Faraday Discussions Cite this: DOI: 10.1039/c5fd00008d
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
PAPER
View Article Online View Journal
Pitting of steam-generator tubing alloys in solutions containing thiosulfate and sulfate or chloride William Zhang, Anatolie G. Carcea and Roger C. Newman*
Received 20th January 2015, Accepted 22nd February 2015 DOI: 10.1039/c5fd00008d
The pitting of nuclear steam generator tubing alloys 600, 690 and 800 was studied at 60 C using dilute thiosulfate solutions containing excess sulfate or (for Alloy 600) chloride. A potentiostatic scratch method was used. In sulfate solutions, all alloys pitted at low potentials, reflecting their lack of protective Mo. The alloys demonstrated the most severe pitting at a sulfate : thiosulfate concentration ratio of 40. Alloy 600 pitted worst at a chloride : thiosulfate ratio of 2000. The results are interpreted through the mutual electromigration of differently charged anions into a pit nucleus, and differences in the major alloy component.
1 Introduction Some corrosion phenomena in nuclear steam generator (SG) tubing have been postulated to occur at low temperature, due to the presence of both oxygen and ‘reduced’ sulfur species (i.e. oxidation state of the sulfur is less than 6). The reduced sulfur could be generated by reaction of sulfate impurity with hydrazine near heat transfer surfaces during SG operation. It is well known that reduced sulfur species are catalytic for the corrosion of iron and nickel, and their alloys; early work focused on pitting of simple stainless steels, such as type 304, in solutions with chloride and various reduced sulfur species.1 Thiosulfate was found to be the most aggressive sulfur anion, much more so than sulde, and the benecial effect of Mo alloying was noted. Later, there were a number of studies of localized corrosion of alloys 600 and 800 in mixed solutions of chloride and thiosulfate, or other reduced sulfur species,2–7 as well as many studies of stress corrosion cracking (SCC) of sensitized Alloy 600 in pure thiosulfate solutions. To our knowledge, there has been no systematic study of the pitting of annealed SG tubing alloys in chloride-free solutions (sulfate plus thiosulfate). This phenomenon was discovered in stainless steel in the early 1980s,8,9 and independently by
University of Toronto, Department of Chemical Engineering and Applied Chemistry, 200 College Street, Toronto, Canada. E-mail: [email protected]; [email protected]; [email protected]; Fax: +1-416-978-8605; Tel: +1-416-946-0604
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Garner in the pulp and paper industry; a number of studies were published in the context of paper machines and microbial corrosion.10,11 Thiosulfate was implicated in practical problems involving fouled fresh-water heat-exchangers,12 although here some chloride was present. In sulfate–thiosulfate pitting, sulfate plays the role of the strong-acid anion – just like chloride in ordinary pitting – that permits, within certain limitations, acidication of the pit cavity by cation hydrolysis. The limitations are the buffer character of sulfate at low pH (pKa z 1.92; evidently this is not a dominant factor, otherwise pitting would not occur), and the tendency of thiosulfate to neutralize the acidity of the pit by disproportionation according to eqn (1): S2O3 + H+ / HSO3 + S
(1)
or by electroreduction as shown in eqn (2): S2O3 + 6H+ + 4e / 2S + 3H2O
(2)
The catalytic effect of adsorbed sulfur (Sads0 or Sadsd) on anodic dissolution of Ni, Fe and NiFe(Cr) alloys is well known from the work of Marcus and others. Essentially, S adsorption in hollow sites weakens the metal–metal bonding in the surface, facilitating dissolution; it also hinders hydrogen adsorption, because the weakening of the metal–metal interaction lowers the hydrogen adsorption energy. On Fe or Ni, S adsorption occurs over a wide range of potential and pH,13,14 and is even believed to occur on Cr, despite the instability of bulk Cr suldes in aqueous solution,15 although no kinetic information is available that shows an aggressive effect of Sads on Cr. Stainless steel is less susceptible than pure Fe or Ni (or FeNi alloys), because the presence of Cr causes some desorption of S.16 This effect is also demonstrated, to a much greater extent, by alloyed Mo, which strongly desorbs sulfur.18 The rst outcome of this, at least for temperatures 99%; sodium thiosulfate was from EM Science, an affiliate of Merck KGaA, Darmstadt, Germany, assay 99.5–100.5%. Sodium chloride was purchased from EMD, assay >99%. The resistivity of the DI water was 18.2 MU cm. A platinum counter electrode and an Hg/Hg2SO4/sat. K2SO4 reference electrode (MSE, 0.65 V vs. SHE at 25 C) were also immersed for the sulfate–thiosulfate experiments; for chloride–thiosulfate, the reference electrode was Ag/AgCl/sat. KCl (0.199 V vs. SHE at 25 C). All experiments reported here used an open beaker as the electrochemical cell, with freshly
Table 1
Compositions of the alloys studied, in wt%
Compositions
Alloy 600
Alloy 690
Alloy 800
Ni Fe Cr C
Balance 9.53 16.1 0.06
Balance 10.1 29.6 0.03
31.2 Balance 20.0 0.07
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
poured solution – the oxidation rate of thiosulfate is slow enough to allow such a procedure, even in highly diluted solutions, which was conrmed by detailed ion chromatography (IC) measurements, shown in Table 2. IC tests were performed using the Dionex ion chronomatographic system (DX500) with a conductivity detector (ED40). A diamond engraving pencil was used for scratching – the scratches were approximately 0.1 mm wide and 9.5 mm long. All the results shown here relate to a solution temperature of 60 C – pitting occurred at 25 C, but it was less stable and required much more effort for interpretation. A similar observation was made in the 1980s on stainless steel – the rst reports of sulfate–thiosulfate pitting used high-purity alloys that were less resistant than ordinary type 304 stainless steel,8,9 and gave the same kind of results at room temperature that were later obtained on commercial 304SS at 50 or 60 C.10,11 The procedure for a sulfate–thiosulfate pitting test was as follows, exploiting the fact that sulfate–thiosulfate pitting ceases at a sufficiently positive potential, where thiosulfate can no longer be electroreduced in the pit.8 First, the sample was passivated at 0 V (MSE) in 0.1 M Na2SO4 for half an hour. Then, it was transferred to the test cell and polarized at 0.91 V (MSE), below any possible pitting onset potential. The current was allowed to stabilize for 30 s. Then a scratch was made at this potential and the current transient (the scratch current) recorded for 180 s. Then, when no pitting occurred, the potential was stepped upwards by 100 or 25 mV, depending on the resolution required, and the process was repeated. If pitting occurred, as indicated by an increase in the current aer initial repassivation (the scratch pitting current), the pits on the scratch were repassivated anodically by applying 0 V (MSE) for 10 s, then the potential was stepped back to the next scratching potential. All this is possible because sulfate– thiosulfate pitting is reluctant to initiate on undisturbed surfaces, although it does do so eventually.
Table 2
Ion chromatography measurements of thiosulfate solutions
Solution concentration 10 mM 10 mM 100 mM 100 mM
120 mM Na2SO4 + 10 mM Na2S2O3 120 mM Na2SO4 + 10 mM Na2S2O3 a
Conditions before IC test
Peak area, nS cm1 min1
Retention time, min
Freshly prepared Heateda and aerated 2 hours Freshly prepared Freshly prepared (repeat) Heated and aerated 2 hours Heated and aerated 2 hours Freshly prepared
241 955 235 573
8.7 8.8
2 072 866 2 031 375
8.7 8.7
2 305 521
8.7
2 127 417
8.7
7 078 476(Na2SO4) 188 775(Na2S2O3) 6 832 029(Na2SO4) 169 861(Na2S2O3)
4.2 8.8 4.3 8.7
Heated and aerated 2 hours
Heating and aeration for 2 hours at 60 C.
Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Paper
Faraday Discussions
Table 3 Lower/upper bounds of pitting with associated sulfate to thiosulfate ratio summarized from Fig. 4a
Ratio alloys
4000
2000
1000
200
100
40
20
10
3.3
A600 A800 A690
NP
LBP
P
P NP LBP
P LBP P
P P UBP
P UBP NP
UBP NP
NP
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
a
NP
NP-no pitting; LBP-lower bound of pitting; UBP-upper bound of pitting; P-pitting.
In order to dene carefully the ranges of the potential and anionic concentration ratio for pitting, various interpolations and repeat scratches were made, and some sequences started at high potential and were worked down to a low potential. The nal charts shown in this paper were the results of a large number of repeat tests. For chloride–thiosulfate pitting, anodic potentials could not be applied to repassivate the pits, because ordinary chloride pitting would have occurred, but an anodic passivation in pure sulfate solution was still used, as before, to minimize the base currents and delay spontaneous pitting away from the scratches. Pitted scratches were examined by SEM and EDX, usually aer allowing a longer time for pit development than was used in the main electrochemical procedure.
3 Results and discussion 3.1 Electrochemical testing – sulfate–thiosulfate pitting Fig. 1 shows typical results of scratching, for cases where pitting did, and did not, occur on Alloy 600. The increase in current due to pitting is obvious in this
Fig. 1 Typical raw data for the scratch experiments: annealed Alloy 600 in 0.1 M Na2SO4 + 0.005 M Na2S2O3 at 60 C. This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Paper
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Dependence of the scratch pitting current after 180 s potential, for tests done in 0.1 M Na2SO4 + 0.0025 M Na2S2O3 on annealed alloys at 60 C: (a) Alloy 600; (b) Alloy 800; (c) Alloy 690. The peak current was always observed at potential 0.51 V in this solution. Fig. 2
Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
Fig. 3 Longer-term exposure of scratched samples at 0.61 V (MSE) showing the relative scratch currents; 0.1 M Na2SO4 + 0.001 M Na2S2O3 at 60 C.
example – but was more subtle for the other alloys, especially Alloy 690, as will be seen later. The current increase was always rather smooth, which is characteristic of thiosulfate-related pitting – the pits are strongly stabilized by sulfur adsorption and by the precipitation of a tight cap of corrosion product, to the point that it is quite easy to study the growth of single pits electrochemically.26
Fig. 4 Pitting ranges (upper and lower bounds of potential and thiosulfate concentration)
for scratch testing of annealed alloys 600, 800 and 690 in 0.1 M Na2SO4 + x M Na2S2O3 at 60 C. Lower/upper bounds of pitting with associated sulfate to thiosulfate ratio are summarized in Table 3. This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Fig. 5 The dependence of pitting rate on potential for annealed Alloy 600 in 1 103 M Na2SO4 + Na2S2O3 with different concentrations at 60 C, showing that the most severe anionic concentration ratio remains at 40.
Fig. 2 shows the scratch pitting current aer 180 s as a function of potential, for each alloy. The initial increase with increasing potential is mainly due to a greater pit growth rate, while the decrease aer the peak is mainly due to a decreased number of pit nuclei on each scratch. The much lower scratch currents for the 800 and 690 alloys are evident in this gure, although in the longer timescale, the current continued to increase at a greater rate than for Alloy 600 (Fig. 3). This was primarily due to the scratches on Alloy 600 having completely pitted away and turned into grooves. Aer exhaustive investigation of the pitting behaviour of the three alloys in relatively concentrated solution, the chart shown in Fig. 4 was obtained. This shows the following main features: 1 Pitting of annealed Alloy 600 occurs over a wide range of thiosulfate concentration and potential. 2 Pitting of Alloys 800 and 690 occurs over very similar, smaller ranges of thiosulfate concentration and potential; these ranges were close or identical to the most severe range for Alloy 600, in terms of scratch current aer 180 s. Depending on one's criterion, either the 800 or the 690 alloy could be considered ‘more susceptible’ according to these data. It may be that with nickel being most affected by sulfur adsorption, more so than iron, the higher (generally benecial) Cr content of Alloy 690 is counteracted by its higher (generally detrimental) Ni content. A maximum susceptibility at a sulfate–thiosulfate concentration ratio of around 40 is evident in Fig. 4, and was rened to 40 10 in further work. Then, ratios around 40 were tested with various degrees of dilution of the solution. As shown in Fig. 5, the special ratio of 40 remained valid aer a 100 dilution.
Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Paper
Faraday Discussions
Finally, a solution with only 1 mM thiosulfate was tested, which gave easy pitting as shown in Fig. 6 – in this poorly conducting solution, the pits spaced themselves out along the scratch in a self-avoiding pattern.
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
3.2 Microscopy and analysis – sulfate–thiosulfate pitting The different propensity of the three alloys to pitting is illustrated in Fig. 7. Alloy 600 has many and large pits. The 800 alloy has fewer pits, but of a similar size to Alloy 600. The 690 alloy clearly has a smaller pit size than the other alloys, for a given time of exposure aer scratching. It could be that these pits have already repassivated spontaneously, but the current–time curves did not indicate any major noise or drops in current that might support such an interpretation. One
Long-term exposure of scratched Alloy 600 in 4 105 M Na2SO4 + 1 106 M Na2S2O3 for 12 hours at 0.435 V vs. MSE and 60 C: (a) current–time curve; (b) pit morphology. Fig. 6
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Fig. 7 Scratch corrosion morphologies of Alloy 600 in 0.1 M Na2SO4 + 0.001 M Na2S2O3, and alloys 800 and 690 in 0.1 M Na2SO4 + 0.0025 M Na2S2O3; all tests were done at 60 C, 0.61 V (MSE) and exposed potentiostatically for 30 minutes after scratching. Marker bars are 100 mm.
factor to be considered is that nickel is more noble than iron, so for a given potential the 690 alloy would have slower dissolution kinetics than Alloy 800 (perhaps), but the 600 alloy is also mostly nickel. The most likely explanation is that the corrosion product cap is denser (less porous) on the 690 alloy, leading to a higher ohmic resistance across the cap (and/or a lower effective cation diffusivity, in the event that pit growth is diffusion controlled), and therefore to a slower pit growth rate. To the extent that the cap contains chromium [oxy]hydroxide, and the alloy has an exceptionally high Cr content, this may be expected. In further work, the pitting currents could be rationalized in terms of area fraction of active surface, pit depth, etc. (or single pits could be created for the study). All pits showed the enrichment of S and depletion of Fe and Ni, relative to Cr, in EDX analyses, as might be expected, see Fig. 8. So it is reasonable to characterize the pit cap as CrOOH containing some oxidized Fe and Ni, and unidentied S species, such as Ni3S2 perhaps.
