Corrosion Resistance of Austenitic and Duplex Stainless Steels in Environments
Corrosion Resistance of Austenitic and Duplex Stainless Steels in Environments
Corrosion Resistance of Austenitic and Duplex Stainless Steels in Environments
Fraser King
April 2009
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Document History
Title: Corrosion Resistance of Austenitic and Duplex Stainless Steels in Environments Related to UK Geological Disposal Subtitle: Client: Document Number: Version Number: Notes: Prepared by: Reviewed by: Version Number: Notes: Fraser King Alan Paulley Version 1.1 Date: January 2009 A Report to NDA RWMD NDA RWMD QRS-1384C-R1 Version 1.0 Date: November 2007
Incorporates changes based on review comments from NDA internal and external reviews.
Incorporates changes based on review comments from additional NDA internal review prior to publication.
Summary
A review has been carried out of the corrosion performance of austenitic and duplex stainless steels as container materials for the storage and disposal of ILW as part of the Phased Geological Repository Concept. Two grades have been selected as representative of each family, namely 304(L) and 316(L) austenitic stainless steels and 2304 and 2205 duplex alloys. Various forms of corrosion are considered, including: general corrosion, localised corrosion in the form of pitting and crevice corrosion, sensitisation-induced intergranular attack, stress corrosion cracking, microbiologically influenced corrosion, atmospheric corrosion, the effects of radiolysis and welding, and galvanic corrosion. In general, the duplex alloys offer a number of advantages over the austenitic stainless steels, including: increased resistance of duplex alloys to localised corrosion due to the higher N content and, in some cases, higher Cr and Mo contents than corresponding austenitic stainless steels, although this benefit is reduced at temperatures >50oC; significantly improved resistance to chloride-induced stress corrosion cracking, with possible immunity of duplex grades at temperatures <100oC; and a higher threshold stress for stress corrosion cracking for duplex alloys which, combined with the significantly higher strength of duplex materials, results in a lower susceptibility to stress corrosion cracking as a result of mechanical damage during storage and handling. Given these advantages of duplex alloys, the most significant difference between the behaviour of austenitic and duplex stainless steel waste containers would be expected (i) on external surfaces during storage and (ii) for internal surfaces in incompletely or non-grouted containers soon after backfilling and in the initial, aerobic, stages of repository saturation. In the latter case, however, duplex stainless steels appear to lose their enhanced resistance to localised corrosion at elevated temperatures (>50oC), so the advantage may be restricted to their lower susceptibility to stress corrosion cracking. Although there are some advantages to the use of duplex alloys over austenitic grades, the expected environmental conditions are such that austenitic stainless steels should perform adequately during the various stages of the Phased Geological Repository Concept. Both families of material offer good corrosion resistance under atmospheric exposure conditions and the presence of cement grout backfill should ensure continued
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excellent performance following repository closure. Duplex alloys provide an extra degree of corrosion resistance during those periods when the probability of localised corrosion and stress corrosion cracking is highest.
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Contents
1 2 Introduction Review of Environmental Conditions and Container Requirements 2.1 Environmental Conditions 2.2 Container Requirements 3 Corrosion of Austenitic and Duplex Stainless Steels 3.1 Introduction to Austenitic and Duplex Stainless Steels 3.2 General Corrosion 3.3 Localised Corrosion 3.4 Intergranular Attack 3.5 Stress Corrosion Cracking 3.6 Microbiologically Influenced Corrosion 3.7 Atmospheric Corrosion 3.8 Effects of Radiolysis 3.9 Effects of Welding 3.10 Galvanic Corrosion 4 5 7 7 8 10 10 11 13 18 19 20 21 22 23 23
Behaviour of Austenitic and Duplex Stainless Steels under Phased Geological Repository Conditions 25 Summary and Conclusions 27 28
References
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Introduction
The Nuclear Decommissioning Authority (NDA) envisages the use of stainless steel containers for the encapsulation of intermediate level waste (ILW) as part of a Phased Geological Repository Concept. Within this concept, the containers are required to provide isolation of the waste during several different phases, including: a period of surface indoor storage, a period of underground atmospheric storage, and eventual disposal in a repository backfilled with cementitious material. The containers will be exposed to a wide range of environmental conditions during their service life and the corrosion behaviour of the stainless steel is, therefore, a key factor in selecting suitable materials for construction. Stainless steels are iron-based alloys characterised by generally good corrosion resistance as a result of a minimum chromium content of 11-13 wt.% (Sedriks 1996). Different families of stainless steel are classified based on their crystallographic structure, e.g., austenitic, ferritic, martensitic, and duplex alloys, the latter containing approximately equal fractions of austenite and ferrite (ASM 1987, 2003, 2005). Whilst all of the stainless steels exhibit good resistance to general corrosion they can be susceptible, to different degrees, to various forms of localised corrosion and stress corrosion cracking. Within a given family of alloys, the resistance to localised corrosion, in particular, can vary depending upon the amount of alloying elements such as Cr, Mo, W, and N that are added. In this report, the corrosion properties of common grades of austenitic and duplex stainless steels are compared and their suitability as container materials for ILW for the Phased Geological Repository Concept considered. The greatest body of available data is for the commonest grades from each family, namely Types 304 and 316 (and their low-carbon variants, 304L and 316L) for the austenitic alloys and alloys 2205, and to a lesser extent, 2304 for the duplex alloys. However, where useful, reference is made to other alloys within these two families of stainless steel. It is not possible to compare the suitability of these various materials without some discussion of the expected environmental conditions and service requirements for the containers. Therefore, the environmental conditions at various stages in the service life of the containers are briefly reviewed, with particular emphasis on those stages during which conditions will be most aggressive and, hence, during which any difference in the corrosion properties of the two families of alloys might be most apparent. Next, the corrosion By behaviour of the austenitic and duplex stainless steels are summarised for each form of corrosion that might be of concern for the Phased Geological Repository Concept. necessity, this review is illustrative of the general properties of these two families of alloy, rather than being an exhaustive review of the respective corrosion properties. Finally, the inferences of the corrosion properties for the behaviour of these two families of alloy as
potential ILW container materials are considered, and the advantages and disadvantages of each material discussed.
period immediately following backfilling, as the repository saturates with groundwater but conditions are still aerobic. The most aggressive forms of corrosion of stainless steels occur in neutral or acidic aerobic chloride environments at elevated temperature. For the external and internal surfaces of the containers, the periods of most concern would appear to be: on the outside of the container, any period during which water and Cl- are present in an aerobic atmosphere and an alkaline interfacial pH has not developed. These conditions could only occur during the operational phase if the proposed controls of the relative humidity and the level of Cl- contamination fail. Alternatively, such conditions could develop immediately after backfilling if Clcontaining groundwater enters the vault prior to the development of the alkaline pore-water pH; and on the inside of the container, the period of most concern for non- or incompletelybackfilled containers will be the early post-backfilled period, when the system is still aerobic but the temperature is increasing due to the curing of the backfill. (Completely backfilled containers should be protected from aggressive forms of internal corrosion at all times because of the pH-conditioning by the cement grout).
