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{{#Wiki_filter: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
{{#Wiki_filter: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.
), Southwest Research Institute
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.
, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA Contact: K. Chiang, kchiang@swri.org, Telephone: +210-522-2308
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
Stainless steel may be considered as a waste package container material in different disposal environments. The objective of this paper is to assess stress corrosion cracking (SCC) susceptibility of 316 stainless steel in possible disposal environments. Factors including (i) material properties affected by fabrication processes such as welding and heat treatment; (ii) environmental conditions including chemistry of aqueous solution surrounding the waste package, temperature, and electrochemical conditions; and (iii) tensile stress in the welded areas as well as tensile stress generated by events 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 information was compiled to define chemical and thermal conditions that could arise in disposal environments.
* Water Chemistry Numerical simulations were conducted to determine
Numerical simulations were conducted to determine electrochemical conditions of the alloy in disposal environments. Literature information on 316 stainless steel's SCC susceptibility in the chemical and thermal conditions similar to those in potential disposal environments was compiled. The susceptibility of the alloy was assessed by comparing the literature information.
* Heat
I. INTRODUCTION Engineered barrier systems for a potential high-level radioactive waste disposal system might include waste 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 degradation process limiting waste container life. One of the potential corrosion degradation modes for the waste container is stress corrosion cracking (SCC). SCC is a phenomenon by which a normally ductile alloy loses its toughness (elongation to rupture time) when it is subject to mechanical stresses under a range of environments.
* Temperature electrochemical conditions of the alloy in disposal                       Treatment
SCC susceptibility of the nuclear waste container materials is dependent on three factors: (i) material-related factors such as metallurgy and microstructure of the material; (ii) environmental conditions including chemical, thermal, and electrochemical conditions; and (iii) magnitude of applied and residual tensile stresses. Welding and heat treatment can influence the microstructure of the material affecting the susceptibility to SCC. The environmental setting is defined by the chemistry of the aqueous solution in contact with the waste package, temperature, and electrochemical variables such as the corrosion potential. Tensile stresses could arise from welding. Fig. 1 illustrates the three factors that must be simultaneously present for SCC to occur.
* 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.
Fig. 1. Factors that lead to SCC of nuclear waste container materials.
I. INTRODUCTION Stress
 
* Residual Stress Engineered barrier systems for a potential high-level
In this paper, the SCC susceptibility of 316 stainless steel in three different possible disposal environments is assessed. The effects of material properties; residual stresses; and chemical, thermal, and electrochemical conditions are considered in regard to the alloy's susceptibility to SCC.
* Seismic Induced radioactive waste disposal system might include waste
StressEnvironmentMaterialSCC*Welding*Heat Treatment*Water Chemistry
* 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.
*Temperature
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.
*ElectrochemicalPotential*Residual Stress
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.
*Seismic Induced
related factors such as metallurgy and microstructure of the material; (ii) environmental conditions including
*Rock Overburden I.A. Material-Related Issues 316 stainless steel is a possible material to be used to construct waste package containers to isolate nuclear waste from the environments in a potential geological disposal system. Fabrication of containers generally requires multiple processes, such as welding and solution annealing.1,2  These processes may alter the microstructure and mechanical properties of the base alloys, and introduce residual tensile stresses. Thermal treatment can cause carbide precipitation; change the grain size and microstructure of the welded areas and heat-affected zone in a 316 stainless steel waste 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 (Mg 2+, 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 in accordance with the ASTM G-129 procedure.
7  A photograph of the experimental test cell for the slow strain rate test is shown in Fig. 2. The same test cell can also be used to conduct the constant load test.


