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Nitridation effect on the oxidation of a austenitic stainless steel AISI 304 at 900 °C
Applied Surface Science 225 (2004) 14–20
Nitridation effect on the oxidation of a austenitic stainless steel AISI 304 at 900 8C Christophe Issartela,*, Henri Buscaila, Eric Caudrona, Re´gis Cueffa, Fre´de´ric Riffarda, Samira El Messkia, Se´bastien Perriera, Philippe Jacquetb, Michel Lambertinb a
Laboratoire Vellave sur l’Elaboration et l’Etude des Mate´riaux, 8 Rue J.B. Fabre, BP 219, 43006 Le Puy en Velay, France b LaBoMap, ENSAM, C.E.R., 71250 Cluny, France Received 17 July 2003; accepted 18 September 2003
Abstract This paper presents a study on the growth of oxides on a commercial chromia-forming alloy (austenitic stainless steel AISI 304), in air at 900 8C. After the nitridation treatment performed, a gN solid solution is observed without any nitride formation in the alloy surface. In situ X-ray diffraction has been used to follow oxides evolution at the testing temperature. At the beginning of the oxidation test, Fe2O3 is formed together with Cr2O3 and Mn1.5Cr1.5O4. Nevertheless, Fe2O3 quickly becomes undetectable and appears again after 30 h oxidation. In the case of untreated specimens, four oxides are observed throughout the oxidation test: Cr2O3, Mn1.5Cr1.5O4, Fe2O3 and Fe7SiO10. Our results show that nitridation increased the high temperature oxidation resistance of the 304 stainless steel at 900 8C. # 2003 Elsevier B.V. All rights reserved. Keywords: Stainless steel; Nitridation; In situ; X-ray diffraction; High temperature oxidation
1. Introduction The high temperature oxidation of 304 steels has already been studied by several authors [1–5]. To improve its wear and corrosion resistance a surface nitridation, can be applied. This technique was already used to improve the alloy hardness and its aqueous corrosion resistance [6–8]. Nevertheless, there is
almost no previous works dealing with the influence of nitridation on the high temperature oxidation of a 304 steel [9]. In order to better understand the effect of the nitridation on the high temperature oxidation of a 304 steel, this work will focus on the study of the oxide evolution on nitrided and reference specimens by use of in situ X-ray diffraction at 900 8C.
2. Experimental details *
Corresponding author. Tel.: þ33-471-0990-43; fax: þ33-471-0990-49. E-mail addresses: [email protected] (C. Issartel), [email protected] (H. Buscail), [email protected] (E. Caudron), [email protected] (R. Cueff), [email protected] (F. Riffard), [email protected] (S. Perrier), [email protected] (M. Lambertin).
The material investigated in this study is a 304 steel. The alloy composition is given in Table 1. The 1 mm thick rectangular specimens of around 5 cm2 total surface area were abraded up to the 800grit SiC paper, then degreased with ethanol and finally
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.09.037
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20 Table 1 The 304 alloy composition 304 steel
wt.%
Fe Cr Ni Mn Si Co Mo Cu C Ti S
Balance 17.9 9.05 1.52 0.475 0.215 0.15 0.096 0.053 0.007 0.004
tered every hour during 40 h. In situ diffractograms were obtained by use of a high temperature Anton PAAR HTK 1200 chamber installed in a Philips X’pert MPD diffractometer (Cu Ka1 ¼ 0:15406 nm radiation).
3. Experimental results 3.1. Oxidation kinetics at high temperature
dried. To study the influence of nitrogen on the corrosion behaviour, a nitridation surface treatment has been applied on some specimens. Nitridation was performed at 430 8C for 8 h. The selected gas mixture is composed of 60% hydrogen and 40% nitrogen. The gas flow was regulated at 3.3 l/ min and a total pressure was fixed at 2 mbar in a BMIB 83 TIC Furnace. The nitridation parameters have been chosen to insure the formation of a gN layer saturated with nitrogen without nitride formation [9,10]. High temperature oxidation testing was performed during 100 h at 900 8C in air at the atmospheric pressure using a Setaram TGDTA 92-1600 microthermobalance. In situ X-ray diffractograms were regis-
m/S (mg.cm-2)
6
15
Fig. 1 presents the mass gain versus time curves (Dm=S ¼ f ðtÞ) of initial and nitrided 304 specimens oxidised during 230 h at 900 8C in air. The comparisons of the kinetic curves show that reference samples have a higher weight gain during the 230 h oxidation test. These results do not reveal a parabolic law for the kinetic of high temperature oxidation. A breakaway is observed for both samples: after 20 h for the reference specimen, after 30 h for the nitrided one. This important weight gain is observed later and is less important for the nitrided sample. The kinetic results show that nitridation has a beneficial effect on the alloy oxidation rate. 3.2. X-ray diffraction studies 3.2.1. Effect of nitridation on the 304 steels structure Results of structural analyses by X-ray diffraction are reported in Fig. 2. As it was observed on untreated
Blank sample
5 4 3 2
Nitrided sample
1 0 0
50
100
150
200
Time (hours) Fig. 1. Mass gain vs. time curves of nitrided and initial specimens at 900 8C, in air.