3.3 Electrochemical testing – chloride–thiosulfate pitting Initial scratch testing on Alloy 600, with no thiosulfate present, demonstrated some degree of pitting resistance up to at least 100 mV (Ag/AgCl) in 0.3 M NaCl at 60 C, and when it did show persistent pitting at potentials around 0 V, the currents were low (sub-mA) and unstable. Given this result, there was no difficulty distinguishing ordinary chloride pitting from thiosulfate-catalyzed pitting, which invariably gave scratch currents of at least 0.5 mA and up to 150 mA in the most severe cases (aer 240 s), with a smooth current increase. There was a subtle peak or shoulder in the rate of thiosulfate-catalyzed chloride pitting with potential, see Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
EDX elemental mapping of a pitted scratch on Alloy 600, exposed to 0.1 M Na2SO4 + 0.001 M Na2S2O3 for 30 minutes after scratching at 0.61 V (MSE) and 60 C, showing S enrichment in the pits and relative depletion of the susceptible elements Ni and Fe compared to Cr. Fig. 8
Fig. 9, but this was not nearly as easy to discern as in the case of sulfate–thiosulfate pitting, where pitting ceased altogether at high potentials. It was immediately apparent, see Fig. 9 and 10, that the amount of thiosulfate required for the pitting of Alloy 600 was much less in (strong) chloride than in a (strong) sulfate solution. More work is required to harmonize this result with the behaviour of type 304 stainless steel, where the original work of Newman, Isaacs and Alman1 suggested an optimal chloride to thiosulfate ratio of no more than 25, while Franz26 also used a ratio of 25 in her work on growth of single pits in 304SS. One possible explanation is that adsorbed S is more catalytic (activating) for a dissolution of Ni than Fe – perhaps due to the latter's greater affinity for oxygen, or to a difference in the degree of charge transfer on to the adsorbed S atoms, as there are several inter-related possibilities. In that case, a lower ux of thiosulfate will be required to maintain the activation of a Ni-based alloy compared to a Febased alloy, with other things being equal (the difference between sulfate and chloride will be discussed further below). Studies of chloride–thiosulfate pitting of Alloy 800, which is a high-alloy stainless steel, are in progress and should clarify this issue. Another possibility is that Alloy 600 has a different behaviour simply This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Fig. 9 Scratch current transients for annealed Alloy 600 in 0.1 M NaCl + 5 105 M Na2S2O3, 60 C, showing that the rate of current increase is not monotonic with the potential when pitting is catalyzed by thiosulfate.
because it is kinetically more noble in acids than type 304SS. Given that electroreduction of thiosulfate is required to stabilize the dissolution in the pit, the rate of this reduction will be lower, at the onset of pitting, in Alloy 600 than in
Fig. 10 Pitting corrosion of annealed Alloy 600 in 0.1 M NaCl + Na2S2O3 with different concentrations at 60 C, showing that pitting is rapid for chloride–thiosulfate concentration ratios in the range from 1000 to 4000. Little or no catalyzed pitting occurred at ratios of 400 or 10 000 – the behaviour reverted to the pure-chloride type (unstable pit growth and higher onset potential of pitting). Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
304SS, due to the displacement of the catalyzed pitting to higher potentials. So for Alloy 600, it is easy to swamp the pit solution with too much thiosulfate, leading to repassivation by the neutralizing effect of disproportionation. It may be (direct demonstration is difficult) that the dissolution kinetics in sulfate, at a given potential and in the absence of passivation, are faster than in chloride. Experiments are in progress using strong HCl and H2SO4 solutions with tetrathionate addition, to model the effect of thiosulfate with a more acid-stable but similarly aggressive anion. The more noble character of Alloy 600, compared to 304SS or the other SG tubing alloys, is apparent in these experiments. No S-catalyzed pitting was observed in solutions with chloride–thiosulfate concentration ratios of 400 or 10 000. Rapid S-catalyzed pitting was conned to the range of ratios from 1000 to 4000. Another difference from the sulfate based system was that pitting was much more difficult to stabilize in diluted solutions, because the ‘optimal’ thiosulfate concentration became absurdly low. So, there are two answers to the question – which is more severe in Alloy 600, sulfate– thiosulfate or chloride–thiosulfate pitting? From the perspective of relatively concentrated systems, chloride is more severe because less thiosulfate is required for pitting. But sulfate–thiosulfate pitting persists in solutions with much lower overall ionic strength, so it is a more plausible practical issue in dilute systems. 3.4 Considerations related to local chemistry and transport In ordinary chloride pitting, the pitting potential, Epit, varies with the bulk concentration of NaCl according to eqn (3): Epit ¼ A B log[Cl]
(3)
where A is a kind of baseline pitting potential determined by the metal dissolution kinetics in the concentrated, acidic metal chloride solution in a pit. B is an IR drop in the pit nucleus. The value of B, for stainless steel at room temperature, is about 90–100 mV. Long ago, Galvele explained this behaviour in terms of reaction and transport in a pit nucleus27,28 (others have speculated on adsorption and transport within the passive lm as the underlying physics leading to this equation). In Galvele's treatment, the change in Epit is just the change in the IR drop within a pit nucleus of particular geometry when the chemistry at the bottom of the pit (hydrolyzed cations; chloride ions) is at a critical value required to stabilize dissolution. Later, strong direct evidence for such an interpretation was provided by Laycock and Newman.29 Galvele's result is obtained by treating Cl as a nonreacting ion in the cavity and equating its (outward) diffusion and (inward) migration uxes at steady state. Now in theory, the value of B should be 2.3RT/ zF, where z is the charge on the anion, i.e. 1. The departure from this value (59 mV at 298 K) is likely due to chloro-complexation of Cr in the pit solution, as pure iron does show a B value close to 60 mV. So, for sulfate–thiosulfate pitting, where sulfate plays the role of chloride, B should be close to 30 mV at 298 K, or 33 mV at 333 K, as shown in the present work. This assumes that SO4, not HSO4, is the appropriate form of sulfate to consider. We justify that by noting that pitting occurs at all – if the buffering effect of the HSO4/SO4 equilibrium was important, pitting would likely not occur. Galvele showed, amongst other things, that when two anions of different charge were present, the anion with the larger charge would accumulate more This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
strongly at the bottom of the pit as the current density in the pit, or the potential, was increased. In other words, the idea that the ratio of anionic concentrations should be preserved in the enriched pit solution is not really correct unless the anions have the same charge (we assume that differences in mobility and diffusivity are small). This immediately explains why less thiosulfate, relative to the predominant anion, is required in a chloride–thiosulfate system than in a sulfate– thiosulfate system, and would also account for a lack of adherence of the former to a strict anionic concentration ratio criterion. When Ezuber11 was studying thiosulfate-related pitting, he paid much more attention to sulfate–thiosulfate than chloride–thiosulfate. Pitting propensity in the sulfate–thiosulfate system showed a very good correlation with the anionic concentration ratio, and this led to the possibly incorrect assumption that the same would be true in chloride– thiosulfate solutions, or mixed solutions with both chloride and sulfate. In fact, the detailed measurements in chloride–thiosulfate were never done. It is possible that this omission can explain the relatively weak correlation of pitting with chloride to thiosulfate ratio observed by Laycock.24 Preliminary measurements of the B value for Alloy 600 in sulfate–thiosulfate solutions show a value of 40–50 mV. This can be seen by comparing the onset potentials for pitting in Fig. 2a and 5. Two orders of magnitude difference in ionic strength cause about a 90 mV shi in the kinetics. This requires more investigation, and is a little blurred by the gradual onset of the catalyzed pitting in these systems. It may be that this is the counterpart of the 90 or 100 mV value seen for stainless steels, i.e. a value that would be 33 mV if it were not for some complexation reaction. Alternatively, the discrepancy could be due to some protonation of sulfate in the pit, so that its charge, on average, is not exactly 2 – we know this must happen to some extent near the bottom of the pit. Anyway, we can certainly say that B is less than 2.3RT/F, which is important. Why is the B value signicant? Because it helps to explain why pitting can occur in such extremely diluted sulfate–thiosulfate solutions. A low B value, such as 50 mV, implies that a 5 orders of magnitude decrease in ionic strength of the solution only elevates the pitting potential, Epit (the lower potential limit of pitting in these data), by 250 mV. Not only does this still leave Epit at a relatively low value by the standards of laboratory testing, it is still well within the range of natural open-circuit or corrosion potential that can be achieved on a passive alloy in a mildly aerated solution of near-neutral pH. Of course the exceptionally stable nature of thiosulfate-related pitting, including the dense pit cover, plays a role as well. Finally we should consider the possibility of SCC in these solutions, as it occurs very easily in simple austenitic stainless steels with chloride instead of sulfate.30 This is unlikely in the Ni-rich alloys but could feasibly occur in Alloy 800, by a de-alloying type of mechanism similar to that proposed for ordinary austenitic steels in chloride, or chloride with reduced sulfur.31 This would likely require an elevated temperature.