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.
Fig. 2. Slow strain rate test apparatus with specimen mounted in the test cell.
Alloy C-22 is comparable to 316 stainless steel with respect to SCC processes. Thus, experiments on SCC for C-22 can yield insights on SCC processes for 316 stainless steel, about direct experimental data. As an example, Fig. 3 shows the time-to-failure ratios (t f/t fair) for the slow strain rate tests of a nickel-based alloy in solutions containing various anionic and cationic species.
8  The ionic species include chloride and bicarbonate ions.
The ratio of time-to-failure in the test environment versus time-to-failure measured in air can be considered as an index of the severity of SCC. Fig. 3 shows that the addition of a small concentration (0.2 molal) of chloride to the 1.1 molal bicarbonate solution significantly decreases the failure time.
Fig. 3. Time-to-failure ratios (t f/t f air) for nickel alloy specimens tested at 95 °C (203 °F) in 1.1 molal and 2.1 molal HCO 3 solutions containing various Cl
- concentrations.
8  The tests were performed at a constant 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 HCO 3 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. 
(a) (b) Fig. 4. Side surface of a nickel alloy specimen s(a) 7.2 molal Cl only and (b) 1.1 molal H Cmolal Cl.8    On the other hand, in a 1.1 molal HCO 3containing 4.2 molal chloride solution, a large nsecondary SCC were present on the side sur faspecimen (Fig. 4b). The elongation of the tes twas reduced to 52.2 percent, with a time-to-f aof 0.51. The effects of the environment (water containing HCO 3 and Cl) in causing SCC are iThus, information about the time-to-failure ratpresence of microcracks on the side surface of tafter the test can be used to determine whether susceptible to SCC in specific environmental co II. DISPOSAL ENVIRONMENTS 
In this paper, potential disposal of staiwaste packages in salt rock, clay, and gconsidered. These three potential disposal en vcan lead to different chemical compositions a nconditions of the aqueous solution in contact w packages. These conditions are discussed addition, the electrochemical conditions that c ato determine SCC susceptibility of the alloy are
II.A. Chemical and Thermal Conditions Stainless steel waste packages placed in could be contacted by sodium chloride or mchloride brines of concentrations of approx i26 and 30 wt %, respectively. The p Hsodium-chloride-rich brines and magnesium-ch lbrines could range from 4-7.
9  The temperatu rbrines could range 90-150 °C (194-302 °F) de pthe design of disposal systems. If waste pac kplaced inside a bentonite clay, an aqueou s predominantly containing sodium, magnes ichloride ions could contact the waste package schemical composition of the aqueous soluti obentonite clay can be found in Table 2-15 opublished by the European Commission maximum temperature of the aqueous solut irange from 50-100 °C (122-212 °F). For packages placed in granite, aqueous solutions csodium chloride could develop. The concentra t strained in C O 3 + 4.2 3 solution number of face of the t specimen ailure ratio chemistry illustrated. io and the the sample an alloy is nditions. nless-steel granite is vironments nd thermal with waste next. In an be used detailed. salt rock magnesium imately of H of the loride-rich re of those pending on kages were s solution ium, and s. Possible on in the of a report
.9  The ion might the waste containing tion of the sodium chloride could cause thconcentration to range from 50-50,00 0 (4.2 x 104 to 0.42 lb/gallon), and th etemperature could be as high as 90 °Cpaper, it is assumed that the aqueous s othe waste packages in a granite rock di scontains NaCl in a concentrati (0.42 lb/gallon) and the aqueous solu t90 °C (194 °F). In the three penvironments, the aqueous solutions c o packages are expected initially to oxygen because the oxygen might b e repository construction and also som eFor deep disposal system with undistu rdissolved oxygen in the solution mig hoxygen reduction reaction and even tsolution might become reducing.
II.B. Electrochemical Conditions The electrochemical conditions fo316 stainless steel waste package potential disposal environments were cOLIAnalyzer Version 3.1 software.
1results have been extensively validate dcompositions of the 316 stainless steTABLE I. The chemical and thermal c ospecifications were input in the softw aresults included polarization curv e potentials. The values of the ca lpotentials were read from the polariz compositions of the aqueous soluti ocorrosion potentials for the 316 stainle sTABLE II. 


