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The influence of implanted silicon on the cyclic oxidation behaviour of two different stainless steels
Surface and Coatings Technology 120–121 (1999) 442–447 www.elsevier.nl/locate/surfcoat
The influence of implanted silicon on the cyclic oxidation behaviour of two different stainless steels F.J. Pe´rez a, *, M.J. Cristo´bal b, M.P. Hierro a, F. Pedraza a a Dpto. de Ciencia de los Materiales, Facultad de Quı´mica, Universidad Complutense, 28040-Madrid, Spain b Dpto. de Ciencia de los Materiales, E.T.S.I.I y M., Universidad de Vigo, Lagoas-Marcosende, 36210-Pontevedra, Spain
Abstract High-temperature alloys are frequently used in power plants, gasification systems, the petrochemical industry, combustion processes and in aerospace applications. Depending on the application, materials are subjected to contaminated atmospheres and/or thermal cycling. Thermal cyclic experiments were carried out in order to study the influence of implanted silicon on the adherence of the scale. The effect of silicon was tested on two different stainless steels in air at 1173 K. The oxidized specimens were characterized by means of X-ray diffraction ( XRD), scanning electron microscopy (SEM ) and energy-dispersive X-ray analysis (EDS ). The experimental results confirm that silicon implantation at a nominal dose of 1016 Si ions cm−2 does not play a significant role on the cyclic oxidation behaviour of the austenitic AISI 304 steel in air at 1173 K. However, it appears to enhance the oxidation resistance of the ferritic AISI 430 at oxidation cycles longer than 250. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Cyclic oxidation; High-temperature oxidation; Silicon implantation; Stainless steels
1. Introduction The ability of the current generation of high-temperature alloys to resist corrosion depends on the capacity to form and maintain a protective oxide scale, such as Cr O , Al O or SiO . The beneficial role of reactive 2 3 2 3 2 elements, such as yttrium, cerium and other rare-earth elements, on the oxidation behaviour of high-temperature alloys has received considerable attention [1–3]. Small amounts ( less than 1%) of these elements added to the bulk alloy or to the surface may decrease the oxidation rate and dramatically improve the scale adherence. Several mechanisms have been proposed [4–6 ] to explain the beneficial influences of rare-earth additions to high-temperature alloys. However, no conclusive mechanisms have yet emerged to explain these effects. The behaviour of cyclic oxidation at high temperature of the surface modified stainless steels has not been studied extensively [7–9]; however, in recent years, a few studies related to the effect of the rare-earth elements on these steels have been reported. For example, Seal * Corresponding author. Tel.: +34-91-39-44215; fax: +34-91-39-33457. E-mail address: [email protected] (F. Pe´rez)
et al. [10] studied the influence of superficial applied CeO coatings on the isothermal and cyclic oxidation 2 behaviour of three grades of austenitic steel (AISI 321, 316 and 304) in dry air, at 1273 K. The results clearly showed that coatings not only reduced the rates of scale growth for all three varieties of steel but also significantly improved the scale adhesion. Stroosnijder et al. [11] investigated the influence of implanted cerium on the corrosion resistance of a wrought austenitic Fe–20Cr– 32Ni steel (Alloy 800 H ) in a simulated coal gasification atmosphere at 973 K. It was shown that the corrosive attack in the H S-containing gas mixture could be 2 reduced for the cerium-implanted material if the implantation dose was sufficiently high. Likewise, Noli et al. [12] investigated the oxidation behaviour of Mg-implanted AISI 321 austenitic stainless steel, and two different mechanisms of Mg ion diffusion and oxidation of the implanted steel were observed. At low temperatures, an improvement in oxidation resistance was found. However at higher temperatures, the implantation of Mg reduced the scale adherence and enhanced the oxidation rate. Nevertheless, little attention has been given to the implantation of silicon ions, although Hou and Stringer recently proposed that this element could act as a
0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 50 3 - 4
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443
Table 1 Compositions of the tested stainless steels (wt%)
AISI 304 AISI 430
Fe
Cr
Ni
Mn
Si
C
Balance Balance
18 16.5
8 0.21
1.42 0.30
0.35 0.32
0.056 0.064
reactive element in high-temperature oxidation [13]. To date, only Amano and co-workers [14–16 ] have studied the high-temperature oxidation behaviour of Ni–20Cr alloys containing 1 and 3% of silicon. According to their results, the addition of silicon reduced the oxidation rate, but had no effect on the adherence of the surface oxide. It is clear from this review that the effect of the rare earth elements on the corrosion resistance of stainless steels is far from complete and may offer a way to analyse possible surface modifications enhancing life time at higher temperatures of operation.
2. Experimental Specimens of AISI 304 steel and AISI 430 steel, whose compositions are given in Table 1, in the form of 10 mm×7 mm×2 mm coupons were cut from coldrolled plates. Before implantation, all the samples were ground on silicon carbide paper to 600 grit and then ultrasonically degreased in acetone and then in ethanol. The uniform implantation of silicon, at a nominal dose of 1016 Si ions cm−2, into one of the principal coupon faces was undertaken using a 150 keV acceleration potential. The oxidation experiments were performed in static laboratory air at atmospheric pressure, up to 320 cycles. Each cycle consisted of a period of 60 min at the oxidation temperature (1173 K ) and 15 min at room temperature, which appeared to be long enough to cool the specimens below 60°C. The specimens were inserted into, and removed from, the heat zone in a few seconds to guarantee rapid heating and cooling. At the initial stages of the experiments, the weight gain of the samples was measured after a few cycles. At a later stage, the specimens were measured at intervals of more cycles. The techniques used to characterize the structure and composition of the oxidation products formed included optical metallography (OM ), X-ray diffraction ( XRD), scanning electron microscopy (SEM ) and energy-dispersive X-ray analysis (EDS ).
Fig. 1. Mass change versus number of 1 h cycles for silicon implanted and un-implanted AISI 304 and AISI 430 steels in air at 1173 K.
mass changes excluded the amount of oxide scale that spalled off during cooling. It should be remembered that these data were not corrected for the non-implanted surface area of specimens, which was equal to approximately 65% of the total surface. Thermogravimetric results show that silicon-implanted AISI 304 steel presents a better oxidation behaviour for up to approximately 200 cycles. However, for a longer oxidation time, the spallation is more significant. Fig. 2 shows in detail the mass gain for the implanted and unimplanted AISI 430 steel. The growth of the oxide follows — after a transient period of faster corrosion rate (from 0 up to 25 h) — linear kinetics. The linear rate constants for unimplanted and siliconimplanted AISI 430 steel approached a constant value of K =6.6×10−10 and K =5.8×10−10 g cm−2 s−1, p p respectively. The mass gain data show a small improvement in the oxidation behaviour for the implanted AISI 430 steel after 250 cycles. After 320 cycles, exposure of both Si-implanted steels exhibited an oxidation behaviour similar to their respective unimplanted steels.
3. Results and discussion 3.1. Oxidation kinetics Fig. 1 shows the mass change measurements after 320 cycles for the implanted and unimplanted steels. The
Fig. 2. Mass change versus number of 1 h cycles for silicon implanted and unimplanted AISI 430 steel in air at 1173 K.
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3.2. Morphology and composition of the oxide scale For the nucleation and growth studies, the behaviours of implanted and unimplanted sides on the same specimen were compared directly, thus excluding the possibility of scatter between various tests. Fig. 3 shows the XRD spectra of the silicon implanted AISI 430 steel oxidized for different 1 h cycles. The X-ray analysis showed that the corrosion products that formed initially on both the un-implanted and the implanted AISI 430 steel were mainly Cr O , together 2 3 with a small amount of a spinel of chromium and manganese, Mn Cr O . Peaks corresponding to both 1.5 1.5 4 phases simultaneously grew with increasing the number of oxidation cycles, but the ratio Cr O /Mn Cr O 2 3 1.5 1.5 4 was smaller after longer oxidation times. The EDS analysis of the top surface oxide scale formed after 2, 48, 80,180 and 320 cycles corroborate these results in both implanted and unimplanted AISI 430 steel. These spectra show that the oxide scales are rich in chromium and manganese and have a small amount of silicon, but again, the ratio Mn/Cr increases with longer oxidation times. The samples of AISI 304 have a thin layer of a∞-martensite (bcc) on the austenitic matrix after being plastically deformed during the mechanical polish. This phase is detected in the XRD spectra on polished unimplanted and implanted AISI 304 samples. This layer of a∞-martensite (bcc) vanishes entirely after a short period of heat treatment, as has been observed by other researchers [17]. XRD spectra obtained on both implanted and unimplanted AISI 304 steel showed that at shorter oxidation times, the peaks correspond to the base alloy with two additional oxides close to Cr O and the cubic Fe O 2 3 3 4 phases. For longer oxidation times, the thickness of the scale increases, as indicated by the absence of the peaks corresponding to the base alloy. Fig. 4 shows the XRD
Fig. 3. XRD spectra of the 1016 Si ions cm−2 implanted AISI 430 steel, after exposure for different cycles in air at 1173 K.
Fig. 4. XRD spectra of the 1016 Si ions cm−2 implanted AISI 304 steel after exposure for different cycles in air at 1173 K.
spectrum of the implanted steel oxide scale after two cycles and the XRD diffractograms of the spalled oxide after 180 and 320 cycles. It is clear from these results that silicon does not inhibit the growing of non-protective and non-adherent iron-rich oxides such as Fe O 3 4 and (Cr,Fe) O 2 3. In all the specimens studied, the scale morphology of the implanted and the unimplanted AISI 304 steel was the same. The oxide surface morphology consisted of a compact base layer of polygonal crystals, many of them with triangular faces whose grain size increases with increasing oxidation time. The samples oxidized for a short time (24–80 cycles) presented scale blistering and cracking in some regions of their surface. After 80 cycles, the surface shows a significant blistering with extensive cracking on the top of the blistered scale [Fig. 5(a)]. The specimens oxidized for longer times (120–180 cycles) showed larger spalled areas. After approximately 200 cycles, spallation of the oxide is complete. Inspection of the spalled surface shows that vertical oblique platelets appeared, which seem to have grown along preferred crystallographic directions [Fig. 5(b)]. EDS analysis showed that iron and oxygen were the major constituents of these platelets. Fig. 6 shows a SEM cross-section of the unimplanted [Fig. 6(a)] and implanted [Fig. 6(b)] AISI 304 after 80 cycles. In both cases, a uniformly thick oxide scale is formed with some internal oxidation. EDS analysis showed the enrichment of silicon in these internally oxidized zones. The internal oxidation could be explained by inward penetration of oxygen through the grain boundaries that are widened by outward diffusion of metallic cations and condensation of vacancies from the surrounding lattice [18]. The SEM micrographs in Fig. 7 show the change in scale morphology that occurred between exposures of 80 cycles and 320 cycles for the implanted AISI 430 steel. Again, the morphology is the same as for the unimplanted steel. At 80 cycles [Fig. 7(a)], the scale is
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(a)
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(b)
Fig. 5. SEM surface morphology of the oxide scale developed on the Si-implanted AISI 304 steel after exposure in air at 1173 K for (a) 80 cycles and (b) 320 cycles.
(a)
(b)
Fig. 6. SEM cross-sections of the oxide scales developed after 80 cycles of oxidation in air at 1173 K for (a) Si-implanted and (b) unimplanted AISI 304 steel.
(a)
(b)
Fig. 7. SEM surface morphology of the oxide scale developed on the Si-implanted AISI 430 steel after exposure in air at 1173 K for (a) 80 cycles and (b) 320 cycles.
fairly uniform. Observation at a higher magnification reveals that the oxide surface morphology consists of a compact layer of small polygonal crystals, with an average diameter between 0.1 and 0.5 mm, in which the compact layer has started to crack. At longer oxidation times [Fig. 7(b)], SEM micrographs show evidence of blistering of the scale together with cracking on top of the blisters.
Cross-sectional micrographs showed that the oxide scale has a non-uniform thickness, with many intrusions at the scale/metal interface (Fig. 8). Some of these intrusions are very large, with a thickness between 4 and 8 mm. It is clear from the SEM micrographs that the scale is divided into two zones. The outer zone has a uniform thickness of about 2–3 mm, and compact morphology, while the inner zone forms intrusions with
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(a)
(b)
Fig. 8. SEM cross-section morphology of the oxide scales developed after 320 cycles of oxidation in air at 1173 K for (a) Si-implanted and (b) unimplanted AISI 430 steel.
an irregular thickness and is very porous. EDS analysis showed that the inner zone underwent chromium and silicon enrichment. Thus, it was concluded that the oxide composition and morphology are the same for the silicon-implanted steels as for the unimplanted steels. Another important aspect concerns the adhesion of the scale to the substrate; neither implanted nor unimplanted AISI 430 showed any appreciable spalling on cooling up to 320 cycles. It is interesting to note that ferritic stainless steel has a better resistance to cyclic oxidation than austenitic stainless steel. This is probably due to the lower coefficient of thermal expansion of the ferritic stainless steel, which then generates lower thermal stresses with cycling [19]. Another factor is that the oxide scale on the AISI 430 is formed by more protective and adherent oxides as chromia and Mn Cr O , whereas, the oxide scale 1.5 1.5 4 on the AISI 304 is formed by less protective and adherent iron-rich oxides as Fe O and (Cr,Fe) O 3 4 2 3. 4. Summary As an overall discussion of the current results, we find that the silicon implantation at the nominal dose of 1016 Si ions cm−2 does not have any significant effect on the kinetic growth of the oxide scales or on the composition and morphology of the oxide scale on either steel. This is not in agreement with similar observations on the effect of reactive elements on the oxidation behaviour of most chromia-forming materials; see, for example, Refs. [2,3,20,21]. Likewise, we have recently published another study [22] in which it was demonstrated that ion implantation of 1×1015 ions cm−2 of silicon at 150 keV improved the oxidation resistance of austenitic AISI 304 steel during isothermal oxidation in air at 1173 K. This disagreement is probably due to the relatively low dose of silicon implanted in the former study. It is interesting to emphasize the different effect of the implanted silicon between isother-
mal and cyclic oxidation. This results invite to continue this work using a higher dose of silicon, in order to determine whether there is a minimum dose that has a beneficial effect on the cyclic oxidation of the AISI 304 at 1173 K.
5. Conclusions On the basis of the results described above, the following conclusions can be drawn: 1. The presence of silicon implanted at a nominal dose of 1016 Si ions cm−2 on the surface layer of the AISI 430 steel appeared to enhance the oxidation resistance to a small degree after longer oxidation times. 2. However, silicon does not have any effect on the composition and morphology of the oxide scale. Thermogravimetric mass changes show that siliconimplanted 304 steel demonstrates a better oxidation behaviour until approximately 200 cycles, but after longer oxidation times, the effect disappears. 3. Silicon-ion implantation does not change the composition and morphology of the oxide scale and consequently does not improve the protective properties of the scale and its adherence to the substrate.
Acknowledgement The authors want to express their gratitude to the Universidad Complutense de Madrid for the financial support of this work: Project No. PR156/97-7169.
References [1] J. Stringer, Mater. Sci. Eng. 120 (1989) 129. [2] Y. Saito, B. Onay, T. Maruyama, J. de Physique IV 3 (1993) 217. [3] K. Przybylski, G.J. Yurek, Mater. Sci. Forum 43 (1989) 1.
F.J. Pe´rez et al. / Surface and Coatings Technology 120–121 (1999) 442–447 [4] J. Stringer, A.Z. Hed, G. Wallwork, B.A. Wilcox, Corros. Sci. 12 (1972) 625. [5] H. Pfeiffer, Werks. Korros. 8 (1957) 574. [6 ] C. Giggins, B. Kear, F.S. Pettit, J.K. Tien, Met. Trans. 5 (1975) 1685. [7] N. Hussain, K.A. Shahid, I.H. Khan, S. Rahman, Oxid. Met. 41 (1994) 215. [8] Z. Kubes, J. Vesela`, Z. Weiss, J. Mater. Sci. Lett. 14 (1995) 876. [9] I. Saeki, H. Konno, R. Furuichi, T. Nakamura, K. Mabuchi, M. Itoh, Corros. Sci. 40 (1998) 191. [10] S. Seal, S.K. Bose, S.K. Roy, Oxid. Met. 41 (1994) 139. [11] M.F. Stroosnijder, J.F. Norton, V. Guttmann, M.J. Bennett, J.H.W. de Wit, Mater. Sci. Eng. A 116 (1989) 110. [12] F. Noli, P. Misaelides, G. Giorginis, H. Baumann, Oxid. Met. 48 (1997) 225.
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[13] P.Y. Hou, J. Stringer, Oxid. Met. 33 (1990) 357. [14] T. Amano, S. Yajima, T. Kimura, Y. Saito, Corros. Eng. 24 (1975) 19. [15] T. Amano, Trans. Japan Inst. Metals 21 (1980) 721. [16 ] Y. Saito, T. Maruyama, T. Amano, Mater. Sci. Eng. 87 (1987) 275. [17] R. Guillament, J. Lopitaux, B. Hannoyer, M. Lenglet, J. de Physique IV C9 (3) (1993) 349. [18] H.J. Grabke, G.H. Meier, Oxid. Met. 8 (1995) 147. [19] A. Godner, Weld. Res. Counc. Bull., (1956) No. 31. [20] R. Guillament, J. Lopitaux, B. Hannoyer, M. Lenglet, J. de Physique IV C9 (3) (1993) 349. [21] M.F. Stroosnijder, J.D. Sunderkotter, M.J. Cristo´bal, H. Jenett, K. Isenbugel, M.A. Baker, Surf. Coat. Technol. 83 (1996) 205. [22] F.J. Pe´rez, E. Otero, M.P. Hierro, C. Go´mez, F. Pedraza, J.L. de Segovia, E. Roma´n, Surf. Coat. Technol. 108–109 (1998) 127.
The influence of implanted silicon on the cyclic oxidation behaviour of two different stainless steels F.J. Pe´rez a, *, M.J. Cristo´bal b, M.P. Hierro a, F. Pedraza a a Dpto. de Ciencia de los Materiales, Facultad de Quı´mica, Universidad Complutense, 28040-Madrid, Spain b Dpto. de Ciencia de los Materiales, E.T.S.I.I y M., Universidad de Vigo, Lagoas-Marcosende, 36210-Pontevedra, Spain
Abstract High-temperature alloys are frequently used in power plants, gasification systems, the petrochemical industry, combustion processes and in aerospace applications. Depending on the application, materials are subjected to contaminated atmospheres and/or thermal cycling. Thermal cyclic experiments were carried out in order to study the influence of implanted silicon on the adherence of the scale. The effect of silicon was tested on two different stainless steels in air at 1173 K. The oxidized specimens were characterized by means of X-ray diffraction ( XRD), scanning electron microscopy (SEM ) and energy-dispersive X-ray analysis (EDS ). The experimental results confirm that silicon implantation at a nominal dose of 1016 Si ions cm−2 does not play a significant role on the cyclic oxidation behaviour of the austenitic AISI 304 steel in air at 1173 K. However, it appears to enhance the oxidation resistance of the ferritic AISI 430 at oxidation cycles longer than 250. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Cyclic oxidation; High-temperature oxidation; Silicon implantation; Stainless steels
1. Introduction The ability of the current generation of high-temperature alloys to resist corrosion depends on the capacity to form and maintain a protective oxide scale, such as Cr O , Al O or SiO . The beneficial role of reactive 2 3 2 3 2 elements, such as yttrium, cerium and other rare-earth elements, on the oxidation behaviour of high-temperature alloys has received considerable attention [1–3]. Small amounts ( less than 1%) of these elements added to the bulk alloy or to the surface may decrease the oxidation rate and dramatically improve the scale adherence. Several mechanisms have been proposed [4–6 ] to explain the beneficial influences of rare-earth additions to high-temperature alloys. However, no conclusive mechanisms have yet emerged to explain these effects. The behaviour of cyclic oxidation at high temperature of the surface modified stainless steels has not been studied extensively [7–9]; however, in recent years, a few studies related to the effect of the rare-earth elements on these steels have been reported. For example, Seal * Corresponding author. Tel.: +34-91-39-44215; fax: +34-91-39-33457. E-mail address: [email protected] (F. Pe´rez)
et al. [10] studied the influence of superficial applied CeO coatings on the isothermal and cyclic oxidation 2 behaviour of three grades of austenitic steel (AISI 321, 316 and 304) in dry air, at 1273 K. The results clearly showed that coatings not only reduced the rates of scale growth for all three varieties of steel but also significantly improved the scale adhesion. Stroosnijder et al. [11] investigated the influence of implanted cerium on the corrosion resistance of a wrought austenitic Fe–20Cr– 32Ni steel (Alloy 800 H ) in a simulated coal gasification atmosphere at 973 K. It was shown that the corrosive attack in the H S-containing gas mixture could be 2 reduced for the cerium-implanted material if the implantation dose was sufficiently high. Likewise, Noli et al. [12] investigated the oxidation behaviour of Mg-implanted AISI 321 austenitic stainless steel, and two different mechanisms of Mg ion diffusion and oxidation of the implanted steel were observed. At low temperatures, an improvement in oxidation resistance was found. However at higher temperatures, the implantation of Mg reduced the scale adherence and enhanced the oxidation rate. Nevertheless, little attention has been given to the implantation of silicon ions, although Hou and Stringer recently proposed that this element could act as a
0257-8972/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 50 3 - 4
F.J. Pe´rez et al. / Surface and Coatings Technology 120–121 (1999) 442–447
443
Table 1 Compositions of the tested stainless steels (wt%)
AISI 304 AISI 430
Fe
Cr
Ni
Mn
Si
C
Balance Balance
18 16.5
8 0.21
1.42 0.30
0.35 0.32
0.056 0.064
reactive element in high-temperature oxidation [13]. To date, only Amano and co-workers [14–16 ] have studied the high-temperature oxidation behaviour of Ni–20Cr alloys containing 1 and 3% of silicon. According to their results, the addition of silicon reduced the oxidation rate, but had no effect on the adherence of the surface oxide. It is clear from this review that the effect of the rare earth elements on the corrosion resistance of stainless steels is far from complete and may offer a way to analyse possible surface modifications enhancing life time at higher temperatures of operation.
2. Experimental Specimens of AISI 304 steel and AISI 430 steel, whose compositions are given in Table 1, in the form of 10 mm×7 mm×2 mm coupons were cut from coldrolled plates. Before implantation, all the samples were ground on silicon carbide paper to 600 grit and then ultrasonically degreased in acetone and then in ethanol. The uniform implantation of silicon, at a nominal dose of 1016 Si ions cm−2, into one of the principal coupon faces was undertaken using a 150 keV acceleration potential. The oxidation experiments were performed in static laboratory air at atmospheric pressure, up to 320 cycles. Each cycle consisted of a period of 60 min at the oxidation temperature (1173 K ) and 15 min at room temperature, which appeared to be long enough to cool the specimens below 60°C. The specimens were inserted into, and removed from, the heat zone in a few seconds to guarantee rapid heating and cooling. At the initial stages of the experiments, the weight gain of the samples was measured after a few cycles. At a later stage, the specimens were measured at intervals of more cycles. The techniques used to characterize the structure and composition of the oxidation products formed included optical metallography (OM ), X-ray diffraction ( XRD), scanning electron microscopy (SEM ) and energy-dispersive X-ray analysis (EDS ).
Fig. 1. Mass change versus number of 1 h cycles for silicon implanted and un-implanted AISI 304 and AISI 430 steels in air at 1173 K.
mass changes excluded the amount of oxide scale that spalled off during cooling. It should be remembered that these data were not corrected for the non-implanted surface area of specimens, which was equal to approximately 65% of the total surface. Thermogravimetric results show that silicon-implanted AISI 304 steel presents a better oxidation behaviour for up to approximately 200 cycles. However, for a longer oxidation time, the spallation is more significant. Fig. 2 shows in detail the mass gain for the implanted and unimplanted AISI 430 steel. The growth of the oxide follows — after a transient period of faster corrosion rate (from 0 up to 25 h) — linear kinetics. The linear rate constants for unimplanted and siliconimplanted AISI 430 steel approached a constant value of K =6.6×10−10 and K =5.8×10−10 g cm−2 s−1, p p respectively. The mass gain data show a small improvement in the oxidation behaviour for the implanted AISI 430 steel after 250 cycles. After 320 cycles, exposure of both Si-implanted steels exhibited an oxidation behaviour similar to their respective unimplanted steels.
3. Results and discussion 3.1. Oxidation kinetics Fig. 1 shows the mass change measurements after 320 cycles for the implanted and unimplanted steels. The
Fig. 2. Mass change versus number of 1 h cycles for silicon implanted and unimplanted AISI 430 steel in air at 1173 K.
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3.2. Morphology and composition of the oxide scale For the nucleation and growth studies, the behaviours of implanted and unimplanted sides on the same specimen were compared directly, thus excluding the possibility of scatter between various tests. Fig. 3 shows the XRD spectra of the silicon implanted AISI 430 steel oxidized for different 1 h cycles. The X-ray analysis showed that the corrosion products that formed initially on both the un-implanted and the implanted AISI 430 steel were mainly Cr O , together 2 3 with a small amount of a spinel of chromium and manganese, Mn Cr O . Peaks corresponding to both 1.5 1.5 4 phases simultaneously grew with increasing the number of oxidation cycles, but the ratio Cr O /Mn Cr O 2 3 1.5 1.5 4 was smaller after longer oxidation times. The EDS analysis of the top surface oxide scale formed after 2, 48, 80,180 and 320 cycles corroborate these results in both implanted and unimplanted AISI 430 steel. These spectra show that the oxide scales are rich in chromium and manganese and have a small amount of silicon, but again, the ratio Mn/Cr increases with longer oxidation times. The samples of AISI 304 have a thin layer of a∞-martensite (bcc) on the austenitic matrix after being plastically deformed during the mechanical polish. This phase is detected in the XRD spectra on polished unimplanted and implanted AISI 304 samples. This layer of a∞-martensite (bcc) vanishes entirely after a short period of heat treatment, as has been observed by other researchers [17]. XRD spectra obtained on both implanted and unimplanted AISI 304 steel showed that at shorter oxidation times, the peaks correspond to the base alloy with two additional oxides close to Cr O and the cubic Fe O 2 3 3 4 phases. For longer oxidation times, the thickness of the scale increases, as indicated by the absence of the peaks corresponding to the base alloy. Fig. 4 shows the XRD
Fig. 3. XRD spectra of the 1016 Si ions cm−2 implanted AISI 430 steel, after exposure for different cycles in air at 1173 K.
Fig. 4. XRD spectra of the 1016 Si ions cm−2 implanted AISI 304 steel after exposure for different cycles in air at 1173 K.
spectrum of the implanted steel oxide scale after two cycles and the XRD diffractograms of the spalled oxide after 180 and 320 cycles. It is clear from these results that silicon does not inhibit the growing of non-protective and non-adherent iron-rich oxides such as Fe O 3 4 and (Cr,Fe) O 2 3. In all the specimens studied, the scale morphology of the implanted and the unimplanted AISI 304 steel was the same. The oxide surface morphology consisted of a compact base layer of polygonal crystals, many of them with triangular faces whose grain size increases with increasing oxidation time. The samples oxidized for a short time (24–80 cycles) presented scale blistering and cracking in some regions of their surface. After 80 cycles, the surface shows a significant blistering with extensive cracking on the top of the blistered scale [Fig. 5(a)]. The specimens oxidized for longer times (120–180 cycles) showed larger spalled areas. After approximately 200 cycles, spallation of the oxide is complete. Inspection of the spalled surface shows that vertical oblique platelets appeared, which seem to have grown along preferred crystallographic directions [Fig. 5(b)]. EDS analysis showed that iron and oxygen were the major constituents of these platelets. Fig. 6 shows a SEM cross-section of the unimplanted [Fig. 6(a)] and implanted [Fig. 6(b)] AISI 304 after 80 cycles. In both cases, a uniformly thick oxide scale is formed with some internal oxidation. EDS analysis showed the enrichment of silicon in these internally oxidized zones. The internal oxidation could be explained by inward penetration of oxygen through the grain boundaries that are widened by outward diffusion of metallic cations and condensation of vacancies from the surrounding lattice [18]. The SEM micrographs in Fig. 7 show the change in scale morphology that occurred between exposures of 80 cycles and 320 cycles for the implanted AISI 430 steel. Again, the morphology is the same as for the unimplanted steel. At 80 cycles [Fig. 7(a)], the scale is
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(a)
445
(b)
Fig. 5. SEM surface morphology of the oxide scale developed on the Si-implanted AISI 304 steel after exposure in air at 1173 K for (a) 80 cycles and (b) 320 cycles.
(a)
(b)
Fig. 6. SEM cross-sections of the oxide scales developed after 80 cycles of oxidation in air at 1173 K for (a) Si-implanted and (b) unimplanted AISI 304 steel.
(a)
(b)
Fig. 7. SEM surface morphology of the oxide scale developed on the Si-implanted AISI 430 steel after exposure in air at 1173 K for (a) 80 cycles and (b) 320 cycles.
fairly uniform. Observation at a higher magnification reveals that the oxide surface morphology consists of a compact layer of small polygonal crystals, with an average diameter between 0.1 and 0.5 mm, in which the compact layer has started to crack. At longer oxidation times [Fig. 7(b)], SEM micrographs show evidence of blistering of the scale together with cracking on top of the blisters.
Cross-sectional micrographs showed that the oxide scale has a non-uniform thickness, with many intrusions at the scale/metal interface (Fig. 8). Some of these intrusions are very large, with a thickness between 4 and 8 mm. It is clear from the SEM micrographs that the scale is divided into two zones. The outer zone has a uniform thickness of about 2–3 mm, and compact morphology, while the inner zone forms intrusions with
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(a)
(b)
Fig. 8. SEM cross-section morphology of the oxide scales developed after 320 cycles of oxidation in air at 1173 K for (a) Si-implanted and (b) unimplanted AISI 430 steel.
an irregular thickness and is very porous. EDS analysis showed that the inner zone underwent chromium and silicon enrichment. Thus, it was concluded that the oxide composition and morphology are the same for the silicon-implanted steels as for the unimplanted steels. Another important aspect concerns the adhesion of the scale to the substrate; neither implanted nor unimplanted AISI 430 showed any appreciable spalling on cooling up to 320 cycles. It is interesting to note that ferritic stainless steel has a better resistance to cyclic oxidation than austenitic stainless steel. This is probably due to the lower coefficient of thermal expansion of the ferritic stainless steel, which then generates lower thermal stresses with cycling [19]. Another factor is that the oxide scale on the AISI 430 is formed by more protective and adherent oxides as chromia and Mn Cr O , whereas, the oxide scale 1.5 1.5 4 on the AISI 304 is formed by less protective and adherent iron-rich oxides as Fe O and (Cr,Fe) O 3 4 2 3. 4. Summary As an overall discussion of the current results, we find that the silicon implantation at the nominal dose of 1016 Si ions cm−2 does not have any significant effect on the kinetic growth of the oxide scales or on the composition and morphology of the oxide scale on either steel. This is not in agreement with similar observations on the effect of reactive elements on the oxidation behaviour of most chromia-forming materials; see, for example, Refs. [2,3,20,21]. Likewise, we have recently published another study [22] in which it was demonstrated that ion implantation of 1×1015 ions cm−2 of silicon at 150 keV improved the oxidation resistance of austenitic AISI 304 steel during isothermal oxidation in air at 1173 K. This disagreement is probably due to the relatively low dose of silicon implanted in the former study. It is interesting to emphasize the different effect of the implanted silicon between isother-
mal and cyclic oxidation. This results invite to continue this work using a higher dose of silicon, in order to determine whether there is a minimum dose that has a beneficial effect on the cyclic oxidation of the AISI 304 at 1173 K.
5. Conclusions On the basis of the results described above, the following conclusions can be drawn: 1. The presence of silicon implanted at a nominal dose of 1016 Si ions cm−2 on the surface layer of the AISI 430 steel appeared to enhance the oxidation resistance to a small degree after longer oxidation times. 2. However, silicon does not have any effect on the composition and morphology of the oxide scale. Thermogravimetric mass changes show that siliconimplanted 304 steel demonstrates a better oxidation behaviour until approximately 200 cycles, but after longer oxidation times, the effect disappears. 3. Silicon-ion implantation does not change the composition and morphology of the oxide scale and consequently does not improve the protective properties of the scale and its adherence to the substrate.
Acknowledgement The authors want to express their gratitude to the Universidad Complutense de Madrid for the financial support of this work: Project No. PR156/97-7169.
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