ML103610331

Estimating the Extent of Damage to Waste Package Surfaces by Localized Corrosion in Potential Disposal Environments

ML103610331
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Issue date:	12/27/2010
From:	Pabalan R, Pensado O, Shukla P NRC/NMSS/DHLWRS, Center for Nuclear Waste Regulatory Analyses (CNWRA), Southwest Research Institute
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Ahn T
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ESTIMATING THE EXTENT OF DAMAGE TO WASTE PACKAGE SURFACES BY LOCALIZED CORROSION IN POTENTIAL DISPOSAL ENVIRONMENTS Pavan Shukla, Roberto Pabalan, and Osvaldo Pensado Center for Nuclear Waste Regulatory Analyses (CNWRA)

Southwest Research Institute San Antonio, Texas 78238 pshukla@cnwra.swri.edu In a performance assessment model, important performance assessments commonly evaluate parameters needed to compute the consequences of radionuclide release consequences associated with radionuclide release to the environment include the localized corrosion as initiating events. In general, failure time of packages enclosing waste forms and the release consequences are a function of the time of waste extent of damage to the waste package surface. package failure and the extent of damage to the waste Radionuclide releases from the waste package are a package surface. Therefore, the extent of damage, function of the extent of damage, generally specified in expressed as a fraction of the waste package area, is an the form of a surface fraction, because the dimension of important parameter.

this damage or breach area constrains radionuclide This paper proposes a model for estimating the extent release by advection and diffusion. This paper proposes of damage to the waste package surface by localized methodologies for estimating the extent of damage to the corrosion in different potential disposal environments.

waste package surface induced by localized corrosion in The model is developed considering physico-chemical different potential disposal systems. The methodologies processes such as availability and viability of cathodic are developed considering physico-chemical processes reactions to sustain anodic reactions that cause localized such as availability and viability of cathodic reactions to corrosion of waste package materials. In particular, the sustain anodic reactions that cause localized corrosion of model accounts for factors such as (i) the type and rate of waste package materials. In particular, the methodologies cathodic reactions available to sustain the anodic account for factors such as (i) the type and rate of reactions, (ii) the rate of metal dissolution due to anodic cathodic reactions available to sustain the anodic reactions, and (iii) the electrochemical driving force reactions, (ii) the rate of metal dissolution due to anodic necessary to initiate and propagate localized corrosion.

reactions, and (iii) the electrochemical driving force The model is applied to estimate the waste package open necessary to initiate and propagate localized corrosion. area, also referred to as breach area, for carbon- and The methodologies are applied to estimate the breach stainless-steel, which have been proposed as waste area for carbon- and stainless-steel waste package package material in different disposal environments. This material in different potential disposal environments. model is based on the anodic propagation of localized corrosion. More work is warranted in the future to relate I. INTRODUCTION damage to the waste package wall with an expected decrease in the total rate of the anodic reactions due to the Estimating the extent of damage to a waste package accumulation of damage (e.g., after penetration through due to localized corrosion is challenging. Information on the wall). Other factors could exist affecting the extent of the extent of damage due to localized corrosion is needed the damage such as surface conditions of cathodic area or in performance assessment models to estimate ionic transport in the active area.

consequences of radionuclide release to the environment.

Radionuclide release can only occur after engineered II. MODEL barriers, such as the waste package, are compromised. An expected degradation mode of metallic waste packages is A localized corrosion model is presented for corrosion. For a wide range of environmental conditions, estimating the breach area on carbon- and stainless-steel chemical degradation takes the form of localized waste packages. A schematic diagram of the localized corrosion, under which waste packages could be corrosion process in the form of crevice corrosion is compromised in a relatively short period. Detailed presented in Fig. 1. As seen in the figure, the anodic and

