atw 2017-09

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atw Vol. 62 (2017) | Issue 8/9 ı August/September DECOMMISSIONING AND WASTE MANAGEMENT 544 • Corrosion tests in “Konrad water” at 50 °C (see Tab. 2). • In-situ tests mimicked the situation during the disposal operations in the galleries at temperatures of about 20 to 33 °C and a relative humidity between 50 to 80 % [14]. • Other tests reproduced conditions in a closed gallery before the complete closure of the Konrad mine. In this case, the temperature at the 1250 m level reached 42 °C and the relative humidity was 82 %. The samples were covered with crushed iron ore. In the in-situ experiments at Asse and Konrad, aerobic conditions prevailed. Samples of DC 01 / St 12 (material number 1.0330), ST 37-2 (material number 1.0038), and nodular cast steel GGG 40.3 (material number 0.7043) were prepared [7, 15]: • Untreated samples • Untreated screwed sample • Untreated welded samples • Samples covered with an epoxy resin coating (150 µm) • Samples covered with an polyurethane resin coating (500 µm) After exposure times up to one year, the samples were recovered, and cleaned mechanically and chemically. The mass loss was determined gravimetrically. The corrosion penetration by non-uniform corrosion such as pitting and shallow pit formation were analyzed using surface profiles, microscopic surface measurements, micrographs and scanning electron micrographs. The performance of the coating systems were also analyzed [7]. Results and discussion The corrosion experiments reported here were performed in salt solutions. Under reducing conditions as they prevail in a deep disposal, the corrosion process of carbon steel consumes water and generates hydrogen. During the corrosion process, dissolved iron reacts with the aqueous medium forming ferrous hydroxides with divalent iron (Fe II ). At 7 < pH < 9, observed solid corrosion products are magnetite (Fe 3 O 4 ) and amorphous iron hydroxides. At sufficiently low redox potentials (absence of oxygen) in chloride solutions, Cl - ions react with amorphous iron hydroxides forming the reaction product “green rusts”. This compound has the formula [Fe II 3Fe III (OH) 8 ]Cl×H 2 O and can be formed at [Cl - ]/[OH - ] > 1 [16]. It consists of both Fe II and trivalent iron (Fe III ). In contact with oxygen, green rust transform quickly to magnetite. In the presence of Mg-rich brines, (Fe,Mg) (OH) 2 and Fe(OH) 2 Cl compounds were found and characterized [17]. The following chapter considers the laboratory test performed at KIT-INE. The corrosion rates determined by in-situ test were significantly lower because of the lacking of water. Canister materials for heat producing wastes Results and the various details of the experiments can be found in more than 100 publications by Smailos et al.. Figure 2 shows the mass loss of fine-grained steel samples as function of time immersed in NaCl solution at several temperatures. Comparing Fig. 2 and Figure 3 higher mass loss and a higher uniform corrosion rate were found for the fine-grained steel samples corroded in MgCl 2 or in NaCl solutions, respectively. The samples experienced the same pretreatments and were corroded under anaerobic conditions in autoclaves at temperatures between 35 °C and 170 °C (200 °C in NaCl solution). In the case of the experiments in MgCl 2 -rich solution, a clear increase of corrosion rates with increasing temperature can be seen. Neglecting the initial corrosion rates which might be influenced by the surface of the samples or by residual oxygen in the autoclaves, the average general corrosion rates for MgCl 2 and NaCl experiments are shown in Figure 4. a) mass loss b) general corrosion rate (normalized to 1 year) | | Fig. 2. Mass loss (a) and general (uniform) corrosion rate (b) of steel 1.0566 in MgCl 2 -rich solution as function of time for different temperatures. a) mass loss b) general corrosion rate (normalized to 1 year) | | Fig. 3. Mass loss (a) and general (uniform) corrosion rate (b) of steel 1.0566 in NaCl solution as function of time for different temperatures. Decommissioning and Waste Management Corrosion of Canister Materials for Radioactive Waste Disposal ı Bernhard Kienzler
atw Vol. 62 (2017) | Issue 8/9 ı August/September a) MgCl 2 -rich solution | | Fig. 4. Comparison of general corrosion rates for fine-grained steel in MgCl 2 and in NaCl solutions. Columns show the average, the line the standard deviation of all experiments using untreated samples. a) Mass loss of steel 1.0566 in MgCl 2 -solution depending on initial pH (not corrected) | | Fig. 5. pH effect on corrosion of steel 1.0566 in MgCl 2 and NaCl solutions. b) NaCl solution b) Proton concentrations (-log m H+ ) as function of temperature for NaCl (blue) and MgCl 2 -rich (red) solutions Lines: calculated [18], Points: average values measured after termination of the experiments. DECOMMISSIONING AND WASTE MANAGEMENT 545 The corrosion rates in MgCl 2 -rich solution showed a significant increase with temperature. At 90 °C the corrosion rate was 56 µm/yr. and increases to 186 µm/yr. at 170 °C. Samples in NaCl solution showed only a minor increase of the corrosion rate between 90 °C and 170 °C or 200 °C, respectively. In this temperature range, the average corrosion rates were determined between 25 and 36 µm/yr. The corrosion rates at 35 °C were the same in both solutions. The different corrosion rates were explained by different pH values of the solutions and by the different characteristics of the corrosion products. The mass loss (corrosion rates) of carbon steel as function of the experimental pH was investigated (Figure 5). The pH dependence of the corrosion rates was investigated in Mg- Cl 2 solution at 170 °C. For this purpose, the solution was acidified with HCl before the experiment. The results for the experiments with starting pH values 1, 2, 3, 4 and 5 are plotted in Fig. 5. Higher pH values could not be adjusted due to precipitation of brucite (Mg(OH) 2 ) at higher pH. The results show no clear trend. The mass losses at the start pH value 1 are clearly below the other measured values, which indicates that in this case, processes other than corrosion dominate the results. Causal studies for clarification of this effect were not performed. pH dependences for other conditions and temperatures were not investigated. Fig. 5b shows the average pH values corrected to the proton concentrations (-log m H+ ) which were measured after termination of the experiments and cooling of the experimental vessels. The calculated pH values (-log m H+ ) for both solutions as functions of temperature are also plotted [18]. The figure shows that within the scatter of measured pH values after cooling did not differ significantly. The influence of the ratio of the sample surface to the solution volume was investigated. Analyzing the complete set of available data (Figure 6), a clear trend cannot be detected. Cast nodular steel was investigated at 150 °C in both salt solutions. Figure 7 shows a constant corrosion rate over a duration of the experiments of 550 days. The rate in MgCl 2 -rich solution is by a factor of two above the rate determined in NaCl solution. Comparison of the general corrosion rates of fine-grained steel with the rates of GGG40.3 in MgCl 2 solutions (see Fig. 4) reveals a higher rate of the fine-grained steel under the same conditions. In NaCl solution the corrosion rates of both steels were comparable. Corrosion products were analyzed by XRD. In the case of NaClsolution, magnetite (Fe 3 O 4 ) and γ-Fe 2 O 3 (maghemites) could be detected. Hematite was also found in the MgCl 2 -rich solution (Figure 8). However, it is not clear if these results show an artifact (possibly an oxidation after completion of the experiment). The diffractograms of the corrosion products in MgCl 2 solution show a series of further reflections which could be assigned to FeSO 4 ⋅6H 2 O and a MgFe-hydroxy carbonate phase (pyroaurite). The latter has a layer structure. The reflexes fit well, but do not correspond to the expectation, since no carbonates were present in the experiments. Other fine grained steels, such as TStE 460 and 15MnNi 6.3 in MgCl 2 solutions showed only hydroxidic corrosion products (Fe, Mg)(OH) 2 and ß-FeOOH (akaganeite). Decommissioning and Waste Management Corrosion of Canister Materials for Radioactive Waste Disposal ı Bernhard Kienzler
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