Conference Paper FORMATION OF CORROSION PRODUCTS OF CARBON STEEL UNDER CONDENSER OPERATING CONDITIONS COMPANY WIDE CW-127420-CONF-001 Revision 0
Prepared by Rédigé par
Turner Carl - Senior Scientist
Reviewed by Vérifié par
Burton Gordon R. - Senior Chemist Approved by Approuvé par
Angell Peter - Manager - Component Life Technology Branch 2013/01/15 2013/01/15 UNRESTRICTED ILLIMITÉ
Atomic Energy of Énergie Atomique du Canada Limited Canada Limitée
Chalk River, Ontario Chalk River (Ontario) Canada K0J 1J0 Canada K0J 1J0 UNRESTRICTED CW-127420-CONF-001 - 1 - Rev. 0
FORMATION OF CORROSION PRODUCTS OF CARBON STEEL UNDER CONDENSER OPERATING CONDITIONS
Carl W. Turner, Atomic Energy of Canada Ltd., Canada, [email protected] Lisheng Chi, Atomic Energy of Canada Ltd., Canada, [email protected]
ABSTRACT Corrosion product transport studies conducted more than a decade ago showed that lepidocrocite, hematite and magnetite are the major components of suspended corrosion products filtered from water collected at the outlet of the Condensate Extraction Pump (CEP), Boiler Feed Pump (BFP), and High-Pressure Heater (HPH) in the steam cycles of pressurized water reactors. The presence of lepidocrocite and hematite in these samples was taken as evidence that the low-pressure section of the feed water system is oxidizing during power operation. It follows that a reduction in the concentration of dissolved oxygen in the condenser and an increase in hydrazine concentration in the feed water would be effective steps to reduce the concentration of fully oxidized corrosion products being transported with the feed water to the steam generators. A review of the literature on the formation of iron oxides under oxidizing conditions, however, has revealed that pH is a more important parameter than dissolved oxygen concentration for determining the phase of iron oxide that forms in the secondary heat-transport system at low temperature, i.e., temperature less than about 100C. Thus, magnetite has been shown to be the dominant phase of iron oxide under oxidizing, alkaline conditions, i.e., pH in the range 9.0 to 11.0, whereas the formation of lepidocrocite and goethite are favoured under mildly acidic to neutral conditions. Goethite also forms under strongly acidic and caustic conditions, but these conditions are not found in the feed water system. Hematite forms under strongly acidic and neutral conditions. It is well known that lepidocrocite and goethite will transform to hematite at elevated temperatures. The only conditions that prevail in the steam cycle during power operation are those that favour the formation of magnetite, yet corrosion product transport studies have consistently shown the presence of hematite and lepidocrocite in samples filtered during power operation. It is proposed that lepidocrocite is formed primarily during maintenance outages when air ingress can lead to the low pH conditions that favour formation of this phase. Once formed, lepidocrocite will transform to hematite at elevated temperatures, thus accounting for the presence of hematite in the corrosion product samples as well. Thus, the evidence from the literature survey of the formation of iron oxides does not support the contention that reducing the concentration of dissolved oxygen in the condenser will reduce the amount of fully oxidized corrosion product that is being transported to the steam generators during power operation. The implications of these findings on outage chemistry control in the secondary system are discussed in this paper.
Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 2 - Rev. 0
INTRODUCTION
Corrosion product transport studies conducted more than a decade ago showed that lepidocrocite, hematite, magnetite and sometimes goethite are the major constituents of suspended corrosion products filtered from water sampled at the discharge of the Condensate Extraction Pump (CEP), Boiler Feed water Pump (BFP), High-Pressure Feed water Heater (FW) and Boiler Blowdown (BD) in the steam cycles of Pressurized (Heavy) Water Reactors, PWRs and PHWRs [1],[2],[3],[4],[5],[6],7],[8]. The concentration of magnetite in the corrosion product samples varied widely from one plant to another and from one sampling location to another in the steam cycle, as shown by the data listed in Table 1 [6]. Note that the fraction of magnetite in the FW and BD at PWR plants tends to be higher than at PHWR plants. The lower fraction of magnetite at PHWRs compared to PWRs is likely a consequence of two main factors: 1) all PHWRs but one1 have a steam-heated direct-contact deaerator between the low-pressure FW heaters and the boiler feed pump that appears to act as a source of fully oxidized corrosion products to the FW, and 2) high-temperature drains contribute a larger fraction of the total feed water iron, mostly in the form of magnetite, at PWRs than at PHWRs. Table 1: The percentage of magnetite in corrosion product samples collected from various sampling points in the steam cycle of PWR and PHWR plants during power operation Reactor Type CEP FW BD % magnetite % magnetite % magnetite PWRa 25-65 30-84 50-96 PHWRb 37-65 14-59 44-83 a: From a database of over 30 measurements b: From a database of over 90 measurements The presence of fully oxidized iron oxides and oxyhydroxides, such as lepidocrocite, goethite and hematite, in the corrosion product samples has generally been taken as evidence that some regions of the condensate/feed water system are oxidizing during power operation [2],[5],[6]. This hypothesis is challenged in this paper, as discussed below. The evidence from corrosion product transport studies shows that the range of composition of corrosion products and, especially, the fraction of magnetite versus fully oxidized corrosion product is not strongly correlated with the concentrations of either dissolved oxygen or hydrazine in the condensate/feed water system during power operation. For example, a comparison of the results from 8 different PWR plants, all of which had < 2.5 g/kg dissolved oxygen at the CEP and reported FW hydrazine concentrations between 110 and 185 g/kg, shows the percentage of magnetite in the FW corrosion product samples ranging from 18 to 100% [5]. Three other plants with CEP dissolved oxygen ranging between 4 and 10 g/kg reported magnetite fractions in the FW between 35 and 72%, which is within the same range reported for plants that operate with lower concentrations of dissolved oxygen in the condensate. The CANDU plant at Embalse, which operates with < 5 g/kg dissolved oxygen and 2 to 3 g/kg hydrazine at the CEP, has reported 41% magnetite in the FW sample (together with 56% hematite and 3% lepidocrocite) [7], similar to the results from the PWR plants cited above which operate with comparable dissolved oxygen and much higher concentrations of hydrazine in their condensate/feed water systems. In addition, CANDU plants operating in Ontario with > 100 g/kg hydrazine in the FW have reported similar fractions of magnetite in the CEP to the Gentilly-2 nuclear power plant [6] that operated from 1984 to 1998 with no hydrazine addition to the steam cycle during operation at power. It is proposed in this paper that the composition of iron-based corrosion products in the secondary system of a nuclear power plant is determined primarily by the pH, and not by the concentration of dissolved oxygen. Furthermore, it is proposed that the fully oxidized phases of iron corrosion products, such as lepidocrocite and goethite, are formed not during power operation but, instead, when the plant is shut down for a maintenance outage, during which time air ingress introduces carbon dioxide to the water that produces the low-pH conditions (i.e., pH 6 to 7) that are conducive to the formation of these phases. Upon return to full power operation, both lepidocrocite and goethite will transform to hematite in the higher-temperature regions of the system, thus introducing hematite to the system. In this paper, we first review the literature on the preparation of iron oxides as a function of pH and dissolved oxygen concentration to determine the chemistry conditions that favour formation of the various phases of iron oxide that have been observed from the corrosion product transport measurements for the secondary side of nuclear power plants. This review will clearly demonstrate that pH is the most important that determines the major phase of iron oxide that precipitates under a given set of conditions, and not the concentration of dissolved oxygen.
1 Nucleoelectrica Argentina Sociedad Anonima (NASA) operates a CANDU PHWR plant at Embalse that does not have a deaerator in the condensate/feed water system.
Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 3 - Rev. 0
The results of past corrosion product transport investigations will be discussed in light of this result, and an argument will be proposed to support the hypothesis that the fully oxidized forms of the iron-based corrosion products are formed only during maintenance outages as the result of air ingress causing pH depression, and therefore their presence on corrosion product filters is not a measure of oxidizing conditions in the condensate/feed water system during power operation.
FORMATION OF IRON OXIDES AND OXYHYDROXIDES FROM SOLUTIONS OF IRON(II) AND IRON(III)
Formation of Magnetite Magnetite is commonly prepared at low temperature (< 100ºC) by oxidizing an Fe(II) solution under alkaline conditions using, for example, air, pure oxygen or KNO3 for the oxidant. If the oxidant is left out of this preparation, the product will be a white suspension of Fe(OH)2. For example, magnetite has been prepared by the drop-wise addition of a solution of KNO3 and KOH to a solution of FeSO4 at room temperature, followed by aging for 2 hours - - - at temperature of 90ºC [9],[10],[11]. Addition of the NO3 /OH solution continues until slightly more than 2 OH ions 2+ have been added per ion of Fe . All solutions are initially deoxygenated with a purge of N2 gas prior to use so that - - KNO3 will be the principal oxidant present in solution. After the addition of the NO3 /OH solution is complete, and the suspension is aged for 2 hours at 90C to produce a final product that is essentially pure magnetite. Sugimoto and Matijevic [11] report that the product is contaminated with goethite if oxygen is not excluded from the preparation. Kim et al. [12] investigated the effect of temperature and the ratio 2OH-/Fe2+ on the formation of magnetite under oxidizing conditions. Suspensions of Fe(OH)2 were prepared by adding controlled amounts of NH4OH to solutions - 2+ of FeSO4 to achieve ratios of 2xOH to Fe ranging from 0.1 to 10. The resulting suspensions were subsequently oxidized using a purge of pure oxygen at temperatures between 30 and 70ºC. The oxidation products were examined by X-ray diffraction (XRD) to determine the phases of iron oxide present. Figure 1 shows that the phases of iron oxide present in the product depend on the ratio 2OH-/Fe2+ and on the temperature. For example, magnetite is reported to be the sole product obtained when the ratio is greater than 1.5 and the temperature between 30 and 70ºC. When the ratio is less than 0.7, the product is a mixture of goethite and lepidocrocite at a temperature of 30ºC. With increasing temperature, the composition changes to goethite at intermediate temperatures and to a mixture of goethite and magnetite at higher temperatures.
Figure 1: Iron Oxides Formed by Oxidizing Solutions of Ferrous Sulphate as a Function of Temperature and the Ratio (2OH-/Fe2+) [12]
Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 4 - Rev. 0
A study conducted by Kiyama [13] gave similar results to those of Kim et al. for a 2OH/Fe of < 1. However, for a ratio > 1, Kiyama found that the phase composition of the reaction product also depends on temperature. With increasing temperature, the composition changes from goethite to a mixture of goethite and magnetite to pure magnetite. Kim et al. [12] attributed the difference at ratios 1.5 to the use of ammonia instead of sodium hydroxide. Both Tsuchia et al. [14] and Yang et al. [15] report the preparation of magnetite by air oxidation of ferrous solutions under alkaline conditions. At a temperature of 30C and pH25C 10, magnetite formation was reported to be complete within 50 minutes [14]. A yield of 92% magnetite was reported for oxidation at 90C for pH25C in the range 9.0 to 11.0 when NaOH was the alkalizing agent, and