16 C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
Fig. 2. X-ray diffractograms obtained on initial and nitrided specimens.
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
Fig. 3. Main in situ X-ray diffractograms obtained on a reference 304 specimen oxidised at 900 8C, in air.
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C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
Fig. 4. Main in situ X-ray diffractograms obtained on nitrided 304 specimens oxidised at 90 8C, in air.
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
specimens, we still note the presence of the martensitic and the austenitic phases. The martensitic presence is related to a mechanical polishing effect. The experimental austenite lattice parameter, calculated from ˚ . It corresponds to (2 0 0) plans, is aexp ¼ 3:5948 A the austenitic stainless steel AISI 304 (JCPDS 33-397) ˚ . In structure with a lattice parameter ath ¼ 3:591 A Fig. 2, the diffractograms show also the effects of nitridation on the sample surface structure. The nitridation treatment leads to the gN solid solution formation shifted to low angles. This gN structure can be described as a nitrogen-saturated structure without any detectable nitride formation. The austenite diffraction peaks enlargement indicates that a finer grain structure is present on the alloy surface. We also observed a more important shift for (2 0 0) plan than for (1 1 1) plan. This is due to a preferential arrangement of the nitrogen atoms into the (2 0 0) oriented grains [11]. The lattice parameter then depends of the nitrogen concentration gradient in the austenite structure. We can calculate the atomic nitrogen concentration CN according to the following relation: a0 ¼ am þ aCN
(1)
with the Ve´ gard constant a ¼ 0:0013 nm/% [12], ˚ and am ¼ 3:5948 A ˚. a0 ¼ 4:0788 A In our study, CN ¼ 37:2% after 8 h nitridation at 430 8C. 3.2.2. In situ high temperature X-ray diffraction studies In reference specimens the diffractograms are reported in Fig. 3. In situ X-ray diffraction shows the growth of a Cr2O3 scale (JCPDS 38-1479) from the first hours of oxidation at 900 8C in air. At the same time, the growth of Mn1.5Cr1.5O4 (JCPDS 33-892), Fe2O3 (JCPDS 33-664) and Fe7SiO10 (JCPDS 221118) are also observed. The Fe2O3 oxide peaks intensities reveal that it is the major phase formed on the alloy surface. Under the same conditions, results obtained on the nitrided specimens are reported in Fig. 4. From the beginning of the test, Cr2O3 peaks appear and grow together with the Mn1.5Cr1.5O4 peaks. During the first hour of oxidation Fe2O3 can be identified on the sample surface. The Fe2O3 peaks disappear during the second hour of oxidation. After 30 h of oxidation, the Fe2O3 peaks appear again. The relative peak
19
intensities evolution demonstrate that Fe2O3 progressively appears while Cr2O3 peaks decrease. The three oxides: Fe2O3, Mn1.5Cr1.5O4 and Cr2O3 can be detected at the end of the test. Compared to untreated specimens, Fe2O3 is less present in the oxide scale formed on nitrided 304 steel.