It is unclear whether the term permanent deformation implies plastic deformation, i.e., a load
exceeding the yield strength of the material, or simply any degree of elastic deformation whilst still under load, i.e., loads less than the yield strength. 8
It is important to note that, unlike high-level waste containers, there is no requirement for absolute containment of the waste form. Indeed, the waste container will be vented for waste forms that can potentially generate gas (Nirex 2005). It is important to bear these waste package specifications in mind when deciding whether austenitic and/or duplex stainless steels are suitable container materials. Because absolute containment is not a requirement, a limited degree of localised corrosion prior to final backfilling of the repository would not necessarily violate the waste package specifications. On the other hand, extensive localised corrosion or a stress corrosion crack along a welded seam might well compromise the structural integrity of a waste package sufficiently that it could not be safely moved or handled.
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Super-duplex stainless steels contain higher concentrations of Cr, Mo, and N for improved resistance to localised corrosion, and have Pitting Resistance Equivalent (PREN) values of greater than 40 (see below).
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Although there are few reports in the literature of the corrosion rate of duplex stainless steels under the conditions of interest, the rates are likely to be of the same order of magnitude as for austenitic alloys. The corrosion resistance of these alloys is provided by the Cr(III)-rich passive film (Clayton and Olefjord 1995) and, since the Cr content of the duplex alloys is typically higher than that for the austenitic materials (Table 1), there is no reason that duplex stainless steels should not also exhibit the excellent general corrosion resistance of the austenitic alloys. Souto et al. (2001) have studied the nature of the passive film on a duplex alloy similar in composition to Uranus 50 (Table 1) and found it to comprise Cr(III), Fe(II)/Fe(III), and Ni(II). Under active conditions, Fe is present as Fe(II), possibly in the form of Fe(OH)2, with Fe(III) (as FeOOH) formed at more-positive potentials. Where direct measurements of the corrosion rates of austenitic and duplex alloys have been made, the duplex alloys are found to have a similar corrosion rate to austenitic alloys. Blanco et al. (2006) compared the behaviour of 304, 316L, and 2205 in saturated Ca(OH)2 solution to simulate cement grout pore water. Figure 2 shows a series of voltammograms for the different alloys (along with a Type 204 austenitic alloy) in the presence of 0.5% NaCl. The duplex alloy clearly has the lowest passive current density, implying that this alloy has the most protective passive film. Evidence from electrochemical impedance spectroscopy (EIS) measurements supports this conclusion (Figure 3), although the difference in the value of the charge-transfer resistance (inversely proportional to the corrosion rate) for the duplex 2205 alloy and that for one of the 316L samples is not significant. Corrosion current densities from short-term tests were of the order of 10-8-10-7 A/cm2, equivalent to corrosion rates of 0.1-1 m/y. The corrosion rate decreases with time as the passive film thickens (Table 3), and these short-term rates would be expected to continue to decrease with increasing exposure time. Although corrosion rates are observed to decrease with exposure time over laboratory timescales (a maximum of 3-5 years), the question remains as to whether long-term aging of the passive film will result in time-dependent changes in the corrosion behaviour. Corrosion potentials typically ennoble with time, a possible result of film thickening, defect annealing, or other structural or chemical changes to the passive film. Some passive films are known to crystallise after long exposure periods (e.g., passive films on Ti alloys, Mattsson and Olefjord 1990, Mattsson et al. 1990), resulting in a less-protective film and possible increase in corrosion rate. The composition and protectiveness of passive films is also generally a strong function of temperature, with the degree of hydration in particular affected by temperature. The question of the long-term stability of passive films remains an area of some uncertainty. Overall, however, the evidence from the literature is that the general corrosion rate of duplex stainless steels is similar to, and possibly slightly lower than, that of the common
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300-series austenitic alloys 304L and 316L. On this basis, there is no apparent advantage to choosing duplex alloys over austenitic stainless steels for the waste container material.
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the pitting resistance equivalent number (PRE or PREN) that accounts for the effect of alloy composition on localised corrosion susceptibility; the critical pitting (CPT) or crevice (CCT) temperature, an experimentally determined threshold temperature dependent on the composition of the test solution; and comparison of the breakdown or repassivation potential for pitting or crevice corrosion and the corrosion potential ECORR. The PREN method is useful for comparing the effect of composition, particularly the beneficial effects of increasing Cr, Mo, and N content of the alloy. The general form of the expression is PREN = %Cr + a%Mo + b%N (1)
The values of a and, in particular, b tend to vary depending on whether corrosion takes the form of pitting or crevice corrosion, on the nature of the environment, and to some degree of the alloy family (Pettersson and Flyg 2004). The value of a tends to be the same for austenitic and duplex alloys and commonly falls in a narrow range between 3 and 3.33, but with extreme values reported of 2.4 to 5.0. There is much greater variability in the b parameter, which depends strongly on the nature of the corrosion and of the environment. The value is typically higher for duplex alloys (having a value of ~30), with a value of between ~10 and 30 for austenitic alloys (generally 16, Szklarska-Smialowska 2005), although some authors have reported the reverse (i.e., 30 for austenitic and 16 for duplex alloys, Oberndorfer et al. 2004). Examination of the nominal alloy compositions in Table 1 indicates that the duplex stainless steels will generally be more resistant to localised corrosion based on the PREN than the equivalent austenitic alloy. For alloys with similar Mo contents (e.g., type 304L and 2304 or type 316L and 2205), the duplex steel will exhibit a higher PREN because of the higher Cr content and, particularly, the presence of N. The common type 304L and 316L austenitic alloys have PREN values in the range 18-25, whereas the duplex alloys have PREN values of 30-35. Super-duplex stainless steels are alloys with PREN values >40. The predicted superior resistance of the duplex alloys is also evident from the critical pitting and crevice temperatures. Figure 5 shows CCT and CPT values (measured in 10% ferric chloride solution according to the ASTM G48 test method) for various austenitic, superaustenitic, duplex, and super-duplex alloys (IMOA 2001). The CCT and CPT values are consistently lower for austenitic stainless steels of a given Mo content, reflecting the beneficial effect of both the higher Cr content and the addition of N to the duplex alloys. Mechanistically, pitting or crevice corrosion should only occur if the ECORR exceeds the critical potential for localised attack. Although the breakdown potential (EP or ECREV for
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pitting and crevice corrosion, respectively) is strictly speaking the potential at which localised corrosion initiates, the corresponding re-passivation potentials (ERP and ERCREV for pitting and crevice corrosion, respectively) are often used as conservative measures of initiation. (The re-passivation potential is the potential below which a propagating pit or crevice will cease to grow). The criterion for localised corrosion is then ECORR > EP, ECREV based on a film breakdown criterion, or ECORR > ERP, ERCREV (2b) (2a)