4 Conclusions Alloys 600, 800 and 690 are susceptible to pitting in sulfate–thiosulfate solutions, including highly diluted versions. The potentials at which pitting Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
occurs are easily attainable in practice with oxygen present. Galvele's B value for sulfate–thiosulfate pitting is low, facilitating pitting in highly diluted systems. Either Alloy 690 or Alloy 800 could be considered the least susceptible of the three alloys, depending on the criterion chosen. The pitting ceases at low or high potential, and at low or high sulfate to thiosulfate concentration ratio. The sulfate to thiosulfate concentration ratio has good predictive power across the range of ionic strengths examined, because sulfate and thiosulfate have the same charge and their concentration ratio should be preserved in the pit cavity. This is not the case for chloride–thiosulfate pitting. In Alloy 600, much less thiosulfate is required for chloride–thiosulfate pitting than for sulfate–thiosulfate pitting. This may not be a general observation across the range of NiCrFe systems. Ni seems to be more sensitive to S adsorption than Fe, so Ni-rich alloys require less thiosulfate. Another factor is that Alloy 600 is kinetically more noble, in the pit or pit-like environments, than the other alloys studied.
Acknowledgements This research was supported by NSERC (Canada) and by UNENE, the University Network of Excellence in Nuclear Engineering. The support, interest and input of K. Sedman (Bruce Power) and P. J. King (B&W Canada) are greatly appreciated. Thanks also to N. Senior for assistance with the preparation of this document.
References 1 R. C. Newman, H. S. Isaacs and B. Alman, Effects of sulfur-compounds on the pitting behavior of type-304 stainless-steel in near-neutral chloride solutions, Corrosion, 1982, 38, 261–265. 2 Z. Fang and R. W. Staehle, Effects of the valence of sulfur on passivation of alloys 600, 690, and 800 at 25 C and 95 C, Corrosion, 1999, 55, 355–379. 3 J. T. Ho and G. P. Yu, Pitting corrosion of Inconel-600 in chloride and thiosulfate anion solutions at low-temperature, Corrosion, 1992, 48, 147–158. 4 J. J. Park and S. I. Pyun, Stochastic approach to the pit growth kinetics of Inconel Alloy 600 in Cl- ion-containing thiosulphate solution at temperatures 25–150 C by analysis of the potentiostatic current transients, Corros. Sci., 2004, 46, 285–296. 5 R. Roberge, Effect of the nickel content in the pitting of stainless-steels in low chloride and thiosulfate solutions, Corrosion, 1988, 44, 274–280. 6 W. T. Tsai, Z. H. Lee, J. T. Lee, M. C. Tsai and P. H. Lo, Pitting and stresscorrosion cracking behavior of Inconel 600 alloy in thiosulfate solution, Mater. Sci. Eng., A, 1989, 118, 121–129. 7 I. J. Yang, localized corrosion of Alloy 600 in thiosulfate solution at 25 C, Corrosion, 1993, 49, 576–584. 8 R. C. Newman, Pitting of stainless alloys in sulfate-solutions containing thiosulfate ions, Corrosion, 1985, 41, 450–453. 9 R. C. Newman and K. Sieradzki, Electrochemical aspects of stress-corrosion cracking of sensitized stainless-steels, Corros. Sci., 1983, 23, 363–378. This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
10 R. C. Newman, W. P. Wong and A. Garner, A mechanism of microbial pitting in stainless-steel, Corrosion, 1986, 42, 489–491. 11 R. C. Newman, W. P. Wong, H. Ezuber and A. Garner, Pitting of stainless-steels by thiosulfate ions, Corrosion, 1989, 45, 282–287. 12 A. M. Brennenstuhl, T. S. Gendron and R. Cleland, Mechanisms of underdeposit corrosion in fresh-water cooled austenitic alloy heatexchangers, Corros. Sci., 1993, 35, 699–711. 13 P. Marcus and E. Protopopoff, Potential-pH diagrams for adsorbed species – application to sulfur adsorbed on iron in water at 25 C and 300 C, J. Electrochem. Soc., 1990, 137, 2709–2712. 14 P. Marcus and E. Protopopoff, Potential-pH diagrams for sulfur and oxygen adsorbed on nickel in water at 25 C and 300 C, J. Electrochem. Soc., 1993, 140, 1571–1575. 15 P. Marcus and E. Protopopoff, Potential-pH diagrams for sulfur and oxygen adsorbed on chromium in water, J. Electrochem. Soc., 1997, 144, 1586–1590. 16 A. Elbiache and P. Marcus, The role of molybdenum in the dissolution and the passivation of stainless-steels with adsorbed sulfur, Corros. Sci., 1992, 33, 261– 269. 17 P. Marcus and E. Protopopoff, Thermodynamics of thiosulfate reduction on surfaces of iron, nickel and chromium in water at 25 and 300 C, Corros. Sci., 1997, 39, 1741–1752. 18 P. Marcus and M. Moscatelli, The role of alloyed molybdenum in the dissolution and the passivation of nickel-molybdenum alloys in the presence of adsorbed sulfur, J. Electrochem. Soc., 1989, 136, 1634–1637. 19 R. C. Newman, The dissolution and passivation kinetics of stainless alloys containing molybdenum .2. Dissolution kinetics in articial pits, Corros. Sci., 1985, 25, 341–350. 20 A. J. Betts and R. C. Newman, The effect of alloyed molybdenum on the activation of anodic dissolution by reduced sulphur compounds, Corros. Sci., 1993, 34, 1551–1555. 21 J. Rossmeisl, E. Skulason, M. E. Bjorketun, V. Tripkovic and J. K. Norskov, Modeling the electried solid–liquid interface, Chem. Phys. Lett., 2008, 466, 68–71. 22 C. D. Taylor and M. Neurock, Theoretical insights into the structure and reactivity of the aqueous/metal interface, Curr. Opin. Solid State Mater. Sci., 2005, 9, 49–65. 23 H. Yang and J. L. Whitten, Adsorption of SH and OH and coadsorption of S, O and H on Ni(111), Surf.Sci., 1997, 370, 136–154. 24 N. J. Laycock, Effects of temperature and thiosulfate on chloride pitting of austenitic stainless steels, Corrosion, 1999, 55, 590–595. 25 E. G. Webb and R. C. Alkire, Pit initiation at single sulde inclusions in stainless steel-II. Detection of local pH, sulde, and thiosulfate, J. Electrochem. Soc., 2002, 149, B280–B285. 26 R. C. Newman and E. M. Franz, Growth and repassivation of single corrosion pits in stainless-steel, Corrosion, 1984, 40, 325–330. 27 J. R. Galvele, Transport processes and mechanism of pitting of metals, J. Electrochem. Soc., 1976, 123, 464–474. 28 S. M. Gravano and J. R. Galvele, Transport processes in passivity breakdown .3. Full hydrolysis plus ion migration plus buffers, Corros. Sci., 1984, 24, 517–534. Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
29 N. J. Laycock and R. C. Newman, Localised dissolution kinetics, salt lms and pitting potentials, Corros. Sci., 1997, 39, 1771–1790. 30 T. Haruna, T. Shibata and R. Toyota, Initiation and propagation of stress corrosion cracks for type 304L stainless steel in chloride solutions containing thiosulfate, Corros. Sci., 1997, 39, 1935–1947. 31 R. C. Newman, R. R. Corderman and K. Sieradzki, Evidence for de-alloying of austenitic stainless steels in simulated stress-corrosion crack environments, Br. Corros. J., 1989, 24, 143–148.
