LiNO3 Effect on Corrosion Prevention of Aluminum with Complex Shapes

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 34, No. 8, p (August 1997) LiNO3 Effect on Corrosion Prevention of Aluminum with Complex Shapes Toshiaki MATSUO*,t, Tatsuo IZUMIDA**, Michihiko HIRONAGA***, Yoshihiko HORIKAWA**** and Takayuki SHIOMI**** * Power and Industrial Systems R _??_ D Division, Hitachi Ltd. ** Hitachi Works, Hitachi Ltd. Back End Project Department, *** Central Research Institute of Electric Power Industry * General Office of Nuclear and Fossil Power Production ***, Kansai Electric Power Co., Inc. (Received March 25, 1997) The LiNO3 effect on aluminum corrosion prevention after land disposal of cement-solidified dry active wastes was examined quantitatively, in the event that the LiH (AlO2)2,5H2O (Li-Al) preservation film was not formed on aluminum surfaces during the solidification process. It is especially probable for these bare surfaces to be left when the wastes include components of complex shapes. LiNO3 dissolves from the waste forms into underground water to form the Li-Al preservation film. So, we thought that the LiNO3 addition would prevent the corrosion. We measured the volume of hydrogen gas generation in mortar-soaked water during the Li-Al preservation film formation, as functions of LiNO3 addition amount, the weight ratio of water to mortar when the mortar-soaked water was produced, and the aluminum surface area, to quantify the effect. We found that aluminum corrosion was inversely proportional to the LiNO3 addition. For the corrosion to be less than 10-5m in 103h, the initially added amount of LiNO3 must be 1.5wt% of the sum of cement and sand. Regardless of the weight ratio of water to mortar when the mortar-soaked water was produced, hydrogen gas generation with LiNO3 was 10% as much as that without it, in 5x103h. Because of the Li-Al preservation film formation reaction, hydrogen gas generation was proportional to the cubic root of the aluminum surface area. KEYWORDS: cement solidification, dry active wastes, aluminum, underground disposal, corrosion protection, lithium nitrate, lithium aluminate double salt, hydrogen, surface area I. INTRODUCTION Treatment methods of dry active wastes generated from nuclear power plants must be developed with consideration given to their applicability to waste land disposal. Such wastes include aluminum materials used as coverings for heat insulating materials, and so on. When conventional cement solidification is applied to wastes containing aluminum materials, alkaline corrosion of the aluminum occurs because of the high ph of cement paste, which is about This corrosion of aluminum is accompanied by generation of hydrogen gas. Generally, dissolution of the Al2O3 surface layer formed on metallic aluminum is responsible for the hydrogen gas generation(1), and the concentration of OH- ions is responsible for the dissolution(2). From the standpoint of land disposal of the waste packages, in addition, the underground * Omika-cho, Hitachi-shi ** S aiwai-cho, Hitachi-shi 317. *** Abik o, Abiko-shi * *** Nakanoshima Corresponding, Kita-ku, Osaka 530.t author, Tel (ext. 5342), Fax , matsuo@erl.hitachi.co.jp water would become alkaline as it penetrated through the waste forms and the engineered concrete (mixture of cement and stones) barrier of the waste repository. Then, the alkaline water may reach the aluminum materials in the wastes and cause aluminum corrosion. We developed a method employing addition of a corrosion inhibitor, LiNO3, to the cement, which successfully prevents aluminum corrosion and hydrogen gas generation during the cement solidification(3). We examined the reaction mechanism of LiNO3 and found there was formation of an insoluble, lithium aluminate double salt (LiH(AlO2)2.5H2O, or Li-Al) preservation film on the metallic aluminum surfaces(3)(4). With the Li-Al preservation film formed on the surface during the solidification process, the aluminum materials do not corrode until it is lost, after the land disposal of the wastes. When it is lost, the alkaline circumstances becomes much milder than just after the land disposal. So, with LiNO3 addition, the aluminum corrosion is about 10-2mm, less than 10% of that without it(5). But, if the wastes include components of complex shapes, it is possible that the Li-Al preservation film is not formed on some parts of the aluminum surface during the cement solidification. In this case, the scenario of hydrogen gas generation is described as follows. When the alkaline underground water reaches this bare aluminum surface, the hydrogen gas generation begins, 823