4. Discussion In Fig. 2, it was observed that after nitridation we still note the presence of the martensitic and the austenitic phases. This was also described by Elmer for the range of AISI 300 steels [13]. These phases can be induced by plastic deformation on the alloy due to the mechanical polishing of the samples [14]. As it was observed by other authors [9,10], the diffractograms also show that the nitridation treatment leads to the gN solid solution formation. No nitrides where observed. These authors also demonstrated that nitrogen atoms occupy octahedral sites of the gN phase, in a statistically disordered way. According to our kinetic results (Fig. 1), it is well established that the nitridation increases the high temperature oxidation resistance of the 304 specimens. This phenomenon was also observed at 1000 8C by Marot et al. [9]. Fig. 1 shows that reference specimens have a higher mass gain than nitrided samples after the 230 h oxidation. The beneficial effect of nitridation on the kinetic regimes observed at 900 8C can be explained by in situ X-ray diffraction (Figs. 3 and 4). The identified oxides are not the same for an initial or a nitrided specimen. We have noticed that Fe2O3 appears at different oxidation times. In reference specimens oxidised at 900 8C, a Cr2O3 scale is immediately formed with Mn1.5Cr1.5O4 at the beginning of the test. At the same time, Fe7SiO10 and Fe2O3 appear and quickly grow. We can envisage that Fe2O3 grows above the other oxides. This result can explain the breakaway in the mass gain registered after 30 h oxidation of untreated specimens because Fe2O3 is known to be less protective than chromia [15,16]. In the case of nitrided 304 specimens, the Fe2O3 peaks quickly appear at the beginning of the oxidation test (Fig. 4). The formation of Fe2O3 initially appears to be due to the fact that chromium is temporarily trapped in a CrN at the very beginning of the test as it
20
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
has been already described by Roux et al. [10]. The subsequent lower oxidation rate of nitrided specimens can be related to the Cr2O3 and Mn1.5Cr1.5O4 growth (Fig. 4). The good Cr2O3 growth can also be a consequence of the alloy structure deformation by the nitridation process (Fig. 2) as it was observed in the case of ion implantation. Pleshivtsev and Krasikov [17] explained that the alloy grain boundary diffusion could be favoured by ion implantation. After 40 h oxidation of nitrided specimens, the Fe2O3 formation is observed again in Fig. 4. It is related to the breakaway observed in the kinetic curve (Fig. 1). The longer delay of the Fe2O3 formation observed on nitrided specimens can be related to the inhibition of the Fe7SiO10 formation. Silicon is then free to form a continuous silicon rich layer at the internal interface which is known to limit the iron diffusion [18–21]. It then permits the formation of a thicker chromia scale, more protective than an iron oxide containing layer.
5. Conclusion After the nitridation treatment performed on the 304 specimens it is found that a gN solid solution is formed on the substrate without any nitride formation. In situ X-ray diffraction has been used to follow the oxide evolution at 900 8C in air. On untreated specimens it is shown that the relatively high oxidation rate is due to the formation of a non-protective iron oxide. This is due to the Fe7SiO10 formation which trapped silicon in a mixed oxide. Then, a continuous silica layer cannot be formed at the internal interface and the iron diffusion is not limited so much. On nitrided specimens Fe2O3 is formed during the first hour of oxidation. Nevertheless, Cr2O3 and Mn1.5Cr1.5O4 quickly appear to form a protective oxide scale leading to a lower scale growth rate compared to initial specimens. It then appears that the nitridation increases the high temperature oxidation resistance of the 304 steel at 900 8C. This is due to
the inhibition of the Fe7SiO10 phase formation which permits to silicon to act as a better iron diffusion barrier.
