Patent Application
20090038712
1. Field of the Invention
The present invention relates to a method of chemically treating aluminum and its alloys in a manner that reduces corrosion thereof, especially under, but not limited to, containment water conditions found in a nuclear power plant loss-of-coolant accident situation and to the treated aluminum and aluminum alloy materials.
2. Description of the Related Art
The Nuclear Regulatory Commission (NRC) has regulatory authority over the safety of nuclear power plants in the United States. A scenario under investigation for many years is the hypothetical loss-of-coolant accident (LOCA), in which a pipe break in the primary cooling loop releases hot pressurized water and steam into the containment building. The collateral damage following a LOCA can be exacerbated by corrosion of metals that are submerged in the pool of cooling water that forms in the containment building. The cooling water in a pressurized water reactor (PWR) is demineralized water containing a low concentration of lithium hydroxide (LiOH) (≦0.7 mg/L) and up to 16,000 mg/L of boric acid (H3BO3). This solution is highly corrosive. Following a LOCA, additional chemicals may be present in the containment pool water depending on the kind of pipe insulation used in the containment building (fiberglass or molded rigid calcium silicate) and the chemicals (sodium hydroxide, trisodium phosphate, or sodium tetraborate) used in the emergency core cooling system (ECCS) to raise pH and minimize volatilization of radioactive iodine. In addition, the corrosion/dissolution of concrete and metals and the suspension of dust and debris from containment surfaces can also contribute to the complexity of the chemical properties of the containment pool water. As a result, the containment pool water following a LOCA has a unique and complex chemistry that contributes to the corrosion of any metals submerged in the pool.
The University of New Mexico and Los Alamos National Laboratory, under a contract with the NRC, conducted an extensive experimental investigation of the corrosion potential of aluminum alloy under conditions representative of the containment pool following a LOCA. The general results of that study are reported elsewhere [see references 1,2,3,4,5,6,7]. The conclusions pertinent to this application relate to the corrosion of aluminum alloy, which was found to be the most corrodible metal under the test conditions. In one test, the corrosion of aluminum was rapid enough to cause precipitation as aluminum hydroxide when the solution cooled after only 24 hours. Precipitate formation is of special concern because precipitates can circulate with the water and, in combination with insulation debris, clog the ECCS recirculation sump screen. A possible consequence of precipitation and clogging could be severe degradation of the ECCS's ability to perform its function of cooling nuclear fuel in the reactor core during controlled shutdown of the plant.
Test results also indicate that differences in test conditions caused substantial differences in the rate of aluminum corrosion. In two tests, the rate of corrosion was orders of magnitude lower than would be predicted based on the solution chemistry.
Aluminum is a reactive metal. In air, it reacts to form a thin, transparent oxide film over the entire exposed aluminum surface [reference 8]. This film controls the rate of corrosion and protects the substrate aluminum metal. In water, however, the air-formed film breaks down [reference 9]. The degree of corrosion depends on the relative rates of film repair/growth and film breakdown [reference 9]. It has been proposed that the dominant mechanism of pure aluminum corrosion is mainly determined by the anodic reaction in alkaline solution [references 10,11]. The mechanism involves consecutive oxide film formation and dissolution and simultaneous water reduction. The reported reaction is:
2Al+6H2O+2OH−→3H2+2Al(OH)4− (1)
The corrosion rate depends on solution pH, temperature, fluid velocity, surface-to-volume ratio, duration of submersion, and presence of anions (e.g., Cl−, NO3−, SO42−, and H3SiO4−) [references 9,11,12,13,14]. The corrosion rate increases as temperature increases [reference 13], as surface-to-volume ratio increases, and as fluid velocity increases up to 4 cm/s [references 12,15]. Aluminum corrosion also increases rapidly as pH increases above 6 [references 11,13,16,17]. After 20 days of immersion at 60° C., it is reported that the corrosion rate at pH=10 (0.048 g/m2·h) was more than five times higher than the corrosion rate at pH=8 (0.009 g/m2·h) [reference 13].
