ppm , 20 ppm , 10 ppm , and 2 ppm F−, all as sodium salts. The solution composition reflected the expected composition of groundwater that may be expected in a HLW repository with an increased chloride concentration. The effect of chloride concentration was studied by adding chloride as sodium chloride (NaCl) to the above solution. However, because of the solubility limitation, solutions with chloride concentration > ∼ 5.5 M were achieved by the use of lithium chloride (LiCl) or magnesium chloride (MgCl2) solutions, without the addition of other anionic and cationic species. The pH of the concentrated MgCl2 solutions were measured to be in the range from 2.0 to 2.7. In tests conducted under air- saturated conditions, a high-purity mixture of 79% N2 + 21% O2 (carbon dioxide [CO2] free air or zero air) gas was used to purge the solution.Five configurations of the various types of specimens used in pitting and crevice corrosion tests in this investigation are shown in Figure 1.FIGURE 1.Schematic of various specimen configurations used in the study of localized corrosion: (a) CPP, (b) lead-in-pencil, (c) longterm test, and (d) crevice.Cylindrical specimens of alloy 825 and type 316L SS measuring 6.2 mm in diameter and 48.6 mm long with a 600-grit finish (Figure 1[a]) were used in cyclic potentiodynamic polarization (CPP) tests. These tests were conducted in an ASTM G-522-type five-neck flask equipped with a calibrated thermometer, a platinum counter electrode, and a salt bridge with a porous silica tip filled with test solution. The salt bridge was connected to a saturated calomel electrode (SCE) that was maintained at room temperature and served as the reference electrode. Test solutions were deaerated thoroughly with high- purity N2 (99.999 %) through a glass frit bubbler. Prior to the start of each test, the Ecorr of the specimen was measured vs the reference electrode using a high-impedance (100 MΩ) electrometer. The potential of the test specimens were increased at a rate of 0.167 mV/s using a computer-controlled potentiostat. Potential scans were started at the Ecorr. The current density of the specimen was monitored continuously. At a current density of 5 mA/cm2, the direction of the potential scan was reversed and the potential of the specimen was decreased using the same scan rate. The potential scan was stopped upon reaching the initial value of Ecorr. Following each scan, the specimen was examined for localized attack. The purpose of the CPP tests was to establish initiation potentials for pitting (Ep) and crevice corrosion (Ecrev), as well as Erp over a wide range of environmental conditions (10−4 M Cl− to 9 M Cl−, 25° C to 95°C). An SCE maintained at room temperature was used as a reference in these tests.Simulated, single-pit propagation tests were conducted on alloy 825 using lead-in-pencil specimens (Figure 1[b]). The advantage of this geometry is that the location of pitting is known and the geometry is amenable to theoretical analyses using relatively simple analytical models. In addition, the repassivation of very deep simulated pits can be investigated with this specimen geometry. The disadvantages of the single-pit specimen are that the morphologies and growth rates of the simulated pit specimens are different than those of naturally formed pits. As a result, the growth rates obtained in single-pit tests cannot be used to predict the penetration of naturally formed pits. Prior to the start of the tests, specimens were weighed to ±0.03 mg. The specimens then were covered with a polyolefin tubing that allowed only one end of the specimen to be exposed to the solution. Electrical contact to the specimen was made outside the test cell. The specimen was oriented so that the opening of the unidirectional pit faced upward. The pit had an area of 0.041 cm2. Corrosion was initiated at potentials of 500 mVSCE to 600 mVSCE. After ≈ 5 h, the anodic current density was constant, indicating that stabilization of the actively growing pit was achieved and the potential was decreased to values from −300 mVSCE to 100 mVSCE at a rate of 5 mV/s. The current and potential were recorded allowing real time calculation of pit depth and propagation rate. This calculation was made using the theoretical weight loss of 2.64 × 10−4 g/coul assuming congruent dissolution of Fe, Cr, and Ni in alloy 825 as Fe2+, Cr3+, and Ni2+, respectively.23 At the conclusion of the tests, specimens were weighed again to confirm the accuracy of the propagation rate calculations. The purpose of these tests was to measure the growth rate and repassivation of single pits and compare them to measurements made on samples where an uncontrollable number of multiple pits formed.Potentiostatic polarization tests were conducted using cubic specimens (Figure 1[c]). These specimens had a total surface area of 15 cm2, including polished and Cr-depleted mill-finished surfaces, as described in previous publications.15,23 Potentiostatic and open- circuit corrosion tests also were conducted using specimens with a controlled crevice (Figure 1[d]).Potentiostatic pitting and crevice corrosion initiation times were measured by polarizing cylindrical and creviced specimens to potentials above and below the Erp measured in CPP tests. Specimens were connected to a computer-controlled multichannel potentiostat. A constant potential was applied for test intervals of 28 days. Between test periods, specimens were examined for visible signs of corrosion and weight loss using an analytical balance with ±0.00005 g reproducibility (standard deviation) while the test solutions were changed. No further exposures were conducted on specimens after localized corrosion occurred. Open-circuit tests were conducted with a setup and solutions similar to those used for the potentiostatic polarization tests. However, instead of controlling the potential of the specimen with a potentiostat, the corrosion potential of alloy 825 was monitored continuously in air-saturated, 1,000 ppm Cl− solutions at 95°C. Periodically, the test was interrupted and the specimen was examined for signs of localized corrosion. These methods allowed the validity of using Erp from short-term CPP tests as a parameter for determining long-term performance to be tested.Since at open circuit, no net current flows in the system, the corrosion kinetics cannot be measured using electrochemical techniques on a single specimen. Thus, a series of open-circuit corrosion tests was conducted with a creviced alloy 825 specimen connected to a large noncreviced alloy 825 plate that served as a cathode through a zero-resistance ammeter (ZRA). If crevice corrosion occurred, the potential of the crevice specimen decreased, resulting in an anodic current in the external circuit. Tests were conducted in solutions containing 1,000 ppm Cl− at 95°C±2°C. The redox potential of the test solution was controlled by using a redox couple such as Cu+/Cu2+ added as chloride salts with hydrochloric acid (HCl) or NaCl additions to achieve the desired total chloride concentration. The area ratio of the alloy 825 cathode plate to the creviced area of the test specimen was at least 10:1. During the tests, the current was measured with a resolution of 1 nA. The potential of the specimens also was recorded with a resolution of 0.1 mVSCE maintained at room temperature. This test method allowed the measurement of corrosion rates under natural exposure conditions at various redox potentials.Pit repassivation tests also were conducted on alloy 825 specimens (Figure 1[a]). Tests were conducted by initiating pits at 600 mVSCE. After initiation, pits were grown at 400 mVSCE. Following the accumulation of a specified charge density, the applied potential of the specimens was decreased to the desired value. The repassivation time was measured as the time required for the current density to decrease < 50 μA/cm2 after the potential of the specimen was decreased rapidly from 400 mVSCE to the desired potential. Details of this technique have been published previously.11,16 These tests allowed the validity of Erp to be tested on specimens with multiple pits.Effects of chloride concentration on Ep and Erp of type 316L SS and alloy 825, as measured in CPP tests, are shown in Figures 2 and 3, respectively. Values of Ep and Erp plotted in these fig
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