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The effect of excimer laser surface treatment on pitting corrosion resistance of 316LS stainless steel
Surface and Coatings Technology 137 Ž2001. 65᎐71
The effect of excimer laser surface treatment on pitting corrosion resistance of 316LS stainless steel T.M. YueU , J.K. Yu, H.C. Man Department of Manufacturing Engineering, The Hong Kong Polytechnic Uni¨ ersity, Hung Hom, Kowloon, Hong Kong, PR China Received 9 May 2000; accepted in revised form 3 October 2000
Abstract Excimer laser surface treatment of 316LS bio-grade stainless steel ŽASTM-F138. was conducted with the aim of enhancing the pitting corrosion resistance of the material. The experiment was performed under two different gas environments: air and nitrogen. The microstructure, phase and compositional changes after laser treatment were characterized by means of XRD, XPS and EDX; the resulting pitting corrosion resistance was evaluated by electrochemical polarization tests. The results show that excimer laser surface melting can effectively eliminate carbides and second phases alike, and also serves the function of homogenizing the microstructure. Nitrogen induced into the laser-treated surface could promote new precipitates and as a result lowered the corrosion resistance. On the other hand, laser treatment in a low partial pressure of nitrogen could enhance the corrosion resistance of 316LS stainless steel in that the active corrosion current was reduced and the passive range was widened. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser surface treatment; Corrosion resistance; Stainless steel; Gas alloying
1. Introduction It is well known that the 300 series austenitic stainless steels are prone to pitting in the presence of halide ions, particularly in chloride ions. This to a certain extent has limited their use in a wide range of engineering applications. Although in many cases w1x highly alloying of stainless steels with chromium, molybdenum and nickel could improve their corrosion resistance, nevertheless these alloying elements are scare and expensive. In fact, corrosion depends strongly on the microstructure and composition of the material at the near-surface region. In this regard, surface modification by means of lasers, electron beams and ion im-
U
Corresponding author. Tel.: q852-23625267. E-mail address: [email protected] ŽT.M. Yue..
plantation w2,3x has become a promising alternative for corrosion resistance improvement of stainless steels. In the last two decades, laser treatment of metals and alloys has emerged as a novel surface modification technique w4,5x. It has been proved that laser surface melting ŽLSM. or alloying ŽLSA. is a valuable method to improve the wear and the corrosion resistance of many metals and alloys. In LSA, the desired alloying elements can be introduced into the molten pool in various forms, and in both solid and gaseous forms. Previous works have shown that LSM and LSA could be employed to improve the corrosion resistance properties of 304 type stainless steels, that was accompanied by a remarkable change in microstructure w6,7x. Anthony and Cline w8x found that LSM could decrease the susceptibility of 304 stainless steel to intergranular attack. However, the work by Parvathavarthini w7x on CO 2 laser surface treatment of stainless steel has shown
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 1 0 4 - X
T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71
66
that the Epp Žcritical pitting potential. of the as lasertreated 304 stainless steel which processed in nitrogen and argon gas was always lower than the untreated material. The reason for this was attributed to the rough and inhomogeneous surface of the as-melted surface. In most of the previous studies, the lasers used were mainly the CO 2 types w7,9x. In fact, among the many surface protection techniques, excimer laser surface treatment is a relatively new method that has been used recently to improve the corrosion resistance of both metallic and composite materials w10᎐12x. The radiation output in the ultra-violet ŽUV. range has given rise to a high material coupling efficiency, this together with the extremely short pulse duration and shallow absorption depth have enabled excimer lasers to make some remarkable surface modifications. Using electrochemical noise measurement technique, Bransden w13x has shown that the pitting corrosion of 304L stainless steel was improved after the material was treated by XeCl UV laser. However, the work did not analyze the phase or micro-structural changes as a result of the UV laser treatment. In the present study, emphasis was placed on the study of the effects of UV laser induced microstructural changes on the pitting corrosion behaviour of 316LS stainless steel.
The surface morphology, chemical composition and phases of the untreated and the laser-treated specimens were analyzed by SEM, EDX, XRD and XPS. For the XRD measurement, the glancing angle was fixed at 0.75⬚, and the intensities of all diffraction peaks were normalized for comparison. The XPS spectra were recorded using a 23.5 eV pass energy, the specimens were analyzed at a take-off angle of 30⬚. The XPS data were fitted using the curve fitting program XPSPEAKS. A Shirley background subtraction and Gaussian peak shape were chosen generally. Corrosion tests were carried out using the 273A potentiostat with M352 software. The standard potentiodynamic polarization test was performed at a sweep rate of 0.5 mV sy1 in neutral 3.5% NaCl solution, all experiments were conducted at 20 " 1⬚C. A saturated calomel electrode ŽSCE. were employed as the reference electrode, potentials was measured via a Luggin᎐ Haber capillary. Two parallel cylindrical graphite rods served as the counter electrode for current measurement. The solution was prepared from analytically pure chemicals and deionized water. The specimens were exposed to the test conditions for 2 h for reaching a steady state open circuit potential ŽOCP. before commencing the test.
2. Experimental methods
3. Results
The as-received austenitic stainless steel 316LS ŽASTM-F138. was in the form of round bars with a diameter of 15 mm. The alloy is of implant quality, and the chemical composition of the alloy is given in Table 1. The specimens for laser treatment were wire-cut into 2-mm-thick discs. All the samples were ground with 600, 800 and 1200-grit emery papers then polished down to 1 m diamond paste. Prior to laser treatment, the specimens were ultrasonically cleaned with alcohol. Laser surface melting and alloying were preformed using a KrF excimer laser ŽLPX 315i., which was operated at an irradiation wavelength of 248 nm. The focussed laser beam size was approximately 1.0 mm in diameter, the pulse duration and frequency was fixed at 25 ns and 5 Hz, respectively. An energy meter was used to measure the energy at the specimen surface, the laser energy intensity used was 5 Jrcm2 . The laser scanning velocity was kept at 1 mmrs, and a 50% track overlap condition was used. Laser surface treatment was performed under an air and nitrogen environment, respectively, at a gas flow rate of 35 lrmin.
3.1. Electrochemical data Potentiodynamic polarization test was conducted for the as-received and the laser-treated specimens. Fig. 1 shows some typical potentiodynamic polarization curves obtained for these materials. It was found that active dissolution, i.e. dissolution in the passive region, and pitting corrosion were totally suppressed for the airtreated specimens. When compared to the untreated specimen a decrease of approximately five times in corrosion current density and passive current density was obtained for the laser-treated specimens, also the passivation region was widened. The corrosion potential measured for the untreated and the laser-treated specimen is as follows: 0.0 mV ŽSCE. for the untreated; y57.4 mV ŽSCE. for treated in air; y138 mV ŽSCE. for treated in N2 . The Epp for the untreated and the air-treated specimen was 509 mV ŽSCE. and 660 mV, respectively. These figures indicate that as far as the Epp is concerned the stainless steel has become more
Table 1 Chemical composition of stainless steel 316LS ŽASTM-F138. Žwt.%. Cr
Ni
Mo
Mn
C
P
S
Si
17.42
14.71
2.81
1.75
0.022
0.014
- 0.01
0.45
T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71
67
noble after excimer laser treatment and pitting corrosion resistance was significantly improved. However, the results also show that the specimen which was treated in nitrogen gas had a higher active current with no distinct passive region observed in dynamic polarization. This indicates that after the stainless steel was laser treated in nitrogen, a marked reduction in corrosion resistance was the result. 3.2. Surface characterization Fig. 2 shows the surface morphology of the untreated and the laser-treated specimens. The surface of the laser-treated specimens was found to be relatively flat and was free from any surface cracking. Inclusions are virtually absent from the surface of the laser-treated specimens, in contrast, some inclusions are clearly observed in the untreated specimen. EDX results show that in the untreated specimen, precipitates of carbide and some other second phases are present: , phase, M 23 C 6 , M 6 C. Although there were still some carbides and second phases found in the laser-treated specimens, the amount was much reduced. The surface of the untreated and laser-treated specimens was characterized by XPS. The results ŽFig. 3. show that when treated in air a single peak of N1s appears at 398.5 eV, this represents dissolved nitrogen in austenite, i.e. in solid solution. For the nitrogen-treated specimen, two peaks exist, one at 398.5 eV and the second one at 396.8 eV, the former represents dissolved nitrogen in the bulk material, whilst the latter represents nitride formation. After polarization, there were four peaks ŽFig. 3c. detected for all the laser-treated specimens. Apart from the two peaks, which represent nitride and solid solution nitrogen, the other two peaks at 399.1 eV and 400.3 eV represent nitrogen bound in NH 3 and NHq 4 , respectively.
Fig. 2. Surface morphology of the Ža. untreated specimen, Žb. airtreated specimen, Žc. N2 -treated specimen.
4. Discussion
Fig. 1. Polarization curves for the untreated and the laser-treated specimens.
A great deal of work w9,14,15x has been performed to elucidate the effects of microstructure and composition on the electrochemical properties of austenitic stainless steels. It is widely accepted that stainless steels would undergo pitting attack whenever damage is caused to the adherent, self-healing and tenacious passive oxide film present on their surface. The damage is mostly due to halide ions, particularly the chloride ions present in the environment. The sites that are prone to such
68
T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71
Fig. 3. XPS analysis showing the nitrogen 1s spectrum of the lasertreated specimens Ža. in air, Žb. N2 , and the N2 -treated specimen after corrosion Žc..