Solution conditions following a LOCA have the potential to dramatically increase the corrosion rate of aluminum. The effect of boric acid and its conjugate base, borate, has been evaluated in experiments by several groups of researchers at pH values ranging from 9.2 to 10 and temperatures ranging from 50 to 130° C., as shown in Table 1 [references 18,19,20].
1Methods: 1 = electrical resistance, 2 = polarization resistance, 3 = current impedance, 4 = weight loss.
A strong dependence on temperature is evident, with increasing temperature resulting in faster aluminum corrosion rates. At 60° C., Jain, et al. [reference 18] reported aluminum corrosion rates ranging from 0.459 to 1.22 g/m2·h, which are 1 to 2 orders of magnitude higher than rates measured at similar temperature and pH without borate. Experimental measurements by Griess and Bacarella [reference 19] and Piippo, et al. [reference 20] generally support these values. Griess and Bacarella [reference 19] conducted experiments on the corrosion of various metals, including six alloys of aluminum, in solutions containing 0.28 M H3BO3 and 0.15 M sodium hydroxide (NaOH) at 55° C. and 100° C. For aluminum alloy 3003, which is the alloy used in the current experiments, the reported corrosion rates were 0.59 g/m2·h at 55° C. and 8.9 g/m2·h at 100° C. In short, although there is variability in the measured corrosion rates from different researchers and with different methods, it is clear that the corrosion rate of aluminum in solution conditions following a PWR LOCA is 1 to 2 orders of magnitude higher than at similar pH and temperature when there is no borate in solution.
Under high pH conditions, the presence of silicate has been observed to minimize corrosion [reference 14]. Labbe and Pagetti [reference 14] indicated that silicate caused an effective inhibition of aluminum corrosion under conditions used for cleaning operations in the dairy industry (60° C. and 0.1 N NaOH, corresponding to pH=13), when the silica concentration was 3,600 mg/L as SiO2. Lee and Babić [ reference21] found that silicon dissolved into 2.3 M KOH solutions (corresponding to pH>14) could passivate aluminum, but only when the concentration was greater than 126,000 mg/L as SiO2. So, while silicate passivation of aluminum has been noted in the literature under high pH and high silicate conditions, it is important to note that there is no existing literature on the ability of silicate to passivate the surface of aluminum or aluminum alloys in solution conditions similar to those present following a LOCA, when high concentrations of borate are present.
The present invention provides a method for chemically treating aluminum and its alloys using an aqueous treatment in a manner that reduces aqueous corrosion thereof. The present invention is especially useful under representative LOCA containment pool cooling water conditions when high contents of borate are present, although the invention is not so limited since the chemical treatment method can be used to reduce corrosion in other service applications or as a pretreatment of aluminum and aluminum alloys materials before exposure to a corrosive environment.
In an illustrative embodiment of the invention, a chemical treatment method involves providing a corrosion-reducing agent comprising silicon in the reactor cooling water following a loss-of-coolant accident or in other aqueous solutions in a manner effective to reduce corrosion of aluminum or aluminum alloys exposed to the cooling water/aqueous solution. The corrosion-reducing agent preferably comprises a dissolvable compound of silicon and oxygen, such as a silicate compound for purposes of illustration and not limitation. Practice of the method can reduce severe corrosion of aluminum and aluminum alloy components loss-of-coolant accident when the aluminum or aluminum alloy surfaces are in contact with the chemicals used for the emergency reactor core cooling system at PWR nuclear power plants. Practice of the invention after a loss-of-coolant accident can reduce aluminum or aluminum alloy corrosion during emergency cooling of the nuclear reactor and thereby reduce transportable chemical products that can exacerbate blockage of the recirculation sump screen.
