Crevice Corrosion Penetration Rates of Alloy 22 In ...corrosion cracking, are considered important processes that may influence the lifetimes of the waste packages. Of these processes,

CNWRA 2006-001

CREVICE CORROSION PENETRATIONRATES OF ALLOY 22 IN

CHLORIDE-CONTAINING WATERS—PROGRESS REPORT

Prepared for

U.S. Nuclear Regulatory CommissionContract NRC–02–02–012

Prepared by

X. HeD.S. Dunn

Center for Nuclear Waste Regulatory AnalysesSan Antonio, Texas

December 2005

ii

PREVIOUS REPORTS IN SERIES

Number Name Date Issued

CNWRA 91-004 A Review of Localized Corrosion of High-Level April 1991Nuclear Waste Container Materials—I

CNWRA 91-008 Hydrogen Embrittlement of Candidate Container June 1991Materials

CNWRA 92-021 A Review of Stress Corrosion Cracking of High-Level August 1992Nuclear Waste Container Materials—I

CNWRA 93-003 Long-Term Stability of High-Level Nuclear Waste February 1993Container Materials: I—Thermal Stability of Alloy 825

CNWRA 93-004 Experimental Investigations of Localized Corrosion of February 1993High-Level Nuclear Waste Container Materials

CNWRA 93-006 Characteristics of Spent Nuclear Fuel and CladdingRelevant to High-Level Waste Source Term May 1993

CNWRA 93-014 A Review of the Potential for Microbially Influenced June 1993Corrosion of High-Level Nuclear Waste Containers

CNWRA 94-010 A Review of Degradation Modes of Alternate Container April 1994Designs and Materials

CNWRA 94-028 Environmental Effects on Stress Corrosion Cracking of October 1994Type 316L Stainless Steel and Alloy 825 as High-LevelNuclear Waste Container Materials

CNWRA 95-010 Experimental Investigations of Failure Processes of May 1995High-Level Radioactive Waste Container Materials

CNWRA 95-020 Expert-Panel Review of the Integrated Waste September 1995Package Experiments Research Project

CNWRA 96-004 Thermal Stability and Mechanical Properties of May 1996High-Level Radioactive Waste Container Materials:Assessment of Carbon and Low-Alloy Steels

CNWRA 97-010 An Analysis of Galvanic Coupling Effects on the August 1997Performance of High-Level Nuclear Waste ContainerMaterials

iii

PREVIOUS REPORTS IN SERIES (continued)


CNWRA 98-004 Effect of Galvanic Coupling Between Overpack Materials March 1998of High-Level Nuclear Waste Containers—Experimentaland Modeling Results

CNWRA 98-008 Effects of Environmental Factors on Container Life July 1998

CNWRA 99-003 Assessment of Performance Issues Related to September 1999Alternate Engineered Barrier System Materials andDesign Options

CNWRA 99-004 Effects of Environmental Factors on the Aqueous September 1999Corrosion of High-Level Radioactive WasteContainers—Experimental Results and Models

CNWRA 2000-06 Assessment of Methodologies to Confirm January 2001Revision 1 Container Performance Model Predictions

CNWRA 2001-003 Effect of Environment on the Corrosion of September 2001Waste Package and Drip Shield Materials

CNWRA 2002-01 Effect of In-Package Chemistry on the Degradation October 2001of Vitrified High-Level Radioactive Waste and SpentNuclear Fuel Cladding

CNWRA 2002-02 Evaluation of Analogs for the Performance Assessment March 2002of High-Level Waste Container Materials

CNWRA 2003-01 Passive Dissolution of Container Materials—Modeling October 2002and Experiments

CNWRA 2003-02 Stress Corrosion Cracking and Hydrogen October 2002Embrittlement of Container and Drip Shield Materials

CNWRA 2003-05 Assessment of Mechanisms for Early Waste March 2003Package Failures

CNWRA 2004-01 Effect of Fabrication Processes on Materials Stability— October 2003Characterization and Corrosion

CNWRA 2004-02 Natural Analogs of High-Level Waste Container January 2004Materials—Experimental Evaluation of Josephinite

iv

PREVIOUS REPORTS IN SERIES (continued)


CNWRA 2004-03 The Effects of Fabrication Processes on the July 2004Mechanical Properties of Waste Packages—Progress Report

CNWRA 2004-08 A Review Report on High Burnup Spent September 2004Nuclear Fuel—Disposal Issues

CNWRA 2005-01 Microbially Influenced Corrosion Studies of October 2004Engineered Barrier System Materials

CNWRA 2005-02 Passive and Localized Corrosion of Overpack November 2005Revision 1 Materials—Modeling and Experiments

CNWRA 2005-03 Microstructural Analyses and Mechanical Properties March 2005of Alloy 22

CNWRA 2006-001 Crevice Corrosion Penetration Rates of December 2005Alloy 22 in Chloride-Containing Waters—Progress Report

v

ABSTRACT

The long lifetime of the waste package is identified by the U.S. Department of Energy (DOE) asa key engineered barrier system attribute for the performance of the potential high-level wasterepository at Yucca Mountain, Nevada. A possible waste package design for the potentialhigh-level waste repository may consist of an outer disposal container made of Alloy 22, whichsurrounds an inner container made of Type 316 nuclear grade stainless steel. Among theimportant corrosion processes to determine the performance of the waste package, crevicecorrosion of Alloy 22 could occur in chloride-containing waters with low concentrations ofinhibiting oxyanions (e.g., nitrate) when the corrosion potential exceeds the crevice corrosionrepassivation potential. This report presents the Center for Nuclear Waste Regulatory Analyses(CNWRA) experimental results of the crevice corrosion propagation rates of Alloy 22 inchloride-containing waters. The results of the investigation indicate that, although crevicecorrosion of Alloy 22 is possible under some environmental conditions and the propagationrates for crevice corrosion are typically greater than the passive uniform corrosion rates, themaximum penetration depth of localized attack is limited to depths significantly less than thecontainer thickness as a result of stifling and the repassivation of crevice corrosion. Becausecrevice corrosion processes are diffusion controlled, using a constant crevice corrosionpropagation rate in the performance assessment calculations is overly conservative. Thestifling and arrest of crevice corrosion significantly decrease the actual values of crevicecorrosion propagation rates even under the aggressive environmental conditions used in theselaboratory tests. A similar behavior can be expected under the range of expectedenvironmental conditions of the potential repository.

vii

CONTENTS

Section Page

PREVIOUS REPORTS IN SERIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vFIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ixTABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiiiEXECUTIVE SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.1 Literature Review on Localized Corrosion Propagation . . . . . . . . . . . . . . . . . 1-21.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-31.3 Scope and Organization of the Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

2 CREVICE CORROSION PROPAGATION TEST METHODS . . . . . . . . . . . . . . . . . . 2-1

3 CREVICE CORROSION PENETRATION RATES AS A FUNCTION OF TIME ANDENVIRONMENTAL CONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3.1 Tests with Single Crevice Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13.2 Tests with Multiple Crevice Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-43.3 Crevice Corrosion Penetration Rates of Alloy 22 as a Function of Time . . . . 3-63.4 Effects of Solution Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-123.5 Effects of Cathode to Anode Area Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12

4 EVALUATION OF THE LOCALIZED CORROSION OF THE WASTE PACKAGEOUTER CONTAINER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

5 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

6 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

ix

FIGURES

Figure Page

2-1 Schematics of the Electrochemical Test Cell with Single Crevice Assembly toStudy the Crevice Corrosion Propagation in Alloy 22 . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

2-2 Illustrations of the Cylindrical Specimen for Single Crevice Assembly andCrevice Corrosion Specimen for Multiple Crevice Assembly . . . . . . . . . . . . . . . . . . . 2-2

3-1 Measured Current Density and Potential Using the Single CreviceAssembly for an Alloy 22 Cylindrical Specimen Galvanically Coupled to aLarge Alloy 22 Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

3-2 Current Density Decay Behavior in the Period of Crevice CorrosionPropagation After the Current Density Peak Indicated in Figure 3-1 . . . . . . . . . . . . . 3-3

3-3 Measured Current Density and Potential Using the Single Crevice Assemblyfor an Alloy 22 Cylindrical Specimen Galvanically Coupled to a LargeAlloy 22 Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

3-4 Measured Current Density and Potential Using the Multiple Crevice Assemblyfor an Alloy 22 Crevice Specimen Galvanically Coupled to a LargeAlloy 22 Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

3-5 Measured Current Density and Potential Using the Multiple Crevice Assemblyfor an Alloy 22 Crevice Specimen Galvanically Coupled to a LargeAlloy 22 Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