III. SCC SUSPECTIBILITY EVAL U III.A. SCC Test Data Literature information was search etest data on 316 stainless steel a nsusceptibility of 316 stainless steel disposal environments. It is assu mconditions, such as the heat affected residual tensile stresses are present. focuses only on the SCC susceptibili tsteel as a function of chemic aelectrochemical conditions. Tsai and slow strain rate tests to determine SC duplex- and 316 stainless steel in 26 wwith a pH equal to 6 and at 90 °C (solution was deaerated, the strain rate  
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


4.1 x 106/sec, and samples were not tests were conducted at the corrosion p oChen 4 reported that transgranular fract ue chloride ion 0 parts per million e aqueous solution C (194 °F).
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#)
9  In this olution surrounding sposal environment on of 50 g/L tion temperature is potential disposal ontacting the waste contain dissolved present during the etime after closure.
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*
rbed groundwaters, ht be consumed by tually the aqueous for the carbon and material in three calculated using the 1 0  The software d.11  The chemical el are provided in onditions, and alloy are. The calculated es and corrosion lculated corrosion ation curves. Both ons and calculated ss steel are listed in UATION ed to compile SCC nd to assess SCC in three potential med that material zone, a nd enough Thus, this paper ty of 316 stainless al, thermal, and Che n 4 conducted CC susceptibility of wt % NaCl solution, (194 °F). The test was selected to be polarized (i.e., the otentials). Tsai and ures were observed 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 Disposal Environment Chemical Conditions Corrosion Potential (E cor r in unit of V vs. SHE
Granite                 50 g/L                     6-9               90 °C (194 °F)                   0.16                       0.32 (0.42 lb/gallon)
#) Chemical Composition pH Range Temperature Oxidizing Condition Reducing Condition Rock salt 26 wt % NaCl solution 4-7 90 °C (194 °F) 0.18 0.40 to 0.14 26 wt % NaCl solution 4-7 150 °C (302 °F) 0.41 to 0.39 0.02 to 0.03 30 wt % MgCl 2 solution 4-5 90 °C (194 °F) 0.18-0.2 0.52 to 0.56 30 wt % MgCl 2 solution 4-5 150 °C (302 °F) 0.39 0.62 to 0.70 Bentonite clay Bentonite clay water*  50 °C (122 °F) 0.21 0.30 Granite 50 g/L (0.42 lb/gallon)
NaCl solution
NaCl solution 6-9 90 °C (194 °F) 0.16 0.32 *The chemical composition of the water in the bentonite clay can be found in Table 2-15 of a report published by the European Commission
*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.
: 9. #SHE stands for standard hydrogen electrode.
#
in 316 stainless steel samples during the posttest examinations, indicating the occurrence of SCC at the open circuit potential. Alyousif and Nishimura 5 conducted constant load experiments to determine SCC susceptibility of 316 stainless steel in boiling saturated MgCl 2 solutions at 135 and 155 °C (275 and 311 °F). The samples were not polarized (i.e., the experiments were conducted at the corrosion potential). The concentrations of the boiling saturated MgCl2 solution at 135 and 155 °C (275 and 311 °F) are approximately equal to 44 and 49 wt %, respectively. The concentrations of the boiling saturated MgCl 2 solutions were determined by OLIAnalyzer Version 3.1 software as Alyousif and Nishimura5 did not provide this information. The tensile stress on the samples was varied between 100 and 500 MPa (1.45 x 10 4 and 7.25 x 10 4 psi) in the constant load experiment. Alyousif and Nishimura 5 reported that the 316 stainless steel underwent intergranular SCC at 135 °C (275 °F) and transgranular SCC at 155 °C (311 °F). Cragnolino et al.
SHE stands for standard hydrogen electrode.
6 conducted slow strain rate tests to determine SCC susceptibility of 316 stainless steel in chloride containing solutions with Mg 2+, Li+, and Na+ cationic species at temperature ranging from 95 to 125 °C (203 to 257 °F). For slow strain rate tests conducted at 95 °C (203 °F), two chloride concentrations were selected: 1,000 and 10,000 parts per million (8.4 x 103 and 8.4 x 102 lb/gallon). In addition, sulfate ions were added in the solution. The sulfate ion concentrations were selected as 20, 1,000, and 10,000 parts per million (1.68 x 104, 8.4 x 103, and 8.4 x 102 lb/gallon). The aqueous solutions were purged with nitrogen, air, and carbon dioxide, and the samples were polarized approximately 200 mV above the corrosion potentials of the 316 stainless steel in the aqueous solutions. The authors reported that the 316 stainless steel did not undergo SCC in any of the solutions. Cragnolino et al.
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.