In another illustrative embodiment of the invention, a chemical pretreatment method involves contacting an aluminum or aluminum alloy material with an aqueous solution having silicon therein in a manner to form a protective silicon-bearing layer or coating thereon before exposure of the material to a corrosive environment.
In still another embodiment of the invention, a treated aluminum or aluminum alloy material is provided having a protective silicon-bearing layer or coating thereon. The protective coating can comprise aluminum and silicon, such as aluminosilicate for purposes of illustration and not limitation.
The present invention is advantageous to reduce aluminum corrosion in an LOCA in that the release of aluminum ions and subsequent precipitation of aluminum hydroxide can be reduced, potentially reducing or eliminating the chemical product contributions to clogging in the ECCS and to reduce aluminum corrosion in other aqueous environments. The present invention also is advantageous as a pretreatment to protect industrial aluminum components and structures before exposure in a variety of adverse chemical environments.
These and other advantages of the present invention will become readily apparent from the following drawings taken with the detailed description.
a shows an SEM (scanning electron microscope) image of new aluminum alloy coupon and
a, 4b, and 4c show SEM images of aluminum alloy coupons after being soaked in the bench experimental solutions at 60° C. for 30 days. All solutions contained 0.259 M H3BO3 and were initially adjusted to pH=9.5 using NaOH. The solution of
a and 5b show the XPS spectra (vertical axis CPS and horizontal axis binding energy (eV)) on the aluminum alloy coupons (
The present invention provides a method for chemically treating aluminum and its alloys using an aqueous treatment in a manner that reduces aqueous corrosion thereof. The present invention can be practiced to reduce corrosion of aluminum metal and aluminum alloys where aluminum is 50 weight % or more of the alloy. Illustrative aluminum alloys include, but are not limited to, the 1000 through 8000 series aluminum and aluminum alloy materials (International Alloy Designation System). The aluminum or aluminum alloys can be made or available in various wrought, cast, or other forms such as including, but not limited to, machined components, forged components, cast components, hot and/or cold rolled plate or sheet components, consolidated powder components, films or layers, and other forms.
The present invention is especially useful to reduce corrosion of aluminum and aluminum alloys under representative LOCA containment pool water conditions when high contents of borate are present. However, practice of the invention is not so limited since the chemical treatment method can be used to reduce corrosion in other service applications or as a pretreatment of aluminum and aluminum alloys materials before exposure to a corrosive environment. For example, the method of the invention is useful as a chemical pretreatment method involving contacting an aluminum or aluminum alloy material with an aqueous solution having a corrosion-reducing agent comprising silicon therein in a manner to form a protective silicon-bearing coating thereon before exposure of the material to a corrosive environment.
Practice of the chemical treatment method of the invention involves providing a corrosion-reducing agent comprising silicon in the reactor cooling water (e.g. the containment pool water) following a loss-of-coolant accident, or in another aqueous solution environment, in a manner effective to reduce corrosion of aluminum or aluminum alloys exposed to the containment pool water or other aqueous solution environment. The corrosion-reducing agent preferably includes a water-dissolvable compound of silicon and oxygen and includes, but is not limited to, a compound such as silicic acid, sodium silicate, disodium metasilicate, calcium silicate, or other silicate-containing compounds. The corrosion-reducing agent can be provided in the reactor cooling water (e.g. containment pool water) during an LOCA scenario by one or more of several techniques including, but not limited to: (a) injecting dissolved silicate solution into emergency cooling water sprays, (b) injecting dissolved silicate solution into cooling water pools, and (c) exposing caches of dry silicate-containing compounds to spilled cooling water.
