OXIDATION OF MATERIALS FOR NUCLEAR WASTE CONTAINERS UNDER LONG TERM DISPOSAL

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1 OXIDATION OF MATERIALS FOR NUCLEAR WASTE CONTAINERS UNDER LONG TERM DISPOSAL A. Terlain, C. Desgranges, D. Gauvain, D. Feron CEA/CEREM/DECM/SCECF GIF SUR YVETTE Cedex, France A. Galtayries, P. Marcus Ecole Nationale Supérieure de Chimie de Paris Laboratoire de Physico Chimie des Surfaces 11 rue Pierre et Marie Curie PARIS Cedex 05, FRANCE ABSTRACT Low alloyed steels or carbon steels are considered as candidate materials for the fabrication of some nuclear waste package containers for intermediate storage. Dry oxidation is one possible degradation mode of such container materials staying typically in the 323 K 573 K temperature range. As the containers being required to remain retrievable for a 100-year period, the understanding of the corrosion attack is the key point to guarantee the retrieval of the waste package in good safety and economical conditions. The estimation of the metal thickness loss by oxidation requires the development of models based upon short-time experimental data for predicting long term container performance. As only few data are available in the literature on dry oxidation of the materials under consideration over periods longer than a few hours in this temperature range, iron and low alloy steel oxidation tests in air with different water contents have been performed. The oxidation kinetics obtained at 573 K up to 700 hours were extended to a 100-year period for the two materials under consideration, showing a low oxide layer thickness (less than 150 µm). It seems that dry oxidation under the tested reference conditions could lead to a very limited container damage. Nevertheless, the basis of these calculation has to be reinforced by a more mechanistic approach. Variation of water content in the tested conditions does not produce a large increase of the oxide layer thickness. Keywords: nuclear waste, container, storage, oxidation 1

2 INTRODUCTION Low alloyed steels or carbon steels are considered as candidate materials for the fabrication of some nuclear waste package containers for intermediate storage, typically for spent fuel. The containers are required to remain retrievable for a secular period. One factor limiting their performance on this time scale is the corrosion. The understanding of the corrosion attack is the key point to guarantee the retrievably of the waste package in good safety and economical conditions. The estimation of the metal thickness loss by oxidation requires to make valuable models from short-time experimental data for predicting long term container performance. This can be done only if the mechanisms and kinetics of corrosion and the main factors which can significantly affect them are known. Containers could undergo different degradation modes depending on the their environment and their temperature. When they contain high level waste, they will be at the beginning at temperature up to 550 K. Thus, in the 325 K 550 K temperature range, dry oxidation is one possible degradation mode of the container materials. As only few data are available in the literature on dry oxidation of the materials under consideration over periods longer than a few hours in this temperature range, we have undertaken an experimental study in order to gain oxidation kinetics data over longer periods and to determine the parameters which could significantly affect them. As the expected kinetics are slow in the considered temperature range, in a first attempt, we have worked at higher temperatures and extrapolated the results at lower temperatures. We present here, some experimental results about the dry oxidation of iron and a low alloyed steel in the 373 K-773 K temperature range under different s. The influence of the water vapor content in the and the kinetics extrapolation to long times are discussed. Materials EXPERIMENTAL PROCEDURE The investigated materials were pure iron and a low alloyed steel. The material chemical compositions are given in the Table 1. Two batches of iron have been used. The tested coupons were rectangular polycrystalline foils of 15 mm x 10 mm or 15 mm x 30 mm with a 2 mm or 1.5 mm thickness. They have a controlled roughness (< 0.8 µm) or they have been polished up to a 1200 SiC grit paper or with a 1 µm diamond paste. After preparation, the specimens have been left at room temperature in the laboratory for 3 days (in order to have a more reproducible initial oxide layer) before the oxidation tests. Experimental Tests Oxidation tests have been performed in quartz reactors heated by electrical furnaces for 260 to 305 hour periods at temperatures between 373 K and 673 K. The s were flowing ( Nm 3 s -1 flow rate) dry air (less than 15 ppm H 2 O), air with 2% vol. H 2 O (named reference ) or wet air (12% vol. H 2 O). After test, the specimens were kept under pure argon or under vacuum. The specimen oxidation has been evaluated by performing weight measurements, observations with optical and electronic microscopes and analyses by means of electronic microprobe, XRD (X-ray 2