References [1] F. Riffard, H. Buscail, E. Caudron, R. Cueff, C. Issartel, S. Perrier, J. Mater. Sci. 37 (2002) 3925–3933. [2] F.J. Pe´ rez, F. Pedraza, M.P. Hierro, J. Balmain, G. Bonnet, Surf. Coat. Technol. 153 (2002) 49–58. [3] V.A.C. Haanappel, M.F. Stroosnijder, Corrosion 57 (6) (2001) 557–565. [4] N.E. Hakiki, M.F. Montemor, M.G.S. Ferreira, M. da Cunha Belo, Corros. Sci. 42 (2000) 687–702. [5] S.K. Mitra, S.K. Roy, S.K. Bose, Oxidation Met. 39 (3–4) (1993) 221–229. [6] N. Bellakhal, K. Draou, J.L. Brisset, J. Chem. Res. (S) (1) (1999) 38–39. [7] J.P. Rivie`re, C. Brin, Ph. Meheust, J.P. Villain, R. Cauvin, J. Phys. IV (81) (2001) 71–78. [8] M.E. Djeghlal, N. Hamedi, L. Barrallier, J. Phys. IV (90) (2001) 141–145. [9] L. Marot, H. Buscail, A. Straboni, F. Riffard, E. Caudron, R. Cueff, J. Phys. IV (12) (2002) 215–221. [10] T. Roux, A. Saker, C. Leroy, C. Frantz, H. Michel, Traitement Thermique 278 (1994) 61–65. [11] C. Blawert, B.L. Mordike, Y. Jiraskova, O. Scheeweiss, Surf. Eng. 16 (6) (1999) 469–475. [12] A. Saker, C. Leroy, H. Michel, C. Frantz, Mater. Sci. Eng. 140 (1991) 702–708. [13] J.W. Elmer, J. Wong, T. Ressler, Scripta Mater. 43 (2000) 751–757. [14] F.J. Pe´ rez, M.J. Cristo´ bal, M.P. Hierro, F. Pedraza, G. Arnau, T.P. Merino, Surf. Coat. Technol. 126 (2000) 116–122. [15] M.J. Capitan, S. Lefebvre, J.P. Dallas, J.L. Pastol, Surf. Coat. Technol. 100–101 (1998) 202–207. [16] F.J. Perez, M.J. Cristobal, M.P. Hierro, F. Pedraza, G. Arnaud, P. Merino, Surf. Coat. Technol. 126 (2000) 116–122. [17] N.V. Pleshivtsev, E.A. Krasikov, Russ. Metall. Metall. 4 (1995) 64–93. [18] F.H. Stott, G.C. Wood, J. Stringer, Oxidation Met. 44 (1–2) (1995) 113–145. [19] H. Fujikawa, T. Morimoto, Y. Nishiyama, Mater. High Temp. 17 (2) (2000) 293–298. [20] S.N. Basu, G.J. Yurek, Oxidation Met. 36 (3–4) (1991) 281– 315. [21] F. Riffard, H. Buscail, E. Caudron, R. Cueff, C. Issartel, S. Perrier, Mater. Charact. 49 (2002) 55–65.
Nitridation effect on the oxidation of a austenitic stainless steel AISI 304 at 900 8C Christophe Issartela,*, Henri Buscaila, Eric Caudrona, Re´gis Cueffa, Fre´de´ric Riffarda, Samira El Messkia, Se´bastien Perriera, Philippe Jacquetb, Michel Lambertinb a
Laboratoire Vellave sur l’Elaboration et l’Etude des Mate´riaux, 8 Rue J.B. Fabre, BP 219, 43006 Le Puy en Velay, France b LaBoMap, ENSAM, C.E.R., 71250 Cluny, France Received 17 July 2003; accepted 18 September 2003
Abstract This paper presents a study on the growth of oxides on a commercial chromia-forming alloy (austenitic stainless steel AISI 304), in air at 900 8C. After the nitridation treatment performed, a gN solid solution is observed without any nitride formation in the alloy surface. In situ X-ray diffraction has been used to follow oxides evolution at the testing temperature. At the beginning of the oxidation test, Fe2O3 is formed together with Cr2O3 and Mn1.5Cr1.5O4. Nevertheless, Fe2O3 quickly becomes undetectable and appears again after 30 h oxidation. In the case of untreated specimens, four oxides are observed throughout the oxidation test: Cr2O3, Mn1.5Cr1.5O4, Fe2O3 and Fe7SiO10. Our results show that nitridation increased the high temperature oxidation resistance of the 304 stainless steel at 900 8C. # 2003 Elsevier B.V. All rights reserved. Keywords: Stainless steel; Nitridation; In situ; X-ray diffraction; High temperature oxidation
1. Introduction The high temperature oxidation of 304 steels has already been studied by several authors [1–5]. To improve its wear and corrosion resistance a surface nitridation, can be applied. This technique was already used to improve the alloy hardness and its aqueous corrosion resistance [6–8]. Nevertheless, there is
almost no previous works dealing with the influence of nitridation on the high temperature oxidation of a 304 steel [9]. In order to better understand the effect of the nitridation on the high temperature oxidation of a 304 steel, this work will focus on the study of the oxide evolution on nitrided and reference specimens by use of in situ X-ray diffraction at 900 8C.
2. Experimental details *
Corresponding author. Tel.: þ33-471-0990-43; fax: þ33-471-0990-49. E-mail addresses: [email protected] (C. Issartel), [email protected] (H. Buscail), [email protected] (E. Caudron), [email protected] (R. Cueff), [email protected] (F. Riffard), [email protected] (S. Perrier), [email protected] (M. Lambertin).