based on re-passivation. Figure 6 illustrates the concept for the pitting of 316L stainless steel in Cl- solutions at 95oC (Dunn et al. 1996). Pitting is possible (based on a comparison of ECORR and ERP) in aerated solution, but not in deaerated solutions. Film breakdown potentials (Eb) have been reported for type 316L and 2205 duplex stainless steels in static seawater (Neville and Hodgkiess 1996). Although the duplex alloy is clearly superior at ambient temperature, the greater temperature sensitivity of alloy 2205 results in a similar susceptibility for the two alloys at temperatures >40oC (Figure 7). The difference between Eb and ECORR is a measure of the susceptibility to localised corrosion. Both Eb and (Eb - ECORR) decrease rapidly with temperature for alloy 2205 below 20oC, whereas the 316L alloy exhibits relatively little temperature dependence. These data suggest that the superiority of duplex over austenitic alloys may be lost at elevated temperatures. It is found that the PREN, CCT and CPT, and critical potentials for pitting and crevice corrosion are related. Figure 8 shows the correlation between PREN and critical pitting temperature (Oberndorfer et al. 2004) and Figure 9 shows a similar dependence between PREN and EP. Note in the latter figure that the duplex (2205) and super-austenitic (20Cb3, 254, 3127) alloys exhibit pitting potentials that are 300-400 mV more positive than those for the 300-series austenitic alloys, again illustrating the beneficial effects of elevated Cr and added N. The threshold conditions for localised corrosion can also be expressed in terms of corrosion maps that define environments in which pitting or crevice corrosion can and cannot be expected. Figure 10 shows one such map for 316L, duplex alloy 2205, and super-austenitic alloys 904L and 254 SMO, defining the threshold temperature for pitting or crevice corrosion as a function of Cl- concentration. The duplex stainless is clearly superior to the austenitic 316L alloy in respect to both pitting and crevice corrosion. It is interesting to note that the Cl- concentration dependence for pitting is greater than that for crevice corrosion for all alloys. This figure refers specifically to O2-saturated systems and would show greater corrosion resistance for lower O2 concentrations, although the relative ranking of the alloys
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would be expected to be the same. Figure 11 shows a similar corrosion map for pitting and SCC as a function of Cl- concentration and temperature (Oberndorfer et al. 2004). Corrosion maps and measurements of the CCT and CPT can be used to assess whether the alloys are immune to localised corrosion at a given service temperature, regardless of the Clconcentration. This type of assessment is useful because it is generally difficult to predict the maximum Cl- concentration that could be generated as a result of evaporation processes, for example during the long-term atmospheric storage period. The data in Figures 5, 8, 10, and 11 clearly show that the duplex alloys 2304 and 2205 exhibit greater resistance to localised corrosion than the corresponding austenitic alloys 304L and 316L. However, based on the CCT and CPT data, it is not possible to claim that these duplex alloys would be immune to crevice or pitting attack during the atmospheric storage phase (maximum temperature 50oC). For example, based on the data in Figure 5, the CPT in 10 wt.% ferric chloride solution (ASTM G48 standard test solution) is 20oC and 33oC for the 2304 and 2205 duplex alloys, respectively, and 3oC and 10oC for the 304L and 316L austenitic alloys, respectively. The critical crevice temperature values are 20-25oC lower. Although the conditions of the ASTM G48 procedure are very aggressive, the data in Figure 10 support the conclusion that even the more-resistant 316L and 2205 alloys would be susceptible to localised corrosion at 50oC at Cl- concentrations of a few thousand ppm, which could conceivably be formed on the surface of a container by repeated dripping/evaporation cycles during the long-term atmospheric storage period. In order to claim immunity to localised corrosion during long-term storage, it would be necessary to consider the use of a super-austenitic or super-duplex alloy containing 4-6 wt.% Mo (e.g, 904L or 254 SMO austenitic grades or 2507 duplex). Although the PREN and critical pitting and crevice corrosion temperatures are useful measures of relative susceptibility, they do not indicate the important role of redox conditions in localised corrosion. The concentration of dissolved O2 (or that of other oxidants present in the system, e.g., Fe(III), radiolytic oxidants, H+, etc.) determines the value of ECORR, the more positive the value the more likely localised corrosion is to occur. In this sense, the most useful indicator of localised corrosion is that based on a comparison of ECORR and EP/ECREV or ERP/ERCREV (Equation (2)). As clearly shown in Figure 6, pitting of 316L stainless steel in O2-containing Cl- solutions is possible in aerated solution, but not under anaerobic conditions. Measurement of ECORR as a function of O2 concentration (or the concentration of other oxidants) would then allow a critical redox potential to be defined below which localised corrosion would not be expected at a given Cl- concentration. In general, however, neither the austenitic nor duplex alloys would be expected to be susceptible to localised corrosion under anaerobic conditions. Bertolini et al. (1996) studied the localised corrosion of austenitic, martensitic, and duplex stainless steels in simulated cement grout porewater solution containing Cl- ions. Based on a
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conservative estimate of the ECORR value for stainless steel in concrete structures of +200 mVSCE, the authors concluded that 304, 304L, 316, 316L, and 2304 alloys would not be susceptible to localised attack in cement environments at 20oC. However, results at 40oC suggested possible localised attack for the austenitic alloys at Cl- concentrations between 4 and 6 wt.%, with attack of the duplex 2304 alloy in 6 wt.% Cl-. It is interesting to note that the higher Cr and N content of the 2304 alloy provides greater resistance to localised corrosion than the Mo added to 316/316L stainless steel. The susceptibility of stainless steels to localised corrosion decreases with increasing pH primarily because of the increase in the threshold potential for pitting (Figure 12). The pitting potential is found to increase significantly at pH >10 for 316 stainless steel and pH 11.5 for the 304 alloy. The resistance of stainless steels to localised corrosion in alkaline solutions typical of the pore water in cement grout is confirmed by tests performed in the Nirex/NDA program. Smart (2002) reports no pitting of 304, 304L, 316, or 316L austenitic stainless steels in cements containing up to 10 wt.% Cl-. The stifling of localised corrosion is currently an area of active research in the corrosion community. The term stifling is used here to describe the observation that the rate of localised penetration decreases with time. Various factors may account for the observation that the rate of propagation of pits and crevices slows with time, including: the ohmic drop in deep pits which shifts the potential at the bottom of the pit or crevice below the repassivation potential; decreasing cathode:anode surface area ratio; mass-transport limitation of aggressive species to the bottom of actively growing pits and crevices; loss of critical pit or crevice chemistry; preferential migration of inhibitive anions into the pit or crevice; or passivation of actively growing pits or crevices due to enrichment of the alloying elements stable at low pH. Of these various factors, two are worthy of more discussion. First, the issue of the necessary cathode:anode surface area ratio is of interest for localised corrosion under atmospheric conditions. In unsaturated systems, the extent of crevice or pit propagation can become cathodically limited since the aerial extent of the cathode is limited because of the limited water availability in unsaturated atmospheres. Mathematical models can be developed to estimate under what conditions a pit or crevice might be anodically or cathodically limited (Cui et al. 2005, Kelly et al. 2006). Factors such as the high resistivity of thin liquid layers
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and suppression of the rate of O2 reduction due to the increase in pH in the cathodic electrolyte can serve to inhibit localised corrosion under atmospheric conditions. The second area of interest here is the role of different alloying elements in promoting passivation within the acidic pit or crevice environment. Typically, alloying elements that are soluble in the acidic chloride environment within the pit or crevice will sustain localised corrosion, whereas those that are less soluble will promote stifling through the formation of either a salt film or protective oxide. The solubility of chromium is affected less by the presence of Cl- than for other metal ions (Strehblow 1995). Molybdenum is relatively These insoluble in acidic solutions (Pourbaix 1974) and there is an apparent synergistic effect of N and Mo in promoting passivity of stainless steels (Clayton and Olefjord 1995). combined effects might result in enrichment of Cr and Mo inside the crevice or pit, which could then promote passivation. These apparently beneficial properties of Cr, Mo, and N in suppressing pit and crevice propagation suggest that duplex alloys might be more prone to the stifling of localised corrosion than the corresponding austenitic alloys. Overall, there is substantial evidence and sufficient mechanistic understanding to support the position that duplex stainless steels, such as 2304 and 2205, are more resistant to localised corrosion than the corresponding austenitic alloys, such as 304(L) and 316(L).