3-6 Photographs Showing Crevice Corrosion Features of Alloy 22Single-Crevice Specimens After Corrosion in 5 M NaCl Solution . . . . . . . . . . . . . . . 3-10

3-7 Measured Crevice Corrosion Penetration Depths as a Function of Time forTests on Alloy 22 at at 95 °C [203 °F] in 5 M NaCl Solution . . . . . . . . . . . . . . . . . . . 3-11

3-8 Calculated Crevice Corrosion Penetration Rates of Alloy 22 According toEq. (3-1) During the Testing Period in the Time Range of 0.5 to 78 Days . . . . . . . . 3-11

3-9 Recorded Galvanic Couple Current Density and Potential for Tests in 5-, 3-, and1-M NaCl Solutions with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F] . . . . . . 3-13

3-10 Measured Current Density and Potential from Single Crevice Assembly foran Alloy 22 Cylindrical Specimen Galvanically Coupled to an Alloy 22 Cathode . . . 3-13

3-11 Photographs of Crevice Corroded Alloy 22 Cylindrical Specimens AfterCrevice Corrosion with Cathode-to-Anode Area Ratios of 4:1 and 2:1 in5 M NaCl Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

4-1 Maximum Localized Corrosion Penetration Depths from Figure 3-7 as aFunction of Exposure Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

xi

TABLES

Table Page

2-1 Composition of Alloy 22 (in Weight Percent) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

3-1 Summary of Alloy 22 Crevice Corrosion Tests Using the Single CreviceAssembly in 5 M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at95 °C [203 °F] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

3-2 Summary of Alloy 22 Crevice Corrosion Tests Using the Multiple CreviceAssembly in 5 M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at95 °C [203 °F] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

xiii

ACKNOWLEDGMENTS

This report was prepared to document work performed by the Center for Nuclear WasteRegulatory Analyses (CNWRA) for the U.S. Nuclear Regulatory Commission (NRC) underContract No. NRC–02–02–012. The activities reported here were performed on behalf of theNRC Office of Nuclear Material Safety and Safeguards, Division of High-Level WasteRepository Safety. This report is an independent product of CNWRA and does not necessarilyreflect the view or regulatory position of NRC.

The authors gratefully acknowledge G. Cragnolino, V. Jain, O. Pensado, and Y.-M. Pan fortechnical discussions of this work, Mr. Brian Derby for conducting the laboratory tests, thetechnical review of G. Cragnolino, the programmatic review of S. Mohanty, and the editorialreview of E. Hanson. Appreciation is due to J. Gonzalez for assistance in the preparation ofthis report.

QUALITY OF DATA, ANALYSES, AND CODE DEVELOPMENT

DATA: All CNWRA-generated original data contained in this report meet the quality assurancerequirements described in the CNWRA Quality Assurance Manual. Experimental data havebeen recorded in CNWRA scientific notebook numbers 668 and 670. Sources for other datashould be consulted for determining the level of quality for those data.

ANALYSES AND CODES: No codes were used in the analyses contained in this report.

xv

EXECUTIVE SUMMARY

The long lifetime of the waste package is identified by the U.S. Department of Energy (DOE) asa key engineered barrier system attribute for the performance of the potential high-level wasterepository at Yucca Mountain, Nevada. A possible waste package design for the potentialhigh-level waste repository may consist of an outer disposal container made of Alloy 22 (Ni-22Cr-13Mo-4Fe-3W), which surrounds an inner container made of Type 316 nuclear gradestainless steel (low C-high N-Fe-18Cr-12Ni-2.5Mo). Degradation processes, including dry-airoxidation, uniform corrosion, disruption of the passive film, localized corrosion, and stresscorrosion cracking, are considered important processes that may influence the lifetimes of thewaste packages. Of these processes, localized corrosion is one of the most insidious forms ofmetal failure mode because when it occurs, it tends to penetrate at a faster rate than uniformcorrosion.

The Center for Nuclear Waste Regulatory Analyses (CNWRA) is conducting an independenttechnical assessment to evaluate plausible degradation processes of Alloy 22. Results of theindependent assessment are used to develop abstractions for waste package performanceimplemented in the U.S. Nuclear Regulatory Commission/CNWRA Total-system PerformanceAssessment (TPA) code. Localized corrosion of Alloy 22 typically in the form of crevicecorrosion can occur in concentrated chloride solutions when the corrosion potential exceeds thecrevice corrosion repassivation potential. The formation of concentrated solutions on the wastepackage surfaces may occur as a result of the evolution of seepage water contacting the wastepackages if the titanium alloy drip shields are damaged or do not function as designed. Thecorrosion potential of Alloy 22 depends on solution pH, temperature, and oxygen reductionkinetics. The crevice corrosion repassivation potential is dependent on the metallurgicalcondition of the alloy, temperature, chloride concentration, and the relative concentration ofinhibiting anions to the chloride concentration. These relationships for localized corrosioninitiation are considered in the TPA code. This report provides results of recent CNWRAexperimental work conducted to determine (i) the crevice corrosion propagation rates of Alloy 22in chloride solutions and (ii) how the rates change with time and environmental conditions. Theresults documented in this report will be incorporated into the TPA code for improved realism.

Previous tests have shown that crevice corrosion of Alloy 22 is only possible in concentratedchloride solutions with low inhibitor concentrations and at temperatures above 60 °C [140 °F].Because the objective of this work was to determine crevice corrosion propagation rates, manyof the tests on Alloy 22 were conducted in concentrated 5 M NaCl solution at 95 °C [203 °F].The test periods were extended from 0.5 to 78 days. The addition of 2 × 10!4 M CuCl2 was usedas an oxidant to increase the corrosion potential above the repassivation potential, which led torapid crevice corrosion initiation. Although cupric solutions are not expected to contact thewaste packages in the potential repository, these solutions were used to initiate crevicecorrosion in a short time to allow the measurement of propagation rates without applyingpotential externally. The subsequent active propagation of crevice corrosion resulted insignificant decreases in the corrosion potential. The current decayed quickly with time as aresult of potential drop. The average corrosion potential drop was determined to be 549 mVwith a standard deviation of 168 mV. The reduction in the corrosion potential, which occurredas a result of crevice corrosion initiation, led to stifling and arrest of crevice corrosion. Thedeepest penetration observed is less than 350 :m [13.8 mils]. The penetration rate decreasedsignificantly with time because of crevice corrosion stifling and arrest.

xvi

Limited tests were conducted in 3 M NaCl and 1 M NaCl solutions at 95 °C [203 °F]. Althoughcrevice corrosion in the 3 M NaCl solution showed a faster propagation rate than that in5 M NaCl solution, a strong tendency to repassivation was evident. Crevice corrosion was notinitiated in the 1 M NaCl solution.

Although crevice corrosion of Alloy 22 is possible under some aggressive environmentalconditions and the propagation rates for crevice corrosion are typically orders of magnitudegreater than the passive uniform corrosion rates, the maximum penetration depth of localizedattack is limited to depths significantly less than the container thickness as a result of stiflingand repassivation of crevice corrosion. Stifling and arrest limit crevice corrosion penetration.These processes significantly reduce the probability for failure as a result of crevice corrosion.

1-1

1 INTRODUCTION

The potential waste package design (Anderson, et al., 2003) for the disposal of high-level wasteat the potential repository in Yucca Mountain, Nevada, may consist of an outer container madefrom Alloy 22 (Ni-22Cr-13Mo-4Fe-3W), and an inner container made of Type 316 nuclear gradestainless steel (low C-high N-Fe-18Cr-12Ni-2.5Mo). For undisturbed repository conditions,corrosion is expected to be the primary degradation process limiting the lifetime of the wastepackage. The loss of containment would allow the release of radionuclides to the environmentimmediately surrounding the waste packages.

The corrosion-related processes considered important to the degradation of the wastepackages include dry-air oxidation, uniform corrosion, disruption of the passive film, localizedcorrosion, and stress corrosion cracking. Of these corrosion processes, localized corrosion isone of the most insidious forms of metal failure because it usually occurs in metals and alloysthat are extremely resistant to uniform corrosion as a result of the formation of passive oxidefilm on the metal surface. When localized corrosion occurs, it tends to penetrate at a faster ratethan general corrosion.

If localized corrosion occurs, it presumably would be in the form of crevice corrosion rather thanpitting corrosion because of the high potentials required to nucleate pits on an openly exposedsurface in nickel-chromium-molybdenum alloys with high chromium and molybdenum (plustungsten) contents (Cragnolino, et al., 1999). In the potential repository, crevices may beformed between the waste package and the emplacement pallet. Crevices may also be formedby deposition of mineral precipitates, corrosion products, dust, contact with rocks, and contactbetween the waste package and the drip shield or ground support materials.