6 also conducted slow strain rate tests in approximately 27 wt % NaCl solutions at 95 °C (203 °F). The corrosion potential of the alloy was approximately -0.12 VSHE (the subscript SHE stands for standard hydrogen reference electrode). The samples were kept at the corrosion potential during the tests. The authors reported that 316 stainless steel only underwent ductile failure or pitting corrosion in the aqueous solution. The authors also conducted the test in 30 and 40 wt% MgCl 2 solutions at 110 and 120 °C (230 and 248 °F). The samples were kept at the corrosion potentials. The authors reported that 316 stainless steel underwent SCC in the 30 and 40 wt % MgCl 2 solutions.
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 Based upon the literature information, it is concluded that 316 stainless steel appears susceptible to SCC in 26 wt % NaCl solution and 30 wt % MgCl 2 solution in the temperature range of 90 to 150 °C (194 to 302 °F) under both oxidizing and reducing environments.
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.
Therefore, 316 stainless steel waste packages appear susceptible to SCC in some salt rock disposal environments. The solution contacting waste packages in bentonite clay might contain approximately 6,550 parts per million chloride (5.46 x 102 lb/gallon), 1,500 parts per million sulfate (1.26 x 102 lb/gallon), 110 parts per million nitrate (9.24 x 104 lb/gallon), and 27 parts per million of bicarbonate ions (2.27 x 104 lb/gallon).
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.
9 Cragnolino et al.
environments. The solution contacting waste packages in bentonite clay might contain approximately 6,550 parts                             REFERENCES per million chloride (5.46 x 102 lb/gallon), 1,500 parts per million sulfate (1.26 x 102 lb/gallon), 110 parts per   1. D. S. DUNN, D. DARUWALLA, and Y.-M. PAN, million nitrate (9.24 x 104 lb/gallon), and 27 parts per         Effect of Fabrication Processes on Material million of bicarbonate ions (2.27 x 104 lb/gallon).9             StabilityCharacterization and Corrosion, CNWRA Cragnolino et al.6 report that SCC of 316 stainless steel         eport 2004-01, San Antonio, Texas (2003).
6 report that SCC of 316 stainless steel was not observed when slow rate experiments were conducted in solutions containing chloride ions at 1,000 and 10,000 parts per million (8.4 x 103 and 8.4 x 102 lb/gallon), and sulfate ions at 20, 1,000, and 10,000 parts per million (1.68 x 104 , 8.4 x 103, and 8.4 x 102 lb/gallon). Previous studies6,8 suggest that the presence of nitrate ions is not likely to cause SCC of the alloy, and concentration of bicarbonate ions is too low to influence SCC of the alloy. It is therefore concluded that SCC of 316 stainless steel is unlikely in a potential bentonite clay disposal environment. The aqueous solutions contacting waste packages in granite could contain chloride ions in the range of 50-50,000 parts per million (4.2 x 104 to 0.42 lb/gallon), whereas experimental data4,6 are available in the aqueous solutions containing chloride ions up to 10,000 parts per million (8.4 x 102 lb/gallon) and at 26 wt % NaCl solution. Based upon this information, SCC susceptibility of the alloy in a potential granite disposal environment cannot be determined.
was not observed when slow rate experiments were             2. D. S. DUNN, Y.-M. PAN, K. T. CHIANG, conducted in solutions containing chloride ions at 1,000         L. YANG, G. A. CRAGNOLINO, and X. HE, The and 10,000 parts per million (8.4 x 103 and 8.4 x 102           Localized Corrosion Resistance and Mechanical lb/gallon), and sulfate ions at 20, 1,000, and 10,000 parts       Properties of Alloy 22 Waste Package Outer per million (1.68 x 104 , 8.4 x 103, and 8.4 x 102             Containers, Journal of Metals, Vol. 57, p. 49 (2005).
IV.
lb/gallon). Previous studies6,8 suggest that the presence of 3. G. A. CRAGNOLINO, H. K. MANAKTALA, and nitrate ions is not likely to cause SCC of the alloy, and         Y.-M. PAN, Thermal Stability and Mechanical concentration of bicarbonate ions is too low to influence         Properties of High-Level Radioactive Waste SCC of the alloy. It is therefore concluded that SCC of           Container Materials Assessment of Carbon and Low-316 stainless steel is unlikely in a potential bentonite clay     Alloy Steels, CNWRA Report 96-004, San Antonio, disposal environment. The aqueous solutions contacting           Texas (1996).
waste packages in granite could contain chloride ions in      4. W.-T. TSAI and M.-S. CHEN, Stress Corrosion Cracking Behavior of 2205 Duplex Stainless Steel in the range of 50-50,000 parts per million (4.2 x 104 to Concentrated NaCl Solutions, Corrosion Science, 0.42 lb/gallon), whereas experimental data4,6 are available Vol. 42, p. 545 (2000).
in the aqueous solutions containing chloride ions up to
: 5. O. M. ALYOUSIF and R. NISHIMURA, The Stress 10,000 parts per million (8.4 x 102 lb/gallon) and at 26 wt Corrosion Behavior of Austenitic Stainless Steels in
% NaCl solution. Based upon this information, SCC Boiling Magnesium Chloride Solutions, Corrosion susceptibility of the alloy in a potential granite disposal Science, Vol. 49, p. 3040 (2007).
environment cannot be determined.
: 6. G. A. CRAGNOLINO, D. S. DUNN, and N. SRIDHAR, Environmental Effect on Stress IV.  