localized damage are some heterogeneities, such as inclusions, grain boundaries and second phase precipitates. The benefit of laser surface treatment on improving corrosion resistance arises primarily from the laser induced microstructrual changes. This could take the form of re-distribution of the major alloy elements, precipitates and second phases as well as changing the characteristics of the surface crystals. In excimer laser surface treatment, since the cooling rate is in the order of 10 9 ⬚Crs, the melt depth is normally very shallow, approximately 3᎐5 m. Under such a condition, epitaxial solidification could occur and introduce crystallographic texture changes to the grains at the surface w16x. The re-solidification process would affect the characteristic of dendrite growth. It would promote crystal growth in the direction of the fast growing crystal planes. In our XRD results ŽFig. 4., a single ␥-phase structure was obtained before and after laser modification, but in comparison with the normalized XRD peak intensities, the relative intensity of ␥Ž111. was increased whilst the intensity for other directions were decreased after laser treatment. Though the effect of this rectification of crystal growth on corrosion resistance is thought to be of second order, it may bring about some beneficial effects on compositional homogenization. This is due to the extremely high cooling rates experienced in excimer surface treatment and therefore limits the time for lateral diffusion, and as a result micro-segregation is reduced. The fact that chromium and molybdenum in 316L are likely to segregate to interdentritic regions in normal solidification mode would mean intermetallics, such as and phases are likely to form. This would lead to depletion of chromium and molybdenum in the surrounding matrix, and as a result cause potential preferential corrosion attack at chromium depleted regions. The reduction of second phases in the surface of the laser-treated stainless steel suggested that segregation was reduced. This should improve the electrochemical behavior of the stainless steel. In the work of Ferreira w17,18x where a CO 2 laser was used to clad superaustenitic stainless steel on mild steel, it was found that the laser cladded alloy has a very similar anodic polarisation performances to that of the commercial steel. Both the laser cladded stainless steel and its commercial counterpart have nearly the same corrosion properties: excellent pitting corrosion resistance in sodium chloride, ferric chloride and hydrochloric acid solutions. That is to say, CO 2 surface treatment did not bring any noticeable improvement on the corrosion resistance of the austenitic stainless steel. This could be attributed to the microsegregation of Cr and Mo at the dendrite boundaries of the surface cladded alloy. The results of Ferreira w17,18x actually show that the regions that were depleted in Cr and Mo were preferentially dis-
T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71
solved when tested in a NaCl solution. Moreover, when the laser-treated sample was immersed in acidified FeCl 3 , a large number of small corrosion pits were found in these regions. Czachor w19x has shown that carbide phases were potential sites for formation of corrosion pits, and this is expected due to depletion of chromium. In the present study, no noticeable segregation of Cr and Mo at dendrite boundaries was detected by EDX. So, it appears that the extremely high cooling rates of excimer surface treatment, some three orders of magnitude higher than that in CO 2 treatment, has successfully reduced segregation to a great extent. Apart from the fast cooling effect, excimer laser is also considered to be very effective in reducing the size and the number of carbides in the laser-treated layer. This is considered to be as a result of ablation by direct irradiation of excimer UV laser. In this regard, the photon energy of excimer laser radiation is some 30 times higher than that of CO 2 lasers. Hibi w20x has reported that excimer laser irradiation of SiC samples could result in photo-chemical dissociation of SiC. This is due to the short wavelength of the excimer laser, and since the bonding energy of Cr᎐C is similar to that of Si᎐C, it is likely that the same ablation effect also happens to Cr-carbides. Fig. 2 and Fig. 5 show the surface morphology of the untreated and the lasertreated specimens prior to and after corrosion test, respectively. As a whole, much less inclusions and smaller inclusions were found in the laser-treated specimens ŽFig. 2., and after the corrosion test less pitting sites were found in the air-treated specimen ŽFig. 5.. The results suggested that excimer laser treatment in air could suppress active corrosion and promote passivation, and this has been reflected in the results of the polarization test ŽFig. 1.. The influence of gas element on the electrochemical behaviour of stainless steel has received much attention previously w14,15,21x. Under normal circumstances, laser gas-alloying can take two forms: Ži. nitrogen andror oxygen dissolve into the melted zone; and Žii. the alloying elements react with nitrogen andror oxygen to form nitrides and oxides. The amount of dis-
69
solved gas has been found to be proportional to the root of the partial gas pressure of the shielding atmosphere. Oxygen has a higher chemical affinity than nitrogen and therefore alloying elements such as chromium will trend to be oxidized during laser processing. However, in the presence of a nitrogen atmosphere, chromium nitride is expected to form. Laser surface melting would cause a sufficient quantity of nitrogen to be dissolved in the molten zone, but the solubility of nitrogen would decrease sharply during re-solidification. As a result, chromium nitride would precipitate out of solution at room temperature, and the chromium depleted regions would be vulnerable to corrosion attack. A typical polarization curve ŽFig. 1. obtained for the nitrogen-treated specimen shows that, no passivation range was found for this sample. The XPS results indicated that the surface of this specimen consists of a nitrogen-contained matrix with nitride precipitates. Vanini w22x also suggested that dispersed particles of chromium nitride in the passive film were responsible for the detrimental effect of nitrogen on passivation. Whilst for the air-treated specimens no nitride phase was detected but the nitrogen exists in solid solution form. This is believed to be attributed to the lower partial pressure of nitrogen gas. In fact, there has been extensive research on the effects of nitrogen on the pitting behaviour of stainless steel. There is a general consensus on the development of corrosion resistance in nitrogen contained steels w21,23᎐25x. Using conventional nitriding and de-nitriding methods, respectively, Mudali w25x and Pettersson w15x found that when the nitrogen level in the steel was within the solid solubility limit, i.e. without nitride precipitates, the pitting corrosion resistance of the steel was significantly improved. Osozawa and Okato w24x proposed the idea that the formation of ammonium ions NHq 4 consumes protons and thereby increases the pH in incipient pits and promotes metallic repassivation. The electrochemical nitrogen dissolution reaction may be represented as: w Nx q 4Hq 3e ª NHq 4
Fig. 4. The results of XRD analysis of the untreated specimen Ža., and the air-treated specimen Žb..