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
PAPER
View Article Online View Journal
Pitting of steam-generator tubing alloys in solutions containing thiosulfate and sulfate or chloride William Zhang, Anatolie G. Carcea and Roger C. Newman*
Received 20th January 2015, Accepted 22nd February 2015 DOI: 10.1039/c5fd00008d
The pitting of nuclear steam generator tubing alloys 600, 690 and 800 was studied at 60 C using dilute thiosulfate solutions containing excess sulfate or (for Alloy 600) chloride. A potentiostatic scratch method was used. In sulfate solutions, all alloys pitted at low potentials, reflecting their lack of protective Mo. The alloys demonstrated the most severe pitting at a sulfate : thiosulfate concentration ratio of 40. Alloy 600 pitted worst at a chloride : thiosulfate ratio of 2000. The results are interpreted through the mutual electromigration of differently charged anions into a pit nucleus, and differences in the major alloy component.
1 Introduction Some corrosion phenomena in nuclear steam generator (SG) tubing have been postulated to occur at low temperature, due to the presence of both oxygen and ‘reduced’ sulfur species (i.e. oxidation state of the sulfur is less than 6). The reduced sulfur could be generated by reaction of sulfate impurity with hydrazine near heat transfer surfaces during SG operation. It is well known that reduced sulfur species are catalytic for the corrosion of iron and nickel, and their alloys; early work focused on pitting of simple stainless steels, such as type 304, in solutions with chloride and various reduced sulfur species.1 Thiosulfate was found to be the most aggressive sulfur anion, much more so than sulde, and the benecial effect of Mo alloying was noted. Later, there were a number of studies of localized corrosion of alloys 600 and 800 in mixed solutions of chloride and thiosulfate, or other reduced sulfur species,2–7 as well as many studies of stress corrosion cracking (SCC) of sensitized Alloy 600 in pure thiosulfate solutions. To our knowledge, there has been no systematic study of the pitting of annealed SG tubing alloys in chloride-free solutions (sulfate plus thiosulfate). This phenomenon was discovered in stainless steel in the early 1980s,8,9 and independently by
University of Toronto, Department of Chemical Engineering and Applied Chemistry, 200 College Street, Toronto, Canada. E-mail: [email protected]; [email protected]; [email protected]; Fax: +1-416-978-8605; Tel: +1-416-946-0604
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Garner in the pulp and paper industry; a number of studies were published in the context of paper machines and microbial corrosion.10,11 Thiosulfate was implicated in practical problems involving fouled fresh-water heat-exchangers,12 although here some chloride was present. In sulfate–thiosulfate pitting, sulfate plays the role of the strong-acid anion – just like chloride in ordinary pitting – that permits, within certain limitations, acidication of the pit cavity by cation hydrolysis. The limitations are the buffer character of sulfate at low pH (pKa z 1.92; evidently this is not a dominant factor, otherwise pitting would not occur), and the tendency of thiosulfate to neutralize the acidity of the pit by disproportionation according to eqn (1): S2O3 + H+ / HSO3 + S
(1)
or by electroreduction as shown in eqn (2): S2O3 + 6H+ + 4e / 2S + 3H2O
(2)
The catalytic effect of adsorbed sulfur (Sads0 or Sadsd) on anodic dissolution of Ni, Fe and NiFe(Cr) alloys is well known from the work of Marcus and others. Essentially, S adsorption in hollow sites weakens the metal–metal bonding in the surface, facilitating dissolution; it also hinders hydrogen adsorption, because the weakening of the metal–metal interaction lowers the hydrogen adsorption energy. On Fe or Ni, S adsorption occurs over a wide range of potential and pH,13,14 and is even believed to occur on Cr, despite the instability of bulk Cr suldes in aqueous solution,15 although no kinetic information is available that shows an aggressive effect of Sads on Cr. Stainless steel is less susceptible than pure Fe or Ni (or FeNi alloys), because the presence of Cr causes some desorption of S.16 This effect is also demonstrated, to a much greater extent, by alloyed Mo, which strongly desorbs sulfur.18 The rst outcome of this, at least for temperatures 99%; sodium thiosulfate was from EM Science, an affiliate of Merck KGaA, Darmstadt, Germany, assay 99.5–100.5%. Sodium chloride was purchased from EMD, assay >99%. The resistivity of the DI water was 18.2 MU cm. A platinum counter electrode and an Hg/Hg2SO4/sat. K2SO4 reference electrode (MSE, 0.65 V vs. SHE at 25 C) were also immersed for the sulfate–thiosulfate experiments; for chloride–thiosulfate, the reference electrode was Ag/AgCl/sat. KCl (0.199 V vs. SHE at 25 C). All experiments reported here used an open beaker as the electrochemical cell, with freshly
Table 1
Compositions of the alloys studied, in wt%
Compositions
Alloy 600
Alloy 690
Alloy 800
Ni Fe Cr C
Balance 9.53 16.1 0.06
Balance 10.1 29.6 0.03
31.2 Balance 20.0 0.07
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
poured solution – the oxidation rate of thiosulfate is slow enough to allow such a procedure, even in highly diluted solutions, which was conrmed by detailed ion chromatography (IC) measurements, shown in Table 2. IC tests were performed using the Dionex ion chronomatographic system (DX500) with a conductivity detector (ED40). A diamond engraving pencil was used for scratching – the scratches were approximately 0.1 mm wide and 9.5 mm long. All the results shown here relate to a solution temperature of 60 C – pitting occurred at 25 C, but it was less stable and required much more effort for interpretation. A similar observation was made in the 1980s on stainless steel – the rst reports of sulfate–thiosulfate pitting used high-purity alloys that were less resistant than ordinary type 304 stainless steel,8,9 and gave the same kind of results at room temperature that were later obtained on commercial 304SS at 50 or 60 C.10,11 The procedure for a sulfate–thiosulfate pitting test was as follows, exploiting the fact that sulfate–thiosulfate pitting ceases at a sufficiently positive potential, where thiosulfate can no longer be electroreduced in the pit.8 First, the sample was passivated at 0 V (MSE) in 0.1 M Na2SO4 for half an hour. Then, it was transferred to the test cell and polarized at 0.91 V (MSE), below any possible pitting onset potential. The current was allowed to stabilize for 30 s. Then a scratch was made at this potential and the current transient (the scratch current) recorded for 180 s. Then, when no pitting occurred, the potential was stepped upwards by 100 or 25 mV, depending on the resolution required, and the process was repeated. If pitting occurred, as indicated by an increase in the current aer initial repassivation (the scratch pitting current), the pits on the scratch were repassivated anodically by applying 0 V (MSE) for 10 s, then the potential was stepped back to the next scratching potential. All this is possible because sulfate– thiosulfate pitting is reluctant to initiate on undisturbed surfaces, although it does do so eventually.
Table 2
Ion chromatography measurements of thiosulfate solutions
Solution concentration 10 mM 10 mM 100 mM 100 mM
120 mM Na2SO4 + 10 mM Na2S2O3 120 mM Na2SO4 + 10 mM Na2S2O3 a
Conditions before IC test
Peak area, nS cm1 min1
Retention time, min
Freshly prepared Heateda and aerated 2 hours Freshly prepared Freshly prepared (repeat) Heated and aerated 2 hours Heated and aerated 2 hours Freshly prepared
241 955 235 573
8.7 8.8
2 072 866 2 031 375
8.7 8.7
2 305 521
8.7
2 127 417
8.7
7 078 476(Na2SO4) 188 775(Na2S2O3) 6 832 029(Na2SO4) 169 861(Na2S2O3)
4.2 8.8 4.3 8.7
Heated and aerated 2 hours
Heating and aeration for 2 hours at 60 C.
Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Paper
Faraday Discussions
Table 3 Lower/upper bounds of pitting with associated sulfate to thiosulfate ratio summarized from Fig. 4a
Ratio alloys
4000
2000
1000
200
100
40
20
10
3.3
A600 A800 A690
NP
LBP
P
P NP LBP
P LBP P
P P UBP
P UBP NP
UBP NP
NP
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
a
NP
NP-no pitting; LBP-lower bound of pitting; UBP-upper bound of pitting; P-pitting.
In order to dene carefully the ranges of the potential and anionic concentration ratio for pitting, various interpolations and repeat scratches were made, and some sequences started at high potential and were worked down to a low potential. The nal charts shown in this paper were the results of a large number of repeat tests. For chloride–thiosulfate pitting, anodic potentials could not be applied to repassivate the pits, because ordinary chloride pitting would have occurred, but an anodic passivation in pure sulfate solution was still used, as before, to minimize the base currents and delay spontaneous pitting away from the scratches. Pitted scratches were examined by SEM and EDX, usually aer allowing a longer time for pit development than was used in the main electrochemical procedure.
3 Results and discussion 3.1 Electrochemical testing – sulfate–thiosulfate pitting Fig. 1 shows typical results of scratching, for cases where pitting did, and did not, occur on Alloy 600. The increase in current due to pitting is obvious in this
Fig. 1 Typical raw data for the scratch experiments: annealed Alloy 600 in 0.1 M Na2SO4 + 0.005 M Na2S2O3 at 60 C. This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Paper
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Dependence of the scratch pitting current after 180 s potential, for tests done in 0.1 M Na2SO4 + 0.0025 M Na2S2O3 on annealed alloys at 60 C: (a) Alloy 600; (b) Alloy 800; (c) Alloy 690. The peak current was always observed at potential 0.51 V in this solution. Fig. 2
Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
Fig. 3 Longer-term exposure of scratched samples at 0.61 V (MSE) showing the relative scratch currents; 0.1 M Na2SO4 + 0.001 M Na2S2O3 at 60 C.
example – but was more subtle for the other alloys, especially Alloy 690, as will be seen later. The current increase was always rather smooth, which is characteristic of thiosulfate-related pitting – the pits are strongly stabilized by sulfur adsorption and by the precipitation of a tight cap of corrosion product, to the point that it is quite easy to study the growth of single pits electrochemically.26
Fig. 4 Pitting ranges (upper and lower bounds of potential and thiosulfate concentration)
for scratch testing of annealed alloys 600, 800 and 690 in 0.1 M Na2SO4 + x M Na2S2O3 at 60 C. Lower/upper bounds of pitting with associated sulfate to thiosulfate ratio are summarized in Table 3. This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Fig. 5 The dependence of pitting rate on potential for annealed Alloy 600 in 1 103 M Na2SO4 + Na2S2O3 with different concentrations at 60 C, showing that the most severe anionic concentration ratio remains at 40.
Fig. 2 shows the scratch pitting current aer 180 s as a function of potential, for each alloy. The initial increase with increasing potential is mainly due to a greater pit growth rate, while the decrease aer the peak is mainly due to a decreased number of pit nuclei on each scratch. The much lower scratch currents for the 800 and 690 alloys are evident in this gure, although in the longer timescale, the current continued to increase at a greater rate than for Alloy 600 (Fig. 3). This was primarily due to the scratches on Alloy 600 having completely pitted away and turned into grooves. Aer exhaustive investigation of the pitting behaviour of the three alloys in relatively concentrated solution, the chart shown in Fig. 4 was obtained. This shows the following main features: 1 Pitting of annealed Alloy 600 occurs over a wide range of thiosulfate concentration and potential. 2 Pitting of Alloys 800 and 690 occurs over very similar, smaller ranges of thiosulfate concentration and potential; these ranges were close or identical to the most severe range for Alloy 600, in terms of scratch current aer 180 s. Depending on one's criterion, either the 800 or the 690 alloy could be considered ‘more susceptible’ according to these data. It may be that with nickel being most affected by sulfur adsorption, more so than iron, the higher (generally benecial) Cr content of Alloy 690 is counteracted by its higher (generally detrimental) Ni content. A maximum susceptibility at a sulfate–thiosulfate concentration ratio of around 40 is evident in Fig. 4, and was rened to 40 10 in further work. Then, ratios around 40 were tested with various degrees of dilution of the solution. As shown in Fig. 5, the special ratio of 40 remained valid aer a 100 dilution.
Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Paper
Faraday Discussions
Finally, a solution with only 1 mM thiosulfate was tested, which gave easy pitting as shown in Fig. 6 – in this poorly conducting solution, the pits spaced themselves out along the scratch in a self-avoiding pattern.
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
3.2 Microscopy and analysis – sulfate–thiosulfate pitting The different propensity of the three alloys to pitting is illustrated in Fig. 7. Alloy 600 has many and large pits. The 800 alloy has fewer pits, but of a similar size to Alloy 600. The 690 alloy clearly has a smaller pit size than the other alloys, for a given time of exposure aer scratching. It could be that these pits have already repassivated spontaneously, but the current–time curves did not indicate any major noise or drops in current that might support such an interpretation. One
Long-term exposure of scratched Alloy 600 in 4 105 M Na2SO4 + 1 106 M Na2S2O3 for 12 hours at 0.435 V vs. MSE and 60 C: (a) current–time curve; (b) pit morphology. Fig. 6
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Fig. 7 Scratch corrosion morphologies of Alloy 600 in 0.1 M Na2SO4 + 0.001 M Na2S2O3, and alloys 800 and 690 in 0.1 M Na2SO4 + 0.0025 M Na2S2O3; all tests were done at 60 C, 0.61 V (MSE) and exposed potentiostatically for 30 minutes after scratching. Marker bars are 100 mm.
factor to be considered is that nickel is more noble than iron, so for a given potential the 690 alloy would have slower dissolution kinetics than Alloy 800 (perhaps), but the 600 alloy is also mostly nickel. The most likely explanation is that the corrosion product cap is denser (less porous) on the 690 alloy, leading to a higher ohmic resistance across the cap (and/or a lower effective cation diffusivity, in the event that pit growth is diffusion controlled), and therefore to a slower pit growth rate. To the extent that the cap contains chromium [oxy]hydroxide, and the alloy has an exceptionally high Cr content, this may be expected. In further work, the pitting currents could be rationalized in terms of area fraction of active surface, pit depth, etc. (or single pits could be created for the study). All pits showed the enrichment of S and depletion of Fe and Ni, relative to Cr, in EDX analyses, as might be expected, see Fig. 8. So it is reasonable to characterize the pit cap as CrOOH containing some oxidized Fe and Ni, and unidentied S species, such as Ni3S2 perhaps.