==SUMMARY==
==SUMMARY==


316 stainless steel is a material that might be used to construct nuclear waste containers because of its corrosion resistance. One degradation mode for 316 stainless steel is SCC. SCC may occur when a susceptible material, environmental conditions, and stresses are present simultaneously. SCC susceptibility of 316 stainless steel is assessed in three potential disposal environments: salt rock, bentonite clay, and granite. Literature information was compiled to assess the SCC susceptibility of the alloy in these three environments. Based upon literature information on chemical and thermal conditions in potential disposal environments and literature data on SCC susceptibility of 316 stainless steel in different aqueous solutions, it is concluded that 316 stainless steel appears susceptible to SCC in a potential rock salt disposal environment but not in studied bentonite clays. Furthermore, there is not enough information available to assess the SCC susceptibility of the alloy in a potential granite disposal environment.
Corrosion Cracking of Type 316L Stainless Steel and Alloy 825 as High-Level Nuclear Waste Container 316 stainless steel is a material that might be used to Materials, CNWRA Report 94-028, San Antonio, construct nuclear waste containers because of its Texas (1994).
ACKNOWLEDGMENTS This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC.
corrosion resistance. One degradation mode for
REFERENCES
: 7. ASTM INTERNATIONAL, Standard Practice for 316 stainless steel is SCC. SCC may occur when a Slow Strain Rate Testing to Evaluate the susceptible material, environmental conditions, and Susceptibility    of    Metallic    Materials    to stresses are present simultaneously.
: 1. D. S. DUNN, D. DARUWALLA, and Y.-M. PAN, "Effect of Fabrication Processes on Material StabilityCharacterization and Corrosion," CNWRA eport 2004-01, San Antonio, Texas (2003). 2. D. S. DUNN, Y.-M. PAN, K. T. CHIANG, L. YANG, G. A. CRAGNOLINO, and X. HE, "The Localized Corrosion Resistance and Mechanical Properties of Alloy 22 Waste Package Outer Containers," Journal of Metals, Vol. 57, p. 49 (2005). 3. G. A. CRAGNOLINO, H. K. MANAKTALA, and Y.-M. PAN, "Thermal Stability and Mechanical Properties of High-Level Radioactive Waste Container Materials Assessment of Carbon and Low-Alloy Steels," CNWRA Report 96-004, San Antonio, Texas (1996). 4. W.-T. TSAI and M.-S. CHEN, "Stress Corrosion Cracking Behavior of 2205 Duplex Stainless Steel in Concentrated NaCl Solutions," Corrosion Science , Vol. 42, p. 545 (2000). 5. O. M. ALYOUSIF and R. NISHIMURA, "The Stress Corrosion Behavior of Austenitic Stainless Steels in Boiling Magnesium Chloride Solutions," Corrosion Science, Vol. 49, p. 3040 (2007). 6. G. A. CRAGNOLINO, D. S. DUNN, and N. SRIDHAR, "Environmental Effect on Stress Corrosion Cracking of Type 316L Stainless Steel and Alloy 825 as High-Level Nuclear Waste Container Materials," CNWRA Report 94-028, San Antonio, Texas  (1994). 7. ASTM INTERNATIONAL, "Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking, G-129: Annual Book of ASTM Standards. Vol. 3.02: Wear and Erosion-Metal Corrosion. West Conshohocken, Pennsylvania (2003). 8. K. T. CHIANG, D. S. DUNN, and G. A. CRAGNOLINO, "Effect of Simulated Groundwater Chemistry on Stress Corrosion Cracking of Alloy 22," Corrosion, Vol. 63, p. 940 (2007).
