Journal of Alloys and Compounds 460 (2008) 614 618 Pulsed nanocrystalline plasma electrolytic carburising for corrosion protection of a -TiAl alloy Part 1. Effect of frequency and duty cycle M. Aliofkhazraee, A. Sabour Rouhaghdam, T. Shahrabi Faculty of Engineering, Materials Engineering Department, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran Received 25 April 2007; received in revised form 31 May 2007; accepted 5 June 2007 Available online 8 June 2007 Abstract Tafel polarization and electrochemical impedance spectroscopy were employed in Ringer s solution to test carburised Ti 48Al 2Cr 2Nb, treated by a novel method called pulsed nanocrystalline plasma electrolytic carburising. The results show excellent corrosion resistance for modified Ti 48Al 2Cr 2Nb. The effect of frequency and duty cycle of pulsed current was investigated. It was found that frequency and duty cycle of pulsed current affect the size and porosity of nanocrystalline carbides. This surface modification with controlled effective parameters can render the Ti 48Al 2Cr 2Nb material extremely corrosion resistant as a biomaterial. 2007 Elsevier B.V. All rights reserved. Keywords: -TiAl alloy; Corrosion; Pulsed nanocrystalline plasma electrolytic carburising; Scanning electron microscopy; Ringer s solution 1. Introduction Plasma carburising is a common surface processing in which carbon is introduced into metal s lattice at elevated temperatures for improving the hardness, wear and corrosion resistance of metals. Various glow discharge processes have rapidly found increasing applications in the surface and heat treatment of tools and machine parts. Faster carbon diffusion allows lower carburising temperatures and shorter treatment times. In recent years, more and more research on developing new technologies used for carburising of metals has been performed. These technologies include plasma carburising, plasma immersion implantation, ECR ion carburising, RF plasma carburising, lowpressure plasma assisted carburising and high current-density ion beam carburising and active screen plasma carburising [1 8]. The only aim of these researches is expected to obtain carburided layer with high hardness and free carbides especially chromium carbides (for stainless steels) with relatively thick on substrate surface. So, the corrosion resistance of metals Corresponding author. Tel.: +98 9126905626; fax: +98 2166960664. E-mail address: sabour01@gmail.com (A. Sabour Rouhaghdam). could be remained or improved while the surface hardness and wear resistance were obviously increased. Pulsed nanocrystalline plasma electrolytic carburising (PNPEC) [9 11], as a novel and effective surface treatment technique developed, has been brought for surface modification of metals and some significant results have been reported. PNPEC process provides Ti 48Al 2Cr 2Nb thick, hard, high corrosion resistant ceramic-like nitride and carbide films composed of a porous nanocrystalline layer. In this particular work, the corrosion behavior of Ti 48Al 2Cr 2Nb in Ringer s solution is investigated for carburised samples at different conditions. Two electrochemical techniques were used for this purpose: Tafel polarization and electrochemical impedance spectroscopy (EIS). Standard corrosion parameters such as corrosion current density (i corr ), corrosion potential (E corr ), polarization resistance (R p ) and corrosion rate (C R ) were calculated from these experiments. The coating s parameters effects on the properties of PNPEC films on Ti 48Al 2Cr 2Nb have not been well studied; the relationship between electronic characteristics and corrosion properties of PNPEC film is also seldom concerned. In this investigation, effects of two different effective parameters (frequency and duty cycle of pulsed current) in glycerol base 0925-8388/$ see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.007