3.3 Electrochemical testing – chloride–thiosulfate pitting Initial scratch testing on Alloy 600, with no thiosulfate present, demonstrated some degree of pitting resistance up to at least 100 mV (Ag/AgCl) in 0.3 M NaCl at 60 C, and when it did show persistent pitting at potentials around 0 V, the currents were low (sub-mA) and unstable. Given this result, there was no difficulty distinguishing ordinary chloride pitting from thiosulfate-catalyzed pitting, which invariably gave scratch currents of at least 0.5 mA and up to 150 mA in the most severe cases (aer 240 s), with a smooth current increase. There was a subtle peak or shoulder in the rate of thiosulfate-catalyzed chloride pitting with potential, see Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
EDX elemental mapping of a pitted scratch on Alloy 600, exposed to 0.1 M Na2SO4 + 0.001 M Na2S2O3 for 30 minutes after scratching at 0.61 V (MSE) and 60 C, showing S enrichment in the pits and relative depletion of the susceptible elements Ni and Fe compared to Cr. Fig. 8
Fig. 9, but this was not nearly as easy to discern as in the case of sulfate–thiosulfate pitting, where pitting ceased altogether at high potentials. It was immediately apparent, see Fig. 9 and 10, that the amount of thiosulfate required for the pitting of Alloy 600 was much less in (strong) chloride than in a (strong) sulfate solution. More work is required to harmonize this result with the behaviour of type 304 stainless steel, where the original work of Newman, Isaacs and Alman1 suggested an optimal chloride to thiosulfate ratio of no more than 25, while Franz26 also used a ratio of 25 in her work on growth of single pits in 304SS. One possible explanation is that adsorbed S is more catalytic (activating) for a dissolution of Ni than Fe – perhaps due to the latter's greater affinity for oxygen, or to a difference in the degree of charge transfer on to the adsorbed S atoms, as there are several inter-related possibilities. In that case, a lower ux of thiosulfate will be required to maintain the activation of a Ni-based alloy compared to a Febased alloy, with other things being equal (the difference between sulfate and chloride will be discussed further below). Studies of chloride–thiosulfate pitting of Alloy 800, which is a high-alloy stainless steel, are in progress and should clarify this issue. Another possibility is that Alloy 600 has a different behaviour simply This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
Fig. 9 Scratch current transients for annealed Alloy 600 in 0.1 M NaCl + 5 105 M Na2S2O3, 60 C, showing that the rate of current increase is not monotonic with the potential when pitting is catalyzed by thiosulfate.
because it is kinetically more noble in acids than type 304SS. Given that electroreduction of thiosulfate is required to stabilize the dissolution in the pit, the rate of this reduction will be lower, at the onset of pitting, in Alloy 600 than in
Fig. 10 Pitting corrosion of annealed Alloy 600 in 0.1 M NaCl + Na2S2O3 with different concentrations at 60 C, showing that pitting is rapid for chloride–thiosulfate concentration ratios in the range from 1000 to 4000. Little or no catalyzed pitting occurred at ratios of 400 or 10 000 – the behaviour reverted to the pure-chloride type (unstable pit growth and higher onset potential of pitting). Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
304SS, due to the displacement of the catalyzed pitting to higher potentials. So for Alloy 600, it is easy to swamp the pit solution with too much thiosulfate, leading to repassivation by the neutralizing effect of disproportionation. It may be (direct demonstration is difficult) that the dissolution kinetics in sulfate, at a given potential and in the absence of passivation, are faster than in chloride. Experiments are in progress using strong HCl and H2SO4 solutions with tetrathionate addition, to model the effect of thiosulfate with a more acid-stable but similarly aggressive anion. The more noble character of Alloy 600, compared to 304SS or the other SG tubing alloys, is apparent in these experiments. No S-catalyzed pitting was observed in solutions with chloride–thiosulfate concentration ratios of 400 or 10 000. Rapid S-catalyzed pitting was conned to the range of ratios from 1000 to 4000. Another difference from the sulfate based system was that pitting was much more difficult to stabilize in diluted solutions, because the ‘optimal’ thiosulfate concentration became absurdly low. So, there are two answers to the question – which is more severe in Alloy 600, sulfate– thiosulfate or chloride–thiosulfate pitting? From the perspective of relatively concentrated systems, chloride is more severe because less thiosulfate is required for pitting. But sulfate–thiosulfate pitting persists in solutions with much lower overall ionic strength, so it is a more plausible practical issue in dilute systems. 3.4 Considerations related to local chemistry and transport In ordinary chloride pitting, the pitting potential, Epit, varies with the bulk concentration of NaCl according to eqn (3): Epit ¼ A B log[Cl]
(3)
where A is a kind of baseline pitting potential determined by the metal dissolution kinetics in the concentrated, acidic metal chloride solution in a pit. B is an IR drop in the pit nucleus. The value of B, for stainless steel at room temperature, is about 90–100 mV. Long ago, Galvele explained this behaviour in terms of reaction and transport in a pit nucleus27,28 (others have speculated on adsorption and transport within the passive lm as the underlying physics leading to this equation). In Galvele's treatment, the change in Epit is just the change in the IR drop within a pit nucleus of particular geometry when the chemistry at the bottom of the pit (hydrolyzed cations; chloride ions) is at a critical value required to stabilize dissolution. Later, strong direct evidence for such an interpretation was provided by Laycock and Newman.29 Galvele's result is obtained by treating Cl as a nonreacting ion in the cavity and equating its (outward) diffusion and (inward) migration uxes at steady state. Now in theory, the value of B should be 2.3RT/ zF, where z is the charge on the anion, i.e. 1. The departure from this value (59 mV at 298 K) is likely due to chloro-complexation of Cr in the pit solution, as pure iron does show a B value close to 60 mV. So, for sulfate–thiosulfate pitting, where sulfate plays the role of chloride, B should be close to 30 mV at 298 K, or 33 mV at 333 K, as shown in the present work. This assumes that SO4, not HSO4, is the appropriate form of sulfate to consider. We justify that by noting that pitting occurs at all – if the buffering effect of the HSO4/SO4 equilibrium was important, pitting would likely not occur. Galvele showed, amongst other things, that when two anions of different charge were present, the anion with the larger charge would accumulate more This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
strongly at the bottom of the pit as the current density in the pit, or the potential, was increased. In other words, the idea that the ratio of anionic concentrations should be preserved in the enriched pit solution is not really correct unless the anions have the same charge (we assume that differences in mobility and diffusivity are small). This immediately explains why less thiosulfate, relative to the predominant anion, is required in a chloride–thiosulfate system than in a sulfate– thiosulfate system, and would also account for a lack of adherence of the former to a strict anionic concentration ratio criterion. When Ezuber11 was studying thiosulfate-related pitting, he paid much more attention to sulfate–thiosulfate than chloride–thiosulfate. Pitting propensity in the sulfate–thiosulfate system showed a very good correlation with the anionic concentration ratio, and this led to the possibly incorrect assumption that the same would be true in chloride– thiosulfate solutions, or mixed solutions with both chloride and sulfate. In fact, the detailed measurements in chloride–thiosulfate were never done. It is possible that this omission can explain the relatively weak correlation of pitting with chloride to thiosulfate ratio observed by Laycock.24 Preliminary measurements of the B value for Alloy 600 in sulfate–thiosulfate solutions show a value of 40–50 mV. This can be seen by comparing the onset potentials for pitting in Fig. 2a and 5. Two orders of magnitude difference in ionic strength cause about a 90 mV shi in the kinetics. This requires more investigation, and is a little blurred by the gradual onset of the catalyzed pitting in these systems. It may be that this is the counterpart of the 90 or 100 mV value seen for stainless steels, i.e. a value that would be 33 mV if it were not for some complexation reaction. Alternatively, the discrepancy could be due to some protonation of sulfate in the pit, so that its charge, on average, is not exactly 2 – we know this must happen to some extent near the bottom of the pit. Anyway, we can certainly say that B is less than 2.3RT/F, which is important. Why is the B value signicant? Because it helps to explain why pitting can occur in such extremely diluted sulfate–thiosulfate solutions. A low B value, such as 50 mV, implies that a 5 orders of magnitude decrease in ionic strength of the solution only elevates the pitting potential, Epit (the lower potential limit of pitting in these data), by 250 mV. Not only does this still leave Epit at a relatively low value by the standards of laboratory testing, it is still well within the range of natural open-circuit or corrosion potential that can be achieved on a passive alloy in a mildly aerated solution of near-neutral pH. Of course the exceptionally stable nature of thiosulfate-related pitting, including the dense pit cover, plays a role as well. Finally we should consider the possibility of SCC in these solutions, as it occurs very easily in simple austenitic stainless steels with chloride instead of sulfate.30 This is unlikely in the Ni-rich alloys but could feasibly occur in Alloy 800, by a de-alloying type of mechanism similar to that proposed for ordinary austenitic steels in chloride, or chloride with reduced sulfur.31 This would likely require an elevated temperature.