The chemical treatment method can be practiced to reduce corrosion of aluminum and its alloys in containment pool water following a loss-of-coolant accident, or in another aqueous solution environment, where the pH is in the range of about 7 to about 11 and borate is initially present in an amount of about 2,000 to about 2,800 mg/L (measured as boron, but present as total borate, including boric acid, conjugate bases of boric acid, and any polyborate species formed during the dissolution of boric acid or borax) of the containment pool water or aqueous solution at a temperature in the range of ambient temperature to 150 degrees C. Containment pool cooling water may be in the temperature range of 50 to 150 degrees C. after an LOCA for example. For example, practice of the method can reduce severe corrosion of aluminum and aluminum alloy components loss-of-coolant accident when the aluminum or aluminum alloy surfaces are in contact with the chemicals used for the emergency reactor core cooling system at PWR nuclear power plants. The chemical treatment method can be effective within 48 hours of immersion time for standard aluminum alloys described above with existing air-induced oxidation present on the aluminum or alloy surface and nominal concentration of 90 mg/L Si in the reactor cooling water (e.g. containment pool cooling water) or aqueous treatment solution. In practice of the invention, the content of silicon in the reactor cooling water or aqueous solution can be relatively low in an effective amount to reduce corrosion in the range of greater than zero to not exceeding about 1,000 mg/L as Si, preferably greater than zero to not exceeding 500 mg/L as Si of the cooling water or aqueous solution, and even more preferably from about 20 mg/L to about 100 mg/L as Si of the cooling water or aqueous solution. A content of silicon in the reactor cooling water or aqueous solution in an amount of about 45 to about 100 mg/L of the cooling water or aqueous solution was especially useful to reduce corrosion.
Practice of the present invention produces a treated aluminum or aluminum alloy material that includes a protective silicon-bearing coating on a surface thereof. The protective coating can comprise aluminum and silicon, such as aluminosilicate for purposes of illustration and not limitation, which protects or passivates the surface to reduce corrosion in containment pool water following a loss-of-coolant accident and in another aqueous solution environments of the types described above.
The following EXAMPLE is offered to further illustrate but not limit the present invention.
Two types of experiments are described below: pilot tests and bench tests. The pilot tests were conducted in a large (946 L) tank and identified the ability of silicate to passivate aluminum under realistic LOCA conditions, when other metals, concrete, insulation, and other dust and debris were present in the aqueous system. The bench tests were conducted in 1 L containers and were controlled experiments that demonstrated that silicate is the specific species that passivates aluminum surfaces under the conditions present in the pilot tests. The experimental setup is described in more detail in the following paragraphs.
The pilot tests simulated the expected containment pool chemistry during the recirculation phase of a LOCA. Five tests, each run for 30 days, explored a range of chemical conditions, as detailed in Table 2. Tests 1 through 4 addressed a 2×2 matrix that varied the pipe insulation material and the chemical used to establish the initial pH. Some operating nuclear power plants primarily use fiberglass pipe insulation, while others use a combination of fiberglass and calcium silicate. During a LOCA, some plants have systems that inject aqueous NaOH into the containment spray water to raise the pH, while others have baskets of dry trisodium phosphate (TSP or Na3PO4·12H2O), which dissolves and raises the pH. Therefore, Tests 1 through 4 evaluated the effect of each pH chemical with each insulation combination. Test 5 represented the chemistry of an ice-condenser plant, where pH in the containment pool is established by the addition of sodium tetraborate (Na2B4O7·10H2O). Other chemical concentrations are lower in ice-condenser plants because of dilution of the pool water by a melting ice column. The chemical doses and pH ranges shown in Table 2 are representative of actual values expected in operating nuclear power plants of each type.
The primary components of the pilot test system were a test tank, a recirculation pump, and instrumentation. The test tank was made of 304 L stainless steel (SS) and was equipped with heating elements and water spray nozzles to simulate the post-LOCA scenario. The volume of the test solution was 946 L (250 gal). The tank contained aluminum alloy, carbon steel, galvanized steel, zinc-primer-topcoated steel, and copper coupons, a formed concrete slab, pulverized concrete, pipe insulation materials, and “latent debris” (dust and debris that was representative of materials found on the surfaces of a containment building). Added chemicals included H3BO3, LiOH, hydrochloric acid (HCl), and the appropriate pH-adjustment chemicals.