electrolyte on the structure, composition and corrosion resistance of Ti 48Al 2Cr 2Nb with PNPEC film were studied. Based on the results, corrosion resistance of the PNPEC films was analyzed. 2. Materials and methods Ti 48Al 2Cr 2Nb (Al 33.5 wt.%, Cr 2.55 wt.%, Nb 2.67 wt.%, Ti balance) (hereafter referred to as -TiAl) was received in the form of rod with a diameter of 20 mm. Disc-shaped specimens with thickness of 5 mm and a diameter of 20 mm were obtained from the center of these rods using usual machining. Prior to surface treatment, each specimen was ground with 240 3000 grit SiC paper and ultrasonically cleaned with alcohol. The corrosion measurements for each experimental condition were carried out using a potentiostat/galvanostat (EG&G 273A, Princeton Applied Research), a standard cell and electrode holders. SCE was used as the reference electrode and the counter electrode consisted of platinum plate. The surface area exposed to the electrolyte was 0.785 cm 2. The corrosion behavior of the treated -TiAl was evaluated at room temperature using Ringer s solution. The Tafel polarization curves for the samples were recorded at a scan rate of 0.1 mv/s. The polarization curves were analyzed to determine E corr (corrosion potential) and i corr (corrosion current). The corrosion rate, C R (rate of metal dissolution), in millimeters per year, was determined with the following standard equation: C R = (i corr A W ) (z F ρ) 1 M. Aliofkhazraee et al. / Journal of Alloys and Compounds 460 (2008) 614 618 615 where A W, z and ρ are characteristic properties of each sample (atomic weight, valence and density, respectively) and F is the Faraday constant (96,500 A s/mol). Equivalent weights based on standard equations were obtained to calculate the corrosion rate. Electrochemical impedance spectroscopy was also used to evaluate the samples. This method was conducted according to the ASTM G-106 standard practice. Samples were left in contact with Ringer s solution at open circuit conditions for 1 h prior to carrying out the EIS tests. The alternating current (ac) impedance spectra for -TiAl were obtained at the open circuit potential, with a scan frequency range of 100 khz to 10 mhz with amplitude of 10 mv to obtain 30 experimental points. Nyquist plots were obtained by curve fitting these data points using a commercial software package called Electrochemistry Power Suite. These plots were analyzed to determine the solution resistance (R ), polarization resistance at the electrode/solution interface (R p ) and the double layer capacitance at this interface (C DL ). Bode plots were also plotted from the same data to better analyze and understand the corrosion phenomena occurring during the impedance tests. -TiAl samples used in this study were modified by different conditions of pulsed nanocrystalline plasma electrolytic carburising. All these samples were carburised at 0.5 A/cm 2 by rectangular pulsed shape method at different frequencies and duty cycles. The temperature of electrolyte was remained constant at 30 C by circulating cold water around electrolyte bath. Electrolyte bath was made from stainless steel and act as anode in electrochemical system of coating process. Other specifications can be found in references [10,12]. The carburised layer formed as a result of the plasma electrolysis process has a thickness around 10 m for all samples regarding to its growth kinetic. The as-received samples were used as control for comparison purposes. All corrosion testing was carried out at 25 C. Four samples were tested for each experimental condition and the average of these results is reported. Outlier data was not included in these calculations. 3. Results and discussion Micrographs of different treated samples were obtained with electron backscattering using scanning electron microscope (SEM) (Philips XL-30). The modified surfaces were observed in SEM by mounting the samples in a polymeric resin. Fig. 1 is an example of three different samples with different frequencies and duty cycles and Fig. 1. Nanocrystalline carbides on the compound layer of sample: (a) 100 Hz, 40%; (b) 1000 Hz, 40%; (c) 5 khz, 15% (60,000 ). shows the difference between the sizes of nanocrystalline carbides. Fig. 2 also illustrates two different samples for comparing their porosity. It seems that by changing conditions that lead to decreasing the size of nanocrystals, the porosity of compound layer also decreases. Fig. 3 illustrates the composition of compound layer for sample treated by 2 khz of frequency and 70% of duty cycle. 3.1. Corrosion experiments Standard techniques were used to extract E corr, i corr and C R values from the Tafel polarization plots, samples of which are shown in Figs. 4 and 5. Table 1 shows the polarization data obtained for -TiAl including Tafel slopes (β a and β c ), E corr and i corr values.