3 diffraction), RBS (Rutherford backscattering spectroscopy) or XPS (emission of photo electrons), depending on the scale thickness. The oxidation kinetics under a Nm 3 s -1 flowing air with 2% H 2 O have been determined by continuously recording the weight gain of a specimen by means of a symmetrical thermobalance. The accuracy is about 4 µg. XPS analyses have been performed with a spectrometer equipped with an AlK α source. In addition to an overview spectrum recorded from 20 to 1100 ev in biding energy, different specific energy region have been recorded at the best resolution corresponding to the Fe 2p, Cr 2p, O 1s and C 1s core levels as well as the Ca 2p, Mo 3d and Pb 4f core levels corresponding to different contaminations. To calculate the oxide thickness, two experimental methodologies have been adopted: (1) if the oxide layer is very thin (less than 60Å), the signal of the metallic iron from the substrate is still visible in the Fe 2p core level. By peak decomposition of the Fe 2p core levels into oxidized and metallic components we can get the oxide layer thickness (d) using to the following equation, assuming that the oxide layer is homogeneously composed of Fe 2 O 3 oxide: λ MeOx d Me 2 p D D M e M e MeOx Me λ λ M e M e 2 p M eox M e 2 p I( MeOx) I( Me) = ln( 1 + ) where D is the density, λ the corresponding mean free path, and I the core level intensity obtained in the peak decomposition; (2) if the oxidized film is too thick (typically > 60 Å), sample depth profiles were recorded by means of argon sputtering in order to get the oxide/metal interface. In this case, the oxide thickness was calculated using the sputtering rate obtained for this type of iron oxide layer. In all cases, the thicknesses are given with an accuracy of ± 10%. The primary ions for RBS analyses were He + ions with energy of 1 MeV obtained from a Van de Graaff accelerator at normal incidence. The samples are analyzed at 165 back scattering angle. In such analysis conditions, this technique is well suited to analyze iron oxide layers with a thickness between 300 and 5000 Å. The accuracy of the measurement is about 200 Å. The energy calibration is obtained from a reference with a very thin gold layer deposited on aluminum. The scale thickness is deduced by successive and iterative simulations of the RBS spectra corresponding to hypothetical descriptions of the target until the simulated spectra fit with the experimental one. An example of experimental spectrum obtained is given in the Figure 1. Microscopic Observations RESULTS After oxidation at 453 K, whatever the, the oxide layers formed on iron and steel are very thin and difficult to observe. The Figure 2 shows the typical surface morphologies of the oxide layers developed on Fe and XC38 steel after oxidation tests at 573 K and 673 K. In all the cases, the oxide morphology is characterized by the presence of fiber or flake shape crystallite entanglement. The crystallite size depend on the material and the oxidation conditions. 3

