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The beneficial role of Y-implantation on the aqueous corrosion of stainless steel
Surface & Coatings Technology 205 (2011) 3506–3511
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
The beneficial role of Y-implantation on the aqueous corrosion of stainless steel F. Noli a,⁎, P. Misaelides a, E. Pavlidou b a b
Department of Chemistry, Aristotle University, GR-54124 Thessaloniki, Greece Department of Physics, Aristotle University, GR-54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 7 October 2010 Accepted in revised form 14 December 2010 Available online 24 December 2010 Keywords: Stainless steel Yttrium Implantation Corrosion NRA RBS
a b s t r a c t The corrosion behaviour of Y-implanted austenitic stainless steel AISI 321 samples was investigated in 0.5 M H2SO4 at ambient temperature using potentiodynamic polarization and cyclic voltammetry. The implantation of 1 × 1016 Y-ions/cm2 of 40 keV energy did not lead to an improvement of the corrosion resistance of the material because of sputtering effects. On the other hand, a significant improvement of the corrosion resistance was observed by increasing of the dose (2 × 1017 Y-ions/cm2 implanted in the presence of oxygen) and the implantation energy (55 and 80 keV). The elemental composition of the near-surface layers of the implanted steel samples prior and after the corrosion attack was determined by Rutherford backscattering spectrometry (RBS) and Nuclear Reaction Analysis (NRA) using alpha particles, protons and deuterons as projectiles. The surface morphology and microstructure of the non-corroded and corroded samples were examined by Scanning Electron Microscopy (SEM). The corrosion resistance of the implanted materials was found to be related with the thickness and the composition of the implanted layer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the last years there is an increased interest in materials showing enhanced mechanical properties (e.g. hardness and wear) as well as oxidation and corrosion resistance. The implantation, using high energy ion-beams is one of the techniques widely used for this purpose. The ion-implantation leads to the modification of the properties of the near-surface layers of metallic materials without considerably affecting the bulk [1–3]. It is also well known that using ion implantation it is possible to alter the chemical and electrochemical properties of the surface of a material and consequently to control the rate of anodic and cathodic reactions. In aqueous corrosion the electrochemical processes at the liquid/solid interface can be inhibited by surface passivation (formation of passive oxides on implanted surface) [4,5]. Nielsen et al. found that the Ta- and Cr-implantation produced a significant improvement in the corrosion resistance of M50 steel [6]. Similar effects were also observed in the case of Mo-implantation [7]. Braun et al. also investigated the influence of several ions (He+, C+, N+, and O+) implanted in iron and steel. Nitrogen was found to show, among the above mentioned implanted species, the most beneficial effect on the reduction of the aqueous corrosion [8]. The implantation of P, Zr and Ti also reduces the corrosion rate of the steels [9,10]. The effect of yttrium implantation on the high temperature oxidation of steels has been extensively investigated [11–15]. It has
⁎ Corresponding author. Tel.: + 30 2310 997997; fax: + 30 2310 997753. E-mail address: [email protected] (F. Noli). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.12.018
been demonstrated that the implanted Y-atoms are segregated along the grain boundaries and, forming a double oxide of the type Y2O3·Cr2O3, protects the material. On the other hand, studies concerning the corrosion behaviour of Y- and so-called “reactive elements” (e.g. La, Ce, and Er) implanted steels are rather limited [16–22]. Zhang et al. investigated the dose influence of 40 keV Y-implantation on the corrosion resistance of steel and found that high-dose (5 × 1017 Y-ions/cm2) and -flux implantation (38 μA/cm2) result in an enhanced corrosion resistance [19]. Weng et al. refer that the change of the surface composition produced by single and multielement implantation (implantation of 1 × 1017 cm− 2 Y of 65 keV energy in combination with Ti- and Mo-ions) as well as the physical mixing produced by ion beams considerably contributes to the improvement of the corrosion resistance of steel [20]. The pitting corrosion of Zr- and Y- as well as multi-element (Y and Cr)-implanted steel has also been studied in NaCl solutions. The results, supported by XPS measurements, have shown that in the first case the implantation causes an invasion of carbon and oxygen atoms resulting in reduction of anodic dissolution current. In the second case the formation, after the implantation, of an Y2O3·Cr2O3 surface layer seems to act as barrier that reduces the contact between the metal matrix and corrosive medium and enhances the corrosion resistance [21,22]. The objective of this work was to investigate the corrosion behaviour of AISI 321 stainless steel implanted with Y-ions of different doses and energies. Yttrium was selected because of its affinity to oxygen (heat of formation in kcal/mol for several oxides: La2O3−407.7, Cr2O3−274.7, and Y2O3−419.6) [23]. On the other hand the selection of two doses (1 × 1016 and 2 × 1017 Y-ions/cm2) and
F. Noli et al. / Surface & Coatings Technology 205 (2011) 3506–3511