The localized corrosion abstraction in the U.S. Nuclear Regulatory Commission (NRC)/Centerfor Nuclear Waste Regulatory Analyses (CNWRA) Total-system Performance Assessment(TPA) code (Mohanty, et al., 2002) is based on a critical potential model. Crevice corrosion isconsidered possible if the corrosion potential (Ecorr) exceeds the repassivation potential forcrevice corrosion (Ercrev). Numerous tests have been conducted to measure the Ecorr and theErcrev as a function of metallurgical and environmental conditions (Dunn, et al., 2005, 2000;Cragnolino, et al., 2004). The Ecorr of Alloy 22 is mainly dependent on the solution pH,temperature, and oxygen reduction kinetics. The Ercrev depends on the metallurgical condition ofthe alloy, temperature, chloride concentration, and the relative concentration of inhibiting anionsto the chloride concentration. The U.S. Department of Energy (DOE) localized corrosionabstraction also uses a similar approach. DOE provided technical information on thisabstraction in a report, Technical Basis Document No. 6: Waste Package and Drip ShieldCorrosion and Appendix O (Bechtel SAIC Company, LLC, 2003, 2004a), in response to theDOE and NRC agreements CLST.1.10 and 1.11.

In the NRC/CNWRA TPA code (Mohanty, et al., 2002), repassivation of localized attack isassumed to occur only if the Ecorr is reduced below the Ercrev. On the other hand, if the Ecorr ofthe container material never decreases below Ercrev, the localized attack is assumed topropagate without repassivation and the localized corrosion penetration relates to time byEq. (1-1).

1Dunn, D.S., O. Pensado, Y.-M. Pan, L.T. Yang, and X. He. “Modeling Corrosion Processes for Alloy 22 WastePackages.” The 29th International Symposium on the Scientific Basis for Waste Management, Ghent, Belgium,September 12–16, 2005. Submitted for publication (2005).

1-2

d kt n= (1-1)

where

d — penetration deptht — timek and n — constants

In Eq. (1-1), n is assumed to be equal to one in the NRC/CNWRA TPA code because no datafor n is available in the literature. The localized corrosion penetration rate of Alloy 22, k, inEq. (1-1) is 0.25 mm/yr [9.8 mpy] and is independent of time since the time exponent is equal toone. The selection of this rate is supported by the localized corrosion penetration rates reportedby Dunn, et al. (1996) for Type 316L stainless steel and Cramer, et al. (1984) for Alloy 625. Alloy 625 was previously considered as a possible waste package material, and it has lowercorrosion resistance than that of Alloy 22 due to lower content of molybdenum and no additionof tungsten. Without protection from the drip shield, recent analyses suggest that some fractionof the waste packages may be susceptible to crevice corrosion1 (Dunn, et al., 2005). Crevicecorrosion may penetrate the Alloy 22 outer container and result in radionuclide release. Additional studies of propagation and repassivation mechanisms were conducted to determinethe effect of localized corrosion on life prediction and the damage evolution determination of thewaste package. This report evaluates (i) the localized corrosion penetration rates of the Alloy22 waste package outer container and (ii) rate changes in respect to time andenvironmental conditions.

1.1 Literature Review on Localized Corrosion Propagation

Localized corrosion propagation studies on Alloy 22 are limited, but the anodic behavior ofAlloy 22 at relatively elevated temperatures appears to be similar to other nickel-chromium-molybdenum alloys. Active propagation requires a highly concentrated and aggressiveenvironment in occluded sites. The alloy composition, solution composition, temperature, andpotential at the localized dissolving interface determine the alloy dissolution rate(Steinsmo, et al., 1993). The fundamental understanding of the dissolution kinetics of complexmulticomponent alloys such as Alloy 22 is not well developed.

When stable crevice or pitting corrosion occurs in stainless steels and other passive alloys suchas titanium and aluminum, propagation rates have been modeled by ohmic, mass, and chargetransfer controlled processes. Localized corrosion growth kinetics typically conform to a powerequation in the same form of Eq. (1-1), where k and n are constants, typically with 0 < n < 1(Frankel, 1998; Szklarska-Smialowska, 2005; Hunkeler and Böhni, 1981; Alkire andWong, 1988; Mughabghab and Sullivan, 1989)

1-3

It has been reported that at fixed applied potentials, the depth of pits grown under either ohmicor diffusion controlled corrosion processes relates to time as d = kt 0.5 (Frankel, 1998; Beck andAlkire,1979). Beck and Alkire (1979) developed a diffusion model to calculate pit growth. Thismodel assumes a hemispherical pit geometry and that the rate of pit growth is controlled by themass transport of corrosion products out of the pit. For sufficiently long times, this modelpredicts that the pit depth and the current vary with t0.5 and t!0.5, respectively. The pit growthrate predicted in this deterministic model was found to be in good agreement with thatdeveloped from empirical experiments (Alkire and Wong, 1988). Hunkeler and Böhni (1981)used the time for pits to perforate aluminum foils of varying thickness as a means to determinethe growth rate of the fastest growing pits. The pit depth evolves with time as t0.5, and thegrowth rate was found to be under ohmic control as the electrolyte resistance had a direct effecton perforation time. In limited potentiostatic studies by Kehler, et al. (2002, 2001), potential,bulk solution chemistry, and alloy composition affected crevice propagation rates, suggestingneither pure ohmic nor pure mass transport control of crevice propagation.

However, for many alloys in different environment systems, stifling and arrest phenomena areoften observed in crevices grown naturally without potentiostatic control, which results in avariety of exponential growth rates characterized by n < 0.5 (Shoesmith, et al., 1995;Marsh, et al., 1985; Priyantha, et al., 2004). Here stifling refers to a decrease in penetrationrate, and arrest refers to a stop in penetration (e.g., repassivation). Stifling and repassivation innatural corrosion systems can arise from several causes including (i) the inability to maintain acritical chemistry for stable crevice propagation due to anodic dissolution rate limitations broughtabout by a variety of causes such as formation of a salt film or change in interface alloycomposition due to incongruent dissolution (Steinsmo, et al., 1993; Okada, 1984a,b),(ii) disruption in the geometric and physical conditions (e.g., corrosion product plugging)required to sustain the occluded site (He, et al., 2005), (iii) evolution in alloy surface compositionin the active crevice over time, and (iv) external cathode limitations and cathodicreaction starvation.

The localized corrosion rates ranging from 0.0127 to 1.27 mm/yr [0.5 to 50 mpy] with a medianvalue of 0.127 mm/yr [5 mpy] (Bechtel SAIC Company, LLC, 2004b) assumed by DOE, whichwere obtained from literature data using acidic chloride and acidic oxidizing chloride solutions,appear to correspond to measured corrosion penetration rates obtained in certain serviceenvironments (Cragnolino, et al., 1999). Smailos (1993) reported a maximum pit depth of0.90 mm [0.035 in] in Alloy 625 after 18 months in 33-percent MgCl2 at 150 °C [272 °F],corresponding to a localized corrosion penetration rate of 0.6 mm/yr [24 mpy]. Carter andCramer (1974) reported that pit penetration rates for Alloy 625 were 0.22 mm/yr [8.7 mpy] after45 days in 105 °C [221 °F] brine containing 155,000-ppm chloride with 30-ppm sulfur. Oldfield(1995) observed crevice corrosion of Alloys 625 and C-276 in both natural and chlorinatedseawater at ambient temperature. The average penetration rate for Alloy 625 following a 2-yearexposure was 0.049 mm/yr [1.9 mpy]. These observations suggest that the propagation ratesused by DOE sufficiently bound the range of propagation rates for similar nickel-chromium-molybdenum alloys.

1.2 Objective

Parameters used to model the propagation rate for crevice corrosion in the NRC/CNWRA TPAcode are selected to provide a constant penetration rate when crevice corrosion is active. Thebasis for these parameter values is limited to a few crevice corrosion penetration depth

1-4

measurements for Alloy 625 and Type 316L stainless steel. The objective of this investigationis to obtain parameters for crevice corrosion propagation using Alloy 22 specimens that can beincorporated into the NRC/CNWRA TPA code. This report summarizes the localized corrosionpropagation models used to assess the potential waste package outer container and presentsthe results of recent experimental work on localized corrosion propagation in Alloy 22 conductedat CNWRA.

1.3 Scope and Organization of the Report

The degradation processes potentially important in the degradation of the engineered barriershave been reviewed in the Integrated Issue Resolution Status Report (NRC, 2005). It is knownthat the failure mode of the waste package is important for determining the amount of water thatcan enter the waste package, affecting the rate of release of radionuclides.