All corrosion coupons, chemicals, and other materials added to the tank were scaled in proportion to actual conditions in operating nuclear power plants using an area:volume ratio, a volume:volume ratio, or a concentration as appropriate. The demineralized water used for the solution was produced by a reverse osmosis system and had a conductivity less than 15 μS/cm. The solution temperature was maintained at 60±3° C. throughout the tests. The solution was circulated at 94.6±5 L/min during the entire test to achieve representative fluid velocities over the submerged corrosion coupons. Monitoring of the tests included collection of grab samples and continuous on-line monitoring of recirculation flow rate, test solution temperature, and pH. Solution samples were collected daily, fiberglass insulation samples were collected periodically during the tests, and metallic coupons were examined after the tests. All sampling procedures and water quality analyses were conducted in accordance with appropriate standard methods and approved project instructions [reference 2 which is incorporated herein by reference].
Aluminum alloy 3003 was used in this study. Each aluminum alloy coupon was 30.5-cm (12-in) square and 0.16-cm ( 1/16-in) thick, with an average weight of about 392 g. In each test, three aluminum alloy coupons were submerged in the solution. A chlorinated polyvinyl chloride (CPVC) coupon rack provided sufficient separation between the test coupons to prevent galvanic interactions.
As is evident from the description above, the pilot tests were conducted in a very complex chemical system that was designed to include many possible interactions between materials that might occur in a containment building during a real LOCA. While this design has clear advantages for identifying unintended interactions, the disadvantage is that it is not always possible to determine which constituents or interactions might be contributing to a particular observed effect. Thus, bench-scale tests were conducted to provide a more controlled environment that could be used to formulate a mechanistic understanding of results observed in the pilot tests.
The bench tests described here focused on identifying conditions that could affect aluminum corrosion under representative LOCA solution conditions. Three 1-L solutions were used for the test. All three contained 16,000 mg/L H3BO3. The first container was a blank for comparison to the pilot tests, the second contained 88.7 mg/L silicon (added as Na2SiO3·9H2O), and the last contained 50 mg/L calcium (added as CaCl2). The selected concentration of silicon or calcium was based on their measured concentrations near the end of pilot Test 4. The test was conducted in capped, 1-L, Nalgene® bottles. All solutions were preheated to 60° C. and pH was adjusted to 9.5 using NaOH before small aluminum coupons were placed into the containers. The solution was semi-open to the atmosphere, since air contact occurred during sampling procedures. The temperature of the solutions was maintained at 60° C. in a constant temperature oven for 30 days.
The 3003 aluminum alloy used in the bench tests was the same as that used in the pilot tests. Each aluminum alloy coupon was 24×13×1.8 mm and weighed about 1.59 g. In contrast to the pilot tests, the solution of the bench tests was mostly stagnant. Prior to the test, the aluminum coupons were ultrasonically cleaned in 95-percent ethanol, rinsed with ultra-pure water (R=18.2 MΩ·cm), and dried in air. The deionized water used in the tests was obtained from a research-grade laboratory water purification system.
Throughout the pilot test series, daily solution samples were obtained for measurements of pH and elemental concentrations. An inductively-coupled plasma / optical emission spectrometer (ICP-OES) (Optima 5300 DV, Perkin Elmer) was used to analyze Al, Ca, and Si concentrations in the solution. The weight loss method provided a second measure of corrosion rates. Prior to weight measurements, the coupons were dried in air but no techniques were employed to remove corrosion products adhered to the surface. Since the presence of corrosion products on the coupon surface could interfere with corrosion rate calculations, weight loss measurements and solution concentrations were used together to evaluate the rate of corrosion in these tests.
Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) (Model JXA-8200, JEOL) analyses were conducted on the metal coupons. The samples were coated with Au/Pd to enhance the conductivity of the surface. X-ray photoelectron spectrometry (XPS) was also used to examine the surface of the aluminum alloy samples before and after corrosion. In the XPS measurements, a Kratos Axis Ultra spectrometer provided the chemical composition of the coupon subsurface down to a depth of 10 nm.
