ML103620293

Assessment of Stress Corrosion Cracking Susceptibility of 316 Stainless Steel in Different Disposal Environments

ML103620293
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Issue date:	12/28/2010
From:	Chiang K, Shukla P NRC/NMSS/DHLWRS, Center for Nuclear Waste Regulatory Analyses (CNWRA)
To:	NRC/NMSS/DHLWRS
Ahn T
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ASSESSMENT OF STRESS CORROSION CRACKING SUSCEPTIBILITY OF 316 STAINLESS STEEL IN DIFFERENT DISPOSAL ENVIRONMENTS K. Chiang and P. Shukla Center for Nuclear Waste Regulatory Analyses (CNWRA), Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA

Contact:

K. Chiang, kchiang@swri.org, Telephone: +210-522-2308 Stainless steel may be considered as a waste package chemical, thermal, and electrochemical conditions; and container material in different disposal environments. The (iii) magnitude of applied and residual tensile stresses.

objective of this paper is to assess stress corrosion Welding and heat treatment can influence the cracking (SCC) susceptibility of 316 stainless steel in microstructure of the material affecting the susceptibility possible disposal environments. Factors including to SCC. The environmental setting is defined by the (i) material properties affected by fabrication processes chemistry of the aqueous solution in contact with the such as welding and heat treatment; (ii) environmental waste package, temperature, and electrochemical conditions including chemistry of aqueous solution variables such as the corrosion potential. Tensile stresses surrounding the waste package, temperature, and could arise from welding. Fig. 1 illustrates the three electrochemical conditions; and (iii) tensile stress in the factors that must be simultaneously present for SCC to welded areas as well as tensile stress generated by events occur.

such as seismic ground motion are of importance to SCC susceptibility of the alloy in disposal environments. The susceptibility of the alloy is assessed considering these factors in potential disposal environments. Literature Material Environment information was compiled to define chemical and thermal conditions that could arise in disposal environments.

• Welding

• Water Chemistry Numerical simulations were conducted to determine

• Heat

• Temperature electrochemical conditions of the alloy in disposal Treatment

• Electrochemical environments. Literature information on 316 stainless Potential steels SCC susceptibility in the chemical and thermal SCC conditions similar to those in potential disposal environments was compiled. The susceptibility of the alloy was assessed by comparing the literature information.

I. INTRODUCTION Stress

• Residual Stress Engineered barrier systems for a potential high-level

• Seismic Induced radioactive waste disposal system might include waste

• Rock Overburden containers made of corrosion-resistant material, such as stainless steel. Fabrication of nuclear waste containers generally will require multiple processes such as welding and solution annealing. Corrosion is expected to be a Fig. 1. Factors that lead to SCC of nuclear waste degradation process limiting waste container life. One of container materials.

the potential corrosion degradation modes for the waste container is stress corrosion cracking (SCC). SCC is a In this paper, the SCC susceptibility of 316 stainless phenomenon by which a normally ductile alloy loses its steel in three different possible disposal environments is toughness (elongation to rupture time) when it is subject assessed. The effects of material properties; residual to mechanical stresses under a range of environments.

stresses; and chemical, thermal, and electrochemical SCC susceptibility of the nuclear waste container conditions are considered in regard to the alloys materials is dependent on three factors: (i) material-susceptibility to SCC.

related factors such as metallurgy and microstructure of the material; (ii) environmental conditions including

Alloy C-22 is comparable to 316 stainless steel with I.A. Material-Related Issues respect to SCC processes. Thus, experiments on SCC for C-22 can yield insights on SCC processes for 316 316 stainless steel is a possible material to be used to stainless steel, about direct experimental data. As an construct waste package containers to isolate nuclear example, Fig. 3 shows the time-to-failure ratios (tf/tfair) for waste from the environments in a potential geological the slow strain rate tests of a nickel-based alloy in disposal system. Fabrication of containers generally solutions containing various anionic and cationic species.8 requires multiple processes, such as welding and solution The ionic species include chloride and bicarbonate ions.

annealing.1,2 These processes may alter the The ratio of time-to-failure in the test environment versus microstructure and mechanical properties of the base time-to-failure measured in air can be considered as an alloys, and introduce residual tensile stresses. Thermal index of the severity of SCC. Fig. 3 shows that the treatment can cause carbide precipitation; change the addition of a small concentration (0.2 molal) of chloride grain size and microstructure of the welded areas and to the 1.1 molal bicarbonate solution significantly heat-affected zone in a 316 stainless steel waste decreases the failure time.

container.1-3 In this paper, it is assumed that fabrication-related defects exist on the waste package.

I.B. Evaluation of Environmental Conditions SCC can occur in a range of the aqueous solution chemistries, temperatures, and polarization potentials of the alloy in the solution. The range of environmental conditions that can be conducive to SCC can be defined using accelerated methods such as slow strain rate tests or constant load experiments. The environmental conditions include the aqueous solution chemistry, pH, electrochemical potential, and temperature.