616 M. Aliofkhazraee et al. / Journal of Alloys and Compounds 460 (2008) 614 618 Fig. 4. Polarization curves for samples treated by different frequencies. Fig. 2. Cross-section micrograph of sample: (a) 1000 Hz, 60%; (b) 10 khz, 20%. Fig. 5. Polarization curves for samples treated by different duty cycles. Fig. 3. XRD pattern of sample 2 khz, 70%. This table indicates low values of corrosion rate for all of the samples. The formation of carbide films on the -TiAl samples, resulting from surface treatment renders these samples passive and highly corrosion resistant in Ringer s solution. Data from EIS was obtained as Nyquist plots and analyzed to determine the solution resistance (R ), polarization resistance at the electrode/solution interface (R p ), and a double layer capacitance at this interface (C DL ). Among these parameters, R p is the factor that determines corrosion resistance of samples. This value is inversely proportional to i corr, and hence, high values of R p correspond to low corrosion rates. This technique was used to compare the results obtained from Tafel polarization tests. Figs. 6 and 8 show Nyquist plots obtained experimentally for -TiAl in Ringer s solution, respectively. The impedance spectra shown in Fig. 6 for -TiAl indicate that the best corrosion resistance was exhibited for samples carburised at high frequen- Table 1 Corrosion parameters from Tafel polarization tests Frequency, duty cycle β a (mv/decade) β c (mv/decade) E corr (mv/sce) i corr ( A/cm 2 ) C R (mm/year) 10 Hz, 20% 328.25 181.30 300 0.64 9.68 10 3 100 Hz, 20% 333.92 195.05 299 0.28 4.24 10 3 1000 Hz, 20% 333.73 306.24 125 0.16 2.44 10 3 1000 Hz, 50% 323.85 135.07 386 0.2 3 10 3 1000 Hz,10% 337.05 155.08 92 0.1 1.6 10 3 -TiAl 316.08 242.15 632 0.68 10.3 10 3
M. Aliofkhazraee et al. / Journal of Alloys and Compounds 460 (2008) 614 618 617 Fig. 6. Nyquist plots for samples treated by different frequencies. Fig. 7. Nyquist plots for samples treated by different duty cycles. cies in comparison with other samples. This behavior can be attributed to the presence of a denser compound layer formed with this treatment by higher frequencies. On the other hand, for samples carburised at lower frequencies, the corrosion resistance was lower, possibly due to the presence of more porous carbide layer, which is not expected to provide significant corrosion resistance for these samples. The Nyquist plots obtained for -TiAl at different duty cycles (Fig. 7) indicate that the best corrosion resistance is obtained for the carburised samples at lower duty cycles in comparison with the samples by higher duty cycles, attributed to the presence of a well performed and lower sizes of nanocrystalline carbides in compound layer on the -TiAl surface and corroborated by the polarization data in Table 1. The presence of continuous protective layer improves the corrosion resistance of this metal, manifested in the high values of R p obtained experimentally for the carburised samples. Table 2 shows the corrosion parameters obtained for -TiAl with EIS. This table shows the values of solution resistance (R ), polarization resistance (R p ) and capacitance of the double layer (C DL ) obtained by curve fitting the EIS data using Electrochemistry Power Suite. Consistent values of electrolyte resistance (R ) are noted for -TiAl in Ringer s solution. The higher value of R for the case of the -TiAl sample carburised at lower frequencies is possibly a manifestation of the porous carburised layer. Large values of polarization resistance (R p ) are also noted for all of the samples. A high R p value is an indication of the working electrode strongly resisting change from its equilibrium state and corresponds to a low rate of ion release. This behavior is clearly noted and is in agreement with the results found with Tafel polarization. From Table 2 it can also be seen that, in general, for all surface treatments in Ringer s solution, high values of polarization resistance (R p ) ranging from 79 to 462 M cm 2 are measured, which implies an excellent corrosion resistance for this metal after surface modification. Also values of C DL in the range of 30 40 Fcm 2 were determined. The R coat values for the two samples treated at different opposite duty cycles and frequencies (1000 Hz, 10%; 10 Hz, 20%) reveal an interesting point. First sample (1000 Hz, 10%) has a lower value of CPE coat and its R coat value formed during the EIS tests is larger than for other sample (10 Hz, 20%) treated at duty cycle of 20%. This reflects the presence of a relatively coherent and stable carbide layer formed on first sample (1000 Hz, 10%) with this treatment, while the obtained layer formed on other sample (10 Hz, 20%) is porous and possibly less stable. During impedance testing, this porous carbide layer is made denser by passivation, reflected in a larger value of CPE coat. Bode phase plot spectra from the EIS data were evaluated to observe changes in the sample surface during EIS testing. From this graph it is possible to deduce the presence of a compact film, if (a) the phase angle is close to 90 over a wide frequency range, and (b) if the spectrum shows linear portions at intermediate frequencies. Fig. 8 shows representative Bode plots for two samples treated at a very opposite frequencies and duty cycles (10 khz, 10%; 10 Hz, 60%) in Ringer s solution as examples. The impedance spectra found for the sample (10 khz, 10%) exhibited a near capacitive response illustrated by a phase angle close to 85, while the sample (10 Hz, 60%) showed a phase angle close to 80 over a wide frequency range. This response and the high values of R p obtained from the Nyquist plot corroborates the presence of a barrier layer formed on this metal Table 2 ac impedance parameters for -TiAl samples Frequency, duty cycle R ( cm 2 ) R p (M cm 2 ) C DL ( Fcm 2 ) R coat (k cm 2 ) CPE coat ( Fcm 2 ) 10 Hz, 20% 8.24 79.24 31.1 742.266 3.2 100 Hz, 20% 7.33 190.94 34.9 1317.51 6.4 1000 Hz, 20% 7.28 433.4 31.4 1936.35 3.6 1000 Hz, 50% 7.39 206.94 30 1190.16 3.5 1000 Hz, 10% 5.5 461.18 39.3 2236.41 1.3 -TiAl 9.82 87.55 17.6
618 M. Aliofkhazraee et al. / Journal of Alloys and Compounds 460 (2008) 614 618 after the surface treatment especially for sample (10 khz, 10%). Sample (10 khz, 10%) has the highest frequency and lowest duty cycle in this study which tends to receive the lowest size of nanocrystalline carbides on its surface. It seems that compact layer with nanocrystalline carbides with smaller dimensions, shows a better corrosion behavior. This fact is obviously clear for all samples treated in this study. So by increasing frequency and decreasing duty cycle of the pulsed current, the corrosion behavior of samples will improve significantly. For all cases, a simple EEC model shown in Fig. 9 is sufficient to explain the observed EIS data. The corrosion behavior of carburised -TiAl in Ringer s solution was experimentally studied and it appears that this material has excellent corrosion resistance based on both direct current and ac techniques. Although the main aim of this study was to investigate the resistance to corrosion of -TiAl in a saline environment, a number of points regarding the type and composition of carbides formed after pulsed nanocrystalline plasma electrolysis and the oxide film growth in Ringer s solution for the samples (10 khz, 10%; 10 Hz, 60%) (different opposite frequencies and duty cycles) need to be further elaborated. Also, parametric studies for optimization of the protective carbide film and subsequent wear tests should be carried out. 4. Conclusions Fig. 8. Bode phase plots for samples (10 khz, 10%; 10 Hz, 60%) in Ringer s solution. The kinetic parameters (E corr, i corr ) obtained by the Tafel polarization technique indicate excellent corrosion resistance for pulsed nanocrystalline plasma electrolytic carburised -TiAl. Both electrochemical techniques show that these samples exhibit a corrosion behavior in Ringer s solution better than raw - TiAl and by increasing frequency and decreasing duty cycle of pulsed current, not only the size of nanocrystalline carbides will decrease, but also the corrosion characteristics will improve significantly. Surface carburising at 1000 Hz significantly increases the corrosion resistance of -TiAl, while carburising at 10 Hz results in the formation of a porous carburised layer. In general, the low values for corrosion rate and the high values of polarization resistance (R p ) obtained experimentally for carburised -TiAl in Ringer s solution; imply that this new surface modification has the potential to be considered as an effective surface treatment for biomaterials. Acknowledgements Fig. 9. Equivalent electrical circuit for all carburised samples. The authors would like to express their thanks to national association of nanoscience and nanotechnology of Iran and Arvandan Oil and Gas Company (TMU 85-09-66) for financial support of this project. References [1] S.P. Hannula, P. Nenonen, J.-P. Hirvonen, Thin Solid Films 181 (1989) 343. [2] M.J. Baldwin, M.P. Fewell, S.C. Haydon, S. Kumar, G.A. Collins, K.T. Short, J. Tendys, Surf. Coat. Technol. 98 (1998) 1187. [3] G.A. Collins, R. Hutchings, J. Tendys, Mater. Sci. Eng. A139 (1991) 171. [4] E. Camps, S. Muhl, S. Romero, Vacuum 51 (1998) 385. [5] D.L. Williamson, J.A. Davis, P.J. Wilbur, J.J. Vajo, R. Eei, J.N. Matossian, Nucl. Instrum. Methods B 127/128 (1997) 930. [6] M.J. Baldwin, S. Kumar, J.M. Priest, M.P. Fewell, K.E. Prince, K.T. Short, Thin Solid Films 345 (1999) 108. [7] P. Smith, R.A. Buchanan, J.R. Roth, S.G. Kamath, J. Vac. Sci. Technol. B 12 (1994) 940; E.I. Meletis, S. Yan, J. Vac. Sci. Technol. A 11 (1993) 25. [8] Sh. Ahangarani, F. Mahboubi, A.R. Sabour, Vacuum 80 (9) (2006) 1032 1037. [9] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73 93. [10] M. Aliofkhazraei, P. Taheri, A.R. Sabour, Ch. Dehghanian, Mater. Sci. (3), in press. [11] M. Tarakci, K. Korkmaz, Y. Gencer, M. Usta, Surf. Coat. Technol. 199 (2/3) (2005) 205 212. [12] M. Aliofkhazraei, P. Taheri, A. Sabour Rouhaghdam, C. Dehghanian, J. Mater. Sci. Technol. 23 (5), in press.