4 Observations of metallographic cross sections have been done on specimens oxidized at 673 K. The metal/oxide interface exhibits a different aspect on iron and steel specimens. In the case of iron, this interface is flat whereas for the steel, it becomes irregular. This phenomenon is classically explained by the increase of thermodynamic degree of freedom due to alloying elements. Moreover, an electronic microprobe analysis of steel oxidized at 673 K in air with 12% H 2 O, has shown that the chemical composition of the layer is not uniform: it has been observed (Figure 3) an enrichment of Si, Mn and Cr at the interface metal/oxide and Mn at the oxide surface. Oxide Layer Thickness In the Table 2 are given the specimen weight gains after the tests. The average uncertainty on the weight gain measurement is about 10 µg cm -2. Each value is the result of an average of the weight gain of two or three specimens. The deviation from the mean value represents between 10 and 50%. From these values, we have calculated the equivalent thickness of a dense and uniform iron oxide layer assuming that all the weight gain is due to the formation of an oxide with a density of 5.2 g cm -3 ( typically Fe 2 O 3 ). In this table are also given the thickness of the oxide scales measured by XPS and RBS analyses. It shows that for thinnest oxide scales obtained at low temperatures (under 573 K), weight gains are too small to deduce any oxide thickness, but it is consistent with XPS analyses. For thicker oxide scales corresponding to upper oxidation temperatures, the thicknesses deduced from weight gain measurements are in good agreement with RBS, XPS results and metallographic cross section observations. Oxide Layer Analyses XPS analyses allow to detect the presence of hydroxyls in an oxide layer due to the difference between the O1s electron energy in these two types of compounds. The Figures 4 and 5 respectively show the XPS O1s spectra of the iron and steel specimen surfaces after oxidation at 373 K and 453 K. A satellite peak at 532 ev energy besides the one at 530 ev, which reveals the presence of hydroxyl species in the superficial layers (in the about 60 Å under the surface), is well developed only for the specimens oxidized at 373 K under air with 12% H 2 O. Figures 6 and 7 show the spectra from the specimens oxidized at 373 K under air with 12% H 2 O after several ionic abrasions, the oxide thickness removed after each abrasion being about 12 Å. These figures suggest that the main part of the hydroxyl species are localized in the first 25 Å of the layer. From the shape of the iron 2p spectra, it can be deduced that iron is in all the cases mainly as Fe 3+ at the oxide/ interface. After ionic erosions, the position and shape of the iron 2p peak have changed: it can be ascribed to the partially reduction of Fe 3+ in Fe 2+ during the successive erosions. In the case of steel analyses, the XPS analyses show that the oxide layer mainly consists of Fe and O but Mn and C have also been detected in addition to impurities like Ca, Pb or Mo. In a profile view of the different elemental concentrations, the Mn signal intensity increases probably up to its nominal concentration in the steel bulk. As far as Cr is concerned, though it is one of the main additives in steel, no significant signal is detected by XPS in the oxide layers developed on the steel after oxidation at 373 K, 453 K or 573 K. Another impurity can be detected also from the overview spectrum which is silicon. Though its detection is not systematic, it seems to be present in the oxide layer. 4

5 XRD analyses of iron and steel samples after oxidation between 453 K and 773 K in the different s have shown the presence of Fe 3 O 4 and Fe 2 O 3. No difference in the lattice parameters of oxides grown under different s has been observed. XRD spectra of steel oxidized under the wet at 453 K and 573 K temperatures show other peaks which could be attributed to another Fe 2 O 3 with a different crystallographic structure (hexagonal instead of rhomboedric). The spectra do not show characteristic peaks of iron hydroxides: these compounds can be present at the surface but in too low quantity to be detected or as a non crystallized form. The presence of an iron manganese hydroxide at the steel surface after oxidation cannot be excluded because the main peaks of this structure are at the same place than some one of hematite. Oxidation Kinetics Iron and steel oxidation kinetics have been measured under the reference at 523 K and 573 K. The curves showing the weight gain variation as a function of the time at 573 K are given on the Figure 8. The steel oxidation curve can be fitted by a logarithmic function. As far as iron is concerned, for the 300 first hours, the best fit is obtained with a parabolic law but a logarithmic function represents better the weight gain variations for 688 hours. The different equations are given in the Table 4. Iron Oxidation Under the Reference Atmosphere DISCUSSION In the 453 K 773 K temperature range of oxidation, whatever the, the oxides which have been identified by XRD are Fe 3 O 4 and αfe 2 O 3. It is the oxides usually reported in the literature for iron oxidation under dry O 2 or air in the 473 K 773 K temperature range 1-3. At a 373 K oxidation temperature, the film is too thin to characterize its structure by XRD. The morphology of the superficial oxide film does not seem to be affected by the water content in the oxidation. Thus, the crystallographic structure of the oxides does not depend on the temperature and in the tested conditions. The weight gain variations for the oxidation at 573 K under the reference are best fitted by a parabolic law for the first 300 hours but by a logarithmic law for a longer period. We can see that the extrapolation of the parabolic law deduced for the 300 first hours over 688 hours leads to overestimated the weight gains. In the literature, the reported weight gain variations of iron during oxidation in dry air or oxygen at temperatures higher than 573 K, often follows parabolic laws 4-6. However, in the 453 K 573 K temperature range, the mentioned laws are parabolic but also logarithmic 7-9. Sometimes, it has also been observed a change of law with the time. One interpretation for this change, is that a Fe 3 O 4 film is first formed and after αfe 2 O 3 nucleates and grows to form a film. As soon as αfe 2 O 3 covers the Fe 3 O 4 film, the oxidation rate decreases. However, this change has been observed in the first hours of oxidation. If we assume that in the 573 K-773 K temperature range the weight gain follows a parabolic law for at least the 300 first hours, we can calculate Kp parabolic constant at different temperatures. On the Figure 9, are plotted the Kp values as a function of 1/T. These data have been obtained with two batches of specimens. The Kp values deduced with the data obtained with the second batch are systematically 5

6 higher than those obtained from the first batch of specimens. The discrepancy between the two batches is probably due to differences in the initial microstructure of the specimens. If we consider separately these two batches, the deduced apparent activation energy is between 90 and 130 kj mol -1. It is not far from that the values obtained from oxidation data under dry air in this study, 140 kj mol -1, but also in the literature 5-6 (between 100 and 150 kj mol -1 ) from shorter experiences. Below 573 K, the weight gain measurements are within the experimental uncertainty and the XPS or RBS data are more reliable. For the same batch of specimens (batch 1), using the extrapolation of Kp values to lower temperatures, it is calculated an oxide thickness of 500 Å at 453 K and 40 Å at 373 K. If we take into account the 50 Å layer initially present at the surface, it can be seen that the calculated values are close to the experimental one (385 Å and 135 Å at 453 K and 373 K respectively). An extrapolation to 100 years of the kinetics data obtained at 573 K leads to the formation of a 60 µm or 5 µm thick oxide layer by using respectively the parabolic or the logarithmic law. However, as the oxide layer grows mechanical stresses build up and leads to the formation of defects like cracking or spalling which decrease the protectiveness of the layer. If we assume that the layer completely spalls away as soon as its thickness reaches a critical value of 10 µm (which is a reasonable value because dense and adherent iron oxide layer with such a thickness has already been observed) and the oxidation kinetics remains the same, the total quantity of oxide formed for 100 years corresponds to a 130 µm oxide thickness. Moreover, from our data, we can expect the formation of an oxide layer less than 1 µm thick due to dry oxidation at 373 K under air with 2% H 2 O. These calculations show that dry oxidation of iron could be very low under 100 year period but they assume that the corrosion mechanism is the same for all the storage period and in particular that there are no changes in the temperature and composition (no pollutants). Oxidation of the Steel under the Reference Atmosphere The oxides which have been identified by XRD on these materials are the same as on the iron ones. Nevertheless, after oxidation at 673 K, the oxide layer formed on the steel is enriched in Si at the interfaces and Mn and Cr at the oxide/ interface. These differences in the chemical composition of the oxide layers formed on iron and the steel may induce differences in oxidation kinetics. For example, it can be expected that high level of Si in the magnetite layer should contribute to a more protective layer. The comparison of the weight gains of the steel and iron shows that at oxidation temperatures lower than 573 K, the weight gain of the two materials are similar. After oxidation at 573 K, the steel weight gains are lower than those of iron whatever the batch. The uncertainties on iron weight gains at 673 K prevents to compare the materials at this temperature. The weight gain variations with the time of this material under the reference at 523 K and 573 K follow a logarithmic law. Extrapolation of this law over a 100-year period for a 573 K temperature leads to the formation of about 1 µm thick layer, which is very low. From the data of Runk and Kim 11, Larose et al 10 have calculated following a similar procedure to ours, the thickness of the oxide scale on carbon steels in dry oxygen at temperatures between 473 K and 573 K for a 1000 year period. They obtained 50 and 3.6 µm thick oxides on a Fe-0.8wt%C steel of for oxidation temperatures respectively equal to 573 K and 373 K. These values become 29 µm and 2.1 µm for a Fe-0.2wt%C steel. These values are higher than those predicted at 573K by our logarithmic law but they have been calculated assuming a parabolic law. In any cases, the expected damage is small. 6

7 Effect of Water Vapor Content Most of the studies devoted to the water vapor effect on the oxidation behavior of alloys concern the high temperature range (higher than 873 K). It is well known than most technical steels oxidize faster in water vapor or in air containing water vapor than in dry oxygen 12,13. The reasons for this are not satisfactorily understood. As far as iron is concerned, the oxide layers formed under dry and wet oxygen have the same composition but their morphology, plasticity and kinetics of growth differ. On the otherwise, the effect of water vapor on iron-based alloys is much more pronounced mainly because the protective scales that are formed in dry oxygen fail to develop or brake down in oxygen containing sufficiently large amounts of water The presence of water vapor in the oxidant complicates the interpretation of the low temperature oxidation. Its effect on the oxidation kinetics differs from one metal to another 15. The few studies devoted to iron 16, 17 have shown differences in the morphology of oxides grown under dry and wet oxygen but no kinetics data are reported. The oxide layers which have grown on these two materials under air with different water contents at 673 K have the same crystallographic structures and exhibit the same surface morphology. However the steel oxidized at 453 or 573 K under wet air shows a supplementary Fe 2 O 3 structure. The effect of water on the weight gains at oxidation temperatures less than 573 K is too small to be detected. At higher oxidation temperature, this effect is visible but limited (increase of about 50%). If this effect is constant over the considered 100 year period, the water content in air is not a crucial parameter as long as no condensation occurs. CONCLUSION Oxidation tests of iron and of a low alloyed steel in air with different water contents have been performed in the 373 K 773 K temperature range in order to gain kinetics data over longer periods (300 to 700 hours) than those mostly reported in the literature. As far as iron is concerned, the apparent activation energy of oxidation under dry air is found to be in the range of values reported from shorter tests. It is a little lower for oxidation under air with 2% H 2 O (90 kj mol -1 compared to 140 kj mol -1 ). To be conservative, we have extrapolated the oxidation kinetics obtained at 573 K over a 100- year period. For the two materials under consideration, the calculated oxide layer thickness are very low: less than 150 µm. It seems that dry oxidation under the tested reference conditions could lead to a very limited container damage. Nevertheless, the validity of this extrapolation has to be reinforced by a more mechanistic approach. It will be done in a further work. For example, the role of intergranular diffusion in the growth of the oxide will be further investigated. In the case of the low alloy steel, a better understanding and maybe a modeling of the evolution of the composition near the interface during oxide growth should be necessary to extrapolate the oxidation kinetics over 100-year period with more confidence. 7

8 Variation of water content in the tested conditions does not produce a large increase of the oxide layer thickness. However, the effect of this parameter over longer periods will be necessary to investigate. Moreover, the influence of pollutants in the has to be considered. ACKOWLEDGEMENTS We gratefully acknowledge Y. SERRUYS (CEA/DTA/SRMP) for the RBS analyses. REFERENCES 1. P. B. Sewell and M. Cohen, J. Electrochem. Soc. 111, 5(1964): p W. E. Boggs, R. H. Kachik and G. E. Pellissier, J. Electrochem. Soc. 114, 1(1967): p R. J. Hussey, D. Caplan, M. J. Graham, Oxidation of Matals 15, 5/6(1981): p D. E. Davies, U. R. Evans and J. N. Agar, Proc. Roy. Soc. London, A225(1954): p E. J. Caule, K. H. Buob and M. Cohen, J. Electrochem. Soc. 108(1961): p H. Sakai, T. Tsuji and K. Naito, J. of Nuclear Science and Technol., 26(1989): p A. B. Winterbottom, J. of the Iron and Steel Institute, 165(1950): P M. J. Graham, S. I. Ali and M. J. Cohen, J. Electrochem. Soc. 117(1970): p A. W. Swanson and H. H. Uhlig, J. Electrochem. Soc. 121(1974): p S. Larose and R. A. Rapp, Review of low-temperature oxidation of carbon steels and low alloyed steels for use as high-level radioactive waste package materials, Nuclear Regulatory Commission, Contract NRC , Report CNWRA R. B. Runk and H. J. Kim, Oxidation of Metals, 2(1970): p P. Kofstad, High Temperature Corrosion, Elsevier Applied Science (1988). 13. D. L. Douglass, P. Kofstad, A. Rahmel, G. C. Wood, Oxid. Met. 45 (1996): p Shen Jianian, Zhou Longjiang, Li Tiefan, Oxid. Met. 48 (1997): p Francis P. Fehlner, Low-Temperature Oxidation, The role of vitreous oxides, John Wiley & Sons Publication (1988). 16. E. A. Gulbransen, T. P. Copan, Discuss. Faraday Soc., 28 (1959): p R. L. Tallman, E.A. Gulbransen, J. Electrochem. Soc., 115 (1968): p TABLE 1 CHEMICAL COMPOSITION OF THE INVESTIGATED MATERIALS (IN WEIGHT PERCENT) C Mn Si S P Ni Cr Mo Cu Sn Al N Fe Iron bal Steel bal. 8

9 TABLE 2 OXIDATION OF IRON UNDER AIR T (K) Duration hours Atmosphere mg cm -2 Weight gain Calculated thickness (µm) Measured Oxide thickness (Å) 293 Room 55 (XPS) 260 dry air (XPS) reference (XPS) 305 wet air (XPS) 260 dry air (XPS) reference 0.00(1) (XPS) 305 wet air 0.00(2) (XPS) 260 dry air 0.04 (1) reference 0.08(1) 0.15 (2) wet air 0.24 (2) dry air 0.35 (1) reference 0.39 (1) 1.25 (2) (RBS) 305 wet air 1.90 (2) µm(meb) dry air 1.8 (1) reference 0.85 (1) 5.7 9

10 TABLE 3 OXIDATION OF STEEL UNDER AIR T (K) Duration hours Atmosphere 293 Room mg cm -2 (µm) Weight gain Calculated thickness Measured Oxide thickness (Å) 50 (XPS) reference (XPS) 305 wet air 120 (XPS) reference (XPS) 305 wet air (RBS) reference (RBS) reference (XPS) 305 wet air > 5000 (RBS) reference wet air µm (MEB) TABLE 4 OXIDATION KINETICS LAWS OF Fe AND STEEL UNDER WET AIR AT 573 K Temperature Duration Fe (W : mg/cm²; t : h) Steel (W : mg/cm²; t : h) 523 K 300 hours - W = Ln(0.022 t + 1) 573 K 300 hours W= t 1/2 W = Ln(1.294 t + 1) 573 K 600 hours W= Ln( t + 1) - 10

11 Not oxidized Oxidized at 523 K e < 100 Å e ~ 1300 Å O Not oxidized Fe Oxidized Energy Channel FIGURE 1: Exemple of RBS Spectra of the steel before and after oxidation in air with 12% vol. H 2 O during 260h at 523K and scale thickness (e) associated obtained from RBS spectra 11

