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High temperature corrosion protection of austenitic AISI 304 stainless steel by Si, Mo and Ce ion implantation
Surface and Coatings Technology 108–109 (1998) 127–131
High temperature corrosion protection of austenitic AISI 304 stainless steel by Si, Mo and Ce ion implantation a ´ a , *, E. Otero a , M.P. Hierro a , C. Gomez ´ ´ c F.J. Perez , F. Pedraza a , J.L. de Segovia b , E. Roman b
a Departamento de Ciencia de los Materiales, Universidad Complutense de Madrid, 28040 -Madrid, Spain ´ ´ , Consejo Superior de Investigaciones Cientıficas ´ , Instituto Torres Quevedo, C /Serrano, 144, 28010 -Madrid, Spain Departamento de Fısica del Vacıo c ´ , Instituto de Ciencia de los Materiales, 28049 Canto Blanco-Madrid, Spain Consejo Superior de Investigaciones Cientıficas
Abstract The influence of implanted silicon, molybdenum and cerium on the oxidation behaviour of a 18Cr8Ni stainless steel was studied at 1173 K up to 144 h in air under isothermal conditions in order to verify the enhanced selective oxidation of chromium by these elements. The implanted surface and the corrosion products formed were characterized by means of AES, SEM, EDS and XRD. Implanted depth profiles were calculated by TRIM96 computational code. The implanted silicon and cerium markedly improved protection against oxidation of the alloy by enhanced chromium transport while molybdenum gave rise to an accelerated oxidation due to the formation of volatile MoO 3 species. Further, ion implantation was found to play a beneficial effect against decarburization of the alloy. 1998 Elsevier Science S.A. All rights reserved. Keywords: Corrosion; Stainless steel; Ion implantation
1. Introduction It is very well known that ion implantation shows several advantages over other surface modification techniques [1,2]. Doses ranging from 10 16 to 10 18 ions / cm 2 are typically directed towards different substrates to improve corrosion properties while fluences from 10 14 to 10 16 ions / cm 2 are more commonly employed in wear and friction tasks [3], but in recent studies it has been demonstrated that low fluences can be effective against oxidation processes [4]. The influence of the addition or ion implantation of Y and rare earths and their oxide dispersions to both Cr 2 O 3 and Al 2 O 3 -forming alloys to improve their high temperature oxidation behaviour has been extensively studied based upon the ‘reactive element effect’ (REE) [5,6]. It has been suggested that the beneficial effects of REs rely on: (a) a physical blocking by RE ions or second-phase particles at grain boundaries; (b) pegging of the oxide to the alloy; (c) vacancy annihilation; (d) reduction of oxidegrowth stresses; (e) enhanced selective oxidation of Cr; and (f) formation of a fine-grain oxide layer [7]. However, the mechanisms are not fully understood and an alternative explanation is that the REs prevent segregation of sulfur to *Corresponding author. Facultad de Ciencias Quimicas, Ciudad Universitaria, 28040 Madrid, Spain. Tel.: 134-1-3944216; fax: 134-13944257.
the scale grain boundaries resulting in a decrease in chromium flux across the scale and improving the adherence of the scale to the alloy [8]. 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 RE in high temperature applications [9], while Mo effects have been related to an improvement of the corrosion resistance in chloride media [10,11]. This paper provides the results of the effect of the oxidation of an austenitic AISI 304 stainless steel in the range of temperatures used and its surface modifications by silicon, molybdenum and cerium ion implantation with low fluences (1310 15 , 1310 14 and 1310 14 ions / cm 2 , respectively) at 1173 K for 144 h under atmospheric pressure of air. Those low fluences applied will give new ideas about the implantation effect against oxidation.
2. Experimental Specimens of 1533.531 mm were cut from cold-rolled AISI-304 pletins. Before implantation, all the specimens were ground to a surface finish of SiC [600. The Si, Mo and Ce ion implantations were carried out at an energy of 150 keV with ion fluences of 1310 15 Si / cm 2 , 1310 14 Mo / cm 2 and 1310 14 Ce / cm 2 . The implanted species distribution (Rp) was accomplished by computer simulation TRIM96 while the implanted surfaces were character-
0257-8972 / 98 / $ – see front matter 1998 Elsevier Science S.A. All rights reserved. PII: S0257-8972( 98 )00685-9
128
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
Fig. 1. Auger spectra of the 1310 Si / cm implanted AISI 304 stainless steel at 150 kV without Ar bombardment (thin line) and with consecutive Ar bombardments (broader lines).