This report focuses on crevice corrosion propagation and is organized into five chapters,including an introduction as Chapter 1. Chapter 2 summarizes the experimental methods usedto study the crevice corrosion propagation in Alloy 22. Chapter 3 provides results of the testsperformed to determine the evolution of penetration by crevice corrosion and the effects ofenvironmental conditions on the penetration rate of Alloy 22. Chapter 4 evaluates the crevicecorrosion penetration of the waste package outer container based on the tests on Alloy 22.Chapter 5 summarizes conclusions.

2-1

Table 2-1. Composition of Alloy 22 (in Weight Percent)

Material Ni* Cr* Mo* W* Fe* Co* Si* Mn* V* P* S* C*

Alloy 22Heat2277-3-3266

Bal 21.40 13.30 2.81 3.75 1.19 0.03 0.23 0.14 0.008 0.004 0.005

Alloy 22Heat2277-1-3133

Bal 21.44 13.27 2.85 4.76 0.65 0.22 0.15 0.030 0.012 0.002 0.005

*Ni—nickel, Cr—chromium, Mo—molybdenum, W—tungsten, Fe—iron, Co—cobalt, Si—silicon, Mn—manganese,V—vanadium, P—phosphorus, S—sulfur, C—carbon

2 CREVICE CORROSION PROPAGATION TEST METHODS

The chemical composition of the Alloy 22 heats used in this study is shown in Table 2-1. Figure 2-1 schematically shows the electrochemical test cell used to study the crevice corrosionpropagation in Alloy 22. The test cell was made of a glass cylinder compressed between polytetrafluoroethylene top and bottom lids. The total volume of the test cell is approximately350 mL [0.093 gal]. An Alloy 22 cylindrical specimen (Heat 2277-3-3266 in Table 2-1)measuring 6.2 mm [0.24 in] in diameter and 48.6 mm [1.89 in] in length (Figure 2-2), wasmounted perpendicularly through the center of the polytetrafluoroethylene bottom lid. One endof the Alloy 22 test specimen with a surface area of approximately 5 cm2 [0.78 in2] was exposedto the test solution. A polytetrafluoroethylene bolt with a cone-shaped plateaued tip wasscrewed through the center of the polytetrafluoroethylene top lid to press against the Alloy 22flat surface, forming an artificial crevice with a crevice area of 0.103 to 0.204 cm2 [0.0160 to0.0316 in2]. A torque of 0.35 N@m [50 in-oz] was applied to create a consistent crevice gap. Test cells were fitted with a water-cooled condenser to minimize solution loss at elevatedtemperatures. A saturated calomel electrode was used as a reference electrode in allexperiments. The reference electrode was connected to the solution through a water-cooledLuggin probe with a porous silica tip to maintain the reference electrode at room temperature. (Luggin probe is a device used in measuring the potential of an electrode with a significantcurrent density imposed on its surface. The probe minimizes the voltage drop that wouldotherwise be included in the measurement and without significantly disturbing the currentdistribution on the specimen.) The Alloy 22 crevice specimen was galvanically coupled to alarge Alloy 22 plate (Heat 2277-1-3133 in Table 2-1) with a surface area of 60.0 cm2 [9.30 in2]through a potentiostat functioning as a zero resistance ammeter. The Alloy 22 cylindricalspecimen and plate were polished to a 600-grit finish, rinsed in deionized water, ultrasonicallycleaned in acetone, and dried.

In this assembly, crevice corrosion can occur as a result of the occluded environment createdby the crevice device and the Alloy 22 cylindrical specimen surface. The large Alloy 22 plate,acting as a cathode, was connected to the specimen through a potentiostat functioning as azero resistance ammeter. For these tests, the area ratio of the Alloy 22 plate to the cylindricalspecimen was approximately 12. Because only a fraction of the cylindrical specimen surfacewas covered by the polytetrafluoroethylene crevice former, the actual Alloy 22 plate to crevicearea ratio was approximately in the range of 300 to 600. The use of a galvanic couple in these

2-2

PTFE

Alloy 22 cylinder

Alloy 22 SCE

e-

VA

Alloy 22 PlateTest cell

Cylindrical Specimen forSingle Crevice Assembly

Figure 2-1. Schematics of the Electrochemical Test Cell with Single Crevice Assembly toStudy the Crevice Corrosion Propagation in Alloy 22

(Note: PTFE—Polytetrafluoroethylene; SCE—Saturated Calomel Electrode)

Figure 2-2. Illustrations of the Cylindrical Specimen for Single Crevice Assembly andCrevice Corrosion Specimen for Multiple Crevice Assembly

Crevice Corrosion Specimen for Multiple CreviceAssembly (Note: PTFE—Polytetrafluoroethylene)

2-3

tests simulates the situation typical for crevice corrosion of a passive alloy under naturalcorroding conditions. Redox conditions that lead to the occurrence of crevice corrosion wereestablished by adding cupric chloride (CuCl2) to the test solution. The potential of the galvaniccouple and the galvanic current density were monitored throughout the tests. The potential andcurrent transients were analyzed to determine the initiation and propagation times for crevicecorrosion. The crevice corrosion propagation rate as a function of time was determined byperforming tests for various durations after the initiation of crevice corrosion and subsequentlyexamining the corroded specimens. The depths of crevice corrosion penetration weredetermined using the microscopic method described in ASTM G46 (ASTM International, 2004a).

Crevice corrosion propagation tests were also conducted in a glass test cell using a multiplecrevice assembly consisting of a number of grooves and plateaus in accordance withASTM G78 (ASTM International, 2004b). In this assembly, the serrated polytetrafluoroethylenecrevice washer contacts with the Alloy 22 crevice specimen (Figure 2-2), forming 24 crevicesites, each with an area of 0.06 cm2 [0.009 in2]. The entire surface area of the crevice corrosionspecimen is 20 cm2 [3.1 in2]. In the test, the multiple crevice assembly was coupled to a largeAlloy 22 plate with a plate to crevice area ratio of approximately 40:1.

Previous tests have shown that crevice corrosion of Alloy 22 is only possible in concentratedchloride solutions with low inhibitor concentrations and at temperatures above 60 °C [140 °F].Because the objective of this work was to determine crevice corrosion propagation rates, mostof the tests were conducted in concentrated 5 M NaCl solution at 95 °C [203 °F] with theaddition of 2 × 10!4 M CuCl2. Different solutions used will be described herein. During all thetests, CO2-free air was flowing through the free space above the test solution in the test cell tomaintain an air saturated solution at the test temperature.

3-1

Figure 3-1. Measured Current Density and Potential Using the Single Crevice Assemblyfor an Alloy 22 Cylindrical Specimen Galvanically Coupled to a Large Alloy 22 Plate in 5 M

NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F] (Test No. 10 inTable 3-1). The Current Density Is the Measured Current Divided by the Entire SpecimenArea Exposed in the Solution. Initial Potential of the Couple after the Addition of CuCl2 Is

429 mVSCE. The Potential at Stable Propagation Is !201 mVSCE.

0 4 8 12Time, Days

1x10-11

1x10-10

1x10-9

1x10-8

1x10-7

1x10-6

1x10-5

1x10-4

1x10-3

Gal

vani

c co

uplin

g cu

rren

t den

sity

, A/c

m2

0

0.4

0.8

1.2

1.6

Gal

vani

c co

uplin

g po

tent

ial,

V SC

E

Current density

Potential

Duration of experiment

Period of crevice corrosion propagation Period when repassivated

Current density peak

630 mV

3 CREVICE CORROSION PENETRATION RATES AS A FUNCTION OF TIMEAND ENVIRONMENTAL CONDITIONS

Many factors can influence crevice corrosion penetration rates, including temperature, solutionchemistry, solution pH, the characteristics of the crevice, alloy composition, metallurgicalfeatures, and metal surface condition. In this chapter, the crevice corrosion penetration rates ofAlloy 22 were studied as a function of time based on tests using single and multiple creviceassemblies. Tests were also performed to examine the effects of solution composition andcathode-to-anode area ratio on crevice corrosion penetration rates.

3.1 Tests with Single Crevice Assembly

Figure 3-1 shows the galvanic coupling current density and potential plots from a crevicecorrosion test under open circuit conditions in 5 M NaCl solution with the addition of 2 × 10!4 MCuCl2 at 95 °C [203 °F]. The assembly had a single crevice with a crevice area of 0.114 cm2

[0.0177 in2]. The current density shown in the figure is the measured current divided by theentire specimen area exposed in the solution, which is approximately 44 times greater than thatof the crevice area.