Ž1.
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T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71
tance of stainless steel and cause the pitting corrosion potential to be more noble. Since, chromium nitride favours the formation of ammonium and this should help to increase the pH of the active sites. 2CrNq 3H 2 O ª Cr2 O 3 q 2NH 3
Ž2.
In the present study, the XPS results of the lasertreated specimens after corrosion show the presence of nitride, NHq 4 and NH 3 , this is in line with the aforementioned theory. Although excimer laser surface treatment and the conventional nitriding method can both be used to improve the pitting corrosion resistance of stainless steel, the latter method has the disadvantage of altering the properties of the bulk material.
5. Conclusions Excimer laser surface treatment of 316LS stainless steel at an energy intensity of 5 Jrcm2 produced a crack-free modified surface. Excimer laser surface treatment in air showed a remarkable improvement in pitting corrosion resistance for 316LS stainless steel. The main reasons for that is the development of a more homogeneous surface composition and the amount of undesirable precipitates and second phases were reduced. The incorporation of nitrogen in the form of solid solution in the matrix also contributed to the improvement. The pitting corrosion resistance of 316LS stainless steel after excimer laser treatment in nitrogen was found to be inferior to the untreated material with no passivation range found. The reason for this is believed to be related to the formation of chromium nitride precipitates.
Acknowledgements
Fig. 5. Surface morphology of the corroded specimens Ža. untreated, Žb. air-treated, Žc. nitrogen-treated.
This mechanism is widely recognized to reconcile with the corrosion current reduction of stainless steel between the active and passive potentials. Newman and Shahrabi w21x pointed out that because nitrogen dissolution as presented in Eq. Ž1. is cathodic, nitrogen will accumulate at the surface active sites of a dissolving surface, such as kinks and steps, particularly at higher potentials, and form a stable nitride. This reaction would block dissolution sites and stifle active dissolution, so nitrogen would enhance the corrosion resis-
The work described in this paper was substantially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region ŽProject No. PolyU 5132r98E.. The authors are grateful to PolyU for infra-structural support. References w1x R. Bany, D. Van Rooyen, Corrosion 39 Ž1983. 227. w2x R.D. Granata, P.G. Moore, Vol. 13, Metals Handbook, 9th ed., ASM International Press, 1978, p. 498᎐505. w3x E. McCafferty, P.G. Moore, J.D. Ayers, G.K. Hubler, in: C.R. Clayton, C.M. Preece ŽEds.., Corrosion of Metals Processed by Directed Energy Beams, The Metallurgical Society, 1982, pp. 1᎐21. w4x C.W. Drapter, J. Metals 35 Ž1982. 24.
T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71 w5x E. McCafferty, G.Y. Hubler, P.M. Natishan, P.G. Moore, R.A. Kant, B.D. Sartwell, Mater. Sci. Eng. 86 Ž1987. 1. w6x E. McCafferty, P.G. Moore, J. Electrochem. Soc. 133 Ž1986. 1090. w7x N. Parvathavarthini, R.K. Dayal, R. Sivakumar, U. Kamachi Mudali, A. Bharati, Mater. Sci. Tech. 8 Ž1992. 1070. w8x T.R. Anthony, H.E. Cline, J. Appl. Phys. 49 Ž1978. 1248. ¨ w9x O.V Akgum, A.F. Cakir, Mater. Sci. Eng. A203 ¨ M. Urgen, Ž1995. 324. w10x A. Weisheit, R. Galun, M. Rehpohler, Lasers Eng. 6 Ž1997. ¨ 127. w11x T.M. Yue, A.H. Wang, H.C. Man, Scr. Mater. 38 Ž1998. 191. w12x T.M. Yue, Y.X. Wu, H.C. Man, Surf. Coating Technol. 114 Ž1999. 13. w13x A.S. Bransden, J.H.P.C. Megaw, P.H. Balkwill, C. Westcott, J. Modern Optics 37 Ž1990. 813. w14x Y.C. Lu, M.B. Ives, Corros. Sci. 35 Ž1993. 89. w15x R.F.A. Jargelius-Pettersson, Corros. Sci. 41 Ž1999. 1639. w16x L.S. Weinman, J.N. DeVault, P.G. Moore, in: E. A. Metzbower ŽEd.., Applications of Lasers in Materials Processing, American Society for Metals, 1979, p. 259.
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w17x R. Li, M.G.S. Ferreira, M.A. Anjos, R. Vilar, Surf. Coat. Technol. 88 Ž1996. 90. w18x R. Li, M.G.S. Ferreira, M.A. Anjos, R. Vilar, Surf. Coat. Technol. 88 Ž1996. 96. w19x M. Janik-Czachor, E. Lunarska, Z. Szlarska, Corrosion 31 Ž1. Ž1975. 394. w20x Y. Hibi, Y. Enomoto, K. Kikuchi, N. Shikata, Appl. Phys. Lett. 66 Ž1995. 817. w21x R.C. Newman, T. Shahrabi, Corros. Sci. 27 Ž1987. 827. w22x A. Sadough Vanini, J.P. Audouard, P. Marcus, Corros. Sci. 36 Ž1994. 1825. w23x R.C. Newman, Y.C. Lu, R. Randy, C.R. Clayton, in: Proc. 9th Int. Conf. Metallic Corrosion, Toronto, vol. 3, 1984, pp. 394᎐399. w24x K. Osozawa, N Okato, in: Proc. USA-Japan Seminar: Passivity and its Breakdown on Iron and Iron-based Alloys, Honolulu, NACE, 1976, pp. 135᎐139. w25x U. Kamachi Mudali, B. Reynders, M. Stratmann, Corros. Sci. 41 Ž1999. 179.
The effect of excimer laser surface treatment on pitting corrosion resistance of 316LS stainless steel T.M. YueU , J.K. Yu, H.C. Man Department of Manufacturing Engineering, The Hong Kong Polytechnic Uni¨ ersity, Hung Hom, Kowloon, Hong Kong, PR China Received 9 May 2000; accepted in revised form 3 October 2000
Abstract Excimer laser surface treatment of 316LS bio-grade stainless steel ŽASTM-F138. was conducted with the aim of enhancing the pitting corrosion resistance of the material. The experiment was performed under two different gas environments: air and nitrogen. The microstructure, phase and compositional changes after laser treatment were characterized by means of XRD, XPS and EDX; the resulting pitting corrosion resistance was evaluated by electrochemical polarization tests. The results show that excimer laser surface melting can effectively eliminate carbides and second phases alike, and also serves the function of homogenizing the microstructure. Nitrogen induced into the laser-treated surface could promote new precipitates and as a result lowered the corrosion resistance. On the other hand, laser treatment in a low partial pressure of nitrogen could enhance the corrosion resistance of 316LS stainless steel in that the active corrosion current was reduced and the passive range was widened. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser surface treatment; Corrosion resistance; Stainless steel; Gas alloying
1. Introduction It is well known that the 300 series austenitic stainless steels are prone to pitting in the presence of halide ions, particularly in chloride ions. This to a certain extent has limited their use in a wide range of engineering applications. Although in many cases w1x highly alloying of stainless steels with chromium, molybdenum and nickel could improve their corrosion resistance, nevertheless these alloying elements are scare and expensive. In fact, corrosion depends strongly on the microstructure and composition of the material at the near-surface region. In this regard, surface modification by means of lasers, electron beams and ion im-
U
Corresponding author. Tel.: q852-23625267. E-mail address: [email protected] ŽT.M. Yue..
plantation w2,3x has become a promising alternative for corrosion resistance improvement of stainless steels. In the last two decades, laser treatment of metals and alloys has emerged as a novel surface modification technique w4,5x. It has been proved that laser surface melting ŽLSM. or alloying ŽLSA. is a valuable method to improve the wear and the corrosion resistance of many metals and alloys. In LSA, the desired alloying elements can be introduced into the molten pool in various forms, and in both solid and gaseous forms. Previous works have shown that LSM and LSA could be employed to improve the corrosion resistance properties of 304 type stainless steels, that was accompanied by a remarkable change in microstructure w6,7x. Anthony and Cline w8x found that LSM could decrease the susceptibility of 304 stainless steel to intergranular attack. However, the work by Parvathavarthini w7x on CO 2 laser surface treatment of stainless steel has shown
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 1 0 4 - X
T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71
66
that the Epp Žcritical pitting potential. of the as lasertreated 304 stainless steel which processed in nitrogen and argon gas was always lower than the untreated material. The reason for this was attributed to the rough and inhomogeneous surface of the as-melted surface. In most of the previous studies, the lasers used were mainly the CO 2 types w7,9x. In fact, among the many surface protection techniques, excimer laser surface treatment is a relatively new method that has been used recently to improve the corrosion resistance of both metallic and composite materials w10᎐12x. The radiation output in the ultra-violet ŽUV. range has given rise to a high material coupling efficiency, this together with the extremely short pulse duration and shallow absorption depth have enabled excimer lasers to make some remarkable surface modifications. Using electrochemical noise measurement technique, Bransden w13x has shown that the pitting corrosion of 304L stainless steel was improved after the material was treated by XeCl UV laser. However, the work did not analyze the phase or micro-structural changes as a result of the UV laser treatment. In the present study, emphasis was placed on the study of the effects of UV laser induced microstructural changes on the pitting corrosion behaviour of 316LS stainless steel.
The surface morphology, chemical composition and phases of the untreated and the laser-treated specimens were analyzed by SEM, EDX, XRD and XPS. For the XRD measurement, the glancing angle was fixed at 0.75⬚, and the intensities of all diffraction peaks were normalized for comparison. The XPS spectra were recorded using a 23.5 eV pass energy, the specimens were analyzed at a take-off angle of 30⬚. The XPS data were fitted using the curve fitting program XPSPEAKS. A Shirley background subtraction and Gaussian peak shape were chosen generally. Corrosion tests were carried out using the 273A potentiostat with M352 software. The standard potentiodynamic polarization test was performed at a sweep rate of 0.5 mV sy1 in neutral 3.5% NaCl solution, all experiments were conducted at 20 " 1⬚C. A saturated calomel electrode ŽSCE. were employed as the reference electrode, potentials was measured via a Luggin᎐ Haber capillary. Two parallel cylindrical graphite rods served as the counter electrode for current measurement. The solution was prepared from analytically pure chemicals and deionized water. The specimens were exposed to the test conditions for 2 h for reaching a steady state open circuit potential ŽOCP. before commencing the test.
2. Experimental methods
3. Results
The as-received austenitic stainless steel 316LS ŽASTM-F138. was in the form of round bars with a diameter of 15 mm. The alloy is of implant quality, and the chemical composition of the alloy is given in Table 1. The specimens for laser treatment were wire-cut into 2-mm-thick discs. All the samples were ground with 600, 800 and 1200-grit emery papers then polished down to 1 m diamond paste. Prior to laser treatment, the specimens were ultrasonically cleaned with alcohol. Laser surface melting and alloying were preformed using a KrF excimer laser ŽLPX 315i., which was operated at an irradiation wavelength of 248 nm. The focussed laser beam size was approximately 1.0 mm in diameter, the pulse duration and frequency was fixed at 25 ns and 5 Hz, respectively. An energy meter was used to measure the energy at the specimen surface, the laser energy intensity used was 5 Jrcm2 . The laser scanning velocity was kept at 1 mmrs, and a 50% track overlap condition was used. Laser surface treatment was performed under an air and nitrogen environment, respectively, at a gas flow rate of 35 lrmin.
3.1. Electrochemical data Potentiodynamic polarization test was conducted for the as-received and the laser-treated specimens. Fig. 1 shows some typical potentiodynamic polarization curves obtained for these materials. It was found that active dissolution, i.e. dissolution in the passive region, and pitting corrosion were totally suppressed for the airtreated specimens. When compared to the untreated specimen a decrease of approximately five times in corrosion current density and passive current density was obtained for the laser-treated specimens, also the passivation region was widened. The corrosion potential measured for the untreated and the laser-treated specimen is as follows: 0.0 mV ŽSCE. for the untreated; y57.4 mV ŽSCE. for treated in air; y138 mV ŽSCE. for treated in N2 . The Epp for the untreated and the air-treated specimen was 509 mV ŽSCE. and 660 mV, respectively. These figures indicate that as far as the Epp is concerned the stainless steel has become more
Table 1 Chemical composition of stainless steel 316LS ŽASTM-F138. Žwt.%. Cr
Ni
Mo
Mn
C
P
S
Si
17.42
14.71
2.81
1.75
0.022
0.014
- 0.01
0.45
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67
noble after excimer laser treatment and pitting corrosion resistance was significantly improved. However, the results also show that the specimen which was treated in nitrogen gas had a higher active current with no distinct passive region observed in dynamic polarization. This indicates that after the stainless steel was laser treated in nitrogen, a marked reduction in corrosion resistance was the result. 3.2. Surface characterization Fig. 2 shows the surface morphology of the untreated and the laser-treated specimens. The surface of the laser-treated specimens was found to be relatively flat and was free from any surface cracking. Inclusions are virtually absent from the surface of the laser-treated specimens, in contrast, some inclusions are clearly observed in the untreated specimen. EDX results show that in the untreated specimen, precipitates of carbide and some other second phases are present: , phase, M 23 C 6 , M 6 C. Although there were still some carbides and second phases found in the laser-treated specimens, the amount was much reduced. The surface of the untreated and laser-treated specimens was characterized by XPS. The results ŽFig. 3. show that when treated in air a single peak of N1s appears at 398.5 eV, this represents dissolved nitrogen in austenite, i.e. in solid solution. For the nitrogen-treated specimen, two peaks exist, one at 398.5 eV and the second one at 396.8 eV, the former represents dissolved nitrogen in the bulk material, whilst the latter represents nitride formation. After polarization, there were four peaks ŽFig. 3c. detected for all the laser-treated specimens. Apart from the two peaks, which represent nitride and solid solution nitrogen, the other two peaks at 399.1 eV and 400.3 eV represent nitrogen bound in NH 3 and NHq 4 , respectively.
Fig. 2. Surface morphology of the Ža. untreated specimen, Žb. airtreated specimen, Žc. N2 -treated specimen.
4. Discussion
Fig. 1. Polarization curves for the untreated and the laser-treated specimens.
A great deal of work w9,14,15x has been performed to elucidate the effects of microstructure and composition on the electrochemical properties of austenitic stainless steels. It is widely accepted that stainless steels would undergo pitting attack whenever damage is caused to the adherent, self-healing and tenacious passive oxide film present on their surface. The damage is mostly due to halide ions, particularly the chloride ions present in the environment. The sites that are prone to such
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T.M. Yue et al. r Surface and Coatings Technology 137 (2001) 65᎐71
Fig. 3. XPS analysis showing the nitrogen 1s spectrum of the lasertreated specimens Ža. in air, Žb. N2 , and the N2 -treated specimen after corrosion Žc..
localized damage are some heterogeneities, such as inclusions, grain boundaries and second phase precipitates. The benefit of laser surface treatment on improving corrosion resistance arises primarily from the laser induced microstructrual changes. This could take the form of re-distribution of the major alloy elements, precipitates and second phases as well as changing the characteristics of the surface crystals. In excimer laser surface treatment, since the cooling rate is in the order of 10 9 ⬚Crs, the melt depth is normally very shallow, approximately 3᎐5 m. Under such a condition, epitaxial solidification could occur and introduce crystallographic texture changes to the grains at the surface w16x. The re-solidification process would affect the characteristic of dendrite growth. It would promote crystal growth in the direction of the fast growing crystal planes. In our XRD results ŽFig. 4., a single ␥-phase structure was obtained before and after laser modification, but in comparison with the normalized XRD peak intensities, the relative intensity of ␥Ž111. was increased whilst the intensity for other directions were decreased after laser treatment. Though the effect of this rectification of crystal growth on corrosion resistance is thought to be of second order, it may bring about some beneficial effects on compositional homogenization. This is due to the extremely high cooling rates experienced in excimer surface treatment and therefore limits the time for lateral diffusion, and as a result micro-segregation is reduced. The fact that chromium and molybdenum in 316L are likely to segregate to interdentritic regions in normal solidification mode would mean intermetallics, such as and phases are likely to form. This would lead to depletion of chromium and molybdenum in the surrounding matrix, and as a result cause potential preferential corrosion attack at chromium depleted regions. The reduction of second phases in the surface of the laser-treated stainless steel suggested that segregation was reduced. This should improve the electrochemical behavior of the stainless steel. In the work of Ferreira w17,18x where a CO 2 laser was used to clad superaustenitic stainless steel on mild steel, it was found that the laser cladded alloy has a very similar anodic polarisation performances to that of the commercial steel. Both the laser cladded stainless steel and its commercial counterpart have nearly the same corrosion properties: excellent pitting corrosion resistance in sodium chloride, ferric chloride and hydrochloric acid solutions. That is to say, CO 2 surface treatment did not bring any noticeable improvement on the corrosion resistance of the austenitic stainless steel. This could be attributed to the microsegregation of Cr and Mo at the dendrite boundaries of the surface cladded alloy. The results of Ferreira w17,18x actually show that the regions that were depleted in Cr and Mo were preferentially dis-
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solved when tested in a NaCl solution. Moreover, when the laser-treated sample was immersed in acidified FeCl 3 , a large number of small corrosion pits were found in these regions. Czachor w19x has shown that carbide phases were potential sites for formation of corrosion pits, and this is expected due to depletion of chromium. In the present study, no noticeable segregation of Cr and Mo at dendrite boundaries was detected by EDX. So, it appears that the extremely high cooling rates of excimer surface treatment, some three orders of magnitude higher than that in CO 2 treatment, has successfully reduced segregation to a great extent. Apart from the fast cooling effect, excimer laser is also considered to be very effective in reducing the size and the number of carbides in the laser-treated layer. This is considered to be as a result of ablation by direct irradiation of excimer UV laser. In this regard, the photon energy of excimer laser radiation is some 30 times higher than that of CO 2 lasers. Hibi w20x has reported that excimer laser irradiation of SiC samples could result in photo-chemical dissociation of SiC. This is due to the short wavelength of the excimer laser, and since the bonding energy of Cr᎐C is similar to that of Si᎐C, it is likely that the same ablation effect also happens to Cr-carbides. Fig. 2 and Fig. 5 show the surface morphology of the untreated and the lasertreated specimens prior to and after corrosion test, respectively. As a whole, much less inclusions and smaller inclusions were found in the laser-treated specimens ŽFig. 2., and after the corrosion test less pitting sites were found in the air-treated specimen ŽFig. 5.. The results suggested that excimer laser treatment in air could suppress active corrosion and promote passivation, and this has been reflected in the results of the polarization test ŽFig. 1.. The influence of gas element on the electrochemical behaviour of stainless steel has received much attention previously w14,15,21x. Under normal circumstances, laser gas-alloying can take two forms: Ži. nitrogen andror oxygen dissolve into the melted zone; and Žii. the alloying elements react with nitrogen andror oxygen to form nitrides and oxides. The amount of dis-
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solved gas has been found to be proportional to the root of the partial gas pressure of the shielding atmosphere. Oxygen has a higher chemical affinity than nitrogen and therefore alloying elements such as chromium will trend to be oxidized during laser processing. However, in the presence of a nitrogen atmosphere, chromium nitride is expected to form. Laser surface melting would cause a sufficient quantity of nitrogen to be dissolved in the molten zone, but the solubility of nitrogen would decrease sharply during re-solidification. As a result, chromium nitride would precipitate out of solution at room temperature, and the chromium depleted regions would be vulnerable to corrosion attack. A typical polarization curve ŽFig. 1. obtained for the nitrogen-treated specimen shows that, no passivation range was found for this sample. The XPS results indicated that the surface of this specimen consists of a nitrogen-contained matrix with nitride precipitates. Vanini w22x also suggested that dispersed particles of chromium nitride in the passive film were responsible for the detrimental effect of nitrogen on passivation. Whilst for the air-treated specimens no nitride phase was detected but the nitrogen exists in solid solution form. This is believed to be attributed to the lower partial pressure of nitrogen gas. In fact, there has been extensive research on the effects of nitrogen on the pitting behaviour of stainless steel. There is a general consensus on the development of corrosion resistance in nitrogen contained steels w21,23᎐25x. Using conventional nitriding and de-nitriding methods, respectively, Mudali w25x and Pettersson w15x found that when the nitrogen level in the steel was within the solid solubility limit, i.e. without nitride precipitates, the pitting corrosion resistance of the steel was significantly improved. Osozawa and Okato w24x proposed the idea that the formation of ammonium ions NHq 4 consumes protons and thereby increases the pH in incipient pits and promotes metallic repassivation. The electrochemical nitrogen dissolution reaction may be represented as: w Nx q 4Hq 3e ª NHq 4
Fig. 4. The results of XRD analysis of the untreated specimen Ža., and the air-treated specimen Žb..
Ž1.
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tance of stainless steel and cause the pitting corrosion potential to be more noble. Since, chromium nitride favours the formation of ammonium and this should help to increase the pH of the active sites. 2CrNq 3H 2 O ª Cr2 O 3 q 2NH 3
Ž2.
In the present study, the XPS results of the lasertreated specimens after corrosion show the presence of nitride, NHq 4 and NH 3 , this is in line with the aforementioned theory. Although excimer laser surface treatment and the conventional nitriding method can both be used to improve the pitting corrosion resistance of stainless steel, the latter method has the disadvantage of altering the properties of the bulk material.
5. Conclusions Excimer laser surface treatment of 316LS stainless steel at an energy intensity of 5 Jrcm2 produced a crack-free modified surface. Excimer laser surface treatment in air showed a remarkable improvement in pitting corrosion resistance for 316LS stainless steel. The main reasons for that is the development of a more homogeneous surface composition and the amount of undesirable precipitates and second phases were reduced. The incorporation of nitrogen in the form of solid solution in the matrix also contributed to the improvement. The pitting corrosion resistance of 316LS stainless steel after excimer laser treatment in nitrogen was found to be inferior to the untreated material with no passivation range found. The reason for this is believed to be related to the formation of chromium nitride precipitates.
Acknowledgements
Fig. 5. Surface morphology of the corroded specimens Ža. untreated, Žb. air-treated, Žc. nitrogen-treated.
This mechanism is widely recognized to reconcile with the corrosion current reduction of stainless steel between the active and passive potentials. Newman and Shahrabi w21x pointed out that because nitrogen dissolution as presented in Eq. Ž1. is cathodic, nitrogen will accumulate at the surface active sites of a dissolving surface, such as kinks and steps, particularly at higher potentials, and form a stable nitride. This reaction would block dissolution sites and stifle active dissolution, so nitrogen would enhance the corrosion resis-
The work described in this paper was substantially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region ŽProject No. PolyU 5132r98E.. The authors are grateful to PolyU for infra-structural support. References w1x R. Bany, D. Van Rooyen, Corrosion 39 Ž1983. 227. w2x R.D. Granata, P.G. Moore, Vol. 13, Metals Handbook, 9th ed., ASM International Press, 1978, p. 498᎐505. w3x E. McCafferty, P.G. Moore, J.D. Ayers, G.K. Hubler, in: C.R. Clayton, C.M. Preece ŽEds.., Corrosion of Metals Processed by Directed Energy Beams, The Metallurgical Society, 1982, pp. 1᎐21. w4x C.W. Drapter, J. Metals 35 Ž1982. 24.
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w17x R. Li, M.G.S. Ferreira, M.A. Anjos, R. Vilar, Surf. Coat. Technol. 88 Ž1996. 90. w18x R. Li, M.G.S. Ferreira, M.A. Anjos, R. Vilar, Surf. Coat. Technol. 88 Ž1996. 96. w19x M. Janik-Czachor, E. Lunarska, Z. Szlarska, Corrosion 31 Ž1. Ž1975. 394. w20x Y. Hibi, Y. Enomoto, K. Kikuchi, N. Shikata, Appl. Phys. Lett. 66 Ž1995. 817. w21x R.C. Newman, T. Shahrabi, Corros. Sci. 27 Ž1987. 827. w22x A. Sadough Vanini, J.P. Audouard, P. Marcus, Corros. Sci. 36 Ž1994. 1825. w23x R.C. Newman, Y.C. Lu, R. Randy, C.R. Clayton, in: Proc. 9th Int. Conf. Metallic Corrosion, Toronto, vol. 3, 1984, pp. 394᎐399. w24x K. Osozawa, N Okato, in: Proc. USA-Japan Seminar: Passivity and its Breakdown on Iron and Iron-based Alloys, Honolulu, NACE, 1976, pp. 135᎐139. w25x U. Kamachi Mudali, B. Reynders, M. Stratmann, Corros. Sci. 41 Ž1999. 179.