up to 95% when the alkalizing agent was ammonia [15]. Formation of Lepidocrocite Lepidocrocite is generally prepared by air-oxidation of an Fe(II) solution at room temperature at a pH close to the neutral point [16],[17],[18]. Typically, the Fe(II) solution is first deoxygenated by purging with nitrogen, followed by the addition of an alkalizing agent such as NaOH to pH25C in the range 6.7 to 6.9. This is followed by air oxidation to produce lepidocrocite. The colour of the solution/suspension changes during the synthesis from dark greenish blue to grey and finally to orange, indicating the formation of the final product. The conditions for the preparation of lepidocrocite correspond to those for which the 2OH-/Fe2+ ratio is less than 1, and are consistent with the results shown in Figure 1. The presence of carbonate and, to a lesser extent, sulphate promotes formation of goethite at the expense of lepidocrocite [19], while the presence of phosphate has the opposite effect [20]. Carlson and Schwertmann observed that the fraction of goethite in the final product increased with increasing concentration of carbonate for air oxidation at pH 7, such that at the product changed from pure lepidocrocite to pure goethite as the concentration of carbonate was increased from zero to 100 mM [21]. Note that, once formed, both lepidocrocite and goethite transform to hematite via dehydration/condensation reactions at temperatures in excess of 100ºC. Formation of Goethite and Hematite Goethite and hematite are both prepared by hydrolysis of an Fe(III) solution via a ferrihydrite intermediate. With time and temperature, the ferrihydrite intermediate transforms to the more thermodynamically stable goethite or hematite, depending on the conditions. For example, forced acid hydrolysis of a Fe(III) solution (OH/Fe 0) at a temperature of 80ºC produces a 6-line ferrihydrite which can be transformed to hematite by ageing the suspension at pH 7 and a temperature of 25C [22],[23]. It has been proposed that the reaction to produce hematite proceeds by an internal structural rearrangement of agglomerated ferrihydrite nuclei since the un-agglomerated particles in suspension remain as unaltered ferrihydrite. Hydrolysis of Fe(III) solutions at room temperature and pH 7 (OH/Fe 3) results in the formation of a 2-line ferrihydrite [22],[23]. The 2-line ferrihydrite can subsequently be converted to goethite by ageing the suspension under either acidic [24],[25],[26] or alkaline 22],[27] conditions via a mechanism involving dissolution and re-precipitation of the 2-line ferrihydrite. In practice, the relative amounts of goethite and hematite that form from 2-line ferrihydrite changes continuously as a function of pH, with the formation of goethite being favoured near pH 4 and pH 12 and formation of hematite being favoured at pH between 7 and 8 [23],[28]. Summary of Results of Literature Review The conditions that favour formation of specific phases of iron oxide by the oxidation and hydrolysis of solutions of Fe(II) and Fe(III) are summarized in Table 2. Note that all preparations involve hydrolysis of iron(II) or iron(III) solutions under oxidizing conditions, i.e., they either start with a solution of Fe(III) and hydrolyse it, or they oxidize the hydrolysis products of an Fe(II) solution. The main parameter that determines the dominant phase of iron oxide in the final product, however, is pH. Of particular interest in the context of this paper is that both lepidocrocite and magnetite form by oxidation of solutions of Fe(II), the only difference being the pH at which the oxidation takes place. Insight into the origin of the dominant influence of pH on the phase of iron oxide formed by hydrolysis and oxidation reactions of Fe(II) is provided in the following Section.
Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 5 - Rev. 0
Table 2: Summary of Conditions for Preparation of Iron Oxides
Formation Formation Conditions Iron Oxides Pathways Environment pH25°C Temp. (ºC) Comment Magnetite Fe2+ Oxidizing, alkaline 9 to 11 25 to 90 - Presence of phosphate promotes Lepidocrocite Fe2+ Oxidizing, neutral 6 to 7 25 formation of lepidocrocite relative to goethite Presence of bicarbonate and sulphate promote Fe2+ Oxidizing, neutral 7 25 formation of goethite Goethite relative to lepidocrocite Hydrolysis at 3+ Hydrolysis, 25 for hydrolysis Fe pH 7. Aging at via 2-line ferrihydrite followed by aging and aging either 4 or 12 Acid hydrolysis 1 to 2 (hydrolysis) 80 (hydrolysis) via 6-line ferrihydrite followed by aging 7 (aging) 25 (aging) 3+ Hematite Fe Aging of 7 for hydrolysis 25 (hydrolysis) neutralized solution via 2-line ferrihydrite and aging 70 (aging) (OH/Fe = 3) COMPARISON OF RESULTS WITH HYDROLYSIS AND OXIDATION BEHAVIOUR OF AQUEOUS FE(II) + Hydrolysis of a solution of Fe(II) results in the formation of range of hydroxide species such as Fe(OH) , Fe(OH)2, - Fe(OH)3 , etc. The relative concentrations of the various hydroxide species are strongly dependent upon the pH, as illustrated in Figure 2 for a 10-3 M solution of Fe(II). At neutral pH where lepidocrocite and goethite (in the presence of carbonate) are the dominant reaction products under oxidizing conditions, the Fe(II) solution is dominated by the species Fe2+ and FeOH+, whereas at an alkaline pH the solution consists of comparable + - concentrations of the species Fe(OH) , Fe(OH)2 and Fe(OH)3 . The ratio of OH to Fe(II) is called the hydroxyl number, and it is evident from the Figure that the average hydroxyl number of an Fe(II) solution increases from essentially 0 at pH 0 to effectively 3 at pH 14.
Figure 2 Log species – pH25°C Diagram of Fe(II) Species in Aqueous Solution [29]
Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 6 - Rev. 0
The strong effect of pH on the relative concentrations of the various hydrolysis products is only part of the story as to why pH has such a dominant effect on the phase of iron oxide formed by hydrolysis and oxidation of Fe(II) solutions. The other part of the story is the significantly different magnitudes of the rate constants for oxidation of the various hydrolysis products, depending on their hydroxyl number. Morgan and Lahav investigated the effect of pH, and thereby hydroxyl number, on the rate of oxidation of aqueous Fe(II) and found that the rate of oxidation increased significantly with hydroxyl number [30]. Upon fitting their reaction rate data to the following overall rate equation:
Equation 1
-5 -1 Morgan and Lahav reported rate constants k0, k1, and k2 equal to 6 × 10 , 1.7 and 4.3 × 105 (min ), respectively. Thus, the rate constants for the oxidation of iron(II) species in solution increase by about 5 orders of magnitude for each integral increase in hydroxyl number from n = 0 to n = 2. For near-neutral conditions, i.e., for pH ranging from about 6 to 7, although the concentration of Fe2+ is from 2 to 3 orders of magnitude greater than the concentration of FeOH+, the rate constant for oxidation of FeOH+ is nearly 5 orders of magnitude greater than the rate constant for the oxidation of Fe2+. Therefore, the phase of iron oxide that forms under near-neutral conditions, i.e., lepidocrocite or goethite in the presence of carbonate, must proceed via a reaction pathway that is dominated by the oxidation of FeOH+, with relatively little contribution from the oxidation of Fe2+. Similarly, oxidation of aqueous Fe(II) under alkaline conditions, i.e., pH25C 10 to 11, that leads to the formation of magnetite must be dominated by oxidation of - Fe(OH)2 and, perhaps, Fe(OH)3 . Equation 1 together with Figure 2 also accounts for why oxidation of aqueous Fe(II) under alkaline conditions does not lead predominantly to the formation of goethite via prior oxidation of Fe2+ to Fe3+. Although a discussion of why the oxidation of Fe(OH)+ should lead to a different phase of iron oxide than oxidation - of Fe(OH)2 or Fe(OH)3 is beyond the scope of this investigation, what this work has provided is a clearer understanding of why pH plays such a decisive role in determining the phase of the iron oxide formed by hydrolysis and oxidation of Fe(II) solutions. COMPARISON WITH CORROSION PRODUCT TRANSPORT MEASUREMENTS Investigations of corrosion product transport in the condensate/feed water systems of PWR and PHWR plants that are summarized in Table 1 show that lepidocrocite, goethite, hematite and magnetite are the major phases of iron oxide found in water samples taken from various locations throughout the steam cycle at locations. All of the