Stress corrosion susceptibility of 316 stainless steel has been studied in chloride-containing aqueous solution in different cations (Mg2+, Li+, and Na+) at temperatures ranging from 90 to 150 °C (194 to 302 °F) using slow strain rate tests.4-6 Slow strain rate testing was conducted Fig. 3. Time-to-failure ratios (tf/tfair) for nickel alloy in accordance with the ASTM G-129 procedure.7 A specimens tested at 95 °C (203 °F) in 1.1 molal and 2.1 photograph of the experimental test cell for the slow molal HCO3 solutions containing various Cl-strain rate test is shown in Fig. 2. The same test cell can concentrations.8 The tests were performed at a constant also be used to conduct the constant load test. strain rate of 3.2 x 106/sec.

Ductile failure, intergranular SCC, or transgranular SCC can be established by posttest examination. An example of a nickel-based alloy sample subjected to the slow strain rate test in the 7.2 molal chloride solution and 1.1 molal HCO3 solution containing 4.2 molal chloride is shown in Fig. 4 (a) and (b). In a solution containing only 7.2 molal chloride, the specimen exhibited significant plastic deformation (87.6 percent elongation) and a time-to-failure ratio close to 1.0. The side surface of the specimen shows ductile failure with no sign of surface cracks.

Fig. 2. Slow strain rate test apparatus with specimen mounted in the test cell.

sodium chloride could cause the chloride ion concentration to range from 50-50,0000 parts per million (4.2 x 104 to 0.42 lb/gallon), and thee aqueous solution temperature could be as high as 90 °C C (194 °F).9 In this paper, it is assumed that the aqueous soolution surrounding the waste packages in a granite rock dissposal environment contains NaCl in a concentration of 50 g/L (0.42 lb/gallon) and the aqueous soluttion temperature is (a) (b) 90 °C (194 °F). In the three ppotential disposal Fig. 4. Side surface of a nickel alloy specimen strained s in environments, the aqueous solutions coontacting the waste (a) 7.2 molal Cl only and (b) 1.1 molal HC CO3 + 4.2 packages are expected initially to contain dissolved molal Cl.8 oxygen because the oxygen might be present during the repository construction and also someetime after closure.

On the other hand, in a 1.1 molal HCO3 solution For deep disposal system with undisturrbed groundwaters, containing 4.2 molal chloride solution, a large number n of dissolved oxygen in the solution mighht be consumed by secondary SCC were present on the side surfa face of the oxygen reduction reaction and eventtually the aqueous specimen (Fig. 4b). The elongation of the testt specimen solution might become reducing.

was reduced to 52.2 percent, with a time-to-faailure ratio of 0.51. The effects of the environment (water chemistry II.B. Electrochemical Conditions containing HCO3 and Cl) in causing SCC are illustrated.

i Thus, information about the time-to-failure ratio and the The electrochemical conditions fo for the carbon and presence of microcracks on the side surface of the t sample 316 stainless steel waste package material in three after the test can be used to determine whether an alloy is potential disposal environments were ccalculated using the susceptible to SCC in specific environmental conditions. OLIAnalyzer Version 3.1 software.110 The software results have been extensively validatedd.11 The chemical II. DISPOSAL ENVIRONMENTS compositions of the 316 stainless steel are provided in TABLE I. The chemical and thermal coonditions, and alloy In this paper, potential disposal of stainless-steel specifications were input in the softwaare. The calculated waste packages in salt rock, clay, and granite g is results included polarization curvees and corrosion considered. These three potential disposal env vironments potentials. The values of the callculated corrosion can lead to different chemical compositions an nd thermal potentials were read from the polarization curves. Both conditions of the aqueous solution in contact with w waste compositions of the aqueous solutioons and calculated packages. These conditions are discussed next. In corrosion potentials for the 316 stainlesss steel are listed in addition, the electrochemical conditions that caan be used TABLE II.

to determine SCC susceptibility of the alloy are detailed.

III. SCC SUSPECTIBILITY EVALU UATION II.A. Chemical and Thermal Conditions III.A. SCC Test Data Stainless steel waste packages placed in salt rock could be contacted by sodium chloride or magnesium m Literature information was searcheed to compile SCC chloride brines of concentrations of approxiimately of test data on 316 stainless steel annd to assess SCC 26 and 30 wt %, respectively. The pH H of the susceptibility of 316 stainless steel in three potential sodium-chloride-rich brines and magnesium-chlloride-rich disposal environments. It is assum med that material brines could range from 4-7.9 The temperaturre of those conditions, such as the heat affected zone, and enough brines could range 90-150 °C (194-302 °F) dep pending on residual tensile stresses are present. Thus, this paper the design of disposal systems. If waste pack kages were focuses only on the SCC susceptibilitty of 316 stainless placed inside a bentonite clay, an aqueouss solution steel as a function of chemicaal, thermal, and predominantly containing sodium, magnesiium, and electrochemical conditions. Tsai and Chen4 conducted chloride ions could contact the waste packagess. Possible slow strain rate tests to determine SC CC susceptibility of chemical composition of the aqueous solutio on in the duplex- and 316 stainless steel in 26 wwt % NaCl solution, bentonite clay can be found in Table 2-15 of o a report with a pH equal to 6 and at 90 °C ((194 °F). The test published by the European Commission.9 The solution was deaerated, the strain rate was selected to be maximum temperature of the aqueous solutiion might 4.1 x 106/sec, and samples were not polarized (i.e., the range from 50-100 °C (122-212 °F). For the waste tests were conducted at the corrosion pootentials). Tsai and packages placed in granite, aqueous solutions containing c Chen4 reported that transgranular fractuures were observed sodium chloride could develop. The concentrattion of the