of Cr in the austenite grain. More significant Cr depletion can occur at austenite-austenite grain boundaries in duplex alloys and elongated austenite grains should be avoided if possible. Femenia et al. (2003, 2004) have reported small galvanic differences between the austenite and ferrite particles in duplex stainless steels. A difference in the Volta potential (the potential of the surface measured in air related to the surface work function, and thought to be linearly related to the corrosion potential measured in solution) of 20-70 mV was reported, with the ferrite grains being more active and the austenite grains more noble. This potential difference was interpreted as indicating possible micro-scale galvanic coupling between the different crystallographic phases which, presumably, could lead to some degree of non-uniform corrosion. The magnitude of this possible micro-scale galvanic coupling and the consequences for the corrosion behaviour of duplex alloys is uncertain. Austenitic alloys, being single phase, would not exhibit this behaviour. Duplex alloys are clearly superior to austenitic stainless steels in terms of resistance to grain boundary sensitisation and IGA. However, the use of low-carbon or carbide-stabilised austenitic grades can sufficiently reduce the susceptibility of austenitic alloys to this form of corrosion.
120oC, the threshold stress for types 316, 2205, and 2507 stainless steels are approximately 4%, 35%, and 75% of the yield strength, respectively. Combined with the higher strength of duplex alloys, the absolute threshold stress value for the duplex stainless steels is significantly higher than for the lower-strength austenitic alloys. Various mechanisms have been proposed for the SCC of duplex stainless steels under the aggressive conditions where cracking is observed (Kangas and Nicholls 1995). Tsai and Chou (2000) report both SCC and hydrogen-assisted cracking of 2205 duplex stainless steel in an aggressive 26 wt.% Cl- solution at pH 2 (Figure 14). The ferrite phase was found to be more susceptible to crack initiation and growth than the austenite grains. Both austenitic and duplex stainless steels are susceptible to SCC in sulphide- and thiosulphate-containing environments. The susceptibility of 304L and 316L austenitic stainless steels has been determined in simulated cementitious environments containing chloride and/or thiosulphate ions for the Nirex/NDA program (Smart 2002). Thiosulphate could be produced in the repository by the microbial reduction of sulphate or be released from cement containing blast-furnace slag. The severity of cracking increased with decreasing pH and with increasing thiosulphate and/or chloride concentrations. Thiosulphate enhances the effect of Cl-. The susceptibility of duplex alloys has not tested over the same range of conditions, but less-severe cracking might be expected given the enhanced resistance of duplex alloys to chloride-induced SCC and their greater pitting resistance, pits often acting as initiation sites for stress corrosion cracks. Overall, there appears to be an advantage to the selection of duplex stainless over the austenitic alloys because of their greater resistance to Cl- SCC. This increased resistance is characterised by higher threshold temperatures for SCC at a given chloride concentration and a higher threshold stress for crack initiation and stress intensity factor for crack growth.
threshold, the diversity of the microbial population decreases with decreasing %RH. In many microbially active systems, it is the diversity of the microbial population that is important, as this diversity allows the consortium to survive adverse external stressors and to act symbiotically. The diversity of the microbial population will be further diminished in cementitious environments as most microbes are not active in alkaline environments at pH >10. Stainless steels are susceptible to MIC in active microbial environments (Little et al. 1991). Although stable Cr-rich passive films minimise the impact of microbial activity (Lloyd et al. 2005), the slight difference in Cr content of the austenitic and duplex stainless steels is insufficient to markedly differentiate their resistance to MIC. The greater overall resistance of the duplex alloys to localised corrosion should translate into a corresponding improved resistance to corrosion under biofilms. However, a deeper understanding of the effect of the expected disposal environments on microbial activity (and, if there is any, on MIC) is warranted.
0.4W, 0.23N; S31254: 19.8Cr, 17.8Ni, 0.013C, 0.51Si, 0.55Mn, 0.023P, 0.001S, 0.62Cu, 6.12Mo, 0.19N). The corrosion observed by Tani and Mayuzumi (2007) is the result of the deliquescence of MgCl2, and possibly CaCl2, because the major component of evaporated seawater (NaCl) does not deliquesce under these test conditions. Figure 16 shows the deliquescence behaviour of a number of chloride salts. Magnesium chloride deliquesces at a relative humidity of ~20-35% at temperatures between 0 and 100oC. Sodium chloride deliquesces at much higher %RH and would not have formed an aqueous phase for the test conditions in Figure 15 (35% RH). It is also interesting to note that although the fractional corroded area increased with temperature for the S31254 and S31260 alloys, this is despite the fact that the adsorbed water film would have been more dilute (as indicated by the decrease in deliquescence %RH with increasing temperature for MgCl2). The architectural applications of austenitic stainless steels have been summarised by Smart and Wood (2004). The use of duplex stainless steels for architectural applications is relatively new and there are few reports in the literature of the extent of atmospheric corrosion following extended periods in service. However, these alloys are being used for architectural purposes, especially in bridge construction (Figure 17). Type 2205 duplex stainless steel was also selected for the upper 120 metres of the towers of the Stonecutters Bridge in Hong Kong harbour (Figure 18). This alloy was selected because of its superior strength compared with austenitic alloys combined with its excellent corrosion resistance.
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be greater than 1 Gy/h for only a limited period of time. Therefore, since the maximum external surface dose rate is ~3 orders of magnitude lower than the threshold above which any effect has been observed, it is concluded that the effects of radiolysis on waste container corrosion can be neglected. (It is assumed that the dose rate for internal surfaces of the waste container are also below the observed threshold for radiolysis effects.)