Fig. 3. Auger spectra of the 1310 14 Ce 1 / cm 2 implanted AISI 304 stainless steel at 150 kV without Ar bombardment (thin line) and with consecutive Ar bombardments (broader lines).
ized by means of Auger electron spectroscopy in a JEOL JAMP-10S Auger microprobe at 3 kV and 50 mA with a 10-mm spot diameter. The implanted species were oxidized in a muffle furnace under isothermal conditions at 1173 K and atmospheric pressure of air. Characterization of the oxidized species was carried out by means of XRD, SEM and EDS.
inner sublayers of the non-oxidized Si-implanted stainless steel while Fig. 2 and Fig. 3 concern those of the nonoxidized Mo- and Ce-implanted AISI 304. The Auger spectra reveal that Si had diffused outwards showing a maximum contribution on the top surface and a small depletion in the sublayers increasing the Si content in the inner region up to that of the as-received steel. The diffusion of silicon is thought to be due to the small size of the element and its avidity to get oxidized as it can be seen from the left shifting of the Si LVV transition (92 eV) and the presence of oxygen (510 eV). Opposite to TRIM calculations molybdenum showed its maximum at the outermost surface while cerium was found to be located preferentially slightly away from the top surface. The results of the isothermal oxidation experiments at 1173 K are shown in Fig. 4. It follows from this plot that the low doses of silocon and cerium reduce by about 50% the parabolic rate of the parent material but Mo increases it. This anomalous behaviour for the Mo-implanted steel can be explained from the characterization for short
15
1
2
3. Results and discussion Table 1 shows the projected ranges calculated by TRIM96 program based upon Monte Carlo calculations where it was assumed that the AISI 304 stainless steel had ˚ thick Cr 2 O 3 passivation layer. It can be readily a 50-A ˚ seen that both Rp(Mo) and Rp(Ce) reach about 300 A depth in the material which represent one-third of the centroid of the Gaussian distribution for the Si-implanted steel. The differences in Rp values arise from the relation between the accelerating voltage, the incidence angle, the type of substrate atoms and their binding energy (which were kept constant) and the type of incidence atom [12]. Fig. 1 represents the Auger spectra of the upper and
Fig. 2. Auger spectra of the 1310 14 Mo 1 / cm 2 implanted AISI 304 stainless steel at 150 kV without Ar bombardment (thin line) and with consecutive Ar bombardments (broader lines).
Fig. 4. Experimental values and kinetic laws for parent (squares), 1310 15 Si 1 / cm 2 (circles), 1310 14 Mo 1 / cm 2 (triangles) and 1310 14 Ce 1 / cm 2 (diamonds) AISI 304 stainless steel oxidated at 1173 K in air for 144.
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
129
Fig. 5. XRD diffraction spectrum of the 1310 14 ions / cm 2 MO-implanted AISI 304 stainless steel after oxidation at 1173 K in air for 48 h.
Fig. 7. Cross-section of the 1310 14 ions / cm 2 Mo-implanted AISI 304 stainless steel after oxidation in air at 1173 K for 48 h.
oxidation times. Figs. 5 and 6 and 7 concern the XRD spectrum, the surface and cross-section photomicrographs for Mo-implanted AISI 304 oxidized at 1173 K for 48 h. The diffractogram reveals the constitution of manganeseenriched spinels, Mn 1.5 Cr 1.5 O 4 and the formation of a poorly protective iron-rich mixed oxide, (Fe 0.6 Cr 0.4 ) 2 O 3 carrying out spalling of the scale (Fig. 6) which is extremely cracked (Fig. 7). However, for maximum exposure times all the specimens showed the same surface morphologies with polygonal crystals of about 0.5 mm in size extending over the surface which were mainly composed of chromium as shown by EDS analysis and hexagonal ‘wafer’ coalesces, where chromium and manganese were the major constituents (Fig. 8). Cross-sections for 144 h at 1173 K showed a convoluted scale both in implanted and non-implanted AISI 304 (Fig. 9) which was mainly composed of chromium with silicon incorporated at the metal–scale interface (Fig. 9a). Moreover, a ribbon between the scale and the inner alloy, where no carbide precipitates, was present. It was observed by EDS microanalysis that silicon, molybdenum, chromium and iron were incorporated to the carbides precipitated along grain and twin boundaries being the ribbons of the implanted specimens less wide than the one of the nonimplanted steel.