After the addition of CuCl2 to the concentrated sodium chloride solution, the Ecorr of the galvaniccouple increased rapidly to values in the range of 420 mVSCE, but the current density was lessthan 10!7 A/cm2 [9.3 × 10!5 A/ft2]. After an incubation period of about 2.5 hours, the galvanic

3-2

coupling current density increased sharply as the potential fell correspondingly, indicating theinitiation of crevice corrosion. Within 3 hours, the current density had increased by 3 orders ofmagnitude, and the potential had dropped by 300 mV. The initial rapid current density increasewas the result of crevice corrosion initiation of an increasingly large surface area of activecorrosion sites, which were galvanically coupled to the reduction reactions on the large Alloy 22plate. After the current density reached a peak value, the potential of the galvanic coupledecreased continuously, leading to a current density decrease. Although the initiation of crevicecorrosion resulted in a large potential drop of the couple, the potential remained above the Ercrev,and the crevice corrosion continued to propagate. Following a significant reduction in the Ecorrby approximately 630 mV compared to the initial value reached immediately after the addition ofCuCl2, crevice corrosion repassivated after 9 days, as indicated by a significant current densitydecrease and a sharp increase in potential. The Ecorr of the galvanic couple increased to valuesnear 360 mV. Although the Ecorr remained high, no crevice corrosion was reinitiated after anadditional period of 4.5 days. The experiment was terminated after a total testing time of13.8 days.

During the crevice corrosion propagation period indicated in Figure 3-1, the current waspredominantly due to metal dissolution in the crevice area. The current density during thatperiod was corrected, taking only the crevice area into account and plotted in Figure 3-2. Thecurrent density decayed in stages, with a fast decay in the beginning and a slower decay after0.5 day. According to Faraday’s Law, if the entire crevice area is uniformly corroded, thepenetration rate will follow the current density profile. The current transient was fitted to anexponential equation, and it was found that the current density decayed with time as t!0.80 (t is indays), which is different from the diffusion controlling process where current decays with time ast!0.5. The time exponent suggests a strong stifling tendency after the initiation of crevicecorrosion presumably associated with the chemical composition of Alloy 22.

The posttest examination of the test specimen revealed severe crevice corrosion around theedge of the crevice area and central region is slightly etched, which is consistent with thepotential drop mechanism proposed by Pickering (1986) (Cho, et al., 1998). Along the edge ofthe crevice, the damage is severe because the potential drop is small. Deeper into creviceregion, damage became less severe due to larger potential drop. The maximum penetrationdepth measured is 200 :m [7.87 mils]. During the 9-day propagation period, the averagepenetration rate is 8.1 × 10!3 m/yr [319 mpy], but after 9 days the penetration rate dropped tovery low values based on the measured current.

In contrast to the short duration of the propagation period observed in the above test, crevicecorrosion persisted for a long time in some tests. Figure 3-3 shows the galvanic couplingcurrent density and potential from one long term test with a duration of 70.9 days. In this test,the crevice area is 11.3 mm2 [0.0175 in2]. As observed in Figure 3-1, crevice corrosion wasinitiated quickly. After the initiation, the current density decayed significantly as a result ofpotential drop, but the current density persisted at a very low value of 6 × 10!7 A/cm2

[5.6 × 10!4 A/ft2] while the potential remained at !222 mVSCE. The test was terminated after70.9 days, and no repassivation was observed. The subsequent examination of the testspecimen shows that there was a single corroded site which persisted and grew to a greaterdepth than the surrounding area. It is possible that this is the site which remained active andcontinued to propagate for a long period, contributing to the observed long-term current densityof 6 × 10!7 A/cm2 [5.6 × 10!4 A/ft2] in Figure 3-3. The maximum crevice corrosion penetration

3-3

0 4 8Time, Days

1x10-4

1x10-3

1x10-2

Cor

rect

ed c

revi

ce a

rea

curr

ent d

ensi

ty, A

/cm

2

Current densityExponential fit

I = 3.6X10-4 t-0.80

Figure 3-3. Measured Current Density and Potential Using the Single Crevice Assemblyfor an Alloy 22 Cylindrical Specimen Galvanically Coupled to a Large Alloy 22 Plate in5-M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F] (Test No. 12 in

Table 3-1). The Current Density Is the Measured Current Divided by the Entire SpecimenArea Exposed in the Solution. Initial Potential of the Couple After the Addition of CuCl2 Is


Figure 3-2. Current Density Decay Behavior in the Period of Crevice CorrosionPropagation After the Current Density Peak Indicated in Figure 3-1 and the Fitted

Exponential Relationship Showing the Strong Repassivating Tendency of Alloy 22 in 5 MNaCl Solution at 95 °C [203 °F]. The Current Density Is the Measured Current Divided by

the Crevice Area.

3-4

Figure 3-4. Measured Current Density and Potential Using the Multiple Crevice Assemblyfor an Alloy 22 Crevice Specimen Galvanically Coupled to a Large Alloy 22 Plate in

5-M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F] (Test No. 17 inTable 3-2). The Current Density Is the Measured Current Divided by the Entire SpecimenArea Exposed in the Solution. Initial Potential of the Couple After the Addition of CuCl2 Is


depth for this specimen was approximately 310 :m [12.2 mils]. A multichannel potentiostat wasused to measure the current density in this test. The oscillation in current density before30 days was due to noise with that channel. It was minimized after switching toanother channel.

3.2 Tests with Multiple Crevice Assembly

Figure 3-4 shows results of a test using the multiple crevice assembly. The addition of CuCl2increased the potential of the galvanic couple from !349 to 210 mVSCE. Crevice corrosion wasinitiated within a few minutes and propagated continuously for more than 5 days. Following theinitiation of crevice corrosion, the potential reached the lowest value of !160 mVSCE. After5.5 days, the current density dropped to values of 10!6 A/cm2 [9.2 × 10!4 A/ft2], and the corrosionpotential increased sharply to values near 300 mVSCE, indicating repassivation of crevicecorrosion. After repassivation for about 1.5 days, crevice corrosion reinitiated, as indicated bythe increase of the current density accompanied by the sudden drop of the corrosion potential. Several additional crevice corrosion repassivation and reinitiation cycles were observedthroughout the test, and each cycle was marked by a noticeable potential and correspondingcurrent density change. Later on in the course of the test, the reinitiation events were veryshort. The repassivation and initiation events observed in this test are presumably related tothe repassivation of the existing corroding sites and the initiation of new sites. After 45 days,the experiment was terminated before complete repassivation occurred.

3-5

Figure 3-5. Measured Current Density and Potential Using the Multiple Crevice Assemblyfor an Alloy 22 Crevice Specimen Galvanically Coupled to a Large Alloy 22 Plate in 5 M

NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F] (Test No. 18 inTable 3-2). The Current Density Is the Measured Current Divided by the Entire SpecimenArea Exposed in the Solution. Initial Potential of the Couple After the Addition of CuCl2

Is 331 mVSCE. The Potential at Stable Propagation Is !203 mVSCE.

An observation of the test specimen after test completion revealed crevice corrosion on 7 of the24 crevice sites created by the serrated polytetrafluoroethylene crevice washer. Corrosion atthe seven corroded sites could be initiated one after another or initiated simultaneously onseveral sites. The average and standard deviation of the penetration depth of the crevice attackwas 197 ± 28 :m [78 ± 1.1 mils]. Because multiple crevice sites were initiated, the penetrationrate cannot be accurately determined.

The above test shows several repassivation and reinitiation events throughout the test. Giventhe observation that the reinitiation events were getting weaker with time, crevice corrosion islikely to repassivate completely after longer time. The galvanic coupling potential and currentdensity data from another test under similar conditions are shown in Figure 3-5. The currentdensity and potential trends observed in this test were similar to those observed in Figure 3-1. After fast initiation, the current decayed over time. After approximately 38 days, crevicecorrosion was stifled and arrested, as indicated by an increase in the galvanic coupling potentialand a decrease in the current density. No additional crevice corrosion events were recordedafter 40 days. The experiment was terminated after 78.0 days. A posttest examination of thespecimen showed that 22 of the 24 crevice sites were corroded, and the average and standarddeviation of the depth of the crevice attack was 124 ± 69 :m [4.9 ± 2.7 mils].