3.10
Galvanic Corrosion
Galvanic corrosion may result from contact between metallic waste forms and the internal surface of the waste container. Passive stainless steels are noble relative to most of the potential metallic waste forms, such as aluminium, and carbon- and low-alloy steels. In a galvanic couple, therefore, the stainless steel will be the cathode and the waste form will be
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the anode, the dissolution of which will effectively cathodically protect the stainless alloy. There is little difference between the relative positions of austenitic and duplex stainless steels in the galvanic series. There are two possible mechanisms by which galvanic coupling could impact the service life of the waste package. First, under aerobic conditions, the oxidation and hydrolysis of dissolved corrosion products can lead to locally acidic and oxidising conditions resulting in an increased probability of localised corrosion of the stainless steel. In the case of the coupling of carbon and stainless steels, the reactions of interest are: Fe Fe(II) + 2e4Fe(II) + O2 + 2H2O 4Fe(III) + 4OHFe(III) + 3H2O Fe(OH)3 + 3H+ (3a) (3b) (3c)
with the cathodic reduction of O2 occurring remotely on the stainless steel surface. Thus, locally aggressive acidic ferric environments can be produced at the point of contact between the carbon and stainless steels. Apart from carbon steel waste forms, this issue can also be caused by the use of carbon or low alloy steel tools for fabrication and handling of the stainless steel containers (so-called smeared- or embedded-iron corrosion), the use of which should be controlled during repository operations. The second mechanism of concern is the hydrogen embrittlement of stainless steel coupled to a more active metal. Because the stainless steel is cathodically polarised by the galvanic couple, the reduction of H2O and the possible absorption of atomic hydrogen can occur under anaerobic conditions. If sufficient hydrogen is absorbed then the material can become embrittled. This problem would tend to be of more concern for the higher-strength duplex alloys, since the potentially higher stress levels would tend to concentrate hydrogen at defects in the material. On the basis of the data in Figure 14, hydrogen-assisted cracking of alloy 2205 occurs at potentials more negative than 800 mVSCE, although a more-negative limit of 900 mVSCE has been proposed by Zucchi et al. (2006) in acidified seawater (pH 6.5). Regardless of the precise potential threshold, such a limit is unlikely to be exceeded by galvanic coupling with carbon steel since the ECORR for the latter material in anaerobic natural waters is approximately 750 mVSCE. Hydrogen embrittlement of the stainless steel container is only possible, therefore, if it becomes galvanically coupled to Al, Mg, or galvanised alloys or, possibly, carbon steel in the presence of sulphide ions. The potential for galvanic corrosion of the stainless steel waste containers is considered to minimal. Even if a galvanic couple was established, it is unlikely to be sufficiently longlived that extensive damage would result. However, it is good design practice to avoid such a possibility in the first place, through appropriate waste form loading and care with the use of carbon steels tools during manufacture and handling.
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Behaviour of Austenitic and Duplex Stainless Steels under Phased Geological Repository Conditions
Based on the above discussion, the expected behaviour of austenitic and duplex stainless steels during various stages of the Phased Geological Repository Concept is summarised in Table 4 (based on the various disposal phases identified by Nirex 2003). The expected rates of general corrosion have not been specifically defined in Table 4, but are expected to be approximately the same for both families of alloys and to be of the order of magnitude defined by Smart et al. (2006). Table 4 does not distinguish between the different grades of austenitic or duplex stainless steels. In general, type 304(L) stainless steel will be more susceptible to localised corrosion and SCC than the 316(L) alloy. The use of the low-carbon grade of each alloy will minimise any differences between the two in terms of sensitisation-induced IGA. For the duplex alloys, the higher Mo content of the 2205 alloy provides greater resistance to localised corrosion and SCC compared with the 2304 stainless steel, despite the higher Cr content of the latter. As discussed in more detail in the previous section, the advantages of the duplex alloys over the austenitic alloys from a corrosion viewpoint are: lower susceptibility of duplex alloys to sensitisation-induced intergranular attack; increased resistance of duplex alloys to localised corrosion due to the higher N content and, in some cases, higher Cr and Mo contents than corresponding austenitic stainless steels, although this benefit is reduced at temperatures >50oC; significantly improved resistance to chloride-induced SCC, with possible immunity of duplex grades at temperatures <100oC; and higher threshold stress for SCC for duplex alloys which, combined with the significantly higher strength of duplex materials, results in a lower susceptibility to SCC caused by mechanical damage during storage and handling. Given these advantages of duplex alloys, the most significant difference between the behaviour of austenitic and duplex stainless steel waste containers would be expected (i) on external surfaces during storage and (ii) for internal surfaces in incompletely or non-grouted containers soon after backfilling and in the initial, aerobic, stages of repository saturation. In the latter case, however, duplex stainless steels appear to lose their enhanced resistance to localised corrosion at elevated temperatures (>50oC), so their advantage may be restricted to their lower susceptibility to SCC.
25
Although there are some advantages to the use of duplex alloys over austenitic grades, the expected environmental conditions are such that austenitic stainless steels should perform adequately during the various stages of the Phased Geological Repository Concept. Both families of material offer good corrosion resistance under atmospheric exposure conditions and the presence of cement grout backfill should ensure continued excellent performance following repository closure. Duplex alloys provide an extra degree of corrosion resistance during those periods when the probability of localised corrosion and SCC is highest.
26
A review has been carried out of the corrosion performance of austenitic and duplex stainless steels as container materials for the storage and disposal of ILW as part of the Phased Geological Repository Concept. Two grades have been selected as representative of each family, namely 304(L) and 316(L) austenitic stainless steels and 2304 and 2205 duplex alloys. Various forms of corrosion have been considered, including: general corrosion, localised corrosion in the form of pitting and crevice corrosion, sensitisation-induced intergranular attack, stress corrosion cracking, microbiologically influenced corrosion, atmospheric corrosion, the effects of radiolysis and welding, and galvanic corrosion. Compared with austenitic stainless steels, the duplex alloys provide significantly higher strength, greater resistance to chloride-induced SCC, and at temperatures up to ~50oC, improved resistance to localised corrosion. These advantages should provide additional assurance of acceptable waste package performance during various stages of the Phased Geological Repository Concept, most notably during the period of surface and belowground storage and during the transient aerobic phase immediately following backfilling and closure of the repository. However, the design of the repository concept and the selection of engineered barriers should ensure acceptable waste package performance for either austenitic or duplex grades of stainless steel. In particular, the presence of cement grout should protect both the internal and, eventually, the external surfaces of the containers from localised corrosion and SCC.