Environmentally Assisted Cracking, G-129: Annual SCC susceptibility of 316 stainless steel is assessed Book of ASTM Standards. Vol. 3.02: Wear and in three potential disposal environments: salt rock, Erosion-Metal Corrosion. West Conshohocken, bentonite clay, and granite. Literature information was Pennsylvania (2003).
: 9. B. KURSTEN, E. SMAILOS, I. AZKARATE, L. WERME, N. R. SMART, and G. SANTARINI, "State-of-the-art Document on the Corrosion Behaviour of Container Materials," 5 th EURATOM FRAMEWORK PROGRAMME 1998-2002 KEY ACTION: NUCLEAR FISSION, European Commission (2004). 10. OLI SYSTEMS, INC., "A Guide to Using the OLI Software for Version 3.1 of the Analyzers," OLI Systems, Inc, Morris Plains, New Jersey: (2010). 11. M. S. GRUSZKIEWICZ, D. A. PALMER, R. D. SPRINGER, P. WANG, and A. ANDERKO, "Phase Behavior of Aqueous Na-K-Mg-Ca-Cl-NO 3 Mixtures: Isopiestic Measurements and Thermodynamic Modeling," Journal of Solution Chemistry, 36, p. 723 (2007).}}
compiled to assess the SCC susceptibility of the alloy in
: 8. K. T. CHIANG,      D. S. DUNN,      and these three environments. Based upon literature G. A. CRAGNOLINO,          Effect    of    Simulated information on chemical and thermal conditions in Groundwater Chemistry on Stress Corrosion potential disposal environments and literature data on Cracking of Alloy 22, Corrosion, Vol. 63, p. 940 SCC susceptibility of 316 stainless steel in different (2007).
aqueous solutions, it is concluded that 316 stainless steel appears susceptible to SCC in a potential rock salt disposal environment but not in studied bentonite clays.
: 9. B. KURSTEN, E. SMAILOS, I. AZKARATE, L. WERME, N. R. SMART, and G. SANTARINI, State-of-the-art Document on the Corrosion Behaviour of Container Materials, 5th EURATOM FRAMEWORK PROGRAMME 1998-2002 KEY ACTION:       NUCLEAR       FISSION,     European Commission (2004).
: 10. OLI SYSTEMS, INC., A Guide to Using the OLI Software for Version 3.1 of the Analyzers, OLI Systems, Inc, Morris Plains, New Jersey: (2010).
: 11. M. S. GRUSZKIEWICZ, D. A. PALMER, R. D.
SPRINGER, P. WANG, and A. ANDERKO, "Phase Behavior of Aqueous Na-K-Mg-Ca-Cl-NO3 Mixtures:       Isopiestic   Measurements       and Thermodynamic Modeling, Journal of Solution Chemistry, 36, p. 723 (2007).}}

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Assessment of Stress Corrosion Cracking Susceptibility of 316 Stainless Steel in Different Disposal Environments
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Issue date: 12/28/2010
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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.

environments. The solution contacting waste packages in bentonite clay might contain approximately 6,550 parts REFERENCES per million chloride (5.46 x 102 lb/gallon), 1,500 parts per million sulfate (1.26 x 102 lb/gallon), 110 parts per 1. D. S. DUNN, D. DARUWALLA, and Y.-M. PAN, million nitrate (9.24 x 104 lb/gallon), and 27 parts per Effect of Fabrication Processes on Material million of bicarbonate ions (2.27 x 104 lb/gallon).9 StabilityCharacterization and Corrosion, CNWRA Cragnolino et al.6 report that SCC of 316 stainless steel eport 2004-01, San Antonio, Texas (2003).

