Yttrium sol–gel coating effects on the cyclic oxidation behaviour of 304 stainless steel

Corrosion Science 45 (2003) 2867–2880 www.elsevier.com/locate/corsci

Yttrium sol–gel coating effects on the cyclic oxidation behaviour of 304 stainless steel F. Riffard *, H. Buscail, E. Caudron, R. Cueff, C. Issartel, S. Perrier Laboratoire Vellave sur l’Elaboration et l’Etude des Mat eriaux, Equipe locale Universit e Blaise Pascal Clermont-Fd II, 8 rue J.B. Fabre, B.P. 219, 43006 Le Puy-en-Velay, France Received 17 September 2002; accepted 2 April 2003

Abstract The present results reveal the interest of sol–gel coating technique to improve 304 steel high temperature oxidation resistance. An yttrium sol–gel coating appears to enhance the oxidation resistance during isothermal oxidation test, to decrease widely the oxide weight gain and to reduce the initial transient oxidation stage generally observed in the case of blank steels. Moreover, the experimental results confirm that yttrium sol–gel coating also plays a significant role on the cyclic oxidation behaviour of the 304 steel. In fact, the yttrium addition promotes remarkably the prolongation of the period during which the oxide scale still remains adherent to the substrate. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Stainless steel; Yttrium; Sol–gel coating; High temperature oxidation; In situ X-ray diffraction

1. Introduction The thermal cycling resistance of catalytic converters is an important criterion particularly if they are subjected to severe temperature fluctuations [1] (from room temperature up to 1273 K). Thus, the use of stainless steels is based on their ability to

*

Corresponding author. Address: Laboratoire Vellave dÕElaboration et dÕEtude des Materiaux, Institut Universitaire de Technologie, Departement de Chimie-Science des Materiaux, 8 rue J.B. Fabre, B.P. 219, 43006 Le Puy en Velay, France. Fax: +33-4-71-09-90-49. E-mail address: riff[email protected] (F. Riffard). 0010-938X/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0010-938X(03)00114-8

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form protective oxide scales. However, under cycling conditions, the chemical and physical properties of the protective oxide scales could be altered. Thus, two main requirements are necessary to obtain high temperature resistant alloys: a low protective oxide scale growing rate and a good adherence of such layer to the alloy especially under thermal cycling conditions. According to literature the best-suited protective oxides are Cr2 O3 and Al2 O3 because diffusion processes through these oxides are relatively slow [2–5]. Thus, most of the commercial refractory alloys (Fe–Cr, Co–Cr, Ni–Cr, Co–Cr–Al, Fe–Ni–Cr, Fe–Cr–Al, etc.) are either chromia- or alumina-forming alloys. Conventional 304 stainless steel develops a Cr2 O3 protective layer against corrosion up to 1100 K. Then predominant outward cationic diffusion induces the protective layer formation. Consequently, the diffusion mechanism is responsible for the parabolic weight gain behaviour observed during isothermal oxidation process [2]. At higher temperatures, and especially during thermal cycling conditions, the stresses generated in the oxide scale induce a spallation phenomenon with loss of chromium from the alloy, which contributes to dramatically decrease the oxidation resistance at high temperature [1]. The beneficial effects of active element additions on the oxidation resistance of many heat resistant alloys are now well known. Small amounts, usually below 1% of reactive elements (Sc, Ti, Y, Zr, Ce, La, etc.) clearly improve the oxidation behaviour of chromia- and alumina-forming alloys [3–12]. Several hypotheses were proposed to explain the reactive element effect (REE) on steel high temperature oxidation resistance such as: modification of the diffusion mechanisms, reduction of vacancies condensation at the internal interface, formation of reactive element oxide inclusions trapping alloy impurities, formation of a perovskite-type phase [13–20, etc.] However, no clear conclusions have been drawn for this effect. Several techniques of surface-depositing reactive elements have been proposed such as chemical vapour deposition methods, electrodeposition methods, ion implantation and application of nitrate-containing solutions. However, most of these methods are too expensive for industrial applications at intermediate temperatures (1150–1350 K). One of the best interesting low cost methods is to deposit yttrium by sol–gel coating. The purpose of this work is to study the correlation between the structure of the compounds present in the oxide scale and the REE under isothermal and 100-cycles high temperature oxidation conditions in air at 1273 K. This study was also performed on blank samples to have reference analyses. This paper constitutes the next stage of our study concerning the oxidation resistance at high temperature of various model materials [21–23].