3-6

d tmax..= 0 0912 0 233 (3-1)

3.3 Crevice Corrosion Penetration Rates of Alloy 22 as a Functionof Time

Multiple crevice corrosion tests similar to those shown in Figures 3-1, 3-3, 3-4, and 3-5 wereconducted for times ranging from 0.5 to 78 days. Tables 3-1 and 3-2 summarize a number ofparameters extracted from these series of crevice corrosion experiments using single creviceand multiple crevice assemblies. For the tests using the single crevice assembly (Table 3-1),the included parameters are the maximum penetration depth; the difference in galvanic couplingpotential between the active and inactive crevice corrosion periods, )Ecorr; and the stifling orrepassivation status for each test. For the tests using the multiple crevice assembly, the totalnumber of sites corroded out of the 24 sites and the maximum penetration depth on each siteare also included in Table 3-2. Figures 3-1, 3-3, 3-4, and 3-5 are the corresponding currentdensity and potential values recorded from tests 10, 12, 17, and 18. Figure 3-6 shows theoptical micrographs of all the corroded Alloy 22 cylindrical specimens using the single creviceassembly. For short-term tests 1, 2, 4, 6, and 7 in Table 3-1 and 13, 14, and 15 in Table 3-2,tests were terminated to obtain penetration depths before completing the entire sequence ofcrevice corrosion initiation, propagation, and repassivation processes.

The common feature noticed from these tests is that initiation of crevice corrosion led tosignificant potential drop. In many of the tests, crevice corrosion stifled and arrested before thetests were terminated. The )Ecorr was estimated to average 619 mV with a standard deviation of126 mV from multiple tests using the single crevice assembly (Table 3-1) and 410 mV with astandard deviation of 163 mV from tests using the multiple crevice assembly (Table 3-2). Thelower value of )Ecorr with a higher standard deviation recorded from tests using the multiplecrevice assembly was presumably associated with multiple sites in this assembly. From all thetests using the single and multiple crevice assemblies, the average )Ecorr was determined to be549 mV with a standard deviation of 168 mV. The potential drop is consistent with previousreports. Isaacs (1987) reported a 500 mV decrease in the Ecorr of iron after the onset of pitting ina chloride solution. He, et al. (2005, 2004) reported a 400 mV potential drop in TitaniumGrade-2 after the initiation of crevice corrosion under open circuit conditions. Sridhar andDunn (1994) measured a potential difference from the outside to the interior of the crevice of300 to 500 mV for Type 316L stainless steel and Alloy 825 during active crevice corrosion. Significant decreases in the Ecorr caused by the active propagation of crevice corrosion may leadto its stifling and arrest as a result of a decrease in the electrochemical driving force required tomaintain the appropriate concentration of soluble corrosion products and aggressive anions inan active crevice (Sridhar, et al., 2001).

Figure 3-7 plots the measured maximum penetration depths, dmax, listed in Table 3-1 and thedeepest penetration depths from each test in Table 3-2 as a function of test durations. Fortest 12 in Table 3-1, there was a single site penetrated to a greater depth that led to a greaterdmax although the test duration in test 12 was shorter than in test 18. A fit of the experimentaldata to d = ktn, Eq. (1-1), yielded Eq. (3-1).

3-7

Table 3-1. Summary of Alloy 22 Crevice Corrosion Tests Using the Single CreviceAssembly in 5 M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F]

Test NumberTest Duration

(Days)

MaximumPenetration Depth

(:m) [mils])Ecorr*(mV)

Did Crevice CorrosionStifle† or Arrest‡ During

the Tests?1 0.5 90 [3.5] 617 Yes, stifled after initiation

2 1.7 105 [4.13] 647 Yes, stifled after initiation

3 3.1 143 [5.63] 400 Yes, stifled and arrestedat 2.5 days










*)Ecorr is the difference in the galvanic couple potential between the active and inactive crevice corrosion periods.†Stifling refers to a decrease in penetration rates.‡Arrest refers to repassivation of crevice corrosion.

where

t — time in daysdmax — maximum penetration depth in mm

The best fit line to the experimental data and the lines showing a 95-percent confidence intervalare also plotted in Figure 3-7. The calculated value of the exponent is much less than 1, whichis the value assumed in the NRC/CNWRA TPA code, and it is also less than 0.5, which wouldbe the value expected for a diffusion or ohmic controlled process. This can be attributed to thestifling and arrest of crevice corrosion observed during the course of several tests and it is alsoconsistent with what was observed from the optical micrographs of the corroded Alloy 22cylindrical specimens shown in Figure 3-6. Crevice corrosion was preferentially started aroundthe edge (i.e., at sites most easily coupled to the external electrode and least likely to be

3-8

Table 3-2. Summary of Alloy 22 Crevice Corrosion Tests Using the Multiple Crevice Assembly in5 M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F]

TestNumber

Test Duration(Days)

Number of SitesCorroded Out of

24 Sites

MaximumPenetration

Depth onEach Site


Did CreviceCorrosion Stifle† orArrest‡ During the

Tests?

13 1.0 8

100 [3.94]93 [3.66]82 [3.23]68 [2.68]62 [2.44]58 [2.28]56 [2.20]30 [1.18]

346 Yes, stifled afterinitiation

14 3.0 8

80 [3.15]80 [3.15]70 [2.76]60 [2.36]60 [2.36]40 [1.58]35 [1.38]30 [1.18]


15 5.0 7

100 [3.94]90 [3.54]85 [3.35]80 [3.15]65 [2.56]62 [2.44]60 [2.36]


16 13.8 17

105 [4.13]95 [3.74]75 [2.95]70 [2.76]70 [2.76]70 [2.76]70 [2.76]70 [2.76]70 [2.76]65 [2.56]60 [2.36]55 [2.16]55 [2.16]55 [2.16]55 [2.16]50 [1.97]50 [1.97]

673 Yes, stifled andarrested at 5.5 days

3-9

( )Penetration Rate mm / yr 0.767= −7 8. t (3-2)

Table 3-2. Summary of Alloy 22 Crevice Corrosion Tests Using the Multiple Crevice Assembly in5 M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F] (continued)

TestNumber

Test Duration(Days)

Number of SitesCorroded Out of

24 Sites

MaximumPenetration

Depth onEach Site


Did CreviceCorrosion Stifle† orArrest‡ During the

Tests?

17 45.0 7

230 [9.06]230 [9.06]210 [8.27]200 [7.87]180 [7.09]170 [6.69]160 [6.30]

370Yes, arrested andreinitiated in severalcycles

18 78.0 22

248 [9.76]180 [7.09]180 [7.09]175 [6.89]168 [6.61]150 [5.91]150 [5.91]126 [4.96]62 [2.44]52 [2.05]50 [1.97]42 [1.65]34 [1.34]

533 Yes, stifled andarrested at 38 days

*)Ecorr is the difference in the galvanic couple potential between the active and inactive crevice corrosion periods.†Stifling refers to a decrease in penetration rates.‡Arrest refers to repassivation of crevice corrosion.

controlled by potential drop effects) and spread immediately following initiation. Fewer changeswere observed with increasing test time.

The derivation of Eq. (3-1) generated the time-dependent penetration rate shown in Eq. (3-2).

where

t — time in days

Figure 3-8 plots the calculated penetration rate according to Eq. (3-2) against testing time. Thepenetration rate decreased from 13 mm/yr [512 mpy] to 0.27 mm/yr [10.6 mpy] when the timeincreased from 0.5 to 78 days. For longer times, the penetration rate could decreaseeven further.

The decrease in penetration rate is not due to the consumption of oxidant in the solution, asdemonstrated by the recovery of the Ecorr after crevice corrosion repassivation shown in

3-10

6.2 mm [0.24 in]

Creviced area

Test 1. 0.5 Day

Creviced area

Test 2. 1.7 Days

Creviced area

Test 3. 3.1 Days

Creviced area

Test 4. 3.7 Days

Creviced area

Test 6. 6.9 Days

Creviced area

Test 7. 8.8 Days

Creviced area

Test 8. 11.9 Days

Creviced area

Test 9. 13.1 Days

Creviced area

Test 10. 13.8 Days

Creviced area

Test 11. 28.0 Days

Creviced area

Test 12. 70.9 Days

Creviced area

Test 5. 4.9 Days

Figure 3-6. Photographs Showing Crevice Corrosion Features of Alloy 22 Single-CreviceSpecimens After Corrosion in 5 M NaCl Solution with the Addition of 2 × 10!4 M CuCl2 at

95 °C [203 °F]. The Diameters of All the Cylindrical Specimens Are 6.2 mm [0.24 in].