The aluminum concentration in solution was used as an indicator of the rate of aluminum corrosion. The aluminum concentration in solution, total mass of aluminum in solution, and predicted aluminum solubility in each test are shown in Table 3. The results show relatively significant corrosion in Tests 1 and 5, but nearly no corrosion in Tests 2, 3, and 4.
In Tests 2, 3, and 4, the aluminum concentration was near or below the detection limit (i.e., 0.5 mg/L) during the entire 30-day test. Aluminum concentrations in daily samples from the remaining two tests are shown in
BDL1
1BDL = below detection limit, i.e., 0.5 mg/L.
2Visual Minteq v. 2.30.
3Measured at the end of the 30-day test.
4The corrosion rate in Test 2 is based on the weight loss after the 30-day test; the minimum corrosion rates for Tests 3 and 4 are based on the minimum detectable weight difference of 0.1 g per coupon over 30 days.
The use of solution concentrations as an indication of corrosion requires that the solution concentration not be controlled by external factors such as solubility. The solubility of aluminum under these conditions is controlled by aluminum hydroxide. While crystalline forms such as gibbsite have lower solubility, it has been shown that solution concentrations are initially controlled by the precipitation of amorphous aluminum hydroxide, the kinetically-favored precipitation product. The solubility of amorphous aluminum hydroxide in aqueous solution increases dramatically with higher pH above 6.3. The solubility of aluminum in each test (i.e., the applied chemical concentrations, measured pH range, and 60° C.) was calculated using Visual Minteq 2.30 [see reference 22]. As shown in Table 3, modeling results indicate that the measured concentrations of aluminum were below the predicted saturation limits in all tests.
Solubility has another vital role in these tests. Modeling indicates that aluminum solubility decreases as temperature decreases. No corrosion-induced precipitates were observed in the test solutions at 60° C. However, in Test 1, a substantial amount of white material precipitated as the solution cooled to room temperature. Precipitation was evident upon cooling as early as the first day of the 30-day test. XRD results indicate that the precipitate was mostly amorphous. In other research, the formation of a similar precipitate clogged simulated components of the ECCS [references 23,24], so these results indicate that aluminum corrosion may have a potentially severe impact on the proper operation of the ECCS. The ability of precipitates to form following corrosion was a key focus of this research. No corrosion-induced precipitates formed either at test temperature or upon cooling in Tests 2, 3, and 4 (it should be noted that precipitates were observed in the pilot test tank at 60° C. during the first hours of Test 4, but this phenomenon was attributed to the reaction between TSP and calcium silicate to form calcium phosphates). A small amount of precipitate formed when the Test 5 solution cooled, starting with the Day 2 sample and proceeding until the end of the test, but the amounts were substantially less than those observed in Test 1. Analyses of the precipitates indicated that the precipitates were mainly amorphous aluminum hydroxide with a significant amount of adsorbed boron.
Weight loss measurements from the corrosion coupons were a second indicator of aluminum corrosion during the pilot tests. As shown in Table 3, there is overall good agreement between weight lost from the coupons and the mass of aluminum measured in solution. In Tests 1 and 5, the mass of aluminum measured in solution was slightly more than the mass lost from the coupons, indicating that some corrosion products adhered to the coupons and interfered slightly with the weight loss calculations. However, generally the weight loss measurements serve as a good confirmation of the solution concentration measurements.
SEM provides a visual picture of corrosion on a microscopic level. SEM images of an unused aluminum coupon and coupons after 30 days of submersion in Tests 1 and 4 are shown in
Together, the aqueous aluminum concentrations, coupon weight measurements, and SEM images present a consistent picture of aluminum corrosion. Corrosion rates based on these results are shown in Table 3. In most cases, the measured corrosion rates are consistent with expectations based on existing literature. The measured value of 1.40 g/m2 h for Test 1 is close to the values identified in Table 1 for corrosion of aluminum at high pH in the presence of borate. The low corrosion rate in Test 2, likewise, is consistent with the low corrosion rate expected at neutral pH. The corrosion rate of 0.25 g/m2 h for Test 5 was lower than Test 1, which is expected based on the lower pH value.