2. Experimental method The AISI 304 steel specimens (15 mm
7.5 mm
2 mm) were cut from cold-rolled plates. Their composition obtained by inductively coupled plasma mass spectrometry (ICPMS) which is a powerful technique for trace multielemental analysis is given in Table 1. After polishing up to 800 SiC grade paper, specimens were washed

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Table 1 Chemical composition (wt.%) of 304 stainless steel obtained by ICPMS Fe

Cr

Ni

Mn

Si

Co

Mo

Cu

C

Ti

S

Bal.

17.9

9.05

1.52

0.48

0.22

0.15

0.1

0.05

0.01

0.01

in water, cleaned in ethanol, and dried. The sol–gel treatment [24–27] is made by dipping the specimen in an yttrium hydroxide colloidal solution at room temperature before a quick drying in air at 50 °C for 5 min. The whole surface layer is covered by this way, but the coating is non-uniform and its average thickness is about 1–2 lm. Isothermal oxidation under laboratory air (1 bar) was performed for 100 h in a Setaram TGTDA-92 thermobalance. Structural evolution of the compounds induced by isothermal oxidation was also characterised by in situ high temperature X-ray diffraction (XRD) under the same experimental conditions. The purpose of the cyclic test was to study the stainless steel oxidation behaviour under extremely severe experimental conditions. Cycling oxidation was performed in static laboratory air at atmospheric pressure using classical tubular furnaces. Three samples were tested to verify the reproducibility of the experiments. The heating/cooling cycle corresponds to a 20-h heating stage, followed by a 4-h cooling stage under ambient air. The furnace temperature was maintained to 1273 K, and the samples were directly inserted into, and removed from in a few seconds to guarantee rapid heating and cooling. Under these conditions, the cooling/heating rates are non-linear. The sample weights were measured after each cycle using a balance (accuracy up to 0.1 mg) to observe the specimen weight evolutions during the 100 cycles. Classical XRD and glancing angle XRD (GAXRD) analyses (CuKa1 radiation, k ¼ 0:15406 nm were also performed on both blank and yttrium-coated samples to observe the main compounds induced after isothermal and cycling oxidation tests.

3. Experimental results 3.1. Isothermal oxidation results 3.1.1. Thermogravimetric analyses Fig. 1 exhibits the mass gain variation of blank and yttrium-coated AISI 304 specimens during 125 h under isothermal oxidation. The growth of the oxide layer formed on blank specimens follows parabolic kinetics after an initial transient linear stage. By contrast, parabolic kinetic behaviours are observed from the very beginning of the oxidation test for all the yttrium-coated specimens. The corresponding parabolic rate constants of blank and yttrium-coated steels are respectively kpðblankÞ ¼ ð2:9  0:2Þ
106 mg2 cm4 s1 and kpðY-coatedÞ ¼ ð0:8  0:1Þ
106 mg2 cm4 s1 . The mass gain isothermal data clearly show the higher temperature oxidation resistance of yttrium-coated steels than blank specimens. Indeed, kinetic studies underline the beneficial effect of yttrium addition, which allows to inhibit the initial

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blank samples

1.2

∆m/S (mg/cm²)

1 0.8 0.6 0.4 Y-coated samples 0.2 0 0

20

40

60

80

100

exposure time (h)

Fig. 1. Weight gain versus time curves of blank and yttrium-coated 304 stainless steels during isothermal oxidation at 1273 K in air.