was not observed when slow rate experiments were 2. D. S. DUNN, Y.-M. PAN, K. T. CHIANG, conducted in solutions containing chloride ions at 1,000 L. YANG, G. A. CRAGNOLINO, and X. HE, The and 10,000 parts per million (8.4 x 103 and 8.4 x 102 Localized Corrosion Resistance and Mechanical lb/gallon), and sulfate ions at 20, 1,000, and 10,000 parts Properties of Alloy 22 Waste Package Outer per million (1.68 x 104 , 8.4 x 103, and 8.4 x 102 Containers, Journal of Metals, Vol. 57, p. 49 (2005).

lb/gallon). Previous studies6,8 suggest that the presence of 3. G. A. CRAGNOLINO, H. K. MANAKTALA, and nitrate ions is not likely to cause SCC of the alloy, and Y.-M. PAN, Thermal Stability and Mechanical concentration of bicarbonate ions is too low to influence Properties of High-Level Radioactive Waste SCC of the alloy. It is therefore concluded that SCC of Container Materials Assessment of Carbon and Low-316 stainless steel is unlikely in a potential bentonite clay Alloy Steels, CNWRA Report 96-004, San Antonio, disposal environment. The aqueous solutions contacting Texas (1996).

waste packages in granite could contain chloride ions in 4. W.-T. TSAI and M.-S. CHEN, Stress Corrosion Cracking Behavior of 2205 Duplex Stainless Steel in the range of 50-50,000 parts per million (4.2 x 104 to Concentrated NaCl Solutions, Corrosion Science, 0.42 lb/gallon), whereas experimental data4,6 are available Vol. 42, p. 545 (2000).

in the aqueous solutions containing chloride ions up to

5. O. M. ALYOUSIF and R. NISHIMURA, The Stress 10,000 parts per million (8.4 x 102 lb/gallon) and at 26 wt Corrosion Behavior of Austenitic Stainless Steels in

% NaCl solution. Based upon this information, SCC Boiling Magnesium Chloride Solutions, Corrosion susceptibility of the alloy in a potential granite disposal Science, Vol. 49, p. 3040 (2007).

environment cannot be determined.

6. G. A. CRAGNOLINO, D. S. DUNN, and N. SRIDHAR, Environmental Effect on Stress IV.

SUMMARY

Corrosion Cracking of Type 316L Stainless Steel and Alloy 825 as High-Level Nuclear Waste Container 316 stainless steel is a material that might be used to Materials, CNWRA Report 94-028, San Antonio, construct nuclear waste containers because of its Texas (1994).

corrosion resistance. One degradation mode for

7. ASTM INTERNATIONAL, Standard Practice for 316 stainless steel is SCC. SCC may occur when a Slow Strain Rate Testing to Evaluate the susceptible material, environmental conditions, and Susceptibility of Metallic Materials to stresses are present simultaneously.

Environmentally Assisted Cracking, G-129: Annual SCC susceptibility of 316 stainless steel is assessed Book of ASTM Standards. Vol. 3.02: Wear and in three potential disposal environments: salt rock, Erosion-Metal Corrosion. West Conshohocken, bentonite clay, and granite. Literature information was Pennsylvania (2003).

compiled to assess the SCC susceptibility of the alloy in

8. K. T. CHIANG, D. S. DUNN, and these three environments. Based upon literature G. A. CRAGNOLINO, Effect of Simulated information on chemical and thermal conditions in Groundwater Chemistry on Stress Corrosion potential disposal environments and literature data on Cracking of Alloy 22, Corrosion, Vol. 63, p. 940 SCC susceptibility of 316 stainless steel in different (2007).

aqueous solutions, it is concluded that 316 stainless steel appears susceptible to SCC in a potential rock salt disposal environment but not in studied bentonite clays.

9. B. KURSTEN, E. SMAILOS, I. AZKARATE, L. WERME, N. R. SMART, and G. SANTARINI, State-of-the-art Document on the Corrosion Behaviour of Container Materials, 5th EURATOM FRAMEWORK PROGRAMME 1998-2002 KEY ACTION: NUCLEAR FISSION, European Commission (2004).
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