cathodic reactions are physically separated in the crevice The following approach is proposed to overcome the lack corrosion process. The metal dissolution (i.e., anodic of information about the electrode potential distribution reactions) predominantly takes place under the crevice outside the crevice. It is widely accepted that localized former). In the region adjacent to the crevice, both anodic corrosion in the form of crevice corrosion can initiate and and cathodic reactions take place. However, the rate of be sustained when the corrosion potential, denoted by anodic and cathodic reactions is dependent upon the Ecorr, is greater than the repassivation potential, denoted electric potential distribution and the presence of by Erp.3-5 Similarly, pitting corrosion can initiate when oxy-hydroxide films. The rate of anodic reactions is generally much lower than the rate of cathodic reactions Ecorr is greater than the breakdown potential, denoted by Ebd.6 A schematic diagram depicting polarization of hypothetical anodic and cathodic reactions as a function of the electrode potential is presented in Fig. 2. According to the polarizations of the anodic and cathodic reactions in Fig. 2, the electrode is susceptible to crevice corrosion because Ecorr is greater than Erp. The electrode potential inside a crevice on the electrode is expected to be more cathodic than Erp, and the electrode potential in the outer region is expected to be more anodic than Erp. Because the electrode potential distribution is not known in the outer region, it can be assumed that the cathodic reactions are occurring at Erp. This assumption will give a Fig. 1. Schematic diagram of the crevice corrosion process. The metal dissolution reactions take place in the crevice region, and conservative estimate of breach area because the the reduction reactions take place in the cathodic region. estimated excess cathodic current will be higher than the actual value as a result of the potential distribution in the immediately outside the crevice region, thus, an excess outer region.

cathodic current is generated because the rate of cathodic Considering the aforementioned assumption in reactions exceeds that of anodic reactions in the outside Eq. (1), the breach area for a waste package surface region. The excess cathodic current is necessary to sustain undergoing crevice corrosion can be estimated according the anodic reactions underneath the crevice.1 to the following equation Under the free corrosion condition, the overall anodic current generated by the anodic reactions underneath the x PR F A

= (1 x) A icat , rp (2) crevice must balance the excess cathodic current.2 This We statement can be represented by the following equation where I A + IC = 0 (1) x breach area fraction of the waste package surface undergoing localized corrosion where PR localized corrosion penetration rate [m/sec]

F Faraday constant (96,485 C/mol)

IA overall anodic current generated by the metal density of the waste package material dissolution reactions underneath the [kg/m3]

crevice [A] A surface area of the waste package [m2]

IC excess cathodic current generated by the We equivalent weight of the waste package cathodic reactions in the outside region [A] material [kg/mol]

icat,rp current density of the cathodic reaction at IC is also referred to as cathodic capacity. The Erp [A/m2]

aforementioned also applies to the pitting corrosion.

Under localized corrosion conditions, if the propagation rate, the rate of the cathodic reactions as a function of the electrode potential, and the electrode potential distribution outside the crevice is known, the maximum area of the waste package surface (i.e., the breach area) that could undergo localized corrosion can be estimated by invoking Eq. (1) and following the approach outlined in Ref. 2.

However, the electrode potential distribution in the outer region is relatively uncertain and, thus, difficult to model.

range from 50-100 °C (122-212 °F). If the waste packages are placed in granite, an aqueous solution containing sodium chloride could develop in the disposal environment. The concentration of the sodium chloride would vary such that the chloride ion concentration could range from 50-50,000 parts per million (4.2 x 10-4 to 0.42 lb/gallon), and the solution temperature could be as high as 90 °C (194 °F).7 In this paper, it is assumed that the solution surrounding the waste packages in a granite rock disposal environment has a NaCl concentration of 50 g/L (0.42 lb/gallon) and the solution temperature is 90 °C (194 °F). In the three representative disposal environments, the chemical solutions surrounding the waste packages will initially have dissolved oxygen because the oxygen is expected to fill in the pores of the excavated disposal site.7 The solutions should eventually Fig. 2. Hypothetical anodic and cathodic reactions taking place become anoxic as dissolved oxygen is consumed by as a function of electrode potential. oxygen reduction reactions.

Eq. (2) can also be used to estimate the breach area of a III. B. Electrochemical Conditions waste package undergoing pitting corrosion by replacing icat,rp with icat,bp, which denotes the current density of the The electrochemical conditions for the carbon- and cathodic reaction at Ebd.. Application of Eq. (2) will 316 stainless-steel waste package materials in the three provide the value of breach area fraction, x, which can be disposal environments were calculated using the further used to estimate the breach area by multiplying x OLIAnalyzer Version 3.1 software.8 The software with the total area of the waste package surface. The generated data for several systems have been extensively model is applied to carbon- and stainless-steels in three validated.9 The chemical compositions of the carbon- and different disposal environments, which are detailed next. 316 stainless-steel are provided in Table I. The chemical, thermal, and alloy specifications were input into the III. DISPOSAL ENVIORNMENTS software. The calculated results included polarization curves and corrosion and repassivation potentials. The Many countries consider disposal of carbon- and values of the cathodic current densities at the calculated stainless-steel waste packages in rock salt, clay, and repassivation potentials were read from the polarization granite.7 Different chemical and thermal conditions could curves.