Exceptions to the expected corrosion rates occurred in Tests 3 and 4, which had measured corrosion rates less than 0.001 g/m2·h. Although the pH range for Test 3 is shown as 7.3 to 8.1 in Table 2, the pH was nearly 8.0 for most of the test (except the first 2 days). A corrosion rate of 0.009 g/m2·h has been reported at pH=8.0 and 60 ° C. without borate [reference 13], and borate has been shown to increase the corrosion rate by at least an order of magnitude. Thus, the observed corrosion rate in Test 3 is at least 1 to 2 orders of magnitude lower than expected. Similarly, corrosion rates of 0.459 to 1.22 g/m2·h at pH=10, 60° C., with borate present are reported in Table 1; thus, the corrosion rate in Test 4 is 2 to 3 orders of magnitude lower than expected based on the literature for similar test conditions. Visual inspection of the coupons as shown in
The key constituent present in Tests 3 and 4 that was not present in the other tests was the calcium silicate insulation (see Table 2). XRF spectrometry revealed that the primary elements present in the calcium silicate insulation were, as expected, calcium and silicon. Thus, the calcium and silicon concentrations in solution were measured in each test. The concentrations at the end of each test are shown in Table 4. As shown in Table 4, the silicon concentration in the Test 3 and 4 solutions reached about 45 mg/L and 82 mg/L, respectively. These concentrations were substantially higher than in Test 1 or 5. The silicon concentration in Test 2 was initially low, but close to Test 3 at the end of the test. Table 4 shows that Tests 3 and 4 also had the highest calcium concentrations, with values of about 110 mg/L and 50 mg/L, respectively. In contrast, lower calcium concentrations were observed in Tests 1, 2, and 5.
Table 4 indicates that the silicon and calcium concentrations were both highest in Tests 3 and 4. Based on these observations, silicon, calcium, or both might be involved in the formation of an insoluble coating or passivation layer on the surface of the submerged aluminum alloy coupons. Water quality modeling using Visual Minteq 2.3 was used to predict the most likely forms of Si and Ca in solution. Silicon was predicted to have been present almost entirely as H4SiO4 in Test 3 but predominantly as H3SiO4− with less than 25% H4SiO4 in Test 4 (the pKa1 of H4SiO4 is around 9.9 at 30° C. [reference 25]). The H3SiO4− species may be more important than H4SiO4 with regard to forming a passivation layer on the aluminum surface, which would explain why the passivation was more notable in Test 4 than in Test 3. Water quality modeling predicted that the calcium was present primarily as Ca2+ and CaH2BO3+ in Test 4 and as Ca2+, CaH2BO3+, and CaHPO4(aq) in Test 3.
Bench tests were conducted to clarify whether silicon or calcium was responsible for passivation of the aluminum surface noted in pilot Tests 3 and 4. The bench tests focused on conditions similar to Test 4, which exhibited the greatest degree of passivation. Bench-scale corrosion tests were prepared with the same stock solution chemistry as Test 4 (Table 2, but without insulation or debris) with the addition of either 88.7 mg/L of Si (added as sodium silicate) or 50 mg/L of Ca (added as calcium chloride); a reference test was also conducted using only the stock solution chemistry without either additive. The measured aluminum concentrations over the duration of the tests are shown in
The introduction of 50 mg/L calcium caused a 2× reduction in aluminum corrosion; i.e., the aluminum concentration increased to only 92 mg/L after 30 days. The corrosion rate was 0.34 g/m2·h in 14 days, about half of the corrosion rate of the coupon in the solution that contained no Si or Ca. One possible mechanism explaining the reduced corrosion is that calcium may react with carbonate to form a protective calcium carbonate layer at the aluminum alloy coupon surface. Carbonate is present in solution from carbon dioxide in the air. Compared to the effect of silicate, however, the effect of calcium is relatively minor.