locations represented by the data listed in Table 1, however, are exposed to alkaline conditions during power operation. Therefore, based on the discussion in the previous Section, corrosion product formed in all of these locations should be predominantly magnetite. In addition, scrape samples taken from the walls of piping and components during maintenance outages show [31],[32]: Lepidocrocite on the piping between the moisture separator reheater and the low-pressure turbine Magnetite, lepdocricite, goethite and hematite on the condenser wall below the water line Hematite in the moisture separator drains tank, deaerator storage tank and downstream of check valves Magnetite, hematite and goethite on the walls of the deaerator storage tank Lepdocrocite on a wet carbon steel surface in the condenser, where it apparently grew during the outage. The presence of the fully-oxidized forms of iron oxide, i.e., lepdocrocite, goethite and hematite, on the surfaces of piping and components implies that the environment under which these corrosion products were formed must have been close to neutral pH. The chemistry environment throughout the steam cycle during power operation is alkaline during power operation, and thus should favour the formation of magnetite by the hydrolysis of Fe2+ under oxidizing conditions. There is no location in the condensate/feed water system where the pH during power operation is low enough to favour formation of lepidocrocite, goethite or hematite. Based on the above considerations, therefore, one might expect to find only magnetite in the condenser, heater shells and piping systems in the steam cycle, and no lepidocrocite, goethite or hematite. During shutdowns, however, sections of the steam cycle are open to the atmosphere for maintenance activities. Air ingress accompanied by dissolution of carbon dioxide in the secondary coolant could possibly establish the near-neutral pH conditions that are conducive to the formation of the more oxidized forms of iron oxide such as lepidocrocite and goethite on the surfaces of carbon steel during a maintenance outage. Sections that have been drained but not fully dried will be particularly susceptible to the establishment of a low-pH environment in the film of water covering the wet surfaces of carbon steel because the carbon dioxide will not have to diffuse very far to reduce the pH of the film to near-neutral conditions. Upon return to power following the maintenance outage, both lepidocrocite and goethite will tend to transform to hematite in the higher-temperature
Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 7 - Rev. 0 sections of the system, contributing further to the formation of hematite. Fully-oxidized corrosion products that have formed under low-pH conditions during the maintenance outage will be transported during full-power operation via heater drains to the condenser hotwell and to the deaerator storage tank where they will contribute to corrosion product transport to the SGs during power operation. IMPACT OF AIR INGRESS ON pH DEPRESSION OF WETTED SURFACES
Figure 3 shows the results of the calculations of pH versus the quantity of CO2 required to reduce the pH of solutions of selected volatile amines from 9.5 to 5.7. Because the calculations were done for an open system where CO2 will continue to dissolve as it is neutralized by the volatile amine, the amount of CO2 required to reduce the pH to a given value is expressed as the amount relative to the total amount of CO2 in an air-saturated solution of given volume at 25ºC. Figure 3 shows that the quantity of CO2 required to reduce the pH to a given value increases with decreasing base strength of the volatile amine, as expected. Thus, the weakest base, morpholine, is the most effective buffer against a reduction in pH caused by air ingress, requiring 160 times the amount of CO2 in air-saturated water to reduce the pH at 25ºC from 9.5 to 5.7. Additional calculations show that the time required to reduce the pH of a film of water on the surface of carbon steel in a system that has been drained but not fully dried is short compared to the duration of a typical maintenance outage. For example, using Fick‟s 2nd law and a diffusion coefficient for carbon dioxide in water equal to 1.88x10-5 cm2/s at 25ºC [33], it can be shown that a 100 micron-thick film of water will reach the saturation concentration of carbon dioxide within about 20 minutes. Figure 3 shows that it requires about 40-times the amount of carbon dioxide found in an air-saturated solution of morpholine, for example, to reduce the pH from 9.5 to 6.7, the pH at which lepidocrocite has been shown to be the major phase of iron oxide that precipitates from a solution of Fe(II). Thus, for any of the amines used for pH-control in the steam cycle listed in Figure 3, it should still take no more than a day or two to reduce the pH of a water film to the region that is conducive to the formation of lepidocrocite and/or goethite by the hydrolysis and oxidation of the Fe2+ ions released by the corrosion of carbon steel. 9.7 25 oC Ammonia 9.2 Morpholine 8.7 Dimethylamine Ethanolamine 8.2