3-11

Figure 3-8. Calculated Crevice Corrosion Penetration Rates of Alloy 22 According toEq. (3-2) During the Testing Period in the Time Range of 0.5 to 78 Days at 95 °C [203 °F]

in 5-M NaCl Solution with the Addition of 2 × 10!4 M CuCl2

Alloy 22, 95 oC5 M NaCl + 2 x 10-4 M CuCl2

20 40 60 80 100Time, Days

0.1

1

10

100

Pen

etra

tion

rate

, mm

/yr

0 20 40 60 80Time, Days

100

200

300

400

500

Max

imum

pen

etra

tion

dept

h, µ

m

Experimental dataExponential fit

R-squared of the fit = 0.65

Figure 3-7. Measured Crevice Corrosion Penetration Depths as a Function of Time forTests on Alloy 22 at 95 °C [203 °F] in 5 M NaCl Solution with the Addition of

2 × 10!4 M CuCl2. The Continuous Line Is the Best Fit [Function of the Form k t n, Eq. (1-1)]to the Experimental Data (Plotted as Circles). The Dotted Lines Are Upper and Lower Fit

Bounds Enclosing a 95 Percent Confidence Interval.

3-12

Figures 3-1, 3-4, and 3-5. Arrest or repassivation of crevice corrosion likely occurs because thepotential drop into the crevice increases with penetration depth. The potential drops for deeppenetrations are large enough to reduce the Ecorr within the crevice close to or below the Ercrevfor crevice corrosion even though the specimen is galvanically coupled to the large areacathode plate. This finding is consistent with the observed significant decreases in the Ecorrcaused by the active propagation of crevice corrosion. A decrease in penetration rate was alsoobserved in localized corrosion of Type 316L stainless steel (Dunn, et al., 1996) and on Alloy625 (Cramer, et al., 1984). Considering the stifling and arrest observed in the tests, the crevicecorrosion penetration rate would eventually approach a very low value. The strong tendency ofrepassivation of Alloy 22 crevice corrosion was also reported by Priyantha, et al. (2004).

3.4 Effects of Solution Composition

The localized corrosion processes of Alloy 22 strongly depend on the pH and the chemicalspecies present in the environment. Studies conducted by Dunn, et al. (2005) showed that Ecorrof Alloy 22 is mainly a function of temperature, pH, and oxygen reduction kinetics, whereas Ercrevdepends on temperature and the concentrations of chloride, nitrate, sulfate, carbonate, andbicarbonate anions present in groundwater. Chloride is known to increase the susceptibility ofAlloy 22 to crevice corrosion, whereas nitrate, sulfate, carbonate, and bicarbonate anions arebeneficial because they inhibit crevice corrosion. The tests shown in Tables 3-1 and 3-2 wereconducted in 5 M NaCl solution at 95 °C [203 °C]. Additional tests in 3 M NaCl and 1 M NaClsolutions were also performed at 95 °C [203 °F] to examine the effect of solution composition onthe crevice corrosion propagation of Alloy 22. To be conservative, tests were conducted insolutions that contained only chloride. One can speculate that with the addition of inhibitingoxyanions, crevice corrosion could propagate even slower and stifle faster.

Figure 3-9 shows the galvanic coupling current density and potential obtained in 3 M NaCl and1-M NaCl solutions as a function of time. The values obtained in 5 M NaCl solution (Figure 3-1)are included for comparison. Similar to the results observed in 5 M NaCl solution, the test in3 M NaCl solution shows a fast initiation of crevice corrosion after the addition of CuCl2. Thecurrent density obtained was even higher than what observed in 5 M NaCl solution, indicating afaster propagation rate. However, the potential remained above 0 mVSCE, which indicates that aless active state developed in the crevice. After 4.1 days, crevice corrosion was repassivated,as indicated by the sudden increase of the galvanic coupling potential and the current densitydecrease. A posttest examination of the test specimen in 3 M NaCl solution revealed severecorrosion in the crevice area, and the maximum penetration depth was 298 :m [11.7 mils]. In1-M NaCl solution, the addition of CuCl2 raised the Ecorr to 450 mV, but crevice corrosion wasnot initiated. During the 12-day test period, the potential remained high and the current densityremained below 10!6 A/cm2 [9.3 ×10!4 A/ft2], indicating that the alloy remained passive.

3.5 Effects of Cathode to Anode Area Ratio

In corrosion processes, metal dissolution (anodic oxidation reaction) is coupled to oxygenreduction or proton reduction reaction at cathodic sites. The availability of a cathodic area andthe surface condition in that area could affect the crevice corrosion propagation rate. Asdescribed above, the tests listed in Table 3-1 were conducted in 5 M NaCl solution at 95 °C[203 °F] using an Alloy 22 cathode with a cathode-to-anode area ratio of approximately 12:1. The effects of the cathode-to-anode area ratio on the crevice corrosion propagation rate of

3-13

0 4 8 12Time, Days

1x10-11

1x10-10

1x10-9

1x10-8

1x10-7

1x10-6

1x10-5

1x10-4

1x10-3

Gal

vani

c co

uplin

g cu

rren

t den

sity

, A/c

m2

0

0.5

1

1.5

2

Gal

vani

c co

uplin

g po

tent

ial,

V SC

E

5 M NaCl3 M NaCl1 M NaCl

Current density

Potential

3 M NaCl

5 M NaCl

1 M NaCl

3 M NaCl1 M NaCl

5 M NaCl

Figure 3-9. Recorded Galvanic Couple Current Density and Potential for Tests in 5-, 3-,and 1-M NaCl Solutions with the Addition of 2 × 10!4 M CuCl2 at 95 °C [203 °F] While

Alloy 22 Was Coupled to a Large Alloy 22 Plate. The Current Density Is the MeasuredCurrent Divided by the Entire Specimen Area Exposed in the Solution.

Alloy 22 were studied by varying the cathode-to-anode area ratio. Figure 3-10 shows the testsconducted with cathode-to-anode area ratios of 4:1 and 2:1 in 5 M NaCl solution at 95 °C[203 °F]. Compared to the values of current density observed in Figure 3-1, no significantdifferences can be noted between tests with cathode-to-anode area ratio of 12:1 and 4:1, whichsuggests that the propagation rate is not affected by reducing the area ratio to 4:1. After thearea ratio was reduced to 2:1, it took about 2 days for crevice corrosion to initiate, but once wasinitiated, the current density remained higher than what was observed in the test with a cathode-to-anode area ratio of 4:1. No repassivation was observed in 22.6 days. Figure 3-11 shows theoptical micrographs of the corroded Alloy 22 specimens with cathode-to-anode area ratios of4:1 and 2:1 in 5 M NaCl solution at 95 °C [203 °F]. The crevice corrosion features are similar inboth tests, with deeper penetration around the edge of the crevice area. The maximumpenetration depths, 224 :m [0.00882 in] and 220 :m [0.00866 in], were also similar in these twotests.

In the single crevice assembly, the crevice area is approximately 0.025 of the area of theAlloy 22 cylindrical specimen exposed to the solution. The noncrevice area of the cylindricalspecimen may also act as a site for the cathodic reaction. This could diminish thecathode-to-anode area ratio effect on crevice corrosion propagation. Tests with a smallerspecimen area exposed to the solution may be necessary to evaluate the effect of thecathode-to-anode area ratio.

3-14

0 5 10 15 20Time, Days

1x10-16

1x10-15

1x10-14

1x10-13

1x10-12

1x10-11

1x10-10

1x10-9

1x10-8

1x10-7

1x10-6

1x10-5

1x10-4

Gal

vani

c co

uplin

g cu

rren

t den

sity

, A/c

m2

0

0.5

1

1.5

2

Gal

vani

c co

uplin

g po

tent

ial,

V SC

E

AreaCathode:AreaAnode = 4 : 1AreaCathode:AreaAnode= 2 : 1

Current density

Potential

Area ratio 4:1

Area ratio 2:1

Area ratio 2:1

Area ratio 4:1

Creviced area

dmax = 224 :m [8.82 mils]AreaCathode: AreaAnode = 4:1

Figure 3-11. Photographs of Alloy 22 Cylindrical Specimens After Crevice Corrosion withCathode-to-Anode Area Ratios of 4:1 and 2:1 in 5 M NaCl Solution with the Addition of

2 × 10!4 M CuCl2 at 95 °C [203 °F]. dmax Is the Maximum Penetration Depth.

Creviced area

1 mm

dmax = 220 :m [8.66 mils]AreaCathode: AreaAnode = 2:1

Figure 3-10. Measured Current Density and Potential from Single Crevice Assembly foran Alloy 22 Cylindrical Specimen Galvanically Coupled to an Alloy 22 Cathode with

Cathode-to-Anode Area Ratios of 4:1 and 2:1 in 5 M NaCl Solution with the Addition of2 × 10!4 M CuCl2. The Current Density Is the Measured Current Divided by the Entire

Specimen Area Exposed in the Solution.