a, 4b, and 4c show SEM images of the aluminum alloy coupons after the bench tests. Consistent with the aluminum concentration results, the roughest and most corroded coupon surface was observed from the solution that contained no Si or Ca. In contrast, the coupon from the solution containing 88.7 mg/L silicon was mostly smooth and uniform. The roughness of the coupon from the 50 mg/L Ca solution was nearly the same as the coupon immersed in the solution without Si or Ca.
Based on the bench experimental results, it appears that an insoluble aluminosilicate compound was predominantly responsible for passivating the aluminum coupons in pilot Tests 3 and 4. In Tests 3 and 4, however, passivation appears to have been achieved at the aluminum surface within the first two days of the tests due to the silicon concentration in the solution.
Direct evidence that a passivation layer formed on the aluminum alloy coupon surface was obtained by analyzing the surface of a pre-cleaned virgin aluminum alloy coupon and a silicate-passivated aluminum alloy using XPS, as shown in
As illustrated in
Although not wishing or intending to be bound by any theory, the principal mechanism of surface passivation of aluminum or its alloys appears to occur whereby the ionic species H3SiO4− reacts with exposed aluminum to form a layer of insoluble Al2OSiO4 which inhibits oxidation reactions from removing aluminum from the solid surface. The treatment methods described above can be applied during a nuclear power plant LOCA scenario, or other industrial facility accident, that exposes aluminum to corrosive aqueous environments, particularly those with elevated temperature and pH.
The bench-scale tests demonstrated that a relatively low dose of silicate (i.e., less than less than 90 mg/L) could passivate the aluminum surface and dramatically reduce the rate of corrosion. XPS examination confirmed that a passivation layer composed of Al2OSiO4 can be formed with a thickness of more than 10 nm. Pilot-scale tests confirmed that this effect would be true even in realistic LOCA conditions, where other metals, concrete, insulation, and other dust and debris are adding complexity to the solution chemistry.
These results indicate that addition of a compound containing silicon, such as a silicate, into the reactor ECCS recirculation water may be an effective way to prevent aluminum corrosion and thereby prevent the precipitation of related chemical products that have the potential to degrade the performance of the ECCS. The bench-scale testing further illustrated that an Al2OSiO4 passivation layer, once formed, is stable with respect to drying and reimmersion in alkaline conditions. This opens a viable opportunity for pretreating industrial aluminum components and structures to enhance their corrosion resistance to a variety of operational and accidentally induced chemical environments.
The present invention is advantageous to reduce aluminum corrosion in an LOCA in that the release of aluminum ions and subsequent precipitation of aluminum hydroxide can be reduced, potentially reducing or eliminating the chemical product contributions to clogging in the ECCS and to reduce aluminum corrosion in other aqueous environments. The present invention also is advantageous as a pretreatment to protect industrial aluminum components and structures before exposure in a variety of adverse chemical environments. The treatment method described above may be used in many industrial applications (particularly those that require immersion in or exposure to water of mechanical structures and components) that include, but are not limited to, (a) surface and subsurface marine environments, (b) water quality treatment, (c) chemical processing, (d) mining and drilling, (e) aviation construction, (f) automotive construction, and (g) liquid process piping and pumping.
Although the present invention has been described above with respect to certain embodiments thereof for purposes of illustration, the invention is not so limited since changes, modifications, and the like can be made thereto within the scope of the invention is set forth in the appended claims.
This application claims benefits and priority of provisional application Ser. No. 60/937,423 filed Jun. 26, 2007, the disclosure of which is incorporated herein by reference.
This invention was made with government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60937423 | Jun 2007 | US |