pH 7.7 7.2 6.7 6.2 5.7 0 20 40 60 80 100 120 140 160
Relative amount of CO2
2 Figure 3: Effect of Relative Amount of CO2 on pH25ºC of an Aqueous Surface Film a Containing Dissolved Volatile Amine
SUMMARY AND IMPLICATIONS FOR CHEMISTRY CONTROL IN THE SECONDARY SYSTEM Corrosion product transport studies conducted more than a decade ago showed that lepidocrocite, goethite, hematite and magnetite were the major components of corrosion products filtered from water taken at the outlet of the CEP, BFP, and HPHO. The presence of lepidocrocite, goethite and hematite in these samples was taken as evidence that some parts of the feed water system were oxidizing during power operation. It followed that a reduction in concentration of dissolved oxygen in the condenser and an increase in hydrazine concentration would be effective ways to reduce the concentration of fully oxidized corrosion products that were being transported with
2 The relative amount of CO2 is the amount of CO2 taken up by the film of water divided by the total amount of CO2 in a film of water that is saturated in air.
Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 8 - Rev. 0 the feed water to the SGs. A recent review of the literature on the formation of iron oxides under oxidizing conditions, however, has revealed that pH is the major parameter that determines the phase of iron oxide that forms by oxidation of hydrolysed solutions of Fe(II). Magnetite has been shown to be the dominant phase of iron oxide when aqueous Fe(II) is oxidized under alkaline conditions, i.e., pH25ºC in the range 9 to 11. Lepidocrocite and goethite are favoured by oxidation of aqueous Fe(II) under near-neutral conditions, i.e., pH25ºC in the range ~ 6 to 7, their relative proportions depending on the presence of impurities, especially carbonate which promotes the formation of goethite at the expense of lepidocrocite. Dehydration/condensation reactions of lepidocrocite and goethite at elevated temperature appear to be the most plausible routes to the formation of hematite that is found in various locations throughout the steam cycles of PWR and PHWR plants. These results are consistent with the effect of pH on the speciation of aqueous Fe(II) combined with the relative rates of oxidation of Fe(II) hydrolysis products as a function of hydroxyl number. The implication of these results is that the relative amount of fully oxidized corrosion product in samples taken from the steam cycle is not influenced by the concentration of dissolved oxygen during power operation when the environment throughout the steam cycle is alkaline. Rather, the relative amount of fully oxidized corrosion product in samples taken from the steam cycle will be determined primarily by the extent to which the pH of the water and of wetted surfaces of carbon steel is depressed by air ingress and dissolution of CO2 during maintenance outages. Therefore, plants that are intent on reducing the amount of fully oxidized corrosion products, i.e., lepidocrocite, goethite and hematite, transported to the SG during power operation need to shift their focus from reducing the concentration of dissolved oxygen during power operation to mitigating pH-depression during maintenance outages. This can be achieved by taking steps to mitigate air ingress to the secondary system and/or ensuring that drained systems are fully dried during a maintenance outage. The conclusions of this investigation are based on the assumption that insights gained from the literature on solution preparations of iron oxides are applicable to the formation of iron oxide corrosion products on the corroding surface of carbon steel. Measurements of the phases of iron oxide that grow on the surface of carbon steel under oxidizing alkaline and oxidizing neutral conditions would be very useful for either confirming or refuting the conclusions drawn in this investigation. ACKNOWLEDGEMENTS The authors thank the CANDU Owners Group for funding this work and AECL for granting permission to publish.
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Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27. UNRESTRICTED CW-127420-CONF-001 - 9 - Rev. 0
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Presented at the Nuclear Plant Chemistry Conference (NPC) 2012 in Paris, France on 2012 September 23 to 27.