4 Conclusions Alloys 600, 800 and 690 are susceptible to pitting in sulfate–thiosulfate solutions, including highly diluted versions. The potentials at which pitting Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
occurs are easily attainable in practice with oxygen present. Galvele's B value for sulfate–thiosulfate pitting is low, facilitating pitting in highly diluted systems. Either Alloy 690 or Alloy 800 could be considered the least susceptible of the three alloys, depending on the criterion chosen. The pitting ceases at low or high potential, and at low or high sulfate to thiosulfate concentration ratio. The sulfate to thiosulfate concentration ratio has good predictive power across the range of ionic strengths examined, because sulfate and thiosulfate have the same charge and their concentration ratio should be preserved in the pit cavity. This is not the case for chloride–thiosulfate pitting. In Alloy 600, much less thiosulfate is required for chloride–thiosulfate pitting than for sulfate–thiosulfate pitting. This may not be a general observation across the range of NiCrFe systems. Ni seems to be more sensitive to S adsorption than Fe, so Ni-rich alloys require less thiosulfate. Another factor is that Alloy 600 is kinetically more noble, in the pit or pit-like environments, than the other alloys studied.
Acknowledgements This research was supported by NSERC (Canada) and by UNENE, the University Network of Excellence in Nuclear Engineering. The support, interest and input of K. Sedman (Bruce Power) and P. J. King (B&W Canada) are greatly appreciated. Thanks also to N. Senior for assistance with the preparation of this document.
References 1 R. C. Newman, H. S. Isaacs and B. Alman, Effects of sulfur-compounds on the pitting behavior of type-304 stainless-steel in near-neutral chloride solutions, Corrosion, 1982, 38, 261–265. 2 Z. Fang and R. W. Staehle, Effects of the valence of sulfur on passivation of alloys 600, 690, and 800 at 25 C and 95 C, Corrosion, 1999, 55, 355–379. 3 J. T. Ho and G. P. Yu, Pitting corrosion of Inconel-600 in chloride and thiosulfate anion solutions at low-temperature, Corrosion, 1992, 48, 147–158. 4 J. J. Park and S. I. Pyun, Stochastic approach to the pit growth kinetics of Inconel Alloy 600 in Cl- ion-containing thiosulphate solution at temperatures 25–150 C by analysis of the potentiostatic current transients, Corros. Sci., 2004, 46, 285–296. 5 R. Roberge, Effect of the nickel content in the pitting of stainless-steels in low chloride and thiosulfate solutions, Corrosion, 1988, 44, 274–280. 6 W. T. Tsai, Z. H. Lee, J. T. Lee, M. C. Tsai and P. H. Lo, Pitting and stresscorrosion cracking behavior of Inconel 600 alloy in thiosulfate solution, Mater. Sci. Eng., A, 1989, 118, 121–129. 7 I. J. Yang, localized corrosion of Alloy 600 in thiosulfate solution at 25 C, Corrosion, 1993, 49, 576–584. 8 R. C. Newman, Pitting of stainless alloys in sulfate-solutions containing thiosulfate ions, Corrosion, 1985, 41, 450–453. 9 R. C. Newman and K. Sieradzki, Electrochemical aspects of stress-corrosion cracking of sensitized stainless-steels, Corros. Sci., 1983, 23, 363–378. This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Faraday Discussions
Paper
10 R. C. Newman, W. P. Wong and A. Garner, A mechanism of microbial pitting in stainless-steel, Corrosion, 1986, 42, 489–491. 11 R. C. Newman, W. P. Wong, H. Ezuber and A. Garner, Pitting of stainless-steels by thiosulfate ions, Corrosion, 1989, 45, 282–287. 12 A. M. Brennenstuhl, T. S. Gendron and R. Cleland, Mechanisms of underdeposit corrosion in fresh-water cooled austenitic alloy heatexchangers, Corros. Sci., 1993, 35, 699–711. 13 P. Marcus and E. Protopopoff, Potential-pH diagrams for adsorbed species – application to sulfur adsorbed on iron in water at 25 C and 300 C, J. Electrochem. Soc., 1990, 137, 2709–2712. 14 P. Marcus and E. Protopopoff, Potential-pH diagrams for sulfur and oxygen adsorbed on nickel in water at 25 C and 300 C, J. Electrochem. Soc., 1993, 140, 1571–1575. 15 P. Marcus and E. Protopopoff, Potential-pH diagrams for sulfur and oxygen adsorbed on chromium in water, J. Electrochem. Soc., 1997, 144, 1586–1590. 16 A. Elbiache and P. Marcus, The role of molybdenum in the dissolution and the passivation of stainless-steels with adsorbed sulfur, Corros. Sci., 1992, 33, 261– 269. 17 P. Marcus and E. Protopopoff, Thermodynamics of thiosulfate reduction on surfaces of iron, nickel and chromium in water at 25 and 300 C, Corros. Sci., 1997, 39, 1741–1752. 18 P. Marcus and M. Moscatelli, The role of alloyed molybdenum in the dissolution and the passivation of nickel-molybdenum alloys in the presence of adsorbed sulfur, J. Electrochem. Soc., 1989, 136, 1634–1637. 19 R. C. Newman, The dissolution and passivation kinetics of stainless alloys containing molybdenum .2. Dissolution kinetics in articial pits, Corros. Sci., 1985, 25, 341–350. 20 A. J. Betts and R. C. Newman, The effect of alloyed molybdenum on the activation of anodic dissolution by reduced sulphur compounds, Corros. Sci., 1993, 34, 1551–1555. 21 J. Rossmeisl, E. Skulason, M. E. Bjorketun, V. Tripkovic and J. K. Norskov, Modeling the electried solid–liquid interface, Chem. Phys. Lett., 2008, 466, 68–71. 22 C. D. Taylor and M. Neurock, Theoretical insights into the structure and reactivity of the aqueous/metal interface, Curr. Opin. Solid State Mater. Sci., 2005, 9, 49–65. 23 H. Yang and J. L. Whitten, Adsorption of SH and OH and coadsorption of S, O and H on Ni(111), Surf.Sci., 1997, 370, 136–154. 24 N. J. Laycock, Effects of temperature and thiosulfate on chloride pitting of austenitic stainless steels, Corrosion, 1999, 55, 590–595. 25 E. G. Webb and R. C. Alkire, Pit initiation at single sulde inclusions in stainless steel-II. Detection of local pH, sulde, and thiosulfate, J. Electrochem. Soc., 2002, 149, B280–B285. 26 R. C. Newman and E. M. Franz, Growth and repassivation of single corrosion pits in stainless-steel, Corrosion, 1984, 40, 325–330. 27 J. R. Galvele, Transport processes and mechanism of pitting of metals, J. Electrochem. Soc., 1976, 123, 464–474. 28 S. M. Gravano and J. R. Galvele, Transport processes in passivity breakdown .3. Full hydrolysis plus ion migration plus buffers, Corros. Sci., 1984, 24, 517–534. Faraday Discuss.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 21 April 2015. Downloaded by Fudan University on 12/05/2015 19:22:05.
Paper
Faraday Discussions
29 N. J. Laycock and R. C. Newman, Localised dissolution kinetics, salt lms and pitting potentials, Corros. Sci., 1997, 39, 1771–1790. 30 T. Haruna, T. Shibata and R. Toyota, Initiation and propagation of stress corrosion cracks for type 304L stainless steel in chloride solutions containing thiosulfate, Corros. Sci., 1997, 39, 1935–1947. 31 R. C. Newman, R. R. Corderman and K. Sieradzki, Evidence for de-alloying of austenitic stainless steels in simulated stress-corrosion crack environments, Br. Corros. J., 1989, 24, 143–148.
This journal is © The Royal Society of Chemistry 2015
Faraday Discuss.