1Dunn, D.S., O. Pensado, Y.-M. Pan, L.T. Yang, and X. He. “Modeling Corrosion Processes for Alloy 22 WastePackages.” The 29th International Symposium on the Scientific Basis for Waste Management, Ghent, Belgium,September 12–16, 2005. Submitted for publication (2005).

4-1

4 EVALUATION OF THE LOCALIZED CORROSION OF THE WASTEPACKAGE OUTER CONTAINER

The potential waste package design for the disposal of high-level waste consists of an outercontainer made of Alloy 22 surrounding an inner container made of Type 316 nuclear gradestainless steel. Initiation and propagation of crevice corrosion is considered to be possible inthe NRC/CNWRA TPA code when the Ecorr is greater than the Ercrev. The calculated Ecorr ofpassive Alloy 22 is dependent on temperature, pH, and oxygen reduction kinetics, but does notconsider the decreases in the corrosion potential resulting from the initiation of crevice corrosion(Dunn, et al., 2005). As shown in the tests described in Chapter 3, significant decrease in theEcorr (i.e., )Ecorr, difference in the galvanic couple potential between the active and inactivecrevice corrosion periods) caused by the active propagation of crevice corrosion led to its stiflingand arrest. If it is assumed that Ecorr ! )Ecorr must exceed Ercrev for steady propagation of thecrevice corrosion front, a lower probability of crevice corrosion propagation susceptibility wouldbe expected for waste packages contacted by seepage water. By including the effects of thereduction in corrosion potential upon the initiation of crevice corrosion in the model1, it wasshown that 0.3 percent of the waste packages contacted by seepage water at temperaturesaround 110 °C [230 °F] would be affected by localized corrosion on the mill-annealed body and3.7 percent on the welded area. Previously it was reported that 3 percent of the wastepackages contacted by seepage water could exhibit localized corrosion on the mill-annealedsurface and 26 percent could exhibit localized corrosion on welded areas (Dunn, et al., 2005).

Based on crevice corrosion penetration depth measurements from multiple tests extended from0.5 to 78 days, the maximum penetration depths are related to time, t, by the equationdmax = 0.0912t0.233, Eq. (3-1). The calculated maximum penetration depths by extrapolating thisequation and its 95 percent confidence interval for a period of 10,000 years are in the range of1.8 to 5.3 mm [0.07 to 0.21 in] or approximately 9 to 26 percent of the possible Alloy 22container thickness, as shown in the shaded area in Figure 4-1. Also shown in Figure 4-1 ascurve 2 is a plot of the calculated depth of penetration for the constant propagation rate of0.25 mm/yr [9.8 mpy], assumed in the NRC/CNWRA TPA code (Mohanty, et al., 2002). Asclearly shown in Figure 4-1, for times beyond 10 years, the use of this constant rate significantlyoverestimates penetration depth by comparison to the values expected from the extrapolation ofEq. (3-1). This calculation suggests that using a constant crevice corrosion propagation rate inthe performance assessment of the waste package crevice corrosion process conservativelybounds the rates derived from experiments.

It should be noted that the extrapolated penetration shown in Figure 4-1 and the penetrationrates given by Eq. (3-2) is based on a limited amount of crevice corrosion data obtained over aperiod of less than 3 months at 95 °C [203 °F] and in a single test solution, 5 M NaCl. Theextrapolation shown in Figure 4-1 does not include penetration due to general corrosion of thewaste package and does not consider the effects of the expected changes to the environmentalconditions within the emplacement drifts. Nevertheless, extrapolating this data to long periodsis likely to be conservative, given multiple observations of stifling and arrest in short-term tests. In addition, even though tests were conducted under a single environmental condition, a

4-2

Figure 4-1. Maximum Localized Corrosion Penetration Depths From Figure 3-7 as aFunction of Exposure Time. Curve 1 is Fitted to the Points Recorded in 5 M NaCl

Solution at 95 °C [203 /F] and Extrapolated to Longer Times according to Eq. (3-1). TheDashed Curves Are Extrapolated Upper and Lower Fits Enclosing a 95-Percent

Confidence Interval. The Shaded Area Shows the Range of Extrapolated PenetrationDepths in 10,000 Years. The Extrapolation Does not Include Penetration due to General

Corrosion of the Waste Package and Does Not Consider the Effects of the ExpectedChanges to the Environmental Conditions Within the Emplacement Drifts. Curve 2 is the

Penetration Depth Calculated From the Constant Penetration Rate of 0.25 mm/yr[9.8 mpy] Assumed in the Total-system Performance Assessment Code

(Mohanty, et al., 2002).

10-3 10-2 10-1 100 101 102 103 104 105 106 107 108 109

Time, Years

0

5

10

15

20

25

Max

imum

pen

etra

tion

dept

h, m

m

Possible outer container full wall thickness 2

1

Penetration depth in 10,000 years

concentrated chloride solution was used with the addition of a strong oxidant without thepresence of inhibiting oxyanions. Seepage water may contact the waste package if the dripshield over the waste package becomes damaged or otherwise fails to function, possibly bycorrosion or mechanical loading processes. In the event these processes occur and crevicecorrosion of the waste package is initiated, the damage to the waste package would be limitedby the strong tendency of Alloy 22 to repassivate. In addition, thousands of years afterrepository closure, when the waste package thermal output is not sufficient to cause significantevaporation of seepage waters, in-drift waters are likely to remain dilute and unlikely to inducecrevice corrosion.

5-1

5 SUMMARY AND CONCLUSIONS

Several of the crevice corrosion propagation tests on Alloy 22 were conducted for periods of 0.5to 78 days in 5 M NaCl solution at 95 °C [203 °F]. The addition of CuCl2 as an oxidant raisedthe corrosion potential above the repassivation potential, which led to rapid crevice corrosioninitiation. The subsequent active propagation of crevice corrosion resulted in significantdecreases in corrosion potential. The current density decayed quickly with time as a result ofthe concurrent potential drop. The average corrosion potential drop was determined to be 549mV with a standard deviation of 168 mV and is comparable to measurements reported in theliterature data. Alloy 22 shows a strong tendency to repassivate even in this aggressiveenvironment deprived of inhibitors. Stifling and arrest of localized corrosion was observed inseveral tests. The deepest penetration occurred near the edge of the crevice former. Thedeepest penetration was observed to be less than 350 :m [13.8 mils]. The penetration ratedecreased significantly with time due to crevice corrosion stifling and arrest.

Limited tests were conducted in 3 M and 1 M NaCl solutions. Although crevice corrosion in3-M NaCl solution showed a faster propagation rate than that in 5 M NaCl solution, a strongtendency to repassivation was evident. Crevice corrosion was not initiated in 1 M NaCl solution. Tests with varying cathode-to-anode area ratios showed that the corrosion process is not limitedby the available cathodic area when the area ratio of the cathode-to-anode was reduced to 2:1. Tests with an even smaller cathode-to-anode area ratio might be needed to observe the effect.

Although crevice corrosion of Alloy 22 is possible under some conditions and the propagationrates for crevice corrosion are typically orders of magnitude greater than the passive uniformcorrosion rates, the maximum penetration depth of localized attack may be limited to depthssignificantly less than the container thickness as a result of stifling and repassivation of crevicecorrosion. If significant decreases in the corrosion potential are considered, a lower probabilityof failure by crevice corrosion would be expected for waste packages contacted byseepage water.

6-1

6 REFERENCES

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ASTM International. “Metals Test Methods and Analytical Procedures.” ASTM G46–94 (2004):Standard Guide for Examination and Evaluation of Pitting Corrosion. Volume 3.02: Wear andErosion—Metal Corrosion. Published on CD-ROM. West Conshohocken, Pennsylvania: ASTM International. 2004a.

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Cragnolino, G.A., D.S. Dunn, C.S. Brossia, V. Jain, and K.S. Chan. “Assessment ofPerformance Issues Related to Alternate Engineered Barrier System Materials and DesignOptions.” CNWRA 99-003. San Antonio, Texas: CNWRA. 1999.

Cramer, S.D., J.P. Carter, and R.K. Conrad. “Corrosion and Scaling of Nickel Alloys in SaltonSea Geothermal Environments.” Proceedings of the International Symposium on SolvingCorrosion and Scaling Problems in Geothermal Systems. Houston, Texas: NACE International. pp. 215–235. 1984.

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Documents

Crevice Corrosion Penetration Rates of Alloy 22 In ...corrosion cracking, are considered important processes that may influence the lifetimes of the waste packages. Of these processes,