transient stage, only observed from 0 up to 10 h in the case of blank samples. Moreover, a significant reduction of the parabolic rate constant is also observed. 3.1.2. Structural analyses Fig. 2 shows the in situ XRD patterns obtained for the blank AISI 304 oxidised during the first 24 h of isothermal treatment. The crystalline compounds identified in the oxide scale can be grouped in the following classes of oxides: Mn1þx M2x O4 and M2 O3 where M stands for chromium and/or iron atoms. In both classes of compounds a whole set of oxides with variable Cr/Fe ratios are well established [28]. These two elements may substitute each other isomorphically resulting in crystalline compounds characterised by minute changes in the cell parameters. However, the diffraction lines corresponding to M2 O3 indicate that Cr2 O3 (Joint Committee Powder Diffraction Standard file: JCPDS no 38-1479) was the main compound ). The within the oxide scale (d spacing of the [1 0 4] diffraction line equal to 2.672 A spinel structure appears to be very close of the ideal formulation Mn1:5 Cr1:5 O4 , JCPDS 33-892). (d ¼ 2:981 A At the beginning of the oxidation test, the patterns reveal the initial nucleation of Cr2 O3 together with a small amount of the spinel structure, Mn1:5 Cr1:5 O4 . The evolution of oxide and metallic characteristic diffraction peak intensities during the first 8 h indicates the formation of a growing layer resulting from the external diffusion of the Mn2þ , Mn3þ and Cr3þ cations towards the oxide–gas interface. After 8 h, a change in the structural composition of the scale appears: iron oxides are detected (i.e. Fe2 O3 (JCPDS 33-664) and FeCr2 O4 (JCPDS 34-140)). Fe2 O3 (hematite) seems to be the most probable phase, but a possible mixed compound (i.e.

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Fig. 2. Initial sample and in situ high temperature 24-h XRD main experimental diffractograms performed on blank 304 steel at 1273 K in air.

Fe1:2 Cr0:8 O3 (JCPDS 34-412)), solid solution of hematite and chromia, must be considered [29]. After the first 10-h oxidation test, the initial oxides (i.e. Cr2 O3 and Mn1:5 Cr1:5 O4 ) and metal substrate diffraction peaks have completely disappeared and only the iron oxide phases can be observed. Fig. 3 shows the in situ XRD patterns obtained for the yttrium-coated AISI 304 oxidised during the first 24 h of isothermal treatment. The high relative intensity of

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Fig. 3. Initial sample and in situ high temperature 24-h XRD main experimental diffractograms performed on yttrium-coated 304 steel at 1273 K in air.

the metal substrate diffraction peaks during the whole experiment suggests that the yttrium-coated specimen is slowly oxidised. The main initial oxides (e.g. Cr2 O3 and Mn1:5 Cr1:5 O4 ) previously observed for the blank samples were also detected all along the oxidation test. By contrast, iron oxide phases were not detected. The most interesting result is the presence of diffraction lines corresponding to yttrium mixed compounds (typically YCrO3 (JCPDS 34-365) and YCrO4 (JCPDS 16-249)) which were previously identified in the case of incorporating methods [30–34].

3.2. Cyclic oxidation results 3.2.1. Thermogravimetric analyses Fig. 4 shows the weight evolution during 100 cycles performed for both blank and yttrium-coated steels. The mass changes include the amount of oxide scale that spalled off during cooling. The present results indicate a rapid spallation process (10 cycles) occurring in the case of blank samples. Yttrium additions have a beneficial effect on the prolongation of the period during which the oxide scale still remains adherent to the substrate. The yttrium introduction significantly increases this period up to about 27 cycles, but leads in the same way to catastrophic oxidation behaviour when spallation is initiated.

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2873

50

number of 20-hour-cycles 0 0

10

20

30

40

50

60

70

80

90

100

-50

Y-coated samples

∆m/S (mg/cm²)

-100 -150 -200 -250 -300 -350 -400 -450 -500

Blank samples

Fig. 4. Weight loss versus number of 20-h cycles under cyclic oxidation conditions at 1273 K in air.

3.2.2. Structural analyses Fig. 5 shows the XRD patterns of the spalled particles of the oxide scale formed on the blank AISI 304 steel oxidised for different cycles. X-ray analyses indicate that the spalled corrosion products formed initially on the blank steel were mainly Fe2 O3 , together with a spinel of chromium and iron, FeCr2 O4 . The same oxidation products were identified in the case of yttrium-coated sample (Fig. 6). After 40 20-h cycles, the catastrophic oxidation behaviour leads to the formation of a nickel-containing spinel oxide such as NiFe2 O4 (JCPDS 10-325). GAXRD patterns obtained on both blank and yttrium-coated 304 steels (Fig. 7) show the composition of the oxide scale still adherent to the substrate before and after spallation phenomena. Before spallation, the oxide scale appears to be mainly composed of chomia and manganese-enriched spinel. After spallation phenomena, the diffractograms reveal the formation of a poorly protective iron-rich mixed oxide FeCr2 O4 , which appears to be formed above hematite.