The material investigated in this study is a 304 steel. The alloy composition is given in Table 1. The 1 mm thick rectangular specimens of around 5 cm2 total surface area were abraded up to the 800grit SiC paper, then degreased with ethanol and finally
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.09.037
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20 Table 1 The 304 alloy composition 304 steel
wt.%
Fe Cr Ni Mn Si Co Mo Cu C Ti S
Balance 17.9 9.05 1.52 0.475 0.215 0.15 0.096 0.053 0.007 0.004
tered every hour during 40 h. In situ diffractograms were obtained by use of a high temperature Anton PAAR HTK 1200 chamber installed in a Philips X’pert MPD diffractometer (Cu Ka1 ¼ 0:15406 nm radiation).
3. Experimental results 3.1. Oxidation kinetics at high temperature
dried. To study the influence of nitrogen on the corrosion behaviour, a nitridation surface treatment has been applied on some specimens. Nitridation was performed at 430 8C for 8 h. The selected gas mixture is composed of 60% hydrogen and 40% nitrogen. The gas flow was regulated at 3.3 l/ min and a total pressure was fixed at 2 mbar in a BMIB 83 TIC Furnace. The nitridation parameters have been chosen to insure the formation of a gN layer saturated with nitrogen without nitride formation [9,10]. High temperature oxidation testing was performed during 100 h at 900 8C in air at the atmospheric pressure using a Setaram TGDTA 92-1600 microthermobalance. In situ X-ray diffractograms were regis-
m/S (mg.cm-2)
6
15
Fig. 1 presents the mass gain versus time curves (Dm=S ¼ f ðtÞ) of initial and nitrided 304 specimens oxidised during 230 h at 900 8C in air. The comparisons of the kinetic curves show that reference samples have a higher weight gain during the 230 h oxidation test. These results do not reveal a parabolic law for the kinetic of high temperature oxidation. A breakaway is observed for both samples: after 20 h for the reference specimen, after 30 h for the nitrided one. This important weight gain is observed later and is less important for the nitrided sample. The kinetic results show that nitridation has a beneficial effect on the alloy oxidation rate. 3.2. X-ray diffraction studies 3.2.1. Effect of nitridation on the 304 steels structure Results of structural analyses by X-ray diffraction are reported in Fig. 2. As it was observed on untreated
Blank sample
5 4 3 2
Nitrided sample
1 0 0
50
100
150
200
Time (hours) Fig. 1. Mass gain vs. time curves of nitrided and initial specimens at 900 8C, in air.
16 C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
Fig. 2. X-ray diffractograms obtained on initial and nitrided specimens.
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
Fig. 3. Main in situ X-ray diffractograms obtained on a reference 304 specimen oxidised at 900 8C, in air.
17
18
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
Fig. 4. Main in situ X-ray diffractograms obtained on nitrided 304 specimens oxidised at 90 8C, in air.
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
specimens, we still note the presence of the martensitic and the austenitic phases. The martensitic presence is related to a mechanical polishing effect. The experimental austenite lattice parameter, calculated from ˚ . It corresponds to (2 0 0) plans, is aexp ¼ 3:5948 A the austenitic stainless steel AISI 304 (JCPDS 33-397) ˚ . In structure with a lattice parameter ath ¼ 3:591 A Fig. 2, the diffractograms show also the effects of nitridation on the sample surface structure. The nitridation treatment leads to the gN solid solution formation shifted to low angles. This gN structure can be described as a nitrogen-saturated structure without any detectable nitride formation. The austenite diffraction peaks enlargement indicates that a finer grain structure is present on the alloy surface. We also observed a more important shift for (2 0 0) plan than for (1 1 1) plan. This is due to a preferential arrangement of the nitrogen atoms into the (2 0 0) oriented grains [11]. The lattice parameter then depends of the nitrogen concentration gradient in the austenite structure. We can calculate the atomic nitrogen concentration CN according to the following relation: a0 ¼ am þ aCN
(1)
with the Ve´ gard constant a ¼ 0:0013 nm/% [12], ˚ and am ¼ 3:5948 A ˚. a0 ¼ 4:0788 A In our study, CN ¼ 37:2% after 8 h nitridation at 430 8C. 3.2.2. In situ high temperature X-ray diffraction studies In reference specimens the diffractograms are reported in Fig. 3. In situ X-ray diffraction shows the growth of a Cr2O3 scale (JCPDS 38-1479) from the first hours of oxidation at 900 8C in air. At the same time, the growth of Mn1.5Cr1.5O4 (JCPDS 33-892), Fe2O3 (JCPDS 33-664) and Fe7SiO10 (JCPDS 221118) are also observed. The Fe2O3 oxide peaks intensities reveal that it is the major phase formed on the alloy surface. Under the same conditions, results obtained on the nitrided specimens are reported in Fig. 4. From the beginning of the test, Cr2O3 peaks appear and grow together with the Mn1.5Cr1.5O4 peaks. During the first hour of oxidation Fe2O3 can be identified on the sample surface. The Fe2O3 peaks disappear during the second hour of oxidation. After 30 h of oxidation, the Fe2O3 peaks appear again. The relative peak
19
intensities evolution demonstrate that Fe2O3 progressively appears while Cr2O3 peaks decrease. The three oxides: Fe2O3, Mn1.5Cr1.5O4 and Cr2O3 can be detected at the end of the test. Compared to untreated specimens, Fe2O3 is less present in the oxide scale formed on nitrided 304 steel.
