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Palladium profiles in titanium treated by high-intensity plasma pulses
Surface and Coatings Technology 158 – 159 (2002) 21–27
Palladium profiles in titanium treated by high-intensity plasma pulses Z. Wernera,*, J. Piekoszewskia,b, W. Szymczyka, F.A. Bonillac, T.S. Ongc, P. Skeldonc, e a ¨ G.E. Thompsonc, S. Zielinskid, A. Chmielewskib, R. Grotzschel , J. Stanislawski { ´ The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 SwierkyOtwock, Poland Institute of Nuclear Chemistry and Technology (ICHTJ), 16 Dorodna str., 03-145 Warsaw, Poland c Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK d ´ 34, 02-668 Warsaw, Poland Institute of Physics, Polish Academy of Sciences (IFPAN), Al. Lotnikow e ¨ Ionenstrahlphysik und Materialforschung, Postfach 51 01 19, D-01314 Dresden, Germany Forschungszentrum Rossendorf (FZR), Institut fur a
b
Abstract Palladium-alloyed surface layers in titanium were formed by deposition of palladium followed by treatment with high-intensity nitrogen plasma pulses (pulsed implantation doping, PID), capable of melting the substrate, and compared with those formed by the deposition by pulse erosion (DPE) process, in which erosion of palladium electrodes of the nitrogen plasma gun provided the deposit. As previously reported, the DPE-processed foils have exhibited excellent corrosion resistance to sulphuric acid. RBS and SIMS were used to examine the palladium profiles in order to gain insight into the palladium transport mechanism in molten titanium. A numerical program for solving heat diffusion equation was adapted to simulate palladium diffusion in molten titanium with depth-dependent diffusion duration accounted for. Corrosion tests in 0.1 M H2 SO4 solution at 80 8C revealed major improvements in the corrosion resistance of titanium foil following either DPE or PID treatments, as indicated by weight change and open-circuit potential, although the surfaces treated by PID can undergo some detachment of coating material in the acid solution. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Pulse plasma treatment; Surface alloys; Corrosion protection
1. Introduction Palladium, both as a coating and as an alloying element, is recognised as being highly effective in preventing acidic corrosion of titanium w1x, which is of practical importance for development of new, environmentally clean technologies of power generation in coalfired power stations. In one of the most promising technologies, currently under development, an electron beam dry scrubber process for reducing the SO2 and NOx concentration in exhaust gases is being examined w2x. In this process, a high-energy (700–800 keV) electron beam is injected into the ammonia-enriched flue gas to create free radicals from the gas constituents.
*Corresponding author. Tel.: q48-22-78-0556; fax: q48-22-77934-81. E-mail address: [email protected] (Z. Werner).
The radiation-induced reactions lead ultimately to formation of useful agricultural fertilisers. A 90% reduction of SO2 content is feasible; regarding the efficiency of NOx reduction (approx. 75%), the process has no serious competitors among alternative wet chemical processes. Currently the process is being tested at a demonstration industrial installation of 270 000 Nm3 yh throughput in the Pomorzany power station in Poland w3x, and at a similar installation in operation in China. The electron beam enters the reaction chamber through a window of 50 mm-thick titanium foil, which must withstand the highly corrosive atmosphere of the products of radiation-induced chemical reactions. Currently, the lifetime of titanium windows is approximately 2000 h, which is too short for long-term industrial operation. The windows fail primarily because of corrosion with possible additional influences of environmentally-assisted cracking and fatigue. Thus, attention is focused on the surface treatment of titanium by palladium to increase foil life.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 0 1 - 3
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Z. Werner et al. / Surface and Coatings Technology 158 – 159 (2002) 21–27
2. Summary of the previous work Dramatic improvement of the corrosion resistance of titanium in reducing acids as a result of noble metal alloying is well documented w4,5x. In particular Tomashov et al. w6x have used thin electroplates in combination with diffusion annealing, while McCafferty and Hubler w7x have employed ion implantation to produce Ti–Pd surface alloys. Draper et al. w8x examined Q-switched laser alloying of vacuum evaporated palladium on titanium, with an increase of corrosion resistance in boiling HCl by a factor of 30. Munn and Wolf w9x found that among electroplated, evaporated, ion implanted, and ion beam mixed evaporated palladium layers, the last exhibited the best stability in sulphuric acid, provided the range of the mixing ions was well in excess of the layer thickness. Recently the subject has been examined by an international team within the European Community INCOCOPERNICUS Programme w10x devoted to the development of a heavy-duty window for industrial scale removal of NOx and SO2 from flue gas by electron beam treatment. The main conclusions of the programme are summarised as follows: (1) Up to 400 nm thick palladium coatings on titanium foil of 50 mm thickness, prepared by vacuum evaporation (VE), ion beam mixing (IBM), ion-beamassisted deposition (IBAD) and plasma-source-ionassisted deposition (PSIAD), resulted in significant improvement of corrosion resistance in hot 0.1 M H2SO4 and 0.1 M HNO3 solutions. The weight loss percent during 100 h tests in the former acid ranged from 0.04 to 1.7% compared with 17% for pure titanium. However, poor adhesion during immersion in 0.1 M H2SO4 at 80 8C was evident, leading to detachment after 1000 h. Interestingly, no significant corrosion of the titanium substrate at detached sites was observed. (2) Detachment, resulting in almost complete loss of palladium, occurred at the coatingysubstrate interface and was probably caused by oxide films andyor interface stresses. (3) Among the coating techniques employed, the IBAD and PSIAD provided more durable coatings w11x. A common feature of the application of these techniques was that of optimising conditions for deposition while leaving the substrate-layer interface relatively sharp. However, none of these approaches produced satisfactory attachment of the palladium layer to the substrate, and therefore a further process leading to a graded transition between the substrate and the coating was examined. The process consisted of alloying palladium with titanium by high intensity plasma pulses that transiently melt the near-surface layers of the substrate. No palladium evaporation was involved and the alloying material was provided by pulse-induced erosion of the tips of palladium electrodes, which accompanies each
plasma discharge (deposition by pulse erosion, DPEmode). After 5–20 plasma pulses, a thin (-20 nm thick) palladium-rich outer layer, followed by much thicker (approx. 1 mm) layer of Ti–Pd alloy, was formed on the titanium; the palladium concentration in the inner layer decreased gradually from 10 to 35 at.% near the surface to zero. The treatment resulted in much improved corrosion resistance of the titanium foil: the percentage weight loss after 100 h tests in H2SO4 at 80 8C ranged from 0.24 to 0.95%. Detachment of the thin palladiumrich surface layer, and possibly part of the inner layer, was also observed following immersion in sulphuric acid, but the corrosion resistance in the exposed regions remained high due to the presence of the residual Ti– Pd alloy w12x. Accumulation of palladium in titanium is proportional to the number of pulses, the deposition rate being in the range 10–20 mgycm2 per pulse, i.e. 0.5–1=1017 atomsy cm2 per pulse, depending on the working gas used. However, the palladium-rich outer layer remains thin regardless of the number of pulses. XRD analysis has revealed the presence of TiPd and TiPd3 phases dispersed in a-Ti. SIMS measurements showed that most of the palladium profiles fit a Gaussian distribution. The experimental palladium profiles were simulated using a model of diffusion, assuming depth independent melt duration, and multiple diffusion from a limited surface source, replenished after each pulse. However, the model predicted a concave shape profile in the log NyN0 vs. x 2 Gaussian coordinates in contrast with experimental results giving a straight line w13x. The general conclusion was that DPE coatings were competitive with those produced by conventional ionbeam methods in terms of the improvement in corrosion resistance. However, preparing plasma discharge electrodes with palladium tips is tedious and the effectiveness of the electrode erosion is low. Therefore, in this work, the DPE process is compared with working-gas plasma-pulse alloying of pre-deposited palladium (pulsed implantation doping, PID). Further, the DPE experiments have been repeated with a reduced number of pulses to clarify the discrepancy between the experimental palladium profiles and theory. 3. Experimental Titanium foil coupons, 50 mm-thick, were palladium coated by e-beam vacuum evaporation to a thickness 20–250 nm (25–300 mgycm2). Subsequently, they were treated with variable number of nitrogen plasma pulses (duration approximately 1 ms, energy density 3.5–4 Jy cm2) generated in the IBIS rod plasma injector equipped with palladium-tipped electrodes and operated in the PID mode (no electrode erosion) w14,15x. Another set of uncoated titanium foil coupons was treated with 1, 2 and 3 nitrogen plasma pulses in the same equipment
Z. Werner et al. / Surface and Coatings Technology 158 – 159 (2002) 21–27
Fig. 1. SIMS profiles of palladium in titanium: directly following evaporation of 120 mgycm2 of palladium (as deposited), and after 1, 2 and 3 nitrogen plasma pulses applied in PID mode.
operated in the DPE mode (palladium electrode erosion) w16x. Mass gain measurements were performed on all samples to determine the retained palladium; EDX measurements were made to confirm the gravimetric results. The palladium profiles were determined by RBS and SIMS. Experimental SIMS profiles were simulated by numerically solving the diffusion equations, with depthdependent diffusion duration considered. The equation was solved for two cases: 1. Diffusion from a limited surface source (corresponding to the process with pre-deposition). 2. Multiple diffusion from limited surface source with replenishment of the source after each cycle (corresponding to the DPE process). Corrosion tests of the foils were carried out by immersion of specimens (two of each kind), of typical dimensions 10=10 mm2, in 0.1 M H2SO4 solution, in contact with laboratory air, at 80 8C for times up to 100 h. The areas of specimens that had not been coated with palladium were masked with lacquer. The weight changes of specimens were determined by a microbalance, with two specimens of each coating condition being exposed to evaluate the reproducibility of behaviour. The weighing were made in the absence of the lacquer. The open-circuit potentials were measured with respect to a saturated calomel electrode (SCE). Surfaces of specimens before and following immersion in the acid were examined by SEM with EDX facilities.
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Fig. 2. RBS spectra of palladium in titanium: directly following evaporation of 25 mgycm2 of palladium (as deposited), and after 1 and 3 nitrogen plasma pulses applied in PID mode.
at the surface, treated with 1, 2 and 3 pulses of nitrogen plasma. Lack of correspondence between the number of pulses and the profile depth reflects the unavoidable fluctuations in the pulse energy and hence in melt (and diffusion) duration. The treatment results in profiles approximately 0.6–0.8 mm deep with a maximum palladium concentration of 70%. Gradual spreading of palladium into the titanium bulk with increasing number of pulses is confirmed by RBS spectra, presented in Fig. 2, for titanium with 25 mgycm2 of pre-deposited palladium, treated with 1 and 3 nitrogen-plasma pulses. SIMS profiles of palladium in samples treated with 1, 2 and 3 plasma pulses in the DPE mode (without pre-deposition) are shown in Fig. 3. The spread of the single-pulse profile reflects merely the depth resolution of the method. The palladium concentrations are lower
4. Results and discussion 4.1. Palladium distributions Fig. 1 presents SIMS profiles of palladium in titanium samples with 120 mgycm2 of palladium pre-deposited
Fig. 3. SIMS profiles of palladium in titanium after 1, 2 and 3 nitrogen plasma pulses applied in DPE mode (electrode erosion).
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Z. Werner et al. / Surface and Coatings Technology 158 – 159 (2002) 21–27
Fig. 4. RBS spectra of palladium in titanium after 1, 2 and 3 nitrogen plasma pulses applied in DPE mode (electrode erosion).
than in the case of pre-deposition. The corresponding RBS spectra of the above samples are shown in Fig. 4; the gradual spreading of palladium in the substrate with the number of pulses is confirmed. Gravimetric, EDX, and SIMS result are generally in mutual agreement as regards the amount of palladium retained in the samples and show no signs of loss of palladium by evaporation. Two main mechanisms have to be considered when discussing impurity distribution in rapidly molten and solidified surface layers: segregation and diffusion. If the segregation coefficient has a low value, an accumulation of impurity close to the surface takes place which results in the terminal concentration spike near the surface w17x. The results presented in Fig. 1 show no sign of such spikes for PID-processes samples. Therefore, we conclude that the segregation mechanism is not significant in this case. This result is not surprising since it is known that for high solidification rates (1– 10 mys) the segregation coefficients in silicon may increase by two orders of magnitude with respect to thermal equilibrium conditions w17x and even more for metals (e.g. Sb in Al w18x). The solidification rate in our case attains 2 mys. The surface spikes visible in the profiles presented in Fig. 3 might suggest that the segregation does indeed take place in this case. However, as it was demonstrated in our previous work w12x, the observed spikes are inherent feature of the DPE processing, in which the last pulse deposits 10–20 nm thick metallic layer on the already solidified substrate surface. In order to simulate the diffusion processes the ETLIT numerical programme, developed for calculating heat evolution in planar targets after delivering a portion of energy to the surface, was employed w19x. A full equivalence between the heat and impurity diffusion equations has been used, provided the diffusion constant D in the impurity diffusion equation is replaced by the
thermal diffusivity KskyrC, (k, the thermal conductivity; r, the density and C, the specific heat) and the amount of impurity pre-deposited in each pulse of the DPE process is replaced by a short (with respect to diffusion duration) pulse of deposited energy. The impurity diffusion time (i.e. molten phase duration) after each pulse is represented as the time intervals between the energy pulses. In previous calculations w13x, it was assumed that the diffusion time is depth-independent. In other words, instantaneous melting of the whole melt depth and its instantaneous solidification was assumed. In reality the melt front is moving with time, and deeper layers remain molten for shorter times than the surface layers (i.e. diffusion time decreases with depth). This feature can be taken into account in our present model if we note that that time occurs always as the (Dt) product in analytical expressions related to diffusion. Thus reduction of the diffusion time with depth should be equivalent to a reduction of the diffusion constant (diffusivity in heat equivalent). Since the ETLIT programme allows division of the subsurface region into five layers with different thermal properties, the depth-dependent decrease of diffusion time has been simulated by gradual decrease of diffusivity to zero in successive layers. It is convenient to present the simulation results in ‘reduced Gaussian coordinates’ i.e. as the logwN(x)y N(xs0)x vs. (xyx0.01)2, where x0.01 denotes the depth, at which concentration drops by 2 orders of magnitude N(x0.01)s0.01N(xs0). This latter selection is arbitrary and is used only to facilitate convenient comparison of profiles extending to different depths. In such coordinates, the standard impurity profile for single, limitedsource diffusion is a straight line joining point (0,0) with point (1,y2). Results of simulations are presented in Fig. 5a and b for samples treated with 3 or 6 pulses, respectively. The simulation 1 is computed for a limited surface source case under assumption of diffusion time decreasing stepwise to zero at a depth exceeding slightly the diffusion range. The simulation 2 is computed for multiple pulses (replenished source) under assumption of depth-independent diffusion time. Finally, the simulation 3 represents superposition of the two effects: 3 (or 6) pulses and depth-dependent diffusion time. Experimental results are shown as open circles. The important conclusions, which are drawn from the simulations, are as follows: a. limited source diffusion, corrected for depth-dependent diffusion time, results always in a convex shape of the profile in reduced Gaussian coordinates, with the curvature of the profile increasing with the number
Z. Werner et al. / Surface and Coatings Technology 158 – 159 (2002) 21–27
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bulk. An analytical solution predicted for such a model is the complementary error function of x erfcwxy2yDtx
Fig. 5. Simulated diffusion profiles plotted in reduced Gaussian coordinates for samples treated with 3 (a) and 6 (b) plasma pulses. Simulation 1—limited surface source (no replenishement), depth-dependant diffusion time; simulation 2—source replenished after each pulse, depth-independent diffusion time; simulation 3— superposition of the two effects; source replenished after each pulse, depth-dependant diffusion time; s, experimental results after 3 nitrogen plasma pulses applied in DPE mode; n, experimental results after 6 pulses.
of pulses (diffusion time) as the diffusion range approaches the melt depth; b. for multiple diffusion with surface source replenished after each pulse, this correction counterbalances the concave form of the profile, resulting in profiles close to straight lines, i.e. Gaussian. This behaviour is apparently only weakly dependent on the number of pulses, maintaining the Gaussian shape of the resultant profile; The profile resulting from single evaporation and subsequent pulse melting should in principle be convex. Experimental palladium profiles in samples with a predeposited palladium layer (Fig. 1) are re-plotted in Fig. 6 in reduced Gaussian coordinates. It is evident that different curvatures are possible. A viable interpretation of these observations is as follows: if the deposited layer is sufficiently thick, diffusion after the first pulse proceeds according to the constant surface concentration model, since the molten palladium may remain for some time at the surface before eventual penetration into the
This function plotted in normalised Gaussian coordinate also has a concave shape. When this negative curvature is compensated by depth-dependent diffusion time, it is likely that a Gaussian shape is obtained. Unfortunately, the ETLIT programme does not allow the surface concentration (i.e. the surface temperature in the heat flow analogy) to be fixed as a constant value. However, the following argument may be considered: in the limit of large number of steps, multiple, limited surface source diffusion with replenishment of the source after each diffusion step is equivalent to unlimited surface source diffusion (constant surface concentration). Therefore, it can be concluded that the solution of this case approaches the erfc function. Thus, instead of solving the constant surface concentration problem with depth-dependent diffusion time, the multiple, limited source diffusion model may be used, with inclusion of the latter dependence. This has been done in the simulation presented in Fig. 5; for such conditions the simulation predicts a Gaussian-like shape. Thus, a concave shape of the profile shown in Fig. 6 may correspond to the case of constant surface concentration, without much influence of depth-dependent diffusion time (which is the case if diffusion depth is much smaller than the melt depth). The other two cases reflect various degrees of the effect of depth-dependent diffusion time on limited surface source diffusion. The results of these simulations apply to small number of pulses, when the diffusion length 6Dt is less than melt depth dm. In the limit of a large number of pulses 6Dt4dm nd the profiles approach a rectangular distribution.
Fig. 6. SIMS profiles from Fig. 1, plotted in reduced Gaussian coordinates. Straight line represents a Gaussian profile.
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Table 1 Results of immersion tests, in 0.1 M H2SO4 solution at 80 8C, under open-circuit conditions of untreated titanium foil and titanium foil treated by DPE and PID processes Material
Immersion time (h)
Rate of weight loss (g my2 dy1)
Weight loss normalized to 100 h immersion time (%)
Open-circuit potential (mV SCE)
Untreated Ti DPE 1 pulse DPE 2 pulses DPE 3 pulses PID 1 pulse PID 2 pulses PID 3 pulses
18 100 50 100 100 100 100
45.1"0.5 y0.01"0.03 0.28"0.07 0.18"0.02 0.36"0.02 0.43"0.02 0.60"0.03
82.6 y0.02 0.51 0.33 0.66 0.79 1.1
y780 455 y290 400 470 470 440
5. Corrosion behaviour The results of immersion tests of the palladium-treated surfaces (Table 1) reveal a major improvement in the corrosion resistance in comparison with untreated titanium foil, with a reduction in the average rate of weight loss by about two orders of magnitude, consistent with previous studies by the authors of DPE-treated foil. The corrosion rate of untreated titanium was relatively rapid such that tests were terminated at short times to avoid disintegration of the specimens. The improved behaviour of the palladium-treated foils was consistent with opencircuit potentials that were significantly higher than that of the untreated titanium, with respective values of approximately 400–470 mV (SCE) and approximately
y780 mV (SCE). These potentials were achieved after approximately 1 h immersion in the acid. Notably, the open-circuit potential of the foil treated by DPE using two pulses, approximately y290 mV (SCE), was somewhat lower than the potentials of other DPE- and PIDtreated foils. The reduced potential correlated with an unexpectedly low amount of deposited palladium, as indicated by ratios of signals from palladium and titanium determined by EDX for the variously treated foils. SEM examinations were carried out on DPE- and PID-treated specimens that revealed the greatest loss of mass (Fig. 7). The surface of the former specimen, treated with two pulses, revealed a relatively compact morphology. However, immersion in the acid resulted in the development of numerous crack-like features or
Fig. 7. Secondary electron scanning electron micrographs of: (a) DPE-treated (2 pulses) titanium foil before immersion in acid; (b) DPE-treated (2 pulses) titanium foil after immersion for 50 h in 0.1 M H2SO4 solution at 80 8C; (c) PID-treated (3 pulses) titanium foil before immersion in acid; (d) PID-treated (3 pulses) titanium foil after immersion for 100 h in 0.1 M H2SO4 solution at 80 8C.
Z. Werner et al. / Surface and Coatings Technology 158 – 159 (2002) 21–27
possibly intergranular penetrations. In contrast, the PIDtreated surface, using three pulses, had an initial finetextured morphology, which appeared to be transformed to a smoother morphology following exposure to the acid. Significantly for this surface, and also for surfaces treated by PID with two pulses, there was loss of coating material in relatively large patches. Such loss was not observed for any DPE-treated specimens. The surfaces treated by PID with a single pulse appeared relatively uniform by visual examination, compared with surfaces treated with either two or three pulses. Whether this observation relates to changed adhesion of the coating material during exposure to the acid requires further study. For specimens revealing detachment, the exposed regions were still protected against corrosion, associated with the high open-circuit potentials sustained by the still attached material at adjacent regions and any palladium remaining on the exposed material. At such potentials, the titanium is in the passive state. In contrast, untreated titanium undergoes relatively rapid general corrosion and intergranular corrosion. Due to the detachment of coating material, the measured changes in mass represent upper limits on the corrosion rate for PIDtreated foil. Further microstructural examination of the coating is needed to understand the transformations of the surface regions and the detachment of material during exposure to the acid, and the precise influence on degradation behaviour of the number of pulses in the DPE and PID processes. 6. Conclusions The presented results can be summarised as follows: ● 3 nitrogen plasma pulses of 3.5 Jycm2 energy density and 1 ms duration applied to a sample with 120 mgy cm2 of evaporated palladium are sufficient to drive palladium to a depth of 0.6 mm with the maximum palladium concentration in alloy at approximately 70 at.% at the surface. ● 3-pulse DPE process produces a palladium profile extending to approximately 0.4 mm but the concentration at the surface is lower and does not exceed 5 at.%. ● Palladium profiles in DPE layers seem to be in agreement with the limited surface source diffusion model with replenishment of the source after each pulse and with depth-dependent diffusion time taken into account. Such model predicts a shape close to Gaussian distribution for small number of pulses. ● Palladium profiles in evaporated and alloyed-in layers show complicated behaviour dependent on the relation between the melt depth and diffusion range. ● Both PID- and DPE-treated surfaces provide good protection against corrosion of the titanium by 0.1 M H2SO4 solution at 80 8C.
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● Surfaces treated by PID can undergo some detachment of coating material which was not encountered with DPE-treated foils. Acknowledgments This work was partially supported by Inco-Copernicus EBOGEM project under Contract No. ICA2-CT2000.1000S. Prof. A. Barcz of the Institute of Physics, PAS is gratefully acknowledged for making his SIMS equipment available for the present analyses. References w1x M. Stern, H. Wissenberg, J. Electrochem. Soc. 106 (1959) 759. w2x A.G. Chmielewski, E. Iller, Z. Zimek, J. Licki, Radiat. Phys. Chem. 40 (1992) 321. w3x A.G. Chmielewski, E. Iller, B. Tyminski, ´ Z. Zimek, J. Licki, Modern Power Syst. (2001) 53–54. w4x M. Stern, H. Wissenberg, J. Electrochem. Soc. 106 (1959) 759. w5x N.D. Tomashov, R.M. Altovsky, G.P. Chernova, J. Electrochem. Soc. 108 (1961) 113. w6x (a) N.D. Tomashov, G.P. Chernova, T.A. Fedoseeva, Prot. Met. (USSR) 13 (1977) 134 (b) N.D. Tomashov, G.P. Chernova, T.A. Fedoseeva, Corrosion 36 (1980) 201. w7x (a) E. McCafferty, G.K. Hubler, J. Electrochem. Soc. 125 (1978) 1892 (b) E. McCafferty, G.K. Hubler, Corrosion Sci. 20 (1980) 103. w8x C.W. Draper, L.S. Meyer, D.C. Jacobson, L. Buene, J.M. Poate, Thin Solid Films 75 (1981) 237. w9x P. Munn, G.K. Wolf, Nucl. Instr. Meth. B7y8 (1985) 205. w10x INCO Copernicus Contract No. IC15-CT97-0711 Final Report (1999). w11x S.D. Barson, P. Skeldon, G.E. Thomson, A. Kolitsch, E. Richter, E. Wieser, J. Piekoszewski, A.G. Chmielewski, Z. Werner, Surf. Coat. Technol. 127 (2000) 179. w12x S.D. Barson, P. Skeldon, G.E. Thomson, J. Piekoszewski, A.G. ¨ Chmielewski, Z. Werner, R. Grotzschel, E. Wieser, Corrosion Sci. 42 (2000) 1213. w13x Z. Werner, J. Piekoszewski, A. Barcz, R. Grotzschel, ¨ F. Pokert, J. Stanislawski, W. Szymczyk, Nucl. Instr. Meth. B175–177 { (2001) 767. w14x J. Piekoszewski, J. Langner, L. Walis, ´ Z. Werner, Surf. Coat. Technol. 93 (1997) 209. w15x J. Piekoszewski, R. Grotzschel, ¨ E. Wieser, J. Stanislawski, Z. { Werner, W. Szymczyk, J. Langner, Surf. Coat. Technol. 128– 129 (2000) 394. w16x J. Piekoszewski, Z. Werner, E. Wieser, J. Langner, R. Grotz¨ schel, H. Reuther, J. Jagielski, Nukleonika 44 (1999) 239. w17x J.M. Poate, G. Foti, D.C. Jacobson (Eds.), Surface Modification and Alloying by Laser, Ion, and Electron Beams, Plenum Press, New York, London, 1983, p. 86. w18x M. von Almen, Laser Beam Interactions with Materials, Physical Principles and Applications, Springer, Berlin, Heidelberg, New York, London, Paris, Tokyo, 1987, p. 118. w19x W. Szyszko, Odzialywanie wiazek jonowych i laserowych z { tarczami cial{ stalych { (Interaction of ion and laser beams with solid targets, in Polish), DSci Thesis, UMCS University, Lublin (1997).
Palladium profiles in titanium treated by high-intensity plasma pulses Z. Wernera,*, J. Piekoszewskia,b, W. Szymczyka, F.A. Bonillac, T.S. Ongc, P. Skeldonc, e a ¨ G.E. Thompsonc, S. Zielinskid, A. Chmielewskib, R. Grotzschel , J. Stanislawski { ´ The Andrzej Soltan Institute for Nuclear Studies (IPJ), 05-400 SwierkyOtwock, Poland Institute of Nuclear Chemistry and Technology (ICHTJ), 16 Dorodna str., 03-145 Warsaw, Poland c Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK d ´ 34, 02-668 Warsaw, Poland Institute of Physics, Polish Academy of Sciences (IFPAN), Al. Lotnikow e ¨ Ionenstrahlphysik und Materialforschung, Postfach 51 01 19, D-01314 Dresden, Germany Forschungszentrum Rossendorf (FZR), Institut fur a
b
Abstract Palladium-alloyed surface layers in titanium were formed by deposition of palladium followed by treatment with high-intensity nitrogen plasma pulses (pulsed implantation doping, PID), capable of melting the substrate, and compared with those formed by the deposition by pulse erosion (DPE) process, in which erosion of palladium electrodes of the nitrogen plasma gun provided the deposit. As previously reported, the DPE-processed foils have exhibited excellent corrosion resistance to sulphuric acid. RBS and SIMS were used to examine the palladium profiles in order to gain insight into the palladium transport mechanism in molten titanium. A numerical program for solving heat diffusion equation was adapted to simulate palladium diffusion in molten titanium with depth-dependent diffusion duration accounted for. Corrosion tests in 0.1 M H2 SO4 solution at 80 8C revealed major improvements in the corrosion resistance of titanium foil following either DPE or PID treatments, as indicated by weight change and open-circuit potential, although the surfaces treated by PID can undergo some detachment of coating material in the acid solution. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Pulse plasma treatment; Surface alloys; Corrosion protection
1. Introduction Palladium, both as a coating and as an alloying element, is recognised as being highly effective in preventing acidic corrosion of titanium w1x, which is of practical importance for development of new, environmentally clean technologies of power generation in coalfired power stations. In one of the most promising technologies, currently under development, an electron beam dry scrubber process for reducing the SO2 and NOx concentration in exhaust gases is being examined w2x. In this process, a high-energy (700–800 keV) electron beam is injected into the ammonia-enriched flue gas to create free radicals from the gas constituents.
*Corresponding author. Tel.: q48-22-78-0556; fax: q48-22-77934-81. E-mail address: [email protected] (Z. Werner).
The radiation-induced reactions lead ultimately to formation of useful agricultural fertilisers. A 90% reduction of SO2 content is feasible; regarding the efficiency of NOx reduction (approx. 75%), the process has no serious competitors among alternative wet chemical processes. Currently the process is being tested at a demonstration industrial installation of 270 000 Nm3 yh throughput in the Pomorzany power station in Poland w3x, and at a similar installation in operation in China. The electron beam enters the reaction chamber through a window of 50 mm-thick titanium foil, which must withstand the highly corrosive atmosphere of the products of radiation-induced chemical reactions. Currently, the lifetime of titanium windows is approximately 2000 h, which is too short for long-term industrial operation. The windows fail primarily because of corrosion with possible additional influences of environmentally-assisted cracking and fatigue. Thus, attention is focused on the surface treatment of titanium by palladium to increase foil life.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 2 0 1 - 3
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2. Summary of the previous work Dramatic improvement of the corrosion resistance of titanium in reducing acids as a result of noble metal alloying is well documented w4,5x. In particular Tomashov et al. w6x have used thin electroplates in combination with diffusion annealing, while McCafferty and Hubler w7x have employed ion implantation to produce Ti–Pd surface alloys. Draper et al. w8x examined Q-switched laser alloying of vacuum evaporated palladium on titanium, with an increase of corrosion resistance in boiling HCl by a factor of 30. Munn and Wolf w9x found that among electroplated, evaporated, ion implanted, and ion beam mixed evaporated palladium layers, the last exhibited the best stability in sulphuric acid, provided the range of the mixing ions was well in excess of the layer thickness. Recently the subject has been examined by an international team within the European Community INCOCOPERNICUS Programme w10x devoted to the development of a heavy-duty window for industrial scale removal of NOx and SO2 from flue gas by electron beam treatment. The main conclusions of the programme are summarised as follows: (1) Up to 400 nm thick palladium coatings on titanium foil of 50 mm thickness, prepared by vacuum evaporation (VE), ion beam mixing (IBM), ion-beamassisted deposition (IBAD) and plasma-source-ionassisted deposition (PSIAD), resulted in significant improvement of corrosion resistance in hot 0.1 M H2SO4 and 0.1 M HNO3 solutions. The weight loss percent during 100 h tests in the former acid ranged from 0.04 to 1.7% compared with 17% for pure titanium. However, poor adhesion during immersion in 0.1 M H2SO4 at 80 8C was evident, leading to detachment after 1000 h. Interestingly, no significant corrosion of the titanium substrate at detached sites was observed. (2) Detachment, resulting in almost complete loss of palladium, occurred at the coatingysubstrate interface and was probably caused by oxide films andyor interface stresses. (3) Among the coating techniques employed, the IBAD and PSIAD provided more durable coatings w11x. A common feature of the application of these techniques was that of optimising conditions for deposition while leaving the substrate-layer interface relatively sharp. However, none of these approaches produced satisfactory attachment of the palladium layer to the substrate, and therefore a further process leading to a graded transition between the substrate and the coating was examined. The process consisted of alloying palladium with titanium by high intensity plasma pulses that transiently melt the near-surface layers of the substrate. No palladium evaporation was involved and the alloying material was provided by pulse-induced erosion of the tips of palladium electrodes, which accompanies each
plasma discharge (deposition by pulse erosion, DPEmode). After 5–20 plasma pulses, a thin (-20 nm thick) palladium-rich outer layer, followed by much thicker (approx. 1 mm) layer of Ti–Pd alloy, was formed on the titanium; the palladium concentration in the inner layer decreased gradually from 10 to 35 at.% near the surface to zero. The treatment resulted in much improved corrosion resistance of the titanium foil: the percentage weight loss after 100 h tests in H2SO4 at 80 8C ranged from 0.24 to 0.95%. Detachment of the thin palladiumrich surface layer, and possibly part of the inner layer, was also observed following immersion in sulphuric acid, but the corrosion resistance in the exposed regions remained high due to the presence of the residual Ti– Pd alloy w12x. Accumulation of palladium in titanium is proportional to the number of pulses, the deposition rate being in the range 10–20 mgycm2 per pulse, i.e. 0.5–1=1017 atomsy cm2 per pulse, depending on the working gas used. However, the palladium-rich outer layer remains thin regardless of the number of pulses. XRD analysis has revealed the presence of TiPd and TiPd3 phases dispersed in a-Ti. SIMS measurements showed that most of the palladium profiles fit a Gaussian distribution. The experimental palladium profiles were simulated using a model of diffusion, assuming depth independent melt duration, and multiple diffusion from a limited surface source, replenished after each pulse. However, the model predicted a concave shape profile in the log NyN0 vs. x 2 Gaussian coordinates in contrast with experimental results giving a straight line w13x. The general conclusion was that DPE coatings were competitive with those produced by conventional ionbeam methods in terms of the improvement in corrosion resistance. However, preparing plasma discharge electrodes with palladium tips is tedious and the effectiveness of the electrode erosion is low. Therefore, in this work, the DPE process is compared with working-gas plasma-pulse alloying of pre-deposited palladium (pulsed implantation doping, PID). Further, the DPE experiments have been repeated with a reduced number of pulses to clarify the discrepancy between the experimental palladium profiles and theory. 3. Experimental Titanium foil coupons, 50 mm-thick, were palladium coated by e-beam vacuum evaporation to a thickness 20–250 nm (25–300 mgycm2). Subsequently, they were treated with variable number of nitrogen plasma pulses (duration approximately 1 ms, energy density 3.5–4 Jy cm2) generated in the IBIS rod plasma injector equipped with palladium-tipped electrodes and operated in the PID mode (no electrode erosion) w14,15x. Another set of uncoated titanium foil coupons was treated with 1, 2 and 3 nitrogen plasma pulses in the same equipment
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Fig. 1. SIMS profiles of palladium in titanium: directly following evaporation of 120 mgycm2 of palladium (as deposited), and after 1, 2 and 3 nitrogen plasma pulses applied in PID mode.
operated in the DPE mode (palladium electrode erosion) w16x. Mass gain measurements were performed on all samples to determine the retained palladium; EDX measurements were made to confirm the gravimetric results. The palladium profiles were determined by RBS and SIMS. Experimental SIMS profiles were simulated by numerically solving the diffusion equations, with depthdependent diffusion duration considered. The equation was solved for two cases: 1. Diffusion from a limited surface source (corresponding to the process with pre-deposition). 2. Multiple diffusion from limited surface source with replenishment of the source after each cycle (corresponding to the DPE process). Corrosion tests of the foils were carried out by immersion of specimens (two of each kind), of typical dimensions 10=10 mm2, in 0.1 M H2SO4 solution, in contact with laboratory air, at 80 8C for times up to 100 h. The areas of specimens that had not been coated with palladium were masked with lacquer. The weight changes of specimens were determined by a microbalance, with two specimens of each coating condition being exposed to evaluate the reproducibility of behaviour. The weighing were made in the absence of the lacquer. The open-circuit potentials were measured with respect to a saturated calomel electrode (SCE). Surfaces of specimens before and following immersion in the acid were examined by SEM with EDX facilities.
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Fig. 2. RBS spectra of palladium in titanium: directly following evaporation of 25 mgycm2 of palladium (as deposited), and after 1 and 3 nitrogen plasma pulses applied in PID mode.
at the surface, treated with 1, 2 and 3 pulses of nitrogen plasma. Lack of correspondence between the number of pulses and the profile depth reflects the unavoidable fluctuations in the pulse energy and hence in melt (and diffusion) duration. The treatment results in profiles approximately 0.6–0.8 mm deep with a maximum palladium concentration of 70%. Gradual spreading of palladium into the titanium bulk with increasing number of pulses is confirmed by RBS spectra, presented in Fig. 2, for titanium with 25 mgycm2 of pre-deposited palladium, treated with 1 and 3 nitrogen-plasma pulses. SIMS profiles of palladium in samples treated with 1, 2 and 3 plasma pulses in the DPE mode (without pre-deposition) are shown in Fig. 3. The spread of the single-pulse profile reflects merely the depth resolution of the method. The palladium concentrations are lower
4. Results and discussion 4.1. Palladium distributions Fig. 1 presents SIMS profiles of palladium in titanium samples with 120 mgycm2 of palladium pre-deposited
Fig. 3. SIMS profiles of palladium in titanium after 1, 2 and 3 nitrogen plasma pulses applied in DPE mode (electrode erosion).
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Fig. 4. RBS spectra of palladium in titanium after 1, 2 and 3 nitrogen plasma pulses applied in DPE mode (electrode erosion).
than in the case of pre-deposition. The corresponding RBS spectra of the above samples are shown in Fig. 4; the gradual spreading of palladium in the substrate with the number of pulses is confirmed. Gravimetric, EDX, and SIMS result are generally in mutual agreement as regards the amount of palladium retained in the samples and show no signs of loss of palladium by evaporation. Two main mechanisms have to be considered when discussing impurity distribution in rapidly molten and solidified surface layers: segregation and diffusion. If the segregation coefficient has a low value, an accumulation of impurity close to the surface takes place which results in the terminal concentration spike near the surface w17x. The results presented in Fig. 1 show no sign of such spikes for PID-processes samples. Therefore, we conclude that the segregation mechanism is not significant in this case. This result is not surprising since it is known that for high solidification rates (1– 10 mys) the segregation coefficients in silicon may increase by two orders of magnitude with respect to thermal equilibrium conditions w17x and even more for metals (e.g. Sb in Al w18x). The solidification rate in our case attains 2 mys. The surface spikes visible in the profiles presented in Fig. 3 might suggest that the segregation does indeed take place in this case. However, as it was demonstrated in our previous work w12x, the observed spikes are inherent feature of the DPE processing, in which the last pulse deposits 10–20 nm thick metallic layer on the already solidified substrate surface. In order to simulate the diffusion processes the ETLIT numerical programme, developed for calculating heat evolution in planar targets after delivering a portion of energy to the surface, was employed w19x. A full equivalence between the heat and impurity diffusion equations has been used, provided the diffusion constant D in the impurity diffusion equation is replaced by the
thermal diffusivity KskyrC, (k, the thermal conductivity; r, the density and C, the specific heat) and the amount of impurity pre-deposited in each pulse of the DPE process is replaced by a short (with respect to diffusion duration) pulse of deposited energy. The impurity diffusion time (i.e. molten phase duration) after each pulse is represented as the time intervals between the energy pulses. In previous calculations w13x, it was assumed that the diffusion time is depth-independent. In other words, instantaneous melting of the whole melt depth and its instantaneous solidification was assumed. In reality the melt front is moving with time, and deeper layers remain molten for shorter times than the surface layers (i.e. diffusion time decreases with depth). This feature can be taken into account in our present model if we note that that time occurs always as the (Dt) product in analytical expressions related to diffusion. Thus reduction of the diffusion time with depth should be equivalent to a reduction of the diffusion constant (diffusivity in heat equivalent). Since the ETLIT programme allows division of the subsurface region into five layers with different thermal properties, the depth-dependent decrease of diffusion time has been simulated by gradual decrease of diffusivity to zero in successive layers. It is convenient to present the simulation results in ‘reduced Gaussian coordinates’ i.e. as the logwN(x)y N(xs0)x vs. (xyx0.01)2, where x0.01 denotes the depth, at which concentration drops by 2 orders of magnitude N(x0.01)s0.01N(xs0). This latter selection is arbitrary and is used only to facilitate convenient comparison of profiles extending to different depths. In such coordinates, the standard impurity profile for single, limitedsource diffusion is a straight line joining point (0,0) with point (1,y2). Results of simulations are presented in Fig. 5a and b for samples treated with 3 or 6 pulses, respectively. The simulation 1 is computed for a limited surface source case under assumption of diffusion time decreasing stepwise to zero at a depth exceeding slightly the diffusion range. The simulation 2 is computed for multiple pulses (replenished source) under assumption of depth-independent diffusion time. Finally, the simulation 3 represents superposition of the two effects: 3 (or 6) pulses and depth-dependent diffusion time. Experimental results are shown as open circles. The important conclusions, which are drawn from the simulations, are as follows: a. limited source diffusion, corrected for depth-dependent diffusion time, results always in a convex shape of the profile in reduced Gaussian coordinates, with the curvature of the profile increasing with the number
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bulk. An analytical solution predicted for such a model is the complementary error function of x erfcwxy2yDtx
Fig. 5. Simulated diffusion profiles plotted in reduced Gaussian coordinates for samples treated with 3 (a) and 6 (b) plasma pulses. Simulation 1—limited surface source (no replenishement), depth-dependant diffusion time; simulation 2—source replenished after each pulse, depth-independent diffusion time; simulation 3— superposition of the two effects; source replenished after each pulse, depth-dependant diffusion time; s, experimental results after 3 nitrogen plasma pulses applied in DPE mode; n, experimental results after 6 pulses.
of pulses (diffusion time) as the diffusion range approaches the melt depth; b. for multiple diffusion with surface source replenished after each pulse, this correction counterbalances the concave form of the profile, resulting in profiles close to straight lines, i.e. Gaussian. This behaviour is apparently only weakly dependent on the number of pulses, maintaining the Gaussian shape of the resultant profile; The profile resulting from single evaporation and subsequent pulse melting should in principle be convex. Experimental palladium profiles in samples with a predeposited palladium layer (Fig. 1) are re-plotted in Fig. 6 in reduced Gaussian coordinates. It is evident that different curvatures are possible. A viable interpretation of these observations is as follows: if the deposited layer is sufficiently thick, diffusion after the first pulse proceeds according to the constant surface concentration model, since the molten palladium may remain for some time at the surface before eventual penetration into the
This function plotted in normalised Gaussian coordinate also has a concave shape. When this negative curvature is compensated by depth-dependent diffusion time, it is likely that a Gaussian shape is obtained. Unfortunately, the ETLIT programme does not allow the surface concentration (i.e. the surface temperature in the heat flow analogy) to be fixed as a constant value. However, the following argument may be considered: in the limit of large number of steps, multiple, limited surface source diffusion with replenishment of the source after each diffusion step is equivalent to unlimited surface source diffusion (constant surface concentration). Therefore, it can be concluded that the solution of this case approaches the erfc function. Thus, instead of solving the constant surface concentration problem with depth-dependent diffusion time, the multiple, limited source diffusion model may be used, with inclusion of the latter dependence. This has been done in the simulation presented in Fig. 5; for such conditions the simulation predicts a Gaussian-like shape. Thus, a concave shape of the profile shown in Fig. 6 may correspond to the case of constant surface concentration, without much influence of depth-dependent diffusion time (which is the case if diffusion depth is much smaller than the melt depth). The other two cases reflect various degrees of the effect of depth-dependent diffusion time on limited surface source diffusion. The results of these simulations apply to small number of pulses, when the diffusion length 6Dt is less than melt depth dm. In the limit of a large number of pulses 6Dt4dm nd the profiles approach a rectangular distribution.
Fig. 6. SIMS profiles from Fig. 1, plotted in reduced Gaussian coordinates. Straight line represents a Gaussian profile.
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Table 1 Results of immersion tests, in 0.1 M H2SO4 solution at 80 8C, under open-circuit conditions of untreated titanium foil and titanium foil treated by DPE and PID processes Material
Immersion time (h)
Rate of weight loss (g my2 dy1)
Weight loss normalized to 100 h immersion time (%)
Open-circuit potential (mV SCE)
Untreated Ti DPE 1 pulse DPE 2 pulses DPE 3 pulses PID 1 pulse PID 2 pulses PID 3 pulses
18 100 50 100 100 100 100
45.1"0.5 y0.01"0.03 0.28"0.07 0.18"0.02 0.36"0.02 0.43"0.02 0.60"0.03
82.6 y0.02 0.51 0.33 0.66 0.79 1.1
y780 455 y290 400 470 470 440
5. Corrosion behaviour The results of immersion tests of the palladium-treated surfaces (Table 1) reveal a major improvement in the corrosion resistance in comparison with untreated titanium foil, with a reduction in the average rate of weight loss by about two orders of magnitude, consistent with previous studies by the authors of DPE-treated foil. The corrosion rate of untreated titanium was relatively rapid such that tests were terminated at short times to avoid disintegration of the specimens. The improved behaviour of the palladium-treated foils was consