Study of high temperature oxidation kinetics of steel using grazing X-ray reflectometry

Transcription

1 170 Study of high temperature oxidation kinetics of steel using grazing X-ray reflectometry A. Knoll, E. Smigiel, N. Broll, A. Cornet Laboratoire de Metallurgic, Corrosion et Materiaux (LMCM), Ecole Nationale Superieure des Arts et Industries de Strasbourg (ENSAIS) 24, boulevard de la Victoire, F STRASBOURG (France) ABSTRACT The oxidation behaviour of metals at high temperatures is of paramount importance in many domains of material science and engineering. In order to know the mechanisms of oxidation several experimental methods, e.g. thermogravimetry, are well established. Nevertheless, X- ray methods are not widely used in this field of research. We introduced X-ray reflectometry to investigate the kinetics of the initial oxidation stage of metal samples. To show the capability of this method we present investigations of the oxidation kinetics of a ST37 steel sample oxidized at a temperature of 270 C. The results will be verified according to different existing theoretical models and give interesting hints on oxidation mechanisms. I INTRODUCTION The oxidation behaviour of materials at elevated temperatures became of paramount importance, since it is of interest in many domains of material science and industrial engineering e.g. high temperature processes in chemical industry or metal production and fabrication. It is now essential to control high temperature oxidation and therefore to investigate the kinetics of the oxide film growth. Actually, the oxidation kinetics are investigated by e.g. thermogravimetry or cathodic reduction. Grazing X-ray methods are not widely used in this field of materials science up to now. Grazing X-Ray Reflectometry for example allows to determine the thicknesses of thin layers on a substrate from several Angstrom up to some hundred nanometers by interference of the reflected X-Rays. Grazing X-ray reflectometry seems therefore to be a suitable tool to investigate the thicknesses of oxide layers grown on metals and so to determine the oxidation kinetics of the material. However, this method is only rarely used in special cases of high temperature oxidation investigation, e.g. oxidation of epitaxial Cr and Fe films [l], oxidation of Ga/Hg systems [2] or oxidation of deposited Al films [3]. We show the interest of this method in the field of high temperature oxidation of bulky steel samples in the form they are normally used in industrial applications. As an exemple we investigated a sample of ST37 steel. After isothermally heating of the samples the thicknesses of the oxide layers were determined as function of the heating time. This allows to determine the kinetic behaviour of the oxidation of ST37 steel in its initial phase.

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3 Copyright (C) JCPDS International Centre for Diffraction Data II HIGH TEMPERATURE OXIDATION In this chapter we give a brief review of mechanisms and models of high temperature oxidation of metals. An extensive treatment of this topic can be found in references [4,5]. The chemical formula for the reaction of a metal M and oxygen gas 02 to form the oxide M,Ob may be written as aa + (b/2)0, = M,O, This reaction can be considered as a special case of a heterogeneous reaction. The reaction path and the oxidation behaviour may depend on a variety of factors, and the reaction mechanisms often prove to be complex. When a metal with a clean surface is exposed to oxygen gas, the initial oxidation phase can be divided into the following three stages: Starting with a clean surface, the initial step in the metal-oxygen reaction is the adsorption of the gas on the metal surface. As the reaction proceeds, individual separated oxide nuclei are formed on the surface which grow laterally to form a continuous oxide film on the metal surface. Thus, the oxide film separates the metal from the gas and the reaction mechanism changes completely. The reaction can only proceed by a solid-state diffusion of one or both of the reactants through the oxide film. Numerous mechanisms of reaction have been considered to account for the many temperature-dependent oxidation phenomena observed. The many models differ in their assumptions about the rate-determining process: the transport mechanism through the oxide film, the driving forces of these transport mechanisms, etc. For thin films the driving force for these transport of reactants due to electric fields in or across the film, whereas for thick films th oxidation kinetic is determined by the chemical potential gradient across the scale. For a particular metal the reaction mechanism will in general be a function of surface characteristics of the sample, temperature, elapsed time of reaction, and gas composition and pressure. When the large variations of the properties of different metals and their oxides are also considered, it is not surprising that a large number of theories and models have been put forward to explain the kinetics of oxidation of metals. Rate equations which are commonly encountered are commonly classified as logarithmic, parabolic and linear. They represent only limiting and ideal cases. Deviations and intermediate rate equations are often encountered. In many cases it is difficult or even impossible to verify the validity and the correctness of the various models and parameters involved in the equations derived. It can be difficult to fit rate data to simple rate equations. In the following a selection of the most important kinetic rate laws for high temperature oxidation will be briefly described. It is charateristic of the oxidation of a large number of metals at low temperatures (< 400 C) that the reaction can be described by logarithmic rate equations: Direct logarithmic. Inverse logarithmic: x=k,,,log(t+t,)+a l/x=b-k,logt

4 172 where x represents the thickness of the oxide film, t denotes the time, ktos and kii represent the rate constants, and A and B are integration constants. A number of theories lead to this type of rate equations considering different reaction mechanisms e.g. rate-determining transport of electrons or ions due to electronic fields in or across the oxide film, cavity formation in the film. At high temperatures the oxidation of many metals is found to follow a parabolic time dependence: x2 = k,t+c where kp represents the parabolic rate constant. High temperature parabolic oxidation signifies that a thermal diffusion process is rate determining. Such a process may include a uniform diffusion of one or both of the reactants through a growing compact scale. Linear oxidation may be described by : x=k,t+d where ki is the linear rate constant. In contrast to the parabolic and logarithmic equations, for which the reaction rates decrease with time, the rate of linear oxidation is constant with time. In this case a surface or phase boundary process or reaction may be rate determining. Oxidation reactions are frequently found to follow of a combination of rate laws. This means, either that the oxidation occurs by two simultaneous mechanisms, one predominating during initial stages and the other after extended oxidation, or that changes may take place in the rate-determining mechanisms as the result of changes in the nature of the oxide scales. In the special case of thin films, the oxide formation at low temperatures involves rapidly changing systems under conditions for which thermal diffusion is slow. Thermodynamic calculations of the defect concentrations in oxides are also questionable. For this reason and because of the experimental diffculties involved, low temperature oxidation is still not well understood. Advances are being made, particulary through the use of carefully prepared specimens and the application of new or advanced instrumental techniques. III X-RAY REFLECTOMETRY In the following chapter we describe first the theoretical basis of the X-ray reflectometry technique used in this examination. A brief description of the experimental procedure follows the theoretical introduction. III.1 THEORETICAL BASIS In Kiessig [6] described the X-ray reflectometry as a method to determine the thickness of thin films on substrates by making use of the total reflection of X-rays. The basic theory of X-ray reflection can be treated just like the reflection of visible light, reflection and transmission being described by the Fresnel equations.

5 173 The principle is based on the fact that the index of refraction n is slightly smaller than 1 for all materials for the regime of X-ray wavelengths. For X-rays the index of refraction can be written as: n = 1- S(1) -i/3(a) With 6 describing the dispersion and p the absorption of a material at a certain wavelength. Both 6 and p are of the order of 10m6. As a consequence total external reflection occurs if X- rays hit a sample under a very small angle of incidence. The exact value for the critical value of total external reflection can be used to determine the density of the deposited films or the substrate according to the value of the critical angle a,: The absorption is neglected in this formula. The penetration depth of the X-ray wave is determined by the angle of incidence of the beam and the wavelength sensible value of the dispersion and absorption correction. The penetration depth varies from 50 A up to several hundred nanometers. For large incidence angles the penetration depth is defined by photoabsorption. The minimal value of the penetration depth is defined by the X-ray density of the material. Therefore, X-ray reflectometry combines surface sensitivity and small penetration depth. L.G. Parr-at [7] extended Kiessig s approach by describing the X-ray reflectivity to more complicated systems containing more than one layer. The reflected intensity is determined by a recursive formula calculating the Fresnel coefficients for all corresponding interfaces. The beams reflected from the different interfaces are added up in respect to their phase conditions. By changing the incidence angle, these phase conditions are changed. Constructional interference results in intensity maxima called Kiessig fringes, whose angular spacing is characteristic for the thickness of the layers. Although Parrat noticed the effect of surface and interface roughness on the reflectivity curves, a mathematically satisfying treatment was first published by Nevot and Croce [8]. During the last decade the grazing angle X-ray reflectometry became a routine tool for nondestructive surface and interface analysis for research and industrial applications, specially in semiconductor and glass industries. Further treatments of X-ray reflectometry can be found in the referenced papers [ III.2 EXPERIMENTAL PROCEDURE In our experiments, we investigated the oxidation behaviour of a ST37 steel sample. The elemental composition of this sample was determined by X-ray fluorescence (XRF) analysis as followed: Element Fe Mn % cu 0.51 Ni 0.25 Si 0.23 C co.18 Table 1: elemental composition of the ST37 steel sample determined by XRF. Traces MO, S, Sn

6 Copyright (C) JCPDS International Centre for Diffraction Data Strong surface rugosity destroys interferences of the reflected X-ray beams. The Kiessig fringes are no longer visible and so thickness determination of the surface layer becomes impossible. Therefore, the preparation of the sample surface is an important point in the experimental procedure. In order to achieve a sufficiently flat surface the sample was first grinded. After that, the sample surface was polished with a diamond paste from a granular size of 9um down to a granular size of a quarter of a micron. The polishing was finished with a SiO2 politure. To grow an oxide film on the sample, the specimen was heated in a furnace in ambient atmosphere. The heating temperature Tn was set to 270 C. After heating, the sample was cooled down and then the thickness of the oxide film was determined by X-ray reflectometry experiments described as followed. After that the same sample was newly heated to continue the oxide formation. To determine the layer thickness of the grown oxide film X-ray reflectometry measurements were performed on a SIEMENS D5000 O/28 powder diffractometer equipped with a special reflectometry sample stage [ 13,141. Figure 1 shows the principle of the reflectometry setup. I ) X-Rays Figure 1: Principle of the experimental reflectometry setup. ( T : X-ray tube, Sl : Slits, S : Sample, C : Controllable Beam Stop, M : Monochromator, D : Detector) All experiments were executed using a copper long fine focus sealed tube. The generator power for all measurements was set to 4OkV/3OmA. A primary slit limits the beam height so that only the sample surface is illuminated. The main components of the reflectometry sample stage are the edge diaphragm opposite to the sample holder and the heigth - adjustable sample holder. Both are mounted on translation stages whose positions are individually controlled by micrometric screws. The absolute position of the edge diaphragm is displayed by a dial indicator. The zero point of this dial indicator was calibrated by making reflectometry measurements with a reference sample, usually a silicon wafer. The edge diaphragm defines both the divergence of the incident beam and the illuminated area on the surface of the sample. Furthermore, it is used to align the surface orientation of the sample and the position of the surface in the centre of the goniometer. For all experiments the distance between edge

7 175 diaphragm and sample surface was set to 10,um. The detector slit determines the angular resolution in 20. For all experiments a 0.03 detector slit was used in combination with a 3 secondary soller slit. A secondary graphite monochromator was used to avoid the influence of the Bremsstrahlung and to suppress the Cu KP radiation and the fluorescence radiation of the sample. IV MEASUREMENTS AND DISCUSSION After each heating phase the X-ray reflectivity was recorded in function of the incidence angle while the angle of the reflected beam is equal to the incidence angle. Figure 2 shows a selection of reflectometry curves measured after different total heating times for an ST37 steel. All curves are scaled to unity and multiplied with an order of magnitude for better comparision. The measurement parameters for all measurements were the same. The total heating times are marked near the corresponding curve. One can clearly see that with increasing heating time the oscillation frequency of the Kiessig fringes increases which means an increasing layer thickness of the formed oxide layer. lo4 tl I I 3 I I I I 1 I! 1 lo3 IO2 m fj 10 z: :E 10 E 4 10-l 2 1o-2 1o-3 1o-4 u,o 02 0,4 ( incidence angle [ I I, I Figure 2 : Reflectivitiy curves recorded after different total heating times

8 176 After the measurements a fourier transform was performed on the collected data to obtain an approximate start value of the layer thickness. Then, the measured data was fitted to simulated data corresponding to the theoretical model above mentioned to get more exact values of the layer thickness. For simulation and fit we used the routines included in the REFSIM Software of SIEMENS Analytical X-Ray Systems. Figure 3a shows the fitted oxide layer thicknesses vs. the total heating time. 300 I I I I I I fj I G 150 E Steel: ST37 T, = 270 C 0 i I I I I I total heating time [min] Figure 3a. The growth of the oxide layer in function of the total heating time. The oxide growth curve can be separated in two main parts. Area 1 from the start of the heating till 50 min heating time shows an oxide growth slowing down till an asymptotic layer thickness of about 78 A. In this area a logarithmic growth model gives best fit results, as predicted in theory for thin oxide films. After 50 min we can observe a strong growth of the oxide layer till about 70 min. Before 50 min heating time Fe304 is formed on the surface. Due a decreasing diffusion rate of oxygen into the sample the oxidation velocity decreases in area 1 up to an asymptotic value of 78 A. After that, in area 2, no more Fe304 could be formed and Fe203 is formed on to the Fe304 layer. Because of the small difference in electron density of Fe304 and Fez03 the two layers could not be distinguished by X-Ray Reflectometry. Therefore, the total oxide layer thickness was measured after a total heating time of 70 min. Although we expected a logarithmic growth one can perceive a repeating structure in the growth curve, which is more clearly visible in the derivative of the growth curve shown in Figure 3b.

9 2.5 I I I I total heating time [min] Figure 3b. Derivative of the oxide layer growth curve in Figure 3a. First, the oxidation velocity increases up to a value of 0.5 A mid. Then the oxidation velocity decreases. When the layer thickness grow another 58 A, this process begins newly. A possible explanation could be the fact that the Fe203 film formation continues after the nucleation phase with a continuous film growth, and then with a new nucleation phase on to the former formed Fe203. For more exact explanation of possible oxidation mechanisms of ST37 steel, we will proceed with further experiments e.g. with different heating temperatures. V CONCLUSION The high temperature oxidation kinetics of a ST37 steel at a temperature of 270 C was studied using grazing X-ray reflectometry. The formation of an oxide film could be followed up to 10 hours total heating time and a total oxide layer thickness of about 28 nm. We observed an oxidation kinetic only partly corresponding to the usual theoretical models and to the expected logarithmic growth. The progress of the film growth give important hints on possible oxidation mechanisms in the initial phase of high temperature oxidation e.g. formation of first Fe203 in area 1 and then Fe304 later on or a succession of nucleation and film growth in area 2. It has be shown that grazing X-ray reflectometry could be a promising tool and an additional technique in the field of high temperature corrosion research. In situ measurements should also be possible. VI ACKNOWLEGDEMENTS One of the author (A.K.) was supported by the European Comission with a Marie Curie Research Training Fellowship.

10 178 VII REFERENCES [l] Stierle A. et al., Surf. Sci., 327 (1995) 9 [2] Plech A., Diploma thesis, University of Munich, 1995 [3] Klimke J. et al., submitted to Mat. Sci.Forum, 1997 [4] Kofstad P., High Temperature Corrosion, Elsevier, London, 1988 [S] Beranger G., Colson J.C., Dabosi F., Corrosion des Materiaux a haute temperature, Les Editions de Physique, Paris, 1985 [6] Kiessig H., Ann. Phys., 10 (1931) 769 [7] Parrat L. G., Phys. Rev. B, 95 (1954) 359 [8] Nevot L., Croce P., Rev. Phys. Appl., 15 (1980) 761 [9] Lengeler B., Adv. in X-ray Anal., 35 (1992) 127 [lo] Bowen D.K., Wormington M., Adv. in X-ray Anal., 36 (1993) 171 [ 1 l] Zabel H., Appl. Phys. A, A58 (1994) 159 [ 121 De Boer D. K. G. et al., X-ray Spectrometry, 24 ( 1995) 91 [ 131 Zabel G., Analytical Application Note No 337, Siemens Corporate Research, Munich, 1994 [14] Knoll A., Brtigemann L., J. de Phys. IV, 7 (1996) C4-385