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three different implantation energies (40, 55 and 80 keV) was decided in order to study the influence of the composition and the width of the modified implanted region. 2. Experimental 2.1. The implantations Mechanically polished AISI 321 stainless steel samples (Fe/Cr18/Ni8/ Ti — supplied by Goodfellows Ltd) of dimensions 10× 20× 0.25 mm were implanted at room temperature with Y-ions (Dosis: 1× 1016 ions/cm2, pressure 10− 6 mbar). By increasing the pressure in the implantation chamber to 10− 5 mbar in the presence of oxygen a saturation dose of 2 ×1017 ions/cm2 was achieved. Implantations with the maximum dose of 2× 1017 ions/cm2 (beam intensity ca. 10 μA/cm2) were performed at the energies of 40, 55 and 80 keV. 2.2. The characterization of the samples The samples were characterized by RBS using protons (Ep: 1.75 MeV, scattering angle: 170°), deuterons (Ed: 1.35 MeV, scattering angle: 170°) and α-particles (Eα: 3.05 MeV, scattering angle: 170°) as projectiles in order to achieve better depth resolution. The oxygen concentration was 16 17 determined by the O(d,p) O nuclear reaction (Ed: 1.35 MeV, scattering angle: 150°). SEM/EDS examination of the samples was performed using a JEOL JSM 840A electron microscope equipped with an energydispersive X-ray (EDS) INCA micro-analytical system (operating conditions were: accelerating voltage 20 kV, probe current 45 nA and counting time 60 s, with ZAF correction being provided on-line). The samples were coated before the examination with carbon, using a JEOL JEE-4X vacuum evaporator. 2.3. The corrosion tests The investigation of the corrosion behaviour was performed in an argon aerated sulphuric acid solution (0.5 M H2SO4) by means of potentiodynamic and cyclovoltammetric techniques according to ASTM Designation G5-82 [24]. The tests were undertaken at ambient temperature using an AUTOLAB Potentio-Galvanostat (ECO CHEMIE, Netherlands) interfaced with a computer and a recorder. A conventional three-electrode cell (EG&G PARR model) used for all measurements. The cell was equipped with a saturated calomel reference electrode, a graphite auxiliary electrode and a holder leaving only the one side of the specimen exposed to the corroding medium. In all cases the electrolyte volume was 800 mL and the sample surface which was in contact with the testing solution was 1 cm2. The open circuit potential or corrosion potential Ecorr was recorded after 30 min stabilisation whereas rapid (50 V/h) and slow scan rates (0.6 V/h) were used. The investigation region was (−800)–(+1500) mV. The scans were starting from the open circuit potential up to −800 mV (cathodic curve) and then up to +1500 mV (1st anodic curve) and back to Ecorr (2nd anodic curve). Subsequent sweeps with a rate of 50 V/h were applied (5 cycles) followed by the slow scan. 3. Results and discussion The depth distribution of the Y-ions implanted at different energies on steel surface is given in Fig. 1. The results, which were obtained by the evaluation of the p-RBS spectra using the computer codes RUMP and SIMNRA, give a maximum distribution range from 80 nm for the 40 keV- up to 110 nm for the 80 keV-implanted Y-ions [17,25,26]. The RBS spectra of the 40 and 80 keV Y-implanted steel (dose: 2 × 1017 ions/cm2) before the corrosion test are given in Fig. 2a. Simulations of the RBS spectra in combination with the NRA data reveal information about the thickness and the elemental composition of the
Fig. 1. Distribution of 40, 55 and 80 keV Y-ions in AISI-321 stainless steel.
near-surface layers. The determined composition of the near-surface region indicated a chromium depletion (also confirmed by SIMS measurements presented in a previous work [15]) especially in the case of the 40 keV Y-ions (dose: 1 × 1016 ions/cm2). This effect, which has also been reported in the literature for other heavy ion implantations (e.g. La and Zr), is most probably due to sputtering [11,27]. Implantations in the presence of oxygen lead to the formation of a protective oxide on the implanted surface as indicated on the RBS spectrum presented in Fig. 2b. The formation of this oxide, which seems to consist of Y2O3·Cr2O3 or YFeO3, has also been proven by several researchers [20,22,30]. In our case, during the implantation in the presence of oxygen, the energetic Y-ions interacted with the absorbed oxygen molecules through a low temperature oxidation process. The thickness of this oxide layer, which was not detected in the case of the implantation at 10− 6 mbar pressure, was estimated to be ca. 70 nm. The RBS spectra after the corrosion treatment are given in Fig. 2c. The effect of the corrosion is obvious as shown by the missing edge of the spectrum in the case of the 40 keV Y-implanted steel. The Y-peak does not appear, as expected, in the RBS spectrum of the corroded samples. This can be explained considering the width of the Y-implanted layer (maximum ca. 90 nm) and the corrosion rate of the implanted steel (ca. 1.15 nm/s) as derived from the potentiodynamic curves [24]. The thickness of the oxygen layer on the corroded implanted samples, determined by NRA, was ca. 85 nm. This can be seen in Fig. 2d, where the oxygen peak on the spectrum of the protons 16 17 emitted by the O(d,p0) O nuclear reaction and the corresponding simulation by the SIMNRA code are presented. Most of the electrochemical corrosion studies in solutions are performed using both rapid and slow scan rates. The rapid scan rates allow measurements under constant conditions of the metal surface and the corrosive media (without or with very thin coating formation). On the other hand, using slow scan rates the corrosion investigation is performed under non-constant conditions of the metal surface (coating formation) and the corrosive media allowing predictions of the general corrosion behaviour of the material [24]. The results of the corrosion experiments are summarized in Table 1. In this Table Ecorr, Epit and Erep respectively represent the corrosion, pitting and repassivation potential and icrit and ipass the critical (or dissolution) and the passivation current [18,24]. The potentiodynamic polarization curve of the non-implanted steel obtained using cyclic voltammetry is given in Fig. 3. The peak at −220 mV corresponds to the iron dissolution, the region between −100 and 1000 mV to the region where the steel surface is passivated under formation of protective oxides (most probably Cr-oxides) and the peak at ca. 1200 mV to the Cr-oxidation. The hysteresis loop during the reverse scan indicates sensitivity to pitting corrosion.