TABLE I. Chemical composition of the carbon- and 316 stainless steel Alloy Mass fraction of the various constituents Fe C Mn Mo Cr Ni Carbon steel 0.967 0.023 9.98 x 103 0 0 0 316 stainless steel 0.671 4.66 x 103 0 0.018 0.183 0.124 TABLE II. Example of chemical and electrochemical conditions for 316 stainless steel in three potential disposal environments Corrosion Potential Chemical Conditions (Ecorr in unit of V vs. SHE#)

Disposal Chemical pH Oxidizing Environment Composition Range Temperature Condition Reducing Condition Rock salt 26 wt % NaCl 4-7 90 °C (194 °F) 0.18 0.40 to 0.14 solution 26 wt % NaCl 4-7 150 °C (302 °F) 0.41 to 0.39 0.02 to 0.03 solution 30 wt % MgCl2 4-5 90 °C (194 °F) 0.18-0.2 0.52 to 0.56 solution 30 wt % MgCl2 4-5 150 °C (302 °F) 0.39 0.62 to 0.70 solution Bentonite clay Bentonite clay 50 °C (122 °F) 0.21 0.30 water*

Granite 50 g/L 6-9 90 °C (194 °F) 0.16 0.32 (0.42 lb/gallon)

NaCl solution

• The chemical composition of the water in the bentonite clay can be found in Table 2-15 of a report published by the European Commission9.

SHE stands for standard hydrogen electrode.

in 316 stainless steel samples during the posttest were selected: 1,000 and 10,000 parts per million examinations, indicating the occurrence of SCC at the (8.4 x 103 and 8.4 x 102 lb/gallon). In addition, sulfate open circuit potential. Alyousif and Nishimura5 conducted ions were added in the solution. The sulfate ion constant load experiments to determine SCC concentrations were selected as 20, 1,000, and susceptibility of 316 stainless steel in boiling saturated 10,000 parts per million (1.68 x 104, 8.4 x 103, and MgCl2 solutions at 135 and 155 °C (275 and 311 °F). The 8.4 x 102 lb/gallon). The aqueous solutions were purged samples were not polarized (i.e., the experiments were with nitrogen, air, and carbon dioxide, and the samples conducted at the corrosion potential). The concentrations were polarized approximately 200 mV above the of the boiling saturated MgCl2 solution at 135 and 155 °C corrosion potentials of the 316 stainless steel in the (275 and 311 °F) are approximately equal to 44 and 49 aqueous solutions. The authors reported that the wt %, respectively. The concentrations of the boiling 316 stainless steel did not undergo SCC in any of the saturated MgCl2 solutions were determined by solutions. Cragnolino et al.6 also conducted slow strain OLIAnalyzer Version 3.1 software as Alyousif and rate tests in approximately 27 wt % NaCl solutions at Nishimura5 did not provide this information. The tensile 95 °C (203 °F). The corrosion potential of the alloy was stress on the samples was varied between 100 and 500 approximately -0.12 VSHE (the subscript SHE stands for MPa (1.45 x 104 and 7.25 x 104 psi) in the constant load standard hydrogen reference electrode). The samples were experiment. Alyousif and Nishimura5 reported that the kept at the corrosion potential during the tests. The 316 stainless steel underwent intergranular SCC at 135 °C authors reported that 316 stainless steel only underwent (275 °F) and transgranular SCC at 155 °C (311 °F). ductile failure or pitting corrosion in the aqueous solution.

Cragnolino et al.6 conducted slow strain rate tests to The authors also conducted the test in 30 and 40 wt%

determine SCC susceptibility of 316 stainless steel in MgCl2 solutions at 110 and 120 °C (230 and 248 °F). The chloride containing solutions with Mg2+, Li+, and Na+ samples were kept at the corrosion potentials. The authors cationic species at temperature ranging from 95 to 125 °C reported that 316 stainless steel underwent SCC in the (203 to 257 °F). For slow strain rate tests conducted at 30 and 40 wt % MgCl2 solutions.

95 °C (203 °F), two chloride concentrations

II.B. SCC Susceptibility of 316 Stainless Steel Furthermore, there is not enough information available to assess the SCC susceptibility of the alloy in a potential Based upon the literature information, it is granite disposal environment.

concluded that 316 stainless steel appears susceptible to SCC in 26 wt % NaCl solution and 30 wt % MgCl2 ACKNOWLEDGMENTS solution in the temperature range of 90 to 150 °C (194 to 302 °F) under both oxidizing and reducing environments. This paper is an independent product of the CNWRA Therefore, 316 stainless steel waste packages appear and does not necessarily reflect the view or regulatory susceptible to SCC in some salt rock disposal position of the NRC.

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