27
References
Alexander, A.L., C.R. Southwell, and B.W. Forgeson. 1961. Corrosion of metals in tropical environments, part 5 stainless steel. Corrosion 17, 345. Anonymous. 2003. Nickel 18(2), p. 5. ASM. 1987. Metals Handbook, Ninth edition, Volume 13, Corrosion. American Society for Metals International, Metals Park, OH. ASM. 2003. ASM Handbook, Volume 13A, Corrosion: Fundamentals, Testing, and
Protection. American Society for Metals International, Metals Park, OH. ASM. 2005. ASM Handbook, Volume 13B, Corrosion: Materials. American Society for Metals International, Metals Park, OH. Bertolini, L., F. Bolzoni, T. Pastore, and P. Pedeferri. 1996. Behaviour of stainless steel in simulated concrete pore solution. Br. Corros. J. 31, 218-222. Blackwood, D.J., L.J. Gould, C.C. Naish, F.M. Porter, A.P. Rance, S.M. Sharland, N.R. Smart, M.I. Thomas, and T. Yates. 2002a. The localised corrosion of carbon steel and stainless steel in simulated repository environments. AEAT/ERRA 0318, Dec 2002. Blackwood, D.J., C.C. Naish, S.M. Sharland and A.M. Thompson. 2002b. An experimental and modelling study to assess the initiation of crevice corrosion in stainless steel containers for radioactive waste. AEAT-ERRA-0300, 2002. Blanco, G., A. Bautista, and H. Takenouti. 2006. EIS study of passivation of austenitic and duplex stainless steels reinforcements in simulated pore solutions. Cement Concrete Composites 28, 212-219. Brown, A.D. 1990. Microbial Water Stress Physiology. John Wiley, Chichester, U.K. BSC (Bechtel SAIC Company). 2004a. Aqueous corrosion rates for waste package materials. Prepared for US DOE, ANL-DSD-MD-000001, Oct 2004. BSC (Bechtel SAIC Company). 2004b. In-drift precipitates/salts model. Report prepared for the U.S. DOE by Bechtel SAIC Company, ANL-EBS-MD-000045 Rev 02. Casteels, F., G. Dresselaars, and H. Tas. 1986. Corrosion behaviour of container materials for geological disposal of high level waste. Communities Report, EUR 10398 EN, p. 3-40. Clayton, C.R. and I. Olefjord. 1995. Passivity of austenitic stainless steels. In Corrosion Mechanisms in Theory and Practice, P. Marcus and J. Oudar (eds.), Marcel Dekker (New York, NY), pp. 175-199. Commission of the European
28
Cui, F., F.J. Presuel-Moreno, and R.G. Kelly. 2005. Computational modeling of cathodic limitations on localized corrosion of wetted SS 316L at room temperature. Corrosion Sci. 47, 2987-3005. Dechema. 1990. Corrosion Handbook: Corrosive Agents and their Interaction with
Materials, Volume 7 Atmosphere. Dunn, D.S., G.A. Cragnolino, and N. Sridhar. 1996. Localized corrosion initiation,
propagation, and repassivation of corrosion resistant high-level nuclear waste container materials. In Proc. CORROSION/96, NACE International (Houston, TX), paper no. 97. Femenia, M., C. Canalias, J. Pan, and C. Leygraf. steels. J. Electrochem. Soc. 150, B274-B281. Femenia, M., J. Pan, and C. Leygraf. 2004. Characterization of ferrite-austenite boundary region of duplex stainless steels by SAES. J. Electrochem. Soc. 151, B581-B585. Fujisawa R., T. Kurashige, U. Inagaki, and M. Senoo. 1999. Gas generation behavior of transuranic waste under disposal conditions. Mat. Res. Soc. Symp. Proc. Vol. 556. Fukaya, Y. and M. Akashi. 2003. Estimation of the cathodic hydrogen evolution rate on radioactive waste packaging materials. International (Houston, TX), paper no. 03680. Garzn, C.M., C.A. Serna, S.D. Brandi, and A.J. Ramirez. 2007. The relationship between atomic partitioning and corrosion resistance in the weld-heat affected zone microstructure of UNS S32304 duplex stainless steel. J. Mater. Sci. 42, 9021-9029. Hoch, A.R., C.C.Naish, S.M. Sharland, A.C. Smith, and K.J. Taylor. 1992. Experiments And Modeling Studies Concerning Localised Corrosion Of Carbon Steel And Stainless Steel Containers For Intermediate And Low Level Radioactive Waste, Materials Research Society Symposia Proceedings, Nov 30, 1992. IMOA (International Molybdenum Association). fabrication of duplex stainless steels. London, U.K. Johnson, M.J. and P.J. Pavlik. 1982. Atmospheric corrosion of stainless steels. In 2001. Practical guidelines for the International Molybdenum Association, In Proc. CORROSION/2003, NACE 2003. Scanning Kelvin probe force
Atmosphere Corrosion, W.H. Ailor (Ed.), p. 461. Kangas, P. and J.M. Nicholls. 1995. Chloride-induced stress corrosion cracking of duplex stainless steels. Models, test methods and experience. Materials and Corrosion 46, 354-365.
29
Kearns, J.R., M.J. Johnson, and I.A. Franson. 1984. The corrosion of stainless steels and nickel alloys in caustic solutions. In Proc. CORROSION/84, NACE International (Houston, TX), paper no. 146. Kelly, R.G., A. Agarwal, F. Cui, Xi Shan, U. Landau, and J. H. Payer. 2006. Considerations of the role of the cathodic region in localized corrosion. In Proc. 11th International High-level Radioactive Waste Management Conference, American Nuclear Society (La Grange Park, IL). King, F. 2006. Review and gap analysis of the corrosion of copper containers under
unsaturated conditions. Ontario Power Generation, Nuclear Waste Management Division Report 06819-REP-01300-10124-R00. Little, B., P. Wagner, and F. Mansfeld. 1991. Microbiologically influenced corrosion of metals and alloys. Int. Mater. Rev. 36, 253-272. Liou, H.-Y., R.-I. Hsieh, and W.-T. Tsai. 2002. Microstructure and stress corrosion cracking in simulated heat-affected zones of duplex stainless steels. Corrosion Sci. 44, 28412856. Lloyd, A.C., R.J. Schuler, J.J. Nel, D.W. Shoesmith, and F. King. 2005. The influence of environmental conditions and passive film properties on the MIC of engineered barriers in the Yucca Mountain Repository. In Scientific Basis for Nuclear Waste Management XXVIII, J.M. Hanchar, S. Stroes-Gascoyne, and L. Browning (eds.), Mat. Res. Soc. Symp. Proc. 824 (Materials Research Society, Warrendale, PA), pps. 3-9. Malik, A. U., N. A. Siddiqi, S. Ahmad, and I. N. Andijani. 1996. The effect of dominant alloy additions on the corrosion behavior of some conventional and high alloy stainless steels in seawater. Corrosion Sci. 37, 1521-1535. Mattsson, H. and I. Olefjord. 1990. Analysis of oxide formed on Ti during exposure in bentonite clay. I. The oxide growth. Werk. Korros. 41, 383-390. Mattsson, H., C. Li, and I. Olefjord. 1990. Analysis of oxide formed on Ti during exposure in bentonite clay. II. The structure of the oxide. Werk. Korros. 41, 578-584. Mcdonald, D.B., M.R. Sherman, D.W. Pfeifer, and Y.P. Virmani. Number 14034, May 1995. McIntyre, D.R. 1987. Experience survey stress corrosion cracking of austenitic stainless steel in water. Materials Technology Institute, St, Louis, MO. MTI Publication No. 27. 1995. Stainless steel
30
Morsy, S.M., S.M. El-Raghy, A.A. Elsaed, and A.E. El-Mehairy. 1979. Effect of cations on the corrosion of stainless alloys in saline water at elevated temperatures. Arab Republic of Egypt Atomic Energy Establishment Report 232. Naish, C.C., D.J. Blackwood, K.J. Taylor, and M.I. Thomas. 1995. The anaerobic corrosion of stainless steels in simulated repository backfill environments. NSS/R307. Neville, A. and T. Hodgkiess. 1996. An assessment of the corrosion behaviour of highgrade alloys in seawater at elevated temperature and under a high velocity impinging flow. Corrosion Sci. 38, 927-956. Newman, R.C. 1995. Stress-corrosion cracking mechanisms. In Corrosion Mechanisms in Theory and Practice, P. Marcus and J. Oudar (eds.), Marcel Dekker (New York, NY), pp. 311-372. Newman, R.C. 1030-1041. Nirex. 2003. Generic Disposal System Specification. UK Nirex Report no. 075 (Vol. 1 and 2). Nirex. 2005. Generic Waste Package Specification. Volume 1 Specification. UK Nirex Report no. N/104. Oberndorfer, M., K. Thayer, and M. Kstenbauer. 2004. Application limits of stainless steels in the petroleum industry. Materials and Corrosion 55, 174-180. Pettersson, R.F.A. and J. Flyg. 2004. Electrochemical evaluation of pitting and crevice corrosion resistance of stainless steels in NaCl and NaBr. Outokumpu report, ACOM 3-2004. Pourbaix, M. 1974. Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd. edition. NACE International, Houston, TX. Rard, J.A., K.J. Staggs, S.D. Day, and S.A. Carroll. 2005. Boiling temperature and reversed deliquescence relative humidity measurements for mineral assemblages in the NaCl + NaNO3 + KNO3 + Ca(NO3)2 + H2O system. Laboratory Report, UCRL-JRNL-217704. Revie, R.W. 2000. Uhligs Corrosion Handbook. Second edition. John Wiley, New York, NY. Rubinshtejn, F.I., D.E. Lashchevskaya, E. Rubinshtejin, L.M. Mamontava, and V.A. Gulyaev. 1977. Accelerated testing of the corrosion resistance of chromium nickel steels in
31
Nirex Report
2001.