2 824 T. MATSUO et al. accompanied by aluminum corrosion. However, LiNO3 in the waste forms dissolves into the underground water, and forms the Li-Al preservation film on the bare aluminum surface at that time. Then, it is expected that the hydrogen gas generation can be prevented. The dissolution of LiNO3 from the waste forms and the formation of the Li-Al preservation film have already been confirmed experimentally(5). But, it is necessary to show the effect of LiNO3 on the corrosion prevention quantitatively. So, one aim of this paper was to show it by carrying out aluminum corrosion experiments using mortar-soaked water, the contents of which are identical with those just after land disposal. We thought that the following factors would influence the quantitative corrosion prevention effect of LiNO3; the initial LiNO3 addition amount to cement, weight ratio of water to mortar when the mortar-soaked water is produced, and content of metallic aluminum in the waste forms. The initially added amount of LiNO3 to cement influences lithium concentration in the underground water reaching the wastes. The least required amount of the initial LiNO3 addition has to be checked, to ensure corrosion during the Li-Al preservation film formation is less than that when it is lost, as is described above. Also, ph and the lithium concentration in the underground water reaching the wastes change together with the total amount of underground water penetration(5), and are thought to have a fluctuation range, in terms of when the Li-Al preservation film is formed after land disposal of the wastes. We thought that the range could be taken into account by relating it to a range of the weight ratio. We took the geometric surface area of the aluminum materials in the wastes as the third parameter, in place of the aluminum content. This is because the hydrogen gas generation was expected to be proportional to the surface area. So, in this paper, the volume of hydrogen gas generation was measured in mortar-soaked water as functions of LiNO3 addition amount, the weight ratio of water to mortar when the mortar-soaked water was produced, and the aluminum surface area. II. EXPERIMENTAL 1. Corrosion Measurements as a Function of LiNO3 Addition Amount At first, the mortar-soaked water was produced. The contents of mortar (mixture of cement and sand) used were listed in Table 1. The cement we used in this experiment was Portland blast-furnace slag cement (slag cement) type C, which is a mixture of ordinary Portland cement (OPC) with blast-furnace slag at the ratio of OPC:slag=35:65. The weight ratio of LiNO3 addition to the mortar was from 0.2 to 20wt%. After one month cure, the mortar was powdered to below 12 mesh size, and soaked in the ion-exchanged water for 3d, with continuous stirring at 500rpm, at 283K(5). The weight ratio of water to mortar was 2/1. The ph value of the mortarsoaked water was The concentration of LiNO3 in it was from 9x10-3 to 1.2M. Then, an aluminum specimen (1.6x10-6m2 surface area) and 10g of filtered mortar powder (i.e. mortar powder used for making the mortar-soaked water) were placed in 4x10-5m3 of the mortar-soaked water. The volume of hydrogen gas generation was measured at 283K, for 103h. Aluminum corrosion d (m) is related to the volume of hydrogen gas generation V (m3) by d={v(h2 gas generation per 1 mol of Al corrosion) (Atomic weight of Al)}/{2.24x10-2(Al, surface) ((Density of Al)}, (1) where H2 gas generation per 1mol of Al corrosion=2/3(mol), atomic weight of Al=27x10-3 (kg/mol), density of Al=2.703x103(kg/m3). 2. Corrosion Measurements as a Function of Weight Ratio of Water to Mortar The volume of hydrogen gas generation was measured for 5x103h, in the same way as described in Sec. II-1, except for the conditions described below. LiNO3 addition to the sum of cement and sand was 1.5wt%. The weight ratio of water to mortar was from 1/1 to 10/1 to produce the mortar-soaked water. The hydrogen gas generation measurements were performed in mortar-soaked water without LiNO3 as the reference case, too. 3. Corrosion Measurements as a Function of Aluminum Surface Area The volume of hydrogen gas generation was measured for 5x103h, in the same way as described in Sec. II-1, except for the conditions described below. LiNO3 addition to the sum of cement and sand was 1.5wt%, and the weight ratio of water to mortar was 2/1 to produce the mortar-soaked water. The ph value of the mortarsoaked water was The concentration of LiNO3 in it was 7x10-2M. Surface areas of aluminum specimens were from 1.5x10-3 to 0.1m2. The aluminum specimens and 10g of the filtered mortar were placed in 8x10-5m3 of the mortar-soaked water. To get these values of surface area, two approaches were used. The Table 1 Composition of mortar JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

3 LiNO3 Effect on Corrosion Prevention of Aluminum with Complex Shapes 825 first one was using plural specimens each with a surface area of 1.5x10-3m2. The second one was using a single, unfolded specimen to check if there was any influence of spaces between overlapped specimens and folds of the specimen, which may prevent OH- ion diffusion partially. The hydrogen gas generation measurements were performed in mortar-soaked water without LiNO3 as the reference case, too. III. RESULTS 1. Dependence on LiNO3 Addition Figure 1 shows the dependence of aluminum corrosion in 103h on the lithium concentration in the mortarsoaked water. The corrosion is inversely proportional to the lithium concentration. From these results, 1.5wt% addition of LiNO3 is required to prevent the aluminum corrosion below 10-5m, which is equivalent to that when the Li-Al preservation film is lost by underground water penetration(5). 2. Dependence on the Weight Ratio of Water to Mortar to Produce Mortar-Soaked Water Figure 2 shows the dependence of the volume of hydrogen gas generation in 5x103h on the weight ratio of water to mortar to produce the mortar-soaked water. In all examined cases, the hydrogen gas generation with LiNO3 is about one tenth as much as that without LiNO3. We thought this is because both the lithium concentration and ph are lowered as the weight ratio is large, and these two effects cancel each other. So, with LiNO3, hydrogen gas generation was 10% as much as that without it, though the fluctuation range of when the Li-Al preservation film is formed after land disposal Fig. 1 Dependence of aluminum corrosion on initial LiNO3 addition amount of the wastes, was taken into account. 3. Dependence of Aluminum Surface Area Figure 3 shows the dependence of the hydrogen gas generation in 5x103h on the surface area of the aluminum specimen. As the surface area is large, the volume of hydrogen gas generation is large, too, regardless of LiNO3 addition. With LiNO3, the volume of hydro- Fig. 2 Dependence of hydrogen gas generation on weight ratio of water to mortar when the mortar-soaked water was produced VOL. 34, NO. 8, AUGUST 1997

4 826 T. MATSUO et al. Fig. 4 Dependence of aluminum corrosion rate on its surface area Fig. 3 Dependence of hydrogen gas generation on aluminum surface area gen gas generation is proportional to the square root or the cubic root of the surface area. On the other hand, without LiNO3, the volume of hydrogen gas generation is proportional to the surface area, as expected. This difference of slopes is discussed later. Figure 4 indicates the time dependence of the aluminum corrosion rate, calculated from the hydrogen gas generation measurement results and Eq. (1). As the surface area is large, the corrosion rate is slightly lowered. IV. DISCUSSION In Sec. III-3, the dependence of hydrogen gas generation on the aluminum surface area with LiNO3 is different from that without LiNO3. We thought the following two factors were possible reasons for this. The first one was the existence of small spaces between overlapped specimens which prevented the diffusion of OH- ions, leading to their partial dilution near some part of the aluminum surface. But, as shown in Fig. 3, there was little difference between the cases using plural specimens and those using a single specimen, regardless of LiNO3 addition. So this did not seem to influence the hydrogen gas generation. The second one was a chemical effect of LiNO3 relating to the formation process of the Li-Al preservation film. With LiNO3, the Li-Al preservation film formation proceeds through the following chemical reaction: Li++2Al(OH)-4+2H2O ->LiH(AlO2)2 5H2O+OH-. (2) The first approximation of the reaction rate constant of reaction (2) can be described as below. K=[Li+][Al(OH)-4]2/[OH-]. (3) We thought that this film formation reaction could prevent the aluminum corrosion, and that the corrosion rate could be proportional to 1/K. With the assumption that variations of ph and lithium concentration are much smaller than the other factors, the corrosion rate dx/dt can be described as below. dx/dtoc([li+][al(oh)-4]2/[oh-])-1 oc1/[al(oh)-4]2. (4) Then, by notating time, aluminum corrosion, surface area, volume of hydrogen gas generation as t, x(t), S, V(t), and taking it into account that [Al(OH)-4] is proportional to dissolution amount of aluminum S,x(t), Eq. (4) can be transformed to dx/dt=b/(s,x)2 (b: constant). (5) JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