Some particular characteristics were observed in base AISI 304 and Si-implanted AISI 304. Oxidation of these specimens for the longest exposure times brought about the formation of disperse sponge-shaped nodules (Fig. 10) where the internal zone showed internal oxidation of the material (Fig. 10a) and the outer zone had the same composition as the hexagonal ‘wafers’. On the basis of the results obtained at 1173 K the present authors suggest the following oxidation mechanisms:
3.1. Non-implanted AISI-304 The Cr 2 O 3 passivation layer cracks when the alloy is put into the furnace at 1173 K due to the differences in thermal expansion coefficients of the metallic alloy and the protective oxide [8]. Oxygen subsequently reaches the nuclei of iron and chromium growing a poorly-protective mixed scale. Diffusion outwards of the smaller cations such as manganese results in the formation of highly protective manganese-enriched spinels Mn 1.5 Cr 1.5 O 4 , but depletion of these spinels occurs around the rosettes. These depleted areas can be considered as diffusion paths for oxygen and the additional surface area for reaction giving rise or increasing the internal oxidation. Fortunately, silicon oxide intrusions at the metal–scale interface peg or key the scale to the metal substrate. Further exposure of the alloy at 1173 K for 144 h leads to a decarburization of the external areas of the alloy while cooling in air carries out the carbide precipitation along high energy areas such as grain boundaries and twins.
3.2. Si-implanted AISI-304
Fig. 6. Surface morphology with oxide spallation of the 1310 14 ions / cm 2 Mo-implanted AISI 304 stainless steel after oxidation in air at 1173 K for 48 h.
During the first stages of oxidation breakaway of the protective scale and transient oxidation as described elsewhere [8] takes place. A convoluted and chemically homogeneous oxide layer is grown and developed. The probable cause of the enhanced selective oxidation of chromium is that silicon oxidizes initially and the resulting oxide particles at the surface act as nucleation sites for
130
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
Fig. 8. (a) Surface morphologies of the parent AISI 304 stainless steel after oxidation in air at 1173 K for 144 h; (b) EDS microanalysis at an hexagonal ‘wafer’.
Fig. 9. (a) Cross-section of the non-implanted and implanted materials for the longest exposure time (144 h) at 1173 K in air (glyceregia etched); (b) EDS microanalysis of the oxide scale.
Cr 2 O 3 [13]. However, Hamikian and Otter [14] proposed an explanation in terms of enhanced transport of chromium to the surface along newly created alloy grain boundaries formed by recrystallization of the amorphous phase that was induced in the surface by the implantation process. In this case, the present authors do not believe such amorphization to occur due to the low doses implanted into the alloy. Further, ion implantation prevents the internal oxidation of the alloy by the formation of the chemically homogeneous oxide layer although a few manganese-enriched rosettes were observed with the corresponding depletion in these protective oxides in their surroundings. Moreover, the low doses of implanted silicon prevents the formation of a continuous layer of silica which could act as a weakening plane due to the a – b transformations in cristobalite [15]. Dispersion of the silcion particles in the external zones of the alloy partially impedes diffusion outwards of the
carbon present in the alloy resulting in less decarburization than in the parent material.
3.3. Mo-implanted AISI 304 Oxidation for the first 8 h takes place as described above for the Si-implanted steel. Exposure at high temperature brings about the formation of molybdenum oxides such as MoO 2 and MoO 3 which turn into volatile species at temperatures above 6008C [16]. These volatile oxides (or hydrated species since moisture is present in the furnace) diffuse out through the grain boundaries of the new formed scale acting as short-circuit paths leading to the formation of cracks and pores which enhance cationic and anionic diffusion thickening the scale. It is well known that for scales above a certain thickness, creation of new surfaces by detachment of the scale may be more favourable than the creep of the oxide required to maintain adhesion [8].
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
131
Fig. 10. (a) Sponge-shaped nodules found in the non-implanted and Si-implanted AISI 304 stainless steel after oxidation at 1173 K in air for 144 h; (b) EDS microanalysis of the inner zone of the nodule.
As oxidation time passes by, depletion of molybdenum from the external layers of the alloy leads to the creation of a new scale in the same manner as in the Si-implanted alloy.
3.4. Ce-implanted AISI 304 Direct oxidation of the alloy takes place up to the first 8 or 9 h probably due to cracking of the Cr 2 O 3 passivation layer of the stainless steel. Later, transient oxidation takes place causing the cerium particles to act as nucleation sites for chromium in such a way that an almost stoichiometric Cr 2 O 3 is formed in the metal–scale interface. This oxide provides a good protection to the alloy especially when manganese-enriched spinels are formed in the outermost layer of the scale. Further, the silicon oxides found at the metal scale interface key the scale to the alloy [17]. The larger cerium atoms located at the metal–air interface before oxidation play the most beneficial role against decarburization of the alloy.
4. Conclusions (1) Ion implantation of 1310 15 ions / cm 2 of silicon at 150 keV improves the oxidation resistance of austenitic AISI 304 stainless steel at 1173 K and atmospheric pressure of air by a chromium-enhanced transport during the first stages of oxidation. (2) Ion implantation of 1310 14 ions / cm 2 of molybdenum at 150 keV results in a catastrophic oxidation at 1173 K and atmospheric pressure of air until the molybdenum implanted at the top surface is triggered out as volatile species. (3) Ion implantation of 1310 14 ions / cm 2 of cerium at 150 keV gives rise to an improvement of the oxidation resistance of the alloy at 1173 K and atmospheric pressure of air by the formation of a very highly protective scale
based on Cr 2 O 3 . The bigger size of cerium blocks cationic diffusion with the subsequent oxygen diffusion. (4) Silicon is always segregated to the metal–scale interface having a keying effect. (5) Ion implantation plays a beneficial role against superficial decarburization of the alloy at 1173 K for 144 h. Acknowledgements The present authors are very much obliged to the ´ ´ Comision Interministerial de Ciencia y Tecnologıa (C.I.C.Y.T.), research project MAT96-0917 and the Universidad Complutense de Madrid, Project No. PR156 / 977169 for supporting the present study. References [1] T. Zhang, J. Xie, C. Ji, J. Chen, H. Xu, J. Li, G. Sun, H.-X. Zhang, Surf. Coat. Technol. 72 (1995) 93–98. [2] J. Townsend, Contemp. Phys. 27 (1987) 241. [3] A. Iskanderova, T.D. Radhhabov, G.R. Rakhimova, Phys. Chem. Mech. Surf. 8(8) (1984) 1081–1101. [4] D. Duday, Ph.D. Thesis, Universite´ de la Rochelle, 1998. [5] S. Seal, S.K. Bose, S.K. Roy, Oxid. Met. 41 (1994) 139–178. ¨ [6] Y. Saito, B. Onay, T. Maruyama, J. Phys. IV (colloque C9; Suppl. J. Phys. III) 3 (1993) 217–230. [7] R.J. Hussey, M.J. Graham, Oxid. Met. 45 (1996) 349–374. [8] F.H. Sttot, G.C. Wood, J. Stringer, Oxid. Met. 8 (1995) 113–145. [9] P.Y. Hou, J. Stringer, Oxid. Met. 33 (1990) 357. [10] H.S. Isaacs, S.M. Huangs, J. Electrochem. Soc. 143(12) (1996) L277–L279. [11] X. de Buchere, P. Andreazza, C. Andreazza-Vignolle, C. Clinard, R. Erre, Surf. Coat. Technol. 80 (1996) 49–52. [12] Kirk-Ohmer Encyclopedia of Chem. Tech., 4th ed., vol. 14, John Wiley & Sons, New York, 1995, pp. 783–814. [13] J. Stringer, B.A. Wilcox, R.I. Jaffee, Oxid. Met. 5 (1972) 11. [14] J.M. Hamikian, D.I. Otter, Oxid. Met. 38 (1992) 139. [15] A.M. Huntz, Mater. Sci. Eng. 87 (1987) 251–260. [16] H.J. Grabke, G.H. Meier, Oxid. Met. 8 (1995) 147–176. [17] Y. Saito, Maruyama, Mater. Sci. Eng. 87 (1987) 275–280.
High temperature corrosion protection of austenitic AISI 304 stainless steel by Si, Mo and Ce ion implantation a ´ a , *, E. Otero a , M.P. Hierro a , C. Gomez ´ ´ c F.J. Perez , F. Pedraza a , J.L. de Segovia b , E. Roman b
a Departamento de Ciencia de los Materiales, Universidad Complutense de Madrid, 28040 -Madrid, Spain ´ ´ , Consejo Superior de Investigaciones Cientıficas ´ , Instituto Torres Quevedo, C /Serrano, 144, 28010 -Madrid, Spain Departamento de Fısica del Vacıo c ´ , Instituto de Ciencia de los Materiales, 28049 Canto Blanco-Madrid, Spain Consejo Superior de Investigaciones Cientıficas
Abstract The influence of implanted silicon, molybdenum and cerium on the oxidation behaviour of a 18Cr8Ni stainless steel was studied at 1173 K up to 144 h in air under isothermal conditions in order to verify the enhanced selective oxidation of chromium by these elements. The implanted surface and the corrosion products formed were characterized by means of AES, SEM, EDS and XRD. Implanted depth profiles were calculated by TRIM96 computational code. The implanted silicon and cerium markedly improved protection against oxidation of the alloy by enhanced chromium transport while molybdenum gave rise to an accelerated oxidation due to the formation of volatile MoO 3 species. Further, ion implantation was found to play a beneficial effect against decarburization of the alloy. 1998 Elsevier Science S.A. All rights reserved. Keywords: Corrosion; Stainless steel; Ion implantation
1. Introduction It is very well known that ion implantation shows several advantages over other surface modification techniques [1,2]. Doses ranging from 10 16 to 10 18 ions / cm 2 are typically directed towards different substrates to improve corrosion properties while fluences from 10 14 to 10 16 ions / cm 2 are more commonly employed in wear and friction tasks [3], but in recent studies it has been demonstrated that low fluences can be effective against oxidation processes [4]. The influence of the addition or ion implantation of Y and rare earths and their oxide dispersions to both Cr 2 O 3 and Al 2 O 3 -forming alloys to improve their high temperature oxidation behaviour has been extensively studied based upon the ‘reactive element effect’ (REE) [5,6]. It has been suggested that the beneficial effects of REs rely on: (a) a physical blocking by RE ions or second-phase particles at grain boundaries; (b) pegging of the oxide to the alloy; (c) vacancy annihilation; (d) reduction of oxidegrowth stresses; (e) enhanced selective oxidation of Cr; and (f) formation of a fine-grain oxide layer [7]. However, the mechanisms are not fully understood and an alternative explanation is that the REs prevent segregation of sulfur to *Corresponding author. Facultad de Ciencias Quimicas, Ciudad Universitaria, 28040 Madrid, Spain. Tel.: 134-1-3944216; fax: 134-13944257.
the scale grain boundaries resulting in a decrease in chromium flux across the scale and improving the adherence of the scale to the alloy [8]. 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 RE in high temperature applications [9], while Mo effects have been related to an improvement of the corrosion resistance in chloride media [10,11]. This paper provides the results of the effect of the oxidation of an austenitic AISI 304 stainless steel in the range of temperatures used and its surface modifications by silicon, molybdenum and cerium ion implantation with low fluences (1310 15 , 1310 14 and 1310 14 ions / cm 2 , respectively) at 1173 K for 144 h under atmospheric pressure of air. Those low fluences applied will give new ideas about the implantation effect against oxidation.
2. Experimental Specimens of 1533.531 mm were cut from cold-rolled AISI-304 pletins. Before implantation, all the specimens were ground to a surface finish of SiC [600. The Si, Mo and Ce ion implantations were carried out at an energy of 150 keV with ion fluences of 1310 15 Si / cm 2 , 1310 14 Mo / cm 2 and 1310 14 Ce / cm 2 . The implanted species distribution (Rp) was accomplished by computer simulation TRIM96 while the implanted surfaces were character-
0257-8972 / 98 / $ – see front matter 1998 Elsevier Science S.A. All rights reserved. PII: S0257-8972( 98 )00685-9
128
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
Fig. 1. Auger spectra of the 1310 Si / cm implanted AISI 304 stainless steel at 150 kV without Ar bombardment (thin line) and with consecutive Ar bombardments (broader lines).
Fig. 3. Auger spectra of the 1310 14 Ce 1 / cm 2 implanted AISI 304 stainless steel at 150 kV without Ar bombardment (thin line) and with consecutive Ar bombardments (broader lines).
ized by means of Auger electron spectroscopy in a JEOL JAMP-10S Auger microprobe at 3 kV and 50 mA with a 10-mm spot diameter. The implanted species were oxidized in a muffle furnace under isothermal conditions at 1173 K and atmospheric pressure of air. Characterization of the oxidized species was carried out by means of XRD, SEM and EDS.
inner sublayers of the non-oxidized Si-implanted stainless steel while Fig. 2 and Fig. 3 concern those of the nonoxidized Mo- and Ce-implanted AISI 304. The Auger spectra reveal that Si had diffused outwards showing a maximum contribution on the top surface and a small depletion in the sublayers increasing the Si content in the inner region up to that of the as-received steel. The diffusion of silicon is thought to be due to the small size of the element and its avidity to get oxidized as it can be seen from the left shifting of the Si LVV transition (92 eV) and the presence of oxygen (510 eV). Opposite to TRIM calculations molybdenum showed its maximum at the outermost surface while cerium was found to be located preferentially slightly away from the top surface. The results of the isothermal oxidation experiments at 1173 K are shown in Fig. 4. It follows from this plot that the low doses of silocon and cerium reduce by about 50% the parabolic rate of the parent material but Mo increases it. This anomalous behaviour for the Mo-implanted steel can be explained from the characterization for short
15
1
2
3. Results and discussion Table 1 shows the projected ranges calculated by TRIM96 program based upon Monte Carlo calculations where it was assumed that the AISI 304 stainless steel had ˚ thick Cr 2 O 3 passivation layer. It can be readily a 50-A ˚ seen that both Rp(Mo) and Rp(Ce) reach about 300 A depth in the material which represent one-third of the centroid of the Gaussian distribution for the Si-implanted steel. The differences in Rp values arise from the relation between the accelerating voltage, the incidence angle, the type of substrate atoms and their binding energy (which were kept constant) and the type of incidence atom [12]. Fig. 1 represents the Auger spectra of the upper and
Fig. 2. Auger spectra of the 1310 14 Mo 1 / cm 2 implanted AISI 304 stainless steel at 150 kV without Ar bombardment (thin line) and with consecutive Ar bombardments (broader lines).
Fig. 4. Experimental values and kinetic laws for parent (squares), 1310 15 Si 1 / cm 2 (circles), 1310 14 Mo 1 / cm 2 (triangles) and 1310 14 Ce 1 / cm 2 (diamonds) AISI 304 stainless steel oxidated at 1173 K in air for 144.
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
129
Fig. 5. XRD diffraction spectrum of the 1310 14 ions / cm 2 MO-implanted AISI 304 stainless steel after oxidation at 1173 K in air for 48 h.
Fig. 7. Cross-section of the 1310 14 ions / cm 2 Mo-implanted AISI 304 stainless steel after oxidation in air at 1173 K for 48 h.
oxidation times. Figs. 5 and 6 and 7 concern the XRD spectrum, the surface and cross-section photomicrographs for Mo-implanted AISI 304 oxidized at 1173 K for 48 h. The diffractogram reveals the constitution of manganeseenriched spinels, Mn 1.5 Cr 1.5 O 4 and the formation of a poorly protective iron-rich mixed oxide, (Fe 0.6 Cr 0.4 ) 2 O 3 carrying out spalling of the scale (Fig. 6) which is extremely cracked (Fig. 7). However, for maximum exposure times all the specimens showed the same surface morphologies with polygonal crystals of about 0.5 mm in size extending over the surface which were mainly composed of chromium as shown by EDS analysis and hexagonal ‘wafer’ coalesces, where chromium and manganese were the major constituents (Fig. 8). Cross-sections for 144 h at 1173 K showed a convoluted scale both in implanted and non-implanted AISI 304 (Fig. 9) which was mainly composed of chromium with silicon incorporated at the metal–scale interface (Fig. 9a). Moreover, a ribbon between the scale and the inner alloy, where no carbide precipitates, was present. It was observed by EDS microanalysis that silicon, molybdenum, chromium and iron were incorporated to the carbides precipitated along grain and twin boundaries being the ribbons of the implanted specimens less wide than the one of the nonimplanted steel.
Some particular characteristics were observed in base AISI 304 and Si-implanted AISI 304. Oxidation of these specimens for the longest exposure times brought about the formation of disperse sponge-shaped nodules (Fig. 10) where the internal zone showed internal oxidation of the material (Fig. 10a) and the outer zone had the same composition as the hexagonal ‘wafers’. On the basis of the results obtained at 1173 K the present authors suggest the following oxidation mechanisms:
3.1. Non-implanted AISI-304 The Cr 2 O 3 passivation layer cracks when the alloy is put into the furnace at 1173 K due to the differences in thermal expansion coefficients of the metallic alloy and the protective oxide [8]. Oxygen subsequently reaches the nuclei of iron and chromium growing a poorly-protective mixed scale. Diffusion outwards of the smaller cations such as manganese results in the formation of highly protective manganese-enriched spinels Mn 1.5 Cr 1.5 O 4 , but depletion of these spinels occurs around the rosettes. These depleted areas can be considered as diffusion paths for oxygen and the additional surface area for reaction giving rise or increasing the internal oxidation. Fortunately, silicon oxide intrusions at the metal–scale interface peg or key the scale to the metal substrate. Further exposure of the alloy at 1173 K for 144 h leads to a decarburization of the external areas of the alloy while cooling in air carries out the carbide precipitation along high energy areas such as grain boundaries and twins.
3.2. Si-implanted AISI-304
Fig. 6. Surface morphology with oxide spallation of the 1310 14 ions / cm 2 Mo-implanted AISI 304 stainless steel after oxidation in air at 1173 K for 48 h.
During the first stages of oxidation breakaway of the protective scale and transient oxidation as described elsewhere [8] takes place. A convoluted and chemically homogeneous oxide layer is grown and developed. The probable cause of the enhanced selective oxidation of chromium is that silicon oxidizes initially and the resulting oxide particles at the surface act as nucleation sites for
130
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
Fig. 8. (a) Surface morphologies of the parent AISI 304 stainless steel after oxidation in air at 1173 K for 144 h; (b) EDS microanalysis at an hexagonal ‘wafer’.
Fig. 9. (a) Cross-section of the non-implanted and implanted materials for the longest exposure time (144 h) at 1173 K in air (glyceregia etched); (b) EDS microanalysis of the oxide scale.
Cr 2 O 3 [13]. However, Hamikian and Otter [14] proposed an explanation in terms of enhanced transport of chromium to the surface along newly created alloy grain boundaries formed by recrystallization of the amorphous phase that was induced in the surface by the implantation process. In this case, the present authors do not believe such amorphization to occur due to the low doses implanted into the alloy. Further, ion implantation prevents the internal oxidation of the alloy by the formation of the chemically homogeneous oxide layer although a few manganese-enriched rosettes were observed with the corresponding depletion in these protective oxides in their surroundings. Moreover, the low doses of implanted silicon prevents the formation of a continuous layer of silica which could act as a weakening plane due to the a – b transformations in cristobalite [15]. Dispersion of the silcion particles in the external zones of the alloy partially impedes diffusion outwards of the
carbon present in the alloy resulting in less decarburization than in the parent material.
3.3. Mo-implanted AISI 304 Oxidation for the first 8 h takes place as described above for the Si-implanted steel. Exposure at high temperature brings about the formation of molybdenum oxides such as MoO 2 and MoO 3 which turn into volatile species at temperatures above 6008C [16]. These volatile oxides (or hydrated species since moisture is present in the furnace) diffuse out through the grain boundaries of the new formed scale acting as short-circuit paths leading to the formation of cracks and pores which enhance cationic and anionic diffusion thickening the scale. It is well known that for scales above a certain thickness, creation of new surfaces by detachment of the scale may be more favourable than the creep of the oxide required to maintain adhesion [8].
´ et al. / Surface and Coatings Technology 108 – 109 (1998) 127 – 131 F. J. Perez
131
Fig. 10. (a) Sponge-shaped nodules found in the non-implanted and Si-implanted AISI 304 stainless steel after oxidation at 1173 K in air for 144 h; (b) EDS microanalysis of the inner zone of the nodule.
As oxidation time passes by, depletion of molybdenum from the external layers of the alloy leads to the creation of a new scale in the same manner as in the Si-implanted alloy.
3.4. Ce-implanted AISI 304 Direct oxidation of the alloy takes place up to the first 8 or 9 h probably due to cracking of the Cr 2 O 3 passivation layer of the stainless steel. Later, transient oxidation takes place causing the cerium particles to act as nucleation sites for chromium in such a way that an almost stoichiometric Cr 2 O 3 is formed in the metal–scale interface. This oxide provides a good protection to the alloy especially when manganese-enriched spinels are formed in the outermost layer of the scale. Further, the silicon oxides found at the metal scale interface key the scale to the alloy [17]. The larger cerium atoms located at the metal–air interface before oxidation play the most beneficial role against decarburization of the alloy.
4. Conclusions (1) Ion implantation of 1310 15 ions / cm 2 of silicon at 150 keV improves the oxidation resistance of austenitic AISI 304 stainless steel at 1173 K and atmospheric pressure of air by a chromium-enhanced transport during the first stages of oxidation. (2) Ion implantation of 1310 14 ions / cm 2 of molybdenum at 150 keV results in a catastrophic oxidation at 1173 K and atmospheric pressure of air until the molybdenum implanted at the top surface is triggered out as volatile species. (3) Ion implantation of 1310 14 ions / cm 2 of cerium at 150 keV gives rise to an improvement of the oxidation resistance of the alloy at 1173 K and atmospheric pressure of air by the formation of a very highly protective scale
based on Cr 2 O 3 . The bigger size of cerium blocks cationic diffusion with the subsequent oxygen diffusion. (4) Silicon is always segregated to the metal–scale interface having a keying effect. (5) Ion implantation plays a beneficial role against superficial decarburization of the alloy at 1173 K for 144 h. Acknowledgements The present authors are very much obliged to the ´ ´ Comision Interministerial de Ciencia y Tecnologıa (C.I.C.Y.T.), research project MAT96-0917 and the Universidad Complutense de Madrid, Project No. PR156 / 977169 for supporting the present study. References [1] T. Zhang, J. Xie, C. Ji, J. Chen, H. Xu, J. Li, G. Sun, H.-X. Zhang, Surf. Coat. Technol. 72 (1995) 93–98. [2] J. Townsend, Contemp. Phys. 27 (1987) 241. [3] A. Iskanderova, T.D. Radhhabov, G.R. Rakhimova, Phys. Chem. Mech. Surf. 8(8) (1984) 1081–1101. [4] D. Duday, Ph.D. Thesis, Universite´ de la Rochelle, 1998. [5] S. Seal, S.K. Bose, S.K. Roy, Oxid. Met. 41 (1994) 139–178. ¨ [6] Y. Saito, B. Onay, T. Maruyama, J. Phys. IV (colloque C9; Suppl. J. Phys. III) 3 (1993) 217–230. [7] R.J. Hussey, M.J. Graham, Oxid. Met. 45 (1996) 349–374. [8] F.H. Sttot, G.C. Wood, J. Stringer, Oxid. Met. 8 (1995) 113–145. [9] P.Y. Hou, J. Stringer, Oxid. Met. 33 (1990) 357. [10] H.S. Isaacs, S.M. Huangs, J. Electrochem. Soc. 143(12) (1996) L277–L279. [11] X. de Buchere, P. Andreazza, C. Andreazza-Vignolle, C. Clinard, R. Erre, Surf. Coat. Technol. 80 (1996) 49–52. [12] Kirk-Ohmer Encyclopedia of Chem. Tech., 4th ed., vol. 14, John Wiley & Sons, New York, 1995, pp. 783–814. [13] J. Stringer, B.A. Wilcox, R.I. Jaffee, Oxid. Met. 5 (1972) 11. [14] J.M. Hamikian, D.I. Otter, Oxid. Met. 38 (1992) 139. [15] A.M. Huntz, Mater. Sci. Eng. 87 (1987) 251–260. [16] H.J. Grabke, G.H. Meier, Oxid. Met. 8 (1995) 147–176. [17] Y. Saito, Maruyama, Mater. Sci. Eng. 87 (1987) 275–280.