Corrosion 57,
model environments simulating the manufacturing conditions of the synthetic resin KFE. Mat. Primen. No. 3, p. 59. Sedriks, A.J. 1996. Corrosion of Stainless Steels. Second Edition. John Wiley, New York, NY. Sharland, S.M. 1991. Proceedings Nuclear Waste Packaging FOCUS91. American Nuclear Society, La Grange Park, Illinois, 60525, USA, pp. 233-240. Shoesmith, D.W. and F. King. 1999. The effects of gamma radiation on the corrosion of candidate materials for the fabrication of nuclear waste packages. Atomic Energy of Canada Limited Report, AECL-11999. Shreir, L.L., R.A. Jarman, and G.T. Burstein. 1994. Corrosion, Third Edition. ButterworthHeinemann, Oxford, UK. Smart, N.R. 2000. Atmospheric corrosion of stainless steel waste containers during storage. AEA Technology Report, AEA-TPD-262, issue C. Smart, N.R. 2002. Review of effect of chloride in cementitious environments on corrosion of stainless steels. Serco Assurance Report for UK Nirex Limited, SA/SIS/14921/R001. Smart, N. 2004. Workshop on atmospheric corrosion of stainless steel. U.K. Nirex Limited, Nirex Technical Note, document number 443937. Smart, N.R. 2005. Atmospheric pitting corrosion of stainless steel radioactive waste
containers. Serco Assurance report to Nirex, Report SA/EIG/14921/C0050. Smart, N.R. and P. Wood. 2004. Corrosion resistance of stainless steel radioactive waste packages. U.K. Nirex Limited, Nirex Report N/110. Smart, N.R., D.J. Blackwood, G.P. Marsh, C.C. Naish, T.M. OBrian, A.P. Rance, and M.I. Thomas. 2004. The Anaerobic Corrosion Of Carbon And Stainless Steels In Simulated Repository Environments: A Summary Review Of Nirex Research. AEAT/ERRA-0313. Smart, N.R., C.C. Naish, and A.M. Pritchard. 2006. Corrosion principles for the assessment of stainless steel radioactive waste containers. Serco Assurance report to Nirex, Report SA/EIG/14921/C010. Souto, R.M., I.C. Mirza Rosca, and S. Gonzlez. 2001. Resistance to localized corrosion of passive films on a duplex stainless steel. Corrosion 57, 300-306.
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Strehblow, H.-H. 1995. Mechanisms of pitting corrosion. . In Corrosion Mechanisms in Theory and Practice, P. Marcus and J. Oudar (eds.), Marcel Dekker (New York, NY), pp. 201-237. Szklarska-Smialowska, Z. Houston, TX. Tani, J.I. and M. Mayuzumi. 2007. Stress corrosion cracking of stainless steel canister for dry storage of spent fuel using concrete cask. Extended abstract in Proc. 3rd Int. Workshop on Long-term Prediction of Corrosion Damage in Nuclear Waste Systems, Pennsylvania State University, State College, PA, May 14-18, 2007. To be submitted to J. Nucl. Mater. Tsai, W.-T. and S.-L. Chou. 2000. Environmentally assisted cracking behavior of duplex stainless steel in concentrated sodium chloride solution. Corrosion Sci. 42, 1741-1762. Wada, R. and T. Nishimura. 1999. Experimental study of hydrogen gas generation rate from corrosion of Zircaloy and stainless steel under anaerobic alkaline condition. Radioactive Waste Management and Environmental Remediation ASME 1999. White, J.H., A.E. Yanic, and H. Schick. 1966. The corrosion of metals in the water of the Dead Sea. Corros. Sci. 6, 447. Zucchi, F., V. Grassi, C. Monticelli, and G. Trabanelli. 2006. Hydrogen embrittlement of duplex stainless steel under cathodic protection in acidic artificial sea water in the presence of sulphide ions. Corrosion Sci. 48, 522-530. 2005. Pitting and Crevice Corrosion, NACE International,
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Table 1: Compositions of Common Austenitic and Duplex Stainless Steels1 Composition (wt.%)2 C Mn Si Austenitic alloys 0.08 2.0 0.03 2.0 0.20 2.0 0.08 2.0 0.03 2.0 0.03 2.0 0.08 2.0 0.08 2.0 Duplex alloys 0.03 2.5 0.03 0.04 2.0 2.0
UNS Number
Common name Cr Ni
Other P S
S30400 S30403 S30900 S31600 S31603 S31703 S32100 S34700 S32304 S31803 S32404
1 2
304 304L 309 316 316L 317L 321 347 SAF 2304 2205 Uranus 50
18-20 18-20 22-24 16-18 16-18 18-20 17-19 17-19 21.524.5 21-23 20.522.5
8-10 8-12 12-15 10-14 10-14 11-15 9-12 9-13 3-5.5 4.5-6.5 5.5-8.5
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.040 0.030 0.030
0.030 0.030 0.030 0.030 0.030 0.030 0.030 0.030 0.040 0.020 0.010
Mo 2-3 Mo 2-3 Mo 3-4 Ti 5 x C3 (Nb+Ta) 10 x C3 N 0.05-0.2, Mo 0.05-0.6 N 0.08-0.2, Mo 2.5-3.5 N 0.2, Mo 2-3, Cu 1-2
After Sedriks (1996) Maximum unless otherwise indicated, balance Fe. 3 Minimum
34
UNS Number
Common name
Elongation (%)
Hardness Rockwell B
S30400 S30403 S30900 S31600 S31603 S31703 S32100 S34700 S32304 S31803 S32404
1 2
304 304L 309 316 316L 317L 321 347 SAF 2304 2205 Uranus 50
290 269 310 290 220 262 241 276 4001 520 386
Austenitic alloys 579 558 620 579 517 593 620 655 Duplex alloys 6001 760 600
55 55 45 50 50 55 45 45 251 27 25
80 79 85 79 79 85 80 85 972 83
35
Table 3: Literature General Corrosion Rates for Austenitic Stainless Steels. (a) Highly Alkaline Solutions. Grade 304 304 304 pH 12.8 10.5 13.3 13 Temperature (oC) 30, 45 Ambient 30 50 80 50 [Cl-] (gg-1) Redox conditions 200 days 60 days 28 days Other Rate (myr-1) 0.0003 0.01 0.3 0.06 0.18 0.82 0.009 0.0055 0.0063 0.055 0.010 0.002 0.0003 0.00006 0.6 0.03 0.4-1.6 1-6 <0.1 Reference Fujisawa et al. 1999 Mcdonald et al. 1995 Blackwood et al. 2002a
18,400
Aerated Deaerated
304
304
Deaerated
230 days
30
Deaerated
Calculated 1 yr 10 yrs 100 yrs 1000 yrs 104 yrs 28 days 0.1 MPa H2 0.1 moldm-3 KOH agar gel, 50 days
18,400 10,000
Mcdonald et al. 1995 Smart et al. 2004 Sharland 1991 Rubinshtejn et al. 1977 Naish et al. 1995
10,000
Deaerated
36
(b) Neutral to Slightly Alkaline Solution. Grade 304 304L 304L 304 pH Ambient Ambient Ambient Ambient Temperature (oC) 90 25-100 27 90 25 50 75 Ambient 30 50-100 27 25 50 75 25-40 25-40 25 80 2,000 [Cl-] (gg-1) 7,000-43,000 Freshwater Saltwater Interstitial clay water Redox conditions Aerated Aerated Aerated Aerated Other 10 hrs Rate (myr-1) 10-130 0.21 11.4 5.82 0.2-0.96 0.22-0.23 0.3-0.35 4 0.01 0.25 1.94 0.1-0.24 0.1-0.34 0.1-0.17 120 days 0.8 0.55 4.8 0.025-0.23 0.01-0.034 Reference Morsy et al. 1979 BSC 2004a BSC 2004a Casteels et al. 1986