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Fig. 2. RBS-spectra of 2 × 1017 ions/cm2, a and c) 40(–) and 80 keV (bold line –) Y-implanted samples before and after the corrosion treatment (p-RBS), b) 40 keV before the corrosion 16 17 treatment (α-RBS) and d) oxygen determination using the O(d,p0) O nuclear reaction.
The potentiodynamic polarization curve of the Y-implanted steel (1×1016 ions/cm2, 40 keV) measured by applying the rapid scan rate is given in Fig. 4. One can observe that the corrosion resistance of the material was not improved after the implantation (reduction of the Ecorr). High currents are passing the surface and the passive region tends to disappear. This indicates that the formation of thin protective layers is somehow hindered and the exposed surface is active and accessible to corrosion attack. This could be attributed to the sputtering effects taking
place during the Y-implantation leading to a low Y-content and a Cr-depleted steel surface layer. After several scans the current value was reduced and the formation of passive region was obvious.
Table 1 Corrosion parameters after the polarization tests. Rapid scan rate investigation No Specimen 1 2 3 4 5
Steel Y-implanted steel Y-implanted steel Y-implanted steel Y-implanted steel
Implantation conditions – 1 × 1016 ions/cm2 40 keV 2 × 1017 ions/cm2 40 keV 2 × 1017 ions/cm2 55 keV 2 × 1017 ions/cm2 80 keV
Ecorr
icrit
ipass
imax
Epitt
Erep
(mV)
(mA)
(mA)
(mA)
(mV)
(mV)
− 320 0.9 − 590 –
0.35 19.2 940 – 50.2 –
+ 150 –
0.3
5.6 940
950
+ 310 –
0.14
6.1 940
960
+ 290 –
0.05
3.7 940
1040
940 –
Fig. 3. Polarization curves of non-implanted steel.
F. Noli et al. / Surface & Coatings Technology 205 (2011) 3506–3511
Fig. 4. Polarization curves of 1 × 1016 ions/cm2 40 keV Y-implanted steel.
The modification of the surface layer of the material using a higher implantation dose in the presence of oxygen leads to a significant improvement of the corrosion resistance because of the reduction of the sputtering effects and to the formation of a thicker protective oxide layer [28,29]. This can be observed in Fig. 5a presenting the polarization curves (rapid scan rate) of the steel samples implanted
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with 2 ⁎ 1017 ions/cm2 of 40 and 80 keV energy. The evaluation of the data presented in Table 1 shows that the potentials especially the Ecorr and Erep are shifted to a nobler direction indicating a delayed initiation of the corrosion on the modified steel surface. The peak of the iron dissolution is not present because of the thin yttrium oxide layer formed on the steel surface. By proceeding corrosion, Y2O3 formation on the surface in addition to Cr2O3 leads to the enhancement of the steel corrosion resistance. As shown in Fig. 5a and b, at higher implantation energies the corrosion protection is more effective attributed to the thickness of the implanted layer and of the protective oxide. Subsequent sweeps also show that after the second scan a surface stabilisation under formation of a thin protective oxide layer (most likely Y2O3·Cr2O3) takes place as indicated by the lower current values in Fig. 5a and Table 1. The role of yttrium and oxygen on the corrosion resistance of stainless steel can be explained in the following way. The increase of the Y-concentration in the implanted region results in an improvement of the corrosion resistance of steel because of chemical effects. The presence of the oxygen in the implantation chamber reduces the sputtering leading to higher Y-concentration in the implanted layer and to the formation of protective yttrium oxide. The formation of a complex oxide inside the sample (interface of surface film and substrate) also promotes the adhesion contributing to the enhancement of the alloy corrosion resistance. The significant reduction of the passive current value, in particular during the slow scan rate investigation (Table 2), indicates a longterm corrosion protection. The highest corrosion resistance was exhibited by the sample implanted with 80 keV Y-ions. This can be attributed to the enhanced thickness of the 80 keV Y-modified surface layers. The values of the imax are found increased in the case of the implanted samples due to the implantation effects (radiation damage, etc). Fig. 6a–c shows the surface microstructure of the non-implanted steel, Y-implanted steel by 1 × 1016 ions/cm2 at 40 keV and Y-implanted steel 2 × 1016 ions/cm2 at 40 keV before the corrosion treatment. The images of the corresponding corroded samples are shown in Fig. 6d–f. In the case of the corroded sample implanted with 1 × 1016 ions/cm2 at 40 keV (Fig. 6e) a different morphology was observed and the surface seems to be accessible to corrosion. On the other hand, coverage with spherical nodules of the surface of the sample implanted with 2 × 1017 ions/cm2 of 55 keV energy can be observed after the corrosion treatment (Fig. 6f). Similar appearances (presence of nodules) on the surface of the sample implanted with 2 × 1017 ions/cm2 of 80 keV energy could be attributed to segregation effects during implantation. SEM cross-sections of the non-implanted and Y-implanted steel samples are given in Fig. 7. One can observe the thick corroded layer (thickness 50–80 μm) on the surface of the non-implanted steel (Fig. 7a) and the limited corroded zone (thickness ca. 2 μm) of the Y-implanted sample (Fig. 7b). The presence of grains of different orientation in the implanted region (Fig. 7b) supports the conclusion that the improvement of the corrosion resistance is not only attributed to chemical effects (presence of a passive layer on steel surface) but also to physical Table 2 Corrosion parameters after the polarization tests. Slow scan rate investigation No
Fig. 5. Polarization curves of a) 40 and 80 keV (2 × 1017 ions/cm2) Y-implanted steel, and b) 55 keV (2 × 1017 ions/cm2) Y-implanted steel.
Specimen
1 2
Steel Y-implanted steel
3
Y-implanted steel
4
Y-implanted steel
Implantation conditions – 2 × 1017 ions/cm2 40 keV 2 × 1017 ions/cm2 55 keV 2 × 1017 ions/cm2 80 keV
ipass
imax
Epitt
Erep
(mA)
(mA)
(mV)
(mV)
0.165 0.080
4.5 8.4
940 950
940 950
0.020
6.5
950
950
0.016
5.0
950
960
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Fig. 6. SEM micrographs a–c) non-implanted steel, Y-implanted steel (40 keV, 1 × 1016 ions/cm2) and Y-implanted steel (40 keV, 2 × 1017 ions/cm2) respectively before the corrosion tests, and d–f) the corresponding samples after the corrosion tests.
and mechanical effects connected with the radiation damage, movement of interstitials and promotion of selective nucleation sites. As reported in the literature, the implanted Y-atoms can also act as nucleation sites for Cr2O3 [11]. The results of the EDS analysis indicate that the iron and chromium contents on the corroded samples were normal while the oxygen content on the surface of all implanted samples was about 2.45% and in the pores about 11.5%. For the non-implanted steel a slight increase
of the oxygen amount was found. These data are in good agreement with the RBS-NRA results. 4. Conclusions The corrosion behaviour of AISI-321 stainless steel can be influenced by changing the implantation parameters (e.g. pressure, dose, and energy). The experimental results showed that the implantation by
F. Noli et al. / Surface & Coatings Technology 205 (2011) 3506–3511
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increasing implantation energy, indicating resistance against corrosion. This beneficial effect can be attributed to the formation of an Y2O3 passive layer on steel surface and to the physical and mechanical effects connected with the radiation damage, movement of interstitials and promotion of selective nucleation sites especially for Cr2O3. Acknowledgements The assistance of the staff of the Nuclear Physics Institute of the University of Frankfurt/D and of the Tandem Accelerator Laboratory of the Nuclear Physics Institute of the NCSR Demokritos, Athens/GR, during the implantations and the RBS-NRA measurements, is thankfully acknowledged. References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] Fig. 7. Cross sections: a) non-implanted and b) Y-implanted steel (55 keV, 2×1017 ions/cm2) after the corrosion tests.
1 × 1016 Y-ions/cm2 of 40 keV energy (under 10− 6 mbar pressure) did not improve the corrosion resistance of Y-implanted steel. On the other hand, the implantation under 10− 5 mbar pressure in the presence of oxygen leads to an increase of the range and concentration of implanted Y-ions and significantly improves the corrosion resistance due to reduction of sputtering effects. The improvement, which increases by increasing implantation dose and energy (from 40 to 80 keV), can be attributed to the enhanced thickness of the modified surface region as derived from the RBS-NRA data. The electrochemical corrosion tests indicated that the values of the characteristic potentials moved to more positive regions and characteristic currents are diminishing by
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
The beneficial role of Y-implantation on the aqueous corrosion of stainless steel F. Noli a,⁎, P. Misaelides a, E. Pavlidou b a b
Department of Chemistry, Aristotle University, GR-54124 Thessaloniki, Greece Department of Physics, Aristotle University, GR-54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 7 October 2010 Accepted in revised form 14 December 2010 Available online 24 December 2010 Keywords: Stainless steel Yttrium Implantation Corrosion NRA RBS
a b s t r a c t The corrosion behaviour of Y-implanted austenitic stainless steel AISI 321 samples was investigated in 0.5 M H2SO4 at ambient temperature using potentiodynamic polarization and cyclic voltammetry. The implantation of 1 × 1016 Y-ions/cm2 of 40 keV energy did not lead to an improvement of the corrosion resistance of the material because of sputtering effects. On the other hand, a significant improvement of the corrosion resistance was observed by increasing of the dose (2 × 1017 Y-ions/cm2 implanted in the presence of oxygen) and the implantation energy (55 and 80 keV). The elemental composition of the near-surface layers of the implanted steel samples prior and after the corrosion attack was determined by Rutherford backscattering spectrometry (RBS) and Nuclear Reaction Analysis (NRA) using alpha particles, protons and deuterons as projectiles. The surface morphology and microstructure of the non-corroded and corroded samples were examined by Scanning Electron Microscopy (SEM). The corrosion resistance of the implanted materials was found to be related with the thickness and the composition of the implanted layer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the last years there is an increased interest in materials showing enhanced mechanical properties (e.g. hardness and wear) as well as oxidation and corrosion resistance. The implantation, using high energy ion-beams is one of the techniques widely used for this purpose. The ion-implantation leads to the modification of the properties of the near-surface layers of metallic materials without considerably affecting the bulk [1–3]. It is also well known that using ion implantation it is possible to alter the chemical and electrochemical properties of the surface of a material and consequently to control the rate of anodic and cathodic reactions. In aqueous corrosion the electrochemical processes at the liquid/solid interface can be inhibited by surface passivation (formation of passive oxides on implanted surface) [4,5]. Nielsen et al. found that the Ta- and Cr-implantation produced a significant improvement in the corrosion resistance of M50 steel [6]. Similar effects were also observed in the case of Mo-implantation [7]. Braun et al. also investigated the influence of several ions (He+, C+, N+, and O+) implanted in iron and steel. Nitrogen was found to show, among the above mentioned implanted species, the most beneficial effect on the reduction of the aqueous corrosion [8]. The implantation of P, Zr and Ti also reduces the corrosion rate of the steels [9,10]. The effect of yttrium implantation on the high temperature oxidation of steels has been extensively investigated [11–15]. It has
⁎ Corresponding author. Tel.: + 30 2310 997997; fax: + 30 2310 997753. E-mail address: [email protected] (F. Noli). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.12.018
been demonstrated that the implanted Y-atoms are segregated along the grain boundaries and, forming a double oxide of the type Y2O3·Cr2O3, protects the material. On the other hand, studies concerning the corrosion behaviour of Y- and so-called “reactive elements” (e.g. La, Ce, and Er) implanted steels are rather limited [16–22]. Zhang et al. investigated the dose influence of 40 keV Y-implantation on the corrosion resistance of steel and found that high-dose (5 × 1017 Y-ions/cm2) and -flux implantation (38 μA/cm2) result in an enhanced corrosion resistance [19]. Weng et al. refer that the change of the surface composition produced by single and multielement implantation (implantation of 1 × 1017 cm− 2 Y of 65 keV energy in combination with Ti- and Mo-ions) as well as the physical mixing produced by ion beams considerably contributes to the improvement of the corrosion resistance of steel [20]. The pitting corrosion of Zr- and Y- as well as multi-element (Y and Cr)-implanted steel has also been studied in NaCl solutions. The results, supported by XPS measurements, have shown that in the first case the implantation causes an invasion of carbon and oxygen atoms resulting in reduction of anodic dissolution current. In the second case the formation, after the implantation, of an Y2O3·Cr2O3 surface layer seems to act as barrier that reduces the contact between the metal matrix and corrosive medium and enhances the corrosion resistance [21,22]. The objective of this work was to investigate the corrosion behaviour of AISI 321 stainless steel implanted with Y-ions of different doses and energies. Yttrium was selected because of its affinity to oxygen (heat of formation in kcal/mol for several oxides: La2O3−407.7, Cr2O3−274.7, and Y2O3−419.6) [23]. On the other hand the selection of two doses (1 × 1016 and 2 × 1017 Y-ions/cm2) and
F. Noli et al. / Surface & Coatings Technology 205 (2011) 3506–3511
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three different implantation energies (40, 55 and 80 keV) was decided in order to study the influence of the composition and the width of the modified implanted region. 2. Experimental 2.1. The implantations Mechanically polished AISI 321 stainless steel samples (Fe/Cr18/Ni8/ Ti — supplied by Goodfellows Ltd) of dimensions 10× 20× 0.25 mm were implanted at room temperature with Y-ions (Dosis: 1× 1016 ions/cm2, pressure 10− 6 mbar). By increasing the pressure in the implantation chamber to 10− 5 mbar in the presence of oxygen a saturation dose of 2 ×1017 ions/cm2 was achieved. Implantations with the maximum dose of 2× 1017 ions/cm2 (beam intensity ca. 10 μA/cm2) were performed at the energies of 40, 55 and 80 keV. 2.2. The characterization of the samples The samples were characterized by RBS using protons (Ep: 1.75 MeV, scattering angle: 170°), deuterons (Ed: 1.35 MeV, scattering angle: 170°) and α-particles (Eα: 3.05 MeV, scattering angle: 170°) as projectiles in order to achieve better depth resolution. The oxygen concentration was 16 17 determined by the O(d,p) O nuclear reaction (Ed: 1.35 MeV, scattering angle: 150°). SEM/EDS examination of the samples was performed using a JEOL JSM 840A electron microscope equipped with an energydispersive X-ray (EDS) INCA micro-analytical system (operating conditions were: accelerating voltage 20 kV, probe current 45 nA and counting time 60 s, with ZAF correction being provided on-line). The samples were coated before the examination with carbon, using a JEOL JEE-4X vacuum evaporator. 2.3. The corrosion tests The investigation of the corrosion behaviour was performed in an argon aerated sulphuric acid solution (0.5 M H2SO4) by means of potentiodynamic and cyclovoltammetric techniques according to ASTM Designation G5-82 [24]. The tests were undertaken at ambient temperature using an AUTOLAB Potentio-Galvanostat (ECO CHEMIE, Netherlands) interfaced with a computer and a recorder. A conventional three-electrode cell (EG&G PARR model) used for all measurements. The cell was equipped with a saturated calomel reference electrode, a graphite auxiliary electrode and a holder leaving only the one side of the specimen exposed to the corroding medium. In all cases the electrolyte volume was 800 mL and the sample surface which was in contact with the testing solution was 1 cm2. The open circuit potential or corrosion potential Ecorr was recorded after 30 min stabilisation whereas rapid (50 V/h) and slow scan rates (0.6 V/h) were used. The investigation region was (−800)–(+1500) mV. The scans were starting from the open circuit potential up to −800 mV (cathodic curve) and then up to +1500 mV (1st anodic curve) and back to Ecorr (2nd anodic curve). Subsequent sweeps with a rate of 50 V/h were applied (5 cycles) followed by the slow scan. 3. Results and discussion The depth distribution of the Y-ions implanted at different energies on steel surface is given in Fig. 1. The results, which were obtained by the evaluation of the p-RBS spectra using the computer codes RUMP and SIMNRA, give a maximum distribution range from 80 nm for the 40 keV- up to 110 nm for the 80 keV-implanted Y-ions [17,25,26]. The RBS spectra of the 40 and 80 keV Y-implanted steel (dose: 2 × 1017 ions/cm2) before the corrosion test are given in Fig. 2a. Simulations of the RBS spectra in combination with the NRA data reveal information about the thickness and the elemental composition of the
Fig. 1. Distribution of 40, 55 and 80 keV Y-ions in AISI-321 stainless steel.
near-surface layers. The determined composition of the near-surface region indicated a chromium depletion (also confirmed by SIMS measurements presented in a previous work [15]) especially in the case of the 40 keV Y-ions (dose: 1 × 1016 ions/cm2). This effect, which has also been reported in the literature for other heavy ion implantations (e.g. La and Zr), is most probably due to sputtering [11,27]. Implantations in the presence of oxygen lead to the formation of a protective oxide on the implanted surface as indicated on the RBS spectrum presented in Fig. 2b. The formation of this oxide, which seems to consist of Y2O3·Cr2O3 or YFeO3, has also been proven by several researchers [20,22,30]. In our case, during the implantation in the presence of oxygen, the energetic Y-ions interacted with the absorbed oxygen molecules through a low temperature oxidation process. The thickness of this oxide layer, which was not detected in the case of the implantation at 10− 6 mbar pressure, was estimated to be ca. 70 nm. The RBS spectra after the corrosion treatment are given in Fig. 2c. The effect of the corrosion is obvious as shown by the missing edge of the spectrum in the case of the 40 keV Y-implanted steel. The Y-peak does not appear, as expected, in the RBS spectrum of the corroded samples. This can be explained considering the width of the Y-implanted layer (maximum ca. 90 nm) and the corrosion rate of the implanted steel (ca. 1.15 nm/s) as derived from the potentiodynamic curves [24]. The thickness of the oxygen layer on the corroded implanted samples, determined by NRA, was ca. 85 nm. This can be seen in Fig. 2d, where the oxygen peak on the spectrum of the protons 16 17 emitted by the O(d,p0) O nuclear reaction and the corresponding simulation by the SIMNRA code are presented. Most of the electrochemical corrosion studies in solutions are performed using both rapid and slow scan rates. The rapid scan rates allow measurements under constant conditions of the metal surface and the corrosive media (without or with very thin coating formation). On the other hand, using slow scan rates the corrosion investigation is performed under non-constant conditions of the metal surface (coating formation) and the corrosive media allowing predictions of the general corrosion behaviour of the material [24]. The results of the corrosion experiments are summarized in Table 1. In this Table Ecorr, Epit and Erep respectively represent the corrosion, pitting and repassivation potential and icrit and ipass the critical (or dissolution) and the passivation current [18,24]. The potentiodynamic polarization curve of the non-implanted steel obtained using cyclic voltammetry is given in Fig. 3. The peak at −220 mV corresponds to the iron dissolution, the region between −100 and 1000 mV to the region where the steel surface is passivated under formation of protective oxides (most probably Cr-oxides) and the peak at ca. 1200 mV to the Cr-oxidation. The hysteresis loop during the reverse scan indicates sensitivity to pitting corrosion.
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Fig. 2. RBS-spectra of 2 × 1017 ions/cm2, a and c) 40(–) and 80 keV (bold line –) Y-implanted samples before and after the corrosion treatment (p-RBS), b) 40 keV before the corrosion 16 17 treatment (α-RBS) and d) oxygen determination using the O(d,p0) O nuclear reaction.
The potentiodynamic polarization curve of the Y-implanted steel (1×1016 ions/cm2, 40 keV) measured by applying the rapid scan rate is given in Fig. 4. One can observe that the corrosion resistance of the material was not improved after the implantation (reduction of the Ecorr). High currents are passing the surface and the passive region tends to disappear. This indicates that the formation of thin protective layers is somehow hindered and the exposed surface is active and accessible to corrosion attack. This could be attributed to the sputtering effects taking
place during the Y-implantation leading to a low Y-content and a Cr-depleted steel surface layer. After several scans the current value was reduced and the formation of passive region was obvious.
Table 1 Corrosion parameters after the polarization tests. Rapid scan rate investigation No Specimen 1 2 3 4 5
Steel Y-implanted steel Y-implanted steel Y-implanted steel Y-implanted steel
Implantation conditions – 1 × 1016 ions/cm2 40 keV 2 × 1017 ions/cm2 40 keV 2 × 1017 ions/cm2 55 keV 2 × 1017 ions/cm2 80 keV
Ecorr
icrit
ipass
imax
Epitt
Erep
(mV)
(mA)
(mA)
(mA)
(mV)
(mV)
− 320 0.9 − 590 –
0.35 19.2 940 – 50.2 –
+ 150 –
0.3
5.6 940
950
+ 310 –
0.14
6.1 940
960
+ 290 –
0.05
3.7 940
1040
940 –
Fig. 3. Polarization curves of non-implanted steel.
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Fig. 4. Polarization curves of 1 × 1016 ions/cm2 40 keV Y-implanted steel.
The modification of the surface layer of the material using a higher implantation dose in the presence of oxygen leads to a significant improvement of the corrosion resistance because of the reduction of the sputtering effects and to the formation of a thicker protective oxide layer [28,29]. This can be observed in Fig. 5a presenting the polarization curves (rapid scan rate) of the steel samples implanted
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with 2 ⁎ 1017 ions/cm2 of 40 and 80 keV energy. The evaluation of the data presented in Table 1 shows that the potentials especially the Ecorr and Erep are shifted to a nobler direction indicating a delayed initiation of the corrosion on the modified steel surface. The peak of the iron dissolution is not present because of the thin yttrium oxide layer formed on the steel surface. By proceeding corrosion, Y2O3 formation on the surface in addition to Cr2O3 leads to the enhancement of the steel corrosion resistance. As shown in Fig. 5a and b, at higher implantation energies the corrosion protection is more effective attributed to the thickness of the implanted layer and of the protective oxide. Subsequent sweeps also show that after the second scan a surface stabilisation under formation of a thin protective oxide layer (most likely Y2O3·Cr2O3) takes place as indicated by the lower current values in Fig. 5a and Table 1. The role of yttrium and oxygen on the corrosion resistance of stainless steel can be explained in the following way. The increase of the Y-concentration in the implanted region results in an improvement of the corrosion resistance of steel because of chemical effects. The presence of the oxygen in the implantation chamber reduces the sputtering leading to higher Y-concentration in the implanted layer and to the formation of protective yttrium oxide. The formation of a complex oxide inside the sample (interface of surface film and substrate) also promotes the adhesion contributing to the enhancement of the alloy corrosion resistance. The significant reduction of the passive current value, in particular during the slow scan rate investigation (Table 2), indicates a longterm corrosion protection. The highest corrosion resistance was exhibited by the sample implanted with 80 keV Y-ions. This can be attributed to the enhanced thickness of the 80 keV Y-modified surface layers. The values of the imax are found increased in the case of the implanted samples due to the implantation effects (radiation damage, etc). Fig. 6a–c shows the surface microstructure of the non-implanted steel, Y-implanted steel by 1 × 1016 ions/cm2 at 40 keV and Y-implanted steel 2 × 1016 ions/cm2 at 40 keV before the corrosion treatment. The images of the corresponding corroded samples are shown in Fig. 6d–f. In the case of the corroded sample implanted with 1 × 1016 ions/cm2 at 40 keV (Fig. 6e) a different morphology was observed and the surface seems to be accessible to corrosion. On the other hand, coverage with spherical nodules of the surface of the sample implanted with 2 × 1017 ions/cm2 of 55 keV energy can be observed after the corrosion treatment (Fig. 6f). Similar appearances (presence of nodules) on the surface of the sample implanted with 2 × 1017 ions/cm2 of 80 keV energy could be attributed to segregation effects during implantation. SEM cross-sections of the non-implanted and Y-implanted steel samples are given in Fig. 7. One can observe the thick corroded layer (thickness 50–80 μm) on the surface of the non-implanted steel (Fig. 7a) and the limited corroded zone (thickness ca. 2 μm) of the Y-implanted sample (Fig. 7b). The presence of grains of different orientation in the implanted region (Fig. 7b) supports the conclusion that the improvement of the corrosion resistance is not only attributed to chemical effects (presence of a passive layer on steel surface) but also to physical Table 2 Corrosion parameters after the polarization tests. Slow scan rate investigation No
Fig. 5. Polarization curves of a) 40 and 80 keV (2 × 1017 ions/cm2) Y-implanted steel, and b) 55 keV (2 × 1017 ions/cm2) Y-implanted steel.
Specimen
1 2
Steel Y-implanted steel
3
Y-implanted steel
4
Y-implanted steel
Implantation conditions – 2 × 1017 ions/cm2 40 keV 2 × 1017 ions/cm2 55 keV 2 × 1017 ions/cm2 80 keV
ipass
imax
Epitt
Erep
(mA)
(mA)
(mV)
(mV)
0.165 0.080
4.5 8.4
940 950
940 950
0.020
6.5
950
950
0.016
5.0
950
960
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Fig. 6. SEM micrographs a–c) non-implanted steel, Y-implanted steel (40 keV, 1 × 1016 ions/cm2) and Y-implanted steel (40 keV, 2 × 1017 ions/cm2) respectively before the corrosion tests, and d–f) the corresponding samples after the corrosion tests.
and mechanical effects connected with the radiation damage, movement of interstitials and promotion of selective nucleation sites. As reported in the literature, the implanted Y-atoms can also act as nucleation sites for Cr2O3 [11]. The results of the EDS analysis indicate that the iron and chromium contents on the corroded samples were normal while the oxygen content on the surface of all implanted samples was about 2.45% and in the pores about 11.5%. For the non-implanted steel a slight increase
of the oxygen amount was found. These data are in good agreement with the RBS-NRA results. 4. Conclusions The corrosion behaviour of AISI-321 stainless steel can be influenced by changing the implantation parameters (e.g. pressure, dose, and energy). The experimental results showed that the implantation by
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increasing implantation energy, indicating resistance against corrosion. This beneficial effect can be attributed to the formation of an Y2O3 passive layer on steel surface and to the physical and mechanical effects connected with the radiation damage, movement of interstitials and promotion of selective nucleation sites especially for Cr2O3. Acknowledgements The assistance of the staff of the Nuclear Physics Institute of the University of Frankfurt/D and of the Tandem Accelerator Laboratory of the Nuclear Physics Institute of the NCSR Demokritos, Athens/GR, during the implantations and the RBS-NRA measurements, is thankfully acknowledged. References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] Fig. 7. Cross sections: a) non-implanted and b) Y-implanted steel (55 keV, 2×1017 ions/cm2) after the corrosion tests.
1 × 1016 Y-ions/cm2 of 40 keV energy (under 10− 6 mbar pressure) did not improve the corrosion resistance of Y-implanted steel. On the other hand, the implantation under 10− 5 mbar pressure in the presence of oxygen leads to an increase of the range and concentration of implanted Y-ions and significantly improves the corrosion resistance due to reduction of sputtering effects. The improvement, which increases by increasing implantation dose and energy (from 40 to 80 keV), can be attributed to the enhanced thickness of the modified surface region as derived from the RBS-NRA data. The electrochemical corrosion tests indicated that the values of the characteristic potentials moved to more positive regions and characteristic currents are diminishing by
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
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