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 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. 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. 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. 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.

Fig. 1. Factors that lead to SCC of nuclear waste container materials. 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.

Stress

EnvironmentMaterial

SCC

• Welding• Heat

Treatment

•Water Chemistry• Temperature• Electrochemical

Potential

• Residual Stress• Seismic Induced• 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 (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 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.

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 (tf/tf

air) 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 (tf/tf

air) for nickel alloy specimens tested at 95 °C (203 °F) in 1.1 molal and 2.1 molal HCO3

− solutions containing various Cl–

concentrations.8 The tests were performed at a constant strain rate of 3.2 × 10−6/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.

(a) (b)

Fig. 4. Side surface of a nickel alloy specimen s(a) 7.2 molal Cl− only and (b) 1.1 molal HCmolal Cl−.8

On the other hand, in a 1.1 molal HCO3

containing 4.2 molal chloride solution, a large nsecondary SCC were present on the side surfaspecimen (Fig. 4b). The elongation of the testwas reduced to 52.2 percent, with a time-to-faof 0.51. The effects of the environment (water containing HCO3

− 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 envcan lead to different chemical compositions anconditions of the aqueous solution in contact wpackages. These conditions are discussed addition, the electrochemical conditions that cato 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 approxi26 and 30 wt %, respectively. The pHsodium-chloride-rich brines and magnesium-chlbrines could range from 4–7.9 The temperaturbrines could range 90–150 °C (194–302 °F) depthe design of disposal systems. If waste packplaced inside a bentonite clay, an aqueouspredominantly containing sodium, magnesichloride ions could contact the waste packageschemical composition of the aqueous solutiobentonite clay can be found in Table 2–15 opublished by the European Commissionmaximum temperature of the aqueous solutirange from 50–100 °C (122–212 °F). For packages placed in granite, aqueous solutions csodium chloride could develop. The concentrat

strained in CO3

− + 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 50–50,000(4.2 × 10−4 to 0.42 lb/gallon), and thetemperature could be as high as 90 °Cpaper, it is assumed that the aqueous sothe waste packages in a granite rock discontains NaCl in a concentrati(0.42 lb/gallon) and the aqueous solut90 °C (194 °F). In the three penvironments, the aqueous solutions copackages are expected initially to oxygen because the oxygen might be repository construction and also someFor deep disposal system with undisturdissolved oxygen in the solution mighoxygen reduction reaction and eventsolution might become reducing. II.B. Electrochemical Conditions

The electrochemical conditions fo316 stainless steel waste package potential disposal environments were cOLIAnalyzer Version 3.1 software.1

results have been extensively validatedcompositions of the 316 stainless steTABLE I. The chemical and thermal cospecifications were input in the softwaresults included polarization curvepotentials. The values of the calpotentials were read from the polarizcompositions of the aqueous solutiocorrosion potentials for the 316 stainlesTABLE II. III. SCC SUSPECTIBILITY EVALU

III.A. SCC Test Data Literature information was searchetest data on 316 stainless steel ansusceptibility of 316 stainless steel disposal environments. It is assumconditions, such as the heat affected residual tensile stresses are present. focuses only on the SCC susceptibilitsteel as a function of chemicaelectrochemical conditions. Tsai and slow strain rate tests to determine SCduplex- and 316 stainless steel in 26 wwith a pH equal to 6 and at 90 °C (solution was deaerated, the strain rate 4.1 × 10−6/sec, and samples were not tests were conducted at the corrosion poChen4 reported that transgranular fractu

e chloride ion 0 parts per million e aqueous solution

C (194 °F).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. rbed groundwaters, ht be consumed by tually the aqueous

for the carbon and material in three

calculated using the 10 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, and enough Thus, this paper

ty of 316 stainless al, thermal, and

Chen4 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 × 10−3 0 0 0

316 stainless steel 0.671 4.66 × 10−3 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

(Ecorr in unit of V vs. SHE#) 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 % MgCl2 solution

4–5 90 °C (194 °F) 0.18–0.2 −0.52 to −0.56

30 wt % MgCl2 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

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 Commission9. #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 Nishimura5 conducted constant load experiments to determine SCC susceptibility of 316 stainless steel in boiling saturated MgCl2 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 MgCl2 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 × 104 and 7.25 × 104 psi) in the constant load experiment. Alyousif and Nishimura5 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.6 conducted slow strain rate tests to determine SCC susceptibility of 316 stainless steel in chloride containing solutions with Mg2+, 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 × 10−3 and 8.4 × 10−2 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 × 10−4, 8.4 × 10−3, and 8.4 × 10−2 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.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% MgCl2 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 % MgCl2 solutions.

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 % MgCl2 solution in the temperature range of 90 to 150 °C (194 to 302 °F) under both oxidizing and reducing environments. 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 × 10−2 lb/gallon), 1,500 parts per million sulfate (1.26 × 10−2 lb/gallon), 110 parts per million nitrate (9.24 × 10−4 lb/gallon), and 27 parts per million of bicarbonate ions (2.27 × 10−4 lb/gallon).9 Cragnolino et al.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 × 10−3 and 8.4 × 10−2

lb/gallon), and sulfate ions at 20, 1,000, and 10,000 parts per million (1.68 × 10−4 , 8.4 × 10−3, and 8.4 × 10−2

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 × 10−4 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 × 10−2 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. IV. 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.

ACKNOWLEDGMENTS

This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC.

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