Alexander et al. 1961 BSC 2004a BSC 2004a Casteels et al. 1986
Seawater 19,000
37
(c) Under Atmosphere Conditions. Grade 304 Temperature (oC) Redox conditions Other Ambient Aerated Urban, 5-15 yrs Urban, 5-15 yrs Marine, 5-15 yrs Industrial/urban, 5-15 yrs Ambient Industrial/urban Ambient Ambient Aerated Aerated Urban, 5-15 yrs Various atmospheres Reference Rate (myr-1) <0.03 Johnson and Pavlik 1982 0.022 0.05-2 0.01 0.03-3 Kearns et al. 1984 <0.03 0.05 Johnson and Pavlik 1982 Dechema 1990
38
Table 4: Expected Corrosion Behaviour of Austenitic and Duplex Stainless Steels During Various Stages of the Phased Geological Repository Concept. Disposal phase Internal or external surface External Austenitic stainless steels Duplex stainless steels
Internal
Low rate of general corrosion under atmospheric conditions. Possibility of localised corrosion and SCC if humidity is high enough to permit deliquescence of surface contaminants. Very low rate of general corrosion and absence of localised corrosion and SCC for grouted waste packages. Possibility of localised corrosion and SCC for incompletely or nongrouted packages at high relative humidity. Protected from localised corrosion and SCC by alkaline grout pH. Low rate of general corrosion. Protected from localised corrosion and SCC by alkaline grout pH. Low rate of general corrosion. Period of susceptibility to localised corrosion and SCC due to elevated temperature (up to 80oC) and ingress of Cl- from groundwater, although surfaces in contact with cement grout should be immune. Very low rate of general corrosion and absence of localised corrosion or SCC.
Low rate of general corrosion under atmospheric conditions. Greater resistance to localised corrosion and SCC than austenitic alloys if humidity is high enough to permit deliquescence of surface contaminants. Very low rate of general corrosion and absence of localised corrosion and SCC for grouted waste packages. For incompletely or non-grouted packages, lower probability of localised corrosion than for austenitic alloys. Immune to SCC at expected storage temperatures. As for austenitic alloys.
As for austenitic alloys. For incompletely or nongrouted packages, increased temperature renders duplex alloys as susceptible to localised corrosion as austenitic alloys. Similar susceptibility to localised corrosion as austenitic alloys if temperature exceeds critical pitting or crevice corrosion temperature for the particular alloy. Immune to SCC at temperatures <100oC. As for austenitic alloys.
39
Figure 1: Effect of pH and temperature on the passive current density (expressed in terms of a corrosion rate) of 316L stainless steel (Blackwood et al. 2002b).
40
Figure 2: Comparison of the voltammetric behaviour of various austenitic and a duplex stainless steel in saturated Ca(OH)2 solution with the addition of 0.5% NaCl at room temperature (Blanco et al. 2006).
41
Figure 3: Time dependence of the charge-transfer resistance for various austenitic and a duplex stainless steel in saturated Ca(OH)2 solution containing 1% NaCl (Blanco et al. 2006).
Figure 4: Crevice corrosion test on duplex alloy 2205 stainless steel in 1 moldm-3 NaCl illustrating preferential attack on austenite phase (Pettersson and Flyg 2004).
42
Temperature (oC)
317LMN
316L
317L
904L
2304
2205
255
Figure 5: Comparison of the critical crevice and pitting temperatures for a range of austenitic and duplex stainless steels in 10 wt.% ferric chloride solution (after IMOA 2001).
2507
43
6Mo
Figure 6: Comparison of the pitting and pit repassivation potentials for type 316L stainless steel to the corrosion potential ECORR in aerated and deaerated chloride solutions at 95oC (Dunn et al. 1996).
44
1500
Eb 2205 Eb-Ecorr 2205
Potential (mVSCE)
1000
500
-500 10 20 30 40 50 60 70
Temperature (oC)
Figure 7: The temperature dependence of the film breakdown potential and the susceptibility to localised corrosion for types 316L and 2205 stainless steels in static seawater (after Neville and Hoghkiess 1996).
45
Figure 8: Correlation between the PREN and critical pitting temperature (Oberndorfer et al. 2004).
Figure 9: Correlation between the pitting resistance equivalent number and pitting potential for various austenitic, duplex, and super-austenitic stainless steels (Malik et al. 1996).
46
Figure 10: Corrosion map illustrating critical pitting and crevice corrosion temperatures as a function of chloride concentration for various austenitic, duplex, and super-austenitic stainless steels (ASM 2005). Critical conditions for pitting and crevice corrosion indicated by solid and dashed lines, respectively.
Figure 11: Domains of immunity and susceptibility to pitting and stress corrosion cracking as a function of chloride concentration and temperature for austenitic, duplex, and super-austenitic alloys (Oberndorfer et al. 2004).
47
Figure 12: Dependence of the pitting potential on pH for types 304 and 316 stainless steel in 3% NaCl solution (from Sedriks 1996).
48
Figure 13: Corrosion map for the susceptibility of various austenitic and duplex stainless steels to stress corrosion cracking in aerated chloride environments as a function of temperature (Sedriks 1996).
49
Figure 14: Potential dependence of SCC and hydrogen-assisted cracking of 2205 duplex stainless steel in 26 wt.% NaCl solution at pH 2 (Tsai and Chou 2000).
50
RH: 35% -2 Chloride: 10g.m as Cl (Synthetic sea salt) Test time: 500h
S31603
S31254 S31260
0.0 20 (68F)
25 (77F)
30 (86F) Temperature / C
o
35
40 (104F)
Figure 15: Dependence of the fractional surface area of different austenitic and duplex stainless steels corroded in a humid atmosphere as a function of exposure temperature (Tani and Mayuzumi 2007).
51
52
Figure 17: Use of duplex stainless steels for bridge applications (Anonymous 2003).
53
54