12 Steel Air with 12% vol. H20 Air with 2% vol. H K 1 µm 573 K 1 µm Iron Air with 12% vol. H20 Air with 2% vol. H K 1 µm 573 K 1 µm FIGURE 2: SEM images of the iron and steel surfaces after oxidation 12

13 Fe Cr M n Si FIGURE 3: Fe, Cr, Mn and Si X rays images of the oxide layer of the steel after oxidation at 673 K under air with 12%H 2 O XPS Intensity (arb. units) (a) (b) (c) (d) 534,5 532,5 530,5 528,5 Binding Energy (ev) FIGURE 4: O 1s spectra of the iron surfaces oxidized,(a) at 373 K under air with 2% H 2 O, (b) at 453 K under air with 2% H 2 O, (c) at 373 K under air with 12% H 2 O, (d) at 453 K under air with 2% H 2 O FIGURE 5: O 1s spectra of the steel surfaces oxidized,(a) at 373 K under air with 2% H 2 O, (b) at 453 K under air with 2% H 2 O, (c) at 373 K under air with 12% H 2 O, (d) at 453 K under air with 2% H 2 O 13

14 FIGURE 6: Iron 2p spectra of iron surface after successive abrasions 0,3 FIGURE 7: Iron 2p spectra of steel surface after successive abrasions 2 mass gain (mg/cm²) 0,15 Iron Steel 1,5 1 0,5 calculated scale thickness (µm) time (h) 0 Iron : oxidation in air with 2% H20 THERMOBALANCE MEASUREMENT Steel (non-polished) : oxidation in air with 2% H2O THERMOBALANCE MEASUREMENT Steel (non polished) : oxidation in air with 2% H2O Steel (polished with Sic1200 grit paper) : oxidation in air with 2% H2O Steel (polished with Sic1200 grit paper) : oxidation in air with 12%H2O Iron : oxidation in air with 2% vol. H2O Iron : oxidation in air with 12% H2O FIGURE 8: Iron (batch 2) and steel weight gain variations with the time during oxidation at 573K 14

15 Iron oxidation 0,0012 0,0014 0,0016 0,0018 0,002 0,0022 1,0E+00 1,0E-01 Kp (mg 2 cm -4 h -1 ) 1,0E-02 1,0E-03 1,0E-04 1,0E-05 1,0E-06 1,0E-07 1,0E-08 1,0E-09 air+2%h2o (batch1) air+2%h2o (batch2) air + 12%H2O 1 / T (K) FIGURE 9: Parabolic constant deduced from iron weight gain measurements after oxidation 15

16 List of the figure captions Figure 1: Exemple of RBS Spectra of the steel before and after oxidation in air with 12% vol. H 2 O during260h at 523K and scale thickness (e) associated obtained from RBS spectra Figure 2: SEM images of the iron and steel surfaces after oxidation Figure 3: Fe, Cr, Mn and Si X rays images of the oxide layer of the steel after oxidation at 673 K under air with 12%H 2 O Figure 4: O 1s spectra of the iron surfaces oxidized,(a) at 373 K under air with 2% H 2 O, (b) at 453 K under air with 2% H 2 O, (c) at 373 K under air with 12% H 2 O, (d) at 453 K under air with 2% H 2 O Figure 5: O 1s spectra of the steel surfaces oxidized,(a) at 373 K under air with 2% H 2 O, (b) at 453 K under air with 2% H 2 O, (c) at 373 K under air with 12% H 2 O, (d) at 453 K under air with 2% H 2 O Figure 6: Iron 2p spectra of iron surface after successive abrasions Figure 7: Iron 2p spectra of steel surface after successive abrasions Figure 8: Iron (batch 2) and steel weight gain variations with the time during oxidation at 573K Figure 9: Parabolic constant deduced from iron weight gain measurements after oxidation 16