4. Discussion In Fig. 2, it was observed that after nitridation we still note the presence of the martensitic and the austenitic phases. This was also described by Elmer for the range of AISI 300 steels [13]. These phases can be induced by plastic deformation on the alloy due to the mechanical polishing of the samples [14]. As it was observed by other authors [9,10], the diffractograms also show that the nitridation treatment leads to the gN solid solution formation. No nitrides where observed. These authors also demonstrated that nitrogen atoms occupy octahedral sites of the gN phase, in a statistically disordered way. According to our kinetic results (Fig. 1), it is well established that the nitridation increases the high temperature oxidation resistance of the 304 specimens. This phenomenon was also observed at 1000 8C by Marot et al. [9]. Fig. 1 shows that reference specimens have a higher mass gain than nitrided samples after the 230 h oxidation. The beneficial effect of nitridation on the kinetic regimes observed at 900 8C can be explained by in situ X-ray diffraction (Figs. 3 and 4). The identified oxides are not the same for an initial or a nitrided specimen. We have noticed that Fe2O3 appears at different oxidation times. In reference specimens oxidised at 900 8C, a Cr2O3 scale is immediately formed with Mn1.5Cr1.5O4 at the beginning of the test. At the same time, Fe7SiO10 and Fe2O3 appear and quickly grow. We can envisage that Fe2O3 grows above the other oxides. This result can explain the breakaway in the mass gain registered after 30 h oxidation of untreated specimens because Fe2O3 is known to be less protective than chromia [15,16]. In the case of nitrided 304 specimens, the Fe2O3 peaks quickly appear at the beginning of the oxidation test (Fig. 4). The formation of Fe2O3 initially appears to be due to the fact that chromium is temporarily trapped in a CrN at the very beginning of the test as it
20
C. Issartel et al. / Applied Surface Science 225 (2004) 14–20
has been already described by Roux et al. [10]. The subsequent lower oxidation rate of nitrided specimens can be related to the Cr2O3 and Mn1.5Cr1.5O4 growth (Fig. 4). The good Cr2O3 growth can also be a consequence of the alloy structure deformation by the nitridation process (Fig. 2) as it was observed in the case of ion implantation. Pleshivtsev and Krasikov [17] explained that the alloy grain boundary diffusion could be favoured by ion implantation. After 40 h oxidation of nitrided specimens, the Fe2O3 formation is observed again in Fig. 4. It is related to the breakaway observed in the kinetic curve (Fig. 1). The longer delay of the Fe2O3 formation observed on nitrided specimens can be related to the inhibition of the Fe7SiO10 formation. Silicon is then free to form a continuous silicon rich layer at the internal interface which is known to limit the iron diffusion [18–21]. It then permits the formation of a thicker chromia scale, more protective than an iron oxide containing layer.
5. Conclusion After the nitridation treatment performed on the 304 specimens it is found that a gN solid solution is formed on the substrate without any nitride formation. In situ X-ray diffraction has been used to follow the oxide evolution at 900 8C in air. On untreated specimens it is shown that the relatively high oxidation rate is due to the formation of a non-protective iron oxide. This is due to the Fe7SiO10 formation which trapped silicon in a mixed oxide. Then, a continuous silica layer cannot be formed at the internal interface and the iron diffusion is not limited so much. On nitrided specimens Fe2O3 is formed during the first hour of oxidation. Nevertheless, Cr2O3 and Mn1.5Cr1.5O4 quickly appear to form a protective oxide scale leading to a lower scale growth rate compared to initial specimens. It then appears that the nitridation increases the high temperature oxidation resistance of the 304 steel at 900 8C. This is due to
the inhibition of the Fe7SiO10 phase formation which permits to silicon to act as a better iron diffusion barrier.
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