4. Discussion Most of industrial plants work under thermal cycling at high temperature. It has been shown that many process parameters, i.e. maximum and minimum temperature, cooling and heating rate, hold-time, play an important role during cyclic oxidation [35] and complicate the understanding of cyclic oxidation mechanism. In fact,

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Fig. 5. h–2h XRD experimental diffractograms performed after several successive 20-h cycles and after cooling of spalled particles obtained from blank specimens.

various mechanisms induce the mass change, such as mass gain due to oxygen uptake, mass loss caused by evaporation and/or spallation and mass gain due to rehealing of the spalled regions. Thus, the thermal cycling resistance of the AISI 304 steel constitutes an important criterion to develop high temperature application systems, especially if these systems were subjected to severe temperature fluctuations. The cycling effect on the materials is accentuated by the fact that the physical properties of the metal and its protective oxide scale are in constant evolution during the oxidation test. The structural analyses show in both cases (uncoated (Fig. 2) or yttrium-coated specimens (Fig. 3)) a layered structure of the oxide scale, with the formation of a manganese-rich spinel subscale over a Cr2 O3 -layer, as it was predicted by other authors [4,11,36–42]. In spite of the low-manganese concentration (1.52%) into the 304 steel, the presence of the Mn1:5 Cr1:5 O4 oxide can be explained by the fact that the lattice diffusivity of Mn is about two orders of magnitude greater than the other lattice-diffusion coefficients (Cr, Fe, Ni), especially in Cr2 O3 grown on chromiaforming alloys [37,43]. Yttrium addition to chromia-forming alloys is known to have a beneficial effect on the oxidation behaviour at high temperature [4–20]. Particularly, the yttrium presence changes the growth mechanism of oxides. Diffusion of O2 anions becomes prevalent in comparison with diffusion of Cr3þ cations. After yttrium-modified chromia-forming alloy oxidation, yttrium is mainly concentrated in grain boundaries [4,7,10,44–47]. This segregation is responsible for a change of the growth mechanism

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Fig. 6. h–2h XRD experimental diffractograms performed after several successive 20-h cycles and after cooling of spalled particles obtained from yttrium-coated specimens.

in oxide scales and the nature of diffusionnal species (O2 instead of Cr3þ ) with an inversion of the diffusion process. Without reactive element, oxide grows by an outward diffusion of Cr3þ from the internal interface to the external interface. Cationic diffusion through grain boundaries is blocked due to yttrium segregation [33]. At the beginning of the oxidation, yttrium is present in the form of Y2 O3 particles, which segregate slowly to grain boundaries to form YCrO3 phase observed after cooling to room temperature [48]. Consequently, yttrium presence improves the oxide scale adherence [7,10,16,49–51]. Cr3þ diffusion is reduced which limits the formation of voids at the metal/oxide interface contributing to the improvement of the oxide scale adherence. Yttrium introduction reduces also the value of stresses in the oxide scale. Yttrium oxides limit the internal oxidation and consequently the stresses resulting of this internal oxidation. Cr2 O3 grains are smaller [52] when yttrium oxides are present which can be explained by a solute-drag effect [53]. This lower grain size is also accompanied by a modification of the scale structure. In the absence of yttrium, the oxide scale microstructure exhibits large and columnar grains

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Fig. 7. h–2h and glancing angle XRD experimental diffractograms performed on yttrium-implanted specimens before and after intensive spallation phenomenon observed under cyclic oxidation at 1273 K in air.

near the oxide–gas interface whereas fine equiaxed grains are observed when yttrium is present [52]. The present results show that an yttrium sol–gel coating promotes a very beneficial effect on the isothermal oxidation resistance, and plays a significant role on the

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cyclic oxidation behaviour of the 304 steel in air at 1273 K. In fact, the yttrium sol– gel coating contributes to a lower oxidation rate (Fig. 1), and to the absence of ironcontaining oxide growth (Fig. 3) during isothermal oxidation tests. Moreover, cyclic oxidation analyses seem to indicate that sol–gel coatings also reduce or delay the intensive spallation phenomenon (Fig. 4). These three major yttrium addition effects were depicted separately below. 4.1. Lower isothermal oxidation rate Yttrium addition enhances the isothermal oxidation resistance of the 304 stainless steel (Fig. 1). The lower mass gain is due to an inhibition of the transient oxidation stage and to a lowering of the parabolic rate constant value by a factor of 4. The yttrium beneficial effect on the isothermal oxidation resistance of the 304 stainless steel is underlined by the reduction of the transient oxidation stage duration and by the decreasing parabolic rate constant value. Moreover, kinetic results show a lowering of the weight gain mass after 100-h oxidation test, which suggests a reduction of the oxide scale thickness, formed on the coated specimens. This lower oxidation rate, which results in a thinner oxide scale, was previously explained by Stringer et al. [54]. In fact, Stringer proposed that the doping elementÕs oxides block the short circuit paths for cation diffusion (diffusion along dislocations). In the most extreme case this may cause a change of the oxide formation mechanism from an outward cation diffusion type to an inward, oxygen, anion diffusion type. In the anion diffusion case, fewer voids form at the metal–oxide interface, which could improve the scale adherence [5,54]. 4.2. Non-iron-containing oxide growth When the stresses are sufficient to crack open the scale, they can cause new cation/ anion diffusion paths to occur. These new short circuit diffusion paths allow the inward diffusion of oxygen to the substrate and cationic outward diffusion of Fe to the scale–atmosphere interface, thereby deteriorating the barrier scale [5]. In situ XRD results of blank 304 steels confirm the growth of FeCr2 O4 and (Cr,Fe)2 O3 oxides during the first 24-h isothermal oxidation treatment. These iron-containing compounds are known such as non-protective and non-adherent oxides [55]. Their presence would induce the spallation phenomena, which would lead to the detrimental oxidation behaviour observed after the initial protective period. Because of the absence of these effects in the presence of yttrium superficially deposited on the 304 steel, the iron-containing oxide formation is inhibited and consequently the scale is more adherent. 4.3. Delay of the spallation phenomenon For both coated and blank steels, the spallation phenomena occur after a protective period where the oxide scale still remains adherent to the substrate. The duration of this period reached 10 cycles in the case of blank samples, and almost 30

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cycles in the case of yttrium-coated specimens. After this initial protective period, the cyclic oxidation behaviour seems to be detrimental since the intensive spallation phenomenon takes place continuously. After this protective period, the alloy is not able any longer to reheal the oxide layer and extensive mass loss occurs. The stresses generated during cyclic oxidation (heating or cooling) are caused by the thermal expansion coefficient difference between the metal and the scale. These thermally generated stresses can be very important (from about 100 MPa to several GPa) [1]. During heating, the base metal expands more than the scale because of its higher thermal expansion coefficient. The scale is subjected to tensile stresses, inducing cracking and consequently spallation process. During the 20-h isothermal hold, damaged areas are protected by a newly oxidised layer on the exposed metal. During subsequent cooling, the metal contracts more than the oxide leading to high compressive thermal stresses in the scale, which may be relaxed by spallation [56]. The damage induced by thermal cycles includes also creep in the metal substrate. This occurs in a narrow zone a few microns deep beneath the scale, and plastic strain occurs throughout the thickness of the oxide, causing cracking and spalling. During isothermal oxidation, the main generated stresses are those resulting from scale growth. They are generally compressive in nature, with fairly low values, and can usually be relaxed by creep of the metal or oxidation at the scale surface [57]. Values of thermal expansion coefficients are typically of about 18
106 K1 between 293 and 1273 K for austenitic stainless steels [1] whereas they are much lower for oxides than for metal (1
106 K1 for SiO2 , 8
106 K1 for Cr2 O3 ) [1,58]. The difference in these thermal expansion coefficients subjects the scale to high thermal stresses during each thermal cycle. Loss of adherence between the oxide scale and the metallic substrate is often due to the difference of dilatation coefficients between the oxide scale and the metallic substrate and the strains resulting during changes in temperature. Yttrium addition is known to promote an increase of adherence between the formed oxide scale and the metallic substrate [7,10,16,49–51,59– 61]. This increase of adherence observed with reactive elements can be explained by the fact that oxides of reactive elements have intermediate expansion coefficients between those of the alloy and those of the oxide scale. Consequently, they can lower the thermal stresses between the oxide scale and the metallic substrate during thermal cycling. Moreover, the stresses are strongly dependent on the oxide scale thickness. Reducing the oxide scale growth, yttrium addition contributes to a finer oxide scale formation and consequently decreases the stresses in the oxide scale. By XRD, Huntz [58] has observed a diminution of compressive stresses in the oxide scale during oxidation tests when yttrium was implanted in steel specimens. Moreover, reactive elements are known to prevent impurity segregation at the metal–oxide interface [10,17,62] by a combination of the reactive element with the impurity. In particular, sulphur and carbon impurities have major effects concerning the oxide scale adherence by segregation at the metal–oxide interface, on the oxidation behaviour segregation at oxide grain boundaries and by changing the oxide growth mechanism with oxygen transport becoming predominant [12,39, 40,63].

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5. Conclusion This study clearly underlines the beneficial effect of yttrium sol–gel coating on high temperature oxidation resistance of the 18%Cr–8%Ni stainless steel under isothermal and cyclic oxidation conditions. The oxidised blank material is not able to withstand the cyclic oxidation at 1273 K for more than 10 cycles, probably because of mechanical failure of the barrier oxide scale. In situ XRD results indicate the iron-containing oxide growth during the isothermal oxidation test of blank specimens. Assumed to be non-protective and non-adherent compounds, these ironcontaining oxides are not able to provide protection. The beneficial effect of yttrium introduction results in the reduction of the initial transient oxidation stage observed during the isothermal treatment. Y-coated 304 steels show also a significant lower parabolic rate constant value resulting in a lowering of the weight gain mass and thus in the reduction of the oxide scale thickness. This beneficial effect could be related to the non-formation of the nonprotective iron-containing oxides during the isothermal oxidation process. The yttrium sol–gel coating constitutes a very effective technique, which allows to extend significantly the life duration of 304 stainless steel under cyclic oxidation conditions in air at 1273 K. References [1] L. Antoni, J.M. Herbelin, Cyclic oxidation of High Temperature Materials, in: Proc. of an EFC Workshop, Frankfurt/Main, 1999, European Federation of Corrosion Publications no. 27, pp. 187– 197. [2] C. Wagner, Z. Phys. Chem. B 21 (1993) 25. [3] M.J. Bennet, D.P. Moon, in: E. Lang (Ed.), The Role of Active Elements in the Oxidation Behaviour of High Temperature Metals and Alloys, Elsevier, Amsterdam, 1989, p. 111. [4] M. Landkof, A.V. Levy, D.H. Boone, R. Gray, E. Yaniv, Corrosion-NACE 41 (1985) 344. [5] D.P. Whittle, J. Stringer, Philos. Trans. R. Soc., London Ser. A 295 (1980) 309. [6] P. Kofstad, A. Rahmel, R.A. Rapp, D.L. Douglass, Oxid. Met. 32 (1989) 125. [7] D.P. Moon, M.J. Bennett, Mater. Sci. Forum 43 (1989) 269. [8] I.M. Allam, D.P. Whittle, J. Stringer, Oxid. Met. 13 (1979) 381. [9] B.A. Pint, Oxid. Met. 45 (1996) 1. [10] A. Strawbridge, P.Y. Hou, Mat. High Temp. 12 (1994) 177. [11] M.F. Stroosnijder, V. Guttman, T. Fransen, J.H.W. De Wit, Oxid. Met. 33 (1990) 371. [12] S.K. Mitra, S.K. Roy, S.K. Bose, Oxid. Met. 39 (1993) 221. [13] J. Stringer, Mater. Sci. Eng. A 120 (1989) 129. [14] F.I. Wei, F.H. Stott, Corros. Sci. 29 (1989) 839. [15] I.M. Allam, D.P. Whittle, J. Stringer, Oxid. Met. 12 (1978) 35. [16] C.H. Yang, P.A. Labun, G. Welsch, T.E. Mitchell, J. Mater. Sci. 22 (1987) 449. [17] P.Y. Hou, J. Stringer, Oxid. Met. 34 (1990) 299. [18] L.V. Ramanathan, in Proceedings of the 11th Int. Corrosion, Florence, vol. 4, 1990, p. 177. [19] E.A. Polman, T. Fransen, P.J. Gellings, Oxid. Met. 33 (1990) 135. [20] T. Zhang, W.W. Gerberich, D.A. Shores, J. Mat. Res. 12 (1997) 697. [21] E. Caudron, H. Buscail, R. Cueff, Surf. Coat. Tech. 126 (2000) 266. [22] E. Caudron, H. Buscail, Corros. Sci. 43 (2001) 1477. [23] Y.P. Jacob, V.A.C. Haanappel, M.F. Stroosnijder, H. Buscail, P. Fielitz, G. Borchardt, Corros. Sci. 44 (2002) 2027.

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