arise around the waste packages when placed in these TABLE I. Chemical Composition of the Carbon- and 316 three representative disposal environments. These Stainless-Steels conditions are discussed next, as are the electrochemical Alloy Mass fraction of the various constituents conditions needed to estimate the breach area fraction. Fe C Mn Mo Cr Ni Carbon 0.967 0.023 9.98e-3 0 0 0 III. A. Chemical and Thermal Conditions steel 316 stainless 0.671 4.66e-3 0 0.018 0.183 0.124 Carbon- and stainless-steel waste packages placed in steel rock salts could be surrounded by sodium chloride or magnesium chloride brines of concentrations Both chemical and calculated electrochemical approximately equal to 26 and 30 wt %, respectively. The conditions for the carbon steel in the three representative pH of the sodium-chloride-rich brines and disposal environments are listed in Table II. Similarly, the magnesium-chloride-rich brines could range from 4-7. chemical and electrochemical conditions for the The temperature of the brines could range from 316 stainless steel are listed in Table I. The data in 90-150 °C (194-302 °F). When the waste packages are Tables II and III indicate that the cathodic current placed inside bentonite clay, a solution predominantly densities at the repassivation potentials of carbon- and containing sodium, magnesium, and chloride ions would 316 stainless-steels could range from 10-4-30 µA/cm2 surround the waste packages. The chemical composition (6.5 x 10-4-194 µA/in2).

of such a solution can be found in Table 2-15 of the European Commission report.7 Based on proposed designs, the maximum temperature of the solution could

IV. RESULTS AND DISCUSSION The model is applied to calculate the breach area stainless-steel are 27.9 and 26 g/mol (0.061 and fraction as a function of cathodic current densities at the 0.057 lb/mol), respectively. Because there is only a repassivation potentials and localized corrosion marginal difference between densities and equivalent penetration rates. The model also needs the values of the weights of the two alloys, the model is applied for carbon densities and equivalent weights of the carbon- and 316 steel only. The calculated values of the breach area stainless-steel. The densities of the carbon- and stainless- fraction as a function of the cathodic current densities and steel are 7,860 and 7,890 kg/m3 (479.5 and 492.5 lb/ft3), penetration rates are presented in Fig. 3. The calculated respectively, and equivalent weights of the carbon- and TABLE II. Chemical and Electrochemical Conditions for Carbon Steel in the Three Representative Disposal Environments Chemical Conditions Electrochemical Conditions Oxic Conditions Anoxic Condition Representative icat,rp Disposal Chemical pH Temperature Erp (V vs. µA/cm2 Erp (V vs. icat,rp µA/cm2 Environments Compositions Range °C (°F) SHE ) (µA/in2) SHE )#

(µA/in2)

Rock salt 26 wt % NaCl 4-7 90 (194) 0.65 10 (64.5) 0.65 10 (64.5) solution 26 wt % NaCl 4-7 150 (302) 0.76 9 (58.1) 0.76 26 (167.7) solution 30 wt % MgCl2 4-5 90 (194) 0.65 1 (6.5) 0.65 12 (77.4) solution 30 wt % MgCl2 4-5 150 (302) 0.78 10 (64.5) 0.78 12 (77.4) solution Bentonite clay Bentonite clay 50 (122) 0.52 20 (129.0) 0.52 2 (12.9) water*

Granite 50 g/L 6-9 90 (194) 0.6 22 (142.0) 0.6 10 (64.5)

(0.42 lb/gallon)

NaCl solution

• The chemical composition of the bentonite clay water (i.e., solution around the waste packages in the bentonite clay) can be found in Table 2-15 of a European Commission report.7 SHE stands for standard hydrogen electrode.

TABLE III. Chemical and Electrochemical Conditions for Stainless Steel in the Three Representative Disposal Environments Chemical Conditions Electrochemical Conditions Oxic Conditions Anoxic Condition Representative icat,rp Disposal Chemical pH Temperature Erp (V vs. µA/cm2 Erp (V vs. icat,rp µA/cm2

1. 2 # 2 Environment Composition Range °C (°F) SHE ) (µA/in ) SHE ) (µA/in )

Rock salt 26 wt % NaCl 4-7 90 (194] 0.14 6 (38.7) 0.14 1 [6.5]

solution 26 wt % NaCl 4-7 150 (302) 0.25 8 (51.6) 0.25 3 (193.5) solution 30 wt % MgCl2 4-5 90 (194) 0.20 1 (6.5) 0.20 0.07 (0.5) solution 30 wt % MgCl2 4-5 150 (302) 0.32 1 (6.5) 0.32 0.08 (0.5) solution Bentonite clay Bentonite clay 50 (122) 0.08 20 (129.0) 0.08 104 water* (6.5 x 10-4)

Granite 50 g/L 6-9 90 (194) 0.05 13 (83.9) 0.05 10-2 (0.42 lb/gallon) (6.5 x 10-2)

NaCl solution

• The chemical composition of the bentonite clay water (i.e., solution around the waste packages in the bentonite clay) can be found in Table 2-15 of a European Commission report.7 SHE stands for standard hydrogen electrode.

results do not account for the fact that the with increasing cathodic current density. It is also dissolved oxygen concentration in the solutions will observed that the breach area fraction decreases with change with time; therefore, the cathodic current densities increasing penetration rate at a fixed value of the cathodic will also change with time when solutions have dissolved current density. This result is also consistent with the fact oxygen. Moreover, it is also assumed that the chemical that the increasing penetration rate would require more and thermal conditions do not change with time; hence, and more cathodic current. Because the waste package the cathodic current density is also constant. Because of area is fixed, only limited crevice sites could be supported the aforementioned assumptions, it is implied that the by the excess cathodic current. Therefore, the breach area breach area fraction in a disposal environment is only a fraction decreases with increasing penetration rate at a function of the cathodic current density, which is fixed value of the cathodic current density. The calculated invariant with time. results are consistent with the physico-chemical model for As seen in Fig. 3, the breach area fraction increases crevice corrosion.

with the cathodic current density. This result is consistent The model results should be used with caution with the fact that more cathodic current is available to especially for carbon-steel waste packages. The pitting support several crevice sites at the waste package surface factor, which is defines as ratio of the penetration depth 1.0 by pitting to the penetration depth by general corrosion, for carbon-steel under oxidizing conditions varies 0.9 between 1 to 3 (e.g., Johnson and King10). Therefore, the 0.8 breach area will be close to unity when oxidizing conditions prevail and carbon-steel is susceptible to Breach Area Fraction 0.7 localized corrosion in a given disposal environment.

0.6 0.5 V. CONCLUSION 0.4 Line Penetration Symbol Rate (µm/year) The proposed model provides consistent values of the 0.3 10 50 breach area fraction as a function of the cathodic current 0.2 100 density and penetration rates. The model can be used to 150 0.1 200 estimate the breach area fraction of a waste package 0.0 susceptible to localized corrosion in a disposal 0 5 10 15 20 25 30 environment. The model can be used as long as relevant icat,rp(µA/cm2) parameters such as penetration rate, repassivation (a) potential, and cathodic current density at the repassivation 1.0 potential are available. Even though the model 0.9 Line applicability has been demonstrated for localized Penetration Symbol Rate (µm/year) corrosion in form of crevice corrosion only, the model can 0.8 250 300 be used also to estimate the breach area fraction for waste Breach Area Fraction 0.7 350 packages susceptible to the pitting corrosion.

400 0.6 450 500 ACKNOWLEDGMENTS 0.5 0.4 This paper describes work performed by the Center for 0.3 Nuclear Waste Regulatory Analyses (CNWRA) and its contractors for the U.S. Nuclear Regulatory Commission 0.2 (USNRC) under Contract No. NRC-02-07-006. The 0.1 activities reported here were performed on behalf of the 0.0 USNRC Office of Nuclear Material Safety and 0 5 10 15 20 25 30 Safeguards, Division of High-Level Waste Repository icat,rp(µA/cm2)

Safety.

This paper is an independent product of CNWRA and (b) does not necessarily reflect the view or regulatory Fig. 3. Calculated breach area fraction as a function of cathodic position of USNRC.

current density at repassivation potential. The values of the localized corrosion penetration rates were varied. Penetration rates range from (a)10-200 µm/year (0.394-7.88 mils/year) and (b) 250-500 µm/year (9.85-19.7 mils/year).

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