5 LiNO3 Effect on Corrosion Prevention of Aluminum with Complex Shapes 827 And by integration with t, we get x3=3bt/s2. Taking it into account that S,x(t) is proportional to V(t), we can get V3=gSt (g: constant). This corresponds to the experimental result that the hydrogen gas generation is proportional to the cubic root of the surface area. If it is true, however, the hydrogen gas generation during the Li-Al preservation film formation has to be proportional to the cubic root of time, too. Figure 5 shows the time dependence of the hydrogen gas generation at each surface area. Except for the steep slopes just after the measurementstarted, and the plateau regions after the Li-Al preservation film formation finished, the above relation is satisfied. So, we thought the reason why the hydrogen gas generation was proportional to the cubic root of the surface area, was due to the chemical effect of LiNO3. We do not think that the tendency of the corrosion prevention effect of LiNO3 grows infinitely as the surface area is large. This is because the added amount of LiNO3 during the solidification process is finite. At the very least, the product of the surface area and the lithium density in the Li-Al preservation film (about 0.5 to 1g/m2) has to be less than the initial LiNO3 addition. If it is larger than the initial addition, the lithium density in the Li-Al preservation film may gradually be lowered to reduce its stability and impervious character. APPENDIX describes how the lithium density was measured in the Li-Al preservation film. For example, when a mortar form of 400kg (assumed specific gravity and volume are 2 and 2x10-1m3) is molded using the mortar listed in Table 1, and the ratio of LiNO3 to the mortar is 1.5wt%, about 5kg of LiNO3, or 0.5kg of lithium is included in this mortar form. With the assumption that the lithium density in the Li-Al preservation film is a uniform 1g/m2, the surface area of aluminum materials included in the waste form should be less than 500m2. V. CONCLUSIONS After land disposal of cement-solidified dry active wastes generated from nuclear power plants, underground water penetrates the waste repository and the waste forms to become alkaline. Then, aluminum materials included in the wastes are corroded by the alkaline water to generate hydrogen gas. When LiNO3 is added to cement but the Li-Al preservation film is not formed on aluminum surface during the solidification process, LiNO3 dissolves from the waste forms to the alkaline water to form the Li-Al preservation film after land disposal of the wastes. We examined the corrosion prevention effect of LiNO3 quantitatively and obtained the following conclusions. (1) Aluminum corrosion was proportional to the inverse of LiNO3 addition. For the corrosion to be less than 10-5m in 103h, the initial addition amount of LiNO3 must be 1.5wt% of the sum of cement and sand. (2) The corrosion prevention effect of LiNO3 was independent of the weight ratio of water to mortar when the mortar-soaked water is produced. With LiNO3, hydrogen gas generation was 10% as much as that without it in 5x103h, though the fluctuation range of when the Li-Al preservation film is formed after land disposal of the wastes, was taken into account. (3) With LiNO3, hydrogen gas generation in 5x103h Fig. 5 Time dependence of hydrogen gas generation with various surface areas VOL. 34, NO. 8, AUGUST 1997

6 828 T. MATSUO et al. was proportional to the cubic root of aluminum surface area. This is because of the chemical effect of LiNO3 relating to the formation reaction of the Li- Al preservation film. -REFERENCES- (1) "Mukikagaku Zensho" (Encyclopedia of Inorganic Chemistry), X-1-1, Al, Maruzen, Tokyo, 158 (1975), [in Japanese]. (2) Pourbaix, M.: "Atlas of Electrochemical Equilibria", Pergamon Press, Oxford, 168, (1966). (3) Matsuo, T., et al.: J. Nucl. Sci. Technol., 32, 912 (1995). (4) Matsuo, T., Kobayashi, K., Tago, K.: J. Phys. Chem., 100, 6531 (1996). (5) Matsuo, T., et al.: J. Nucl. Sci. Technol., 33, 852 (1996). [APPENDEX] 1. Lithium Density Measurements in the Li-Al Preservation Film At first, two sheets of aluminum foil (1x2cm, 1x 10-3m thickness) and two aluminum wires (1x10-3mp, 6.4x10-2m) were prepared. Then, these aluminum specimens were coated with the Li-Al preservation film by soaking it in 5x10-4m3 of 0.5M KOH aqueous solution with 0.5mol of dissolved LiNO3 for one week. After that, each specimen was soaked in 2.5x10-5m3 of 1M HCl aqueous solution to dissolve the Li-Al preservation film on the specimen surface. Lithium ion concentration in each solution was measured by ICP analysis. Lithium density in the Li-Al preservation film s was obtained as =(Lithium ion concentration)x(2 /(Specime surface area)..5x10-5m3) The values of lithium density obtained from the above measurements were 0.7 and 0.8g/m2 for the wires, 0.7 and 1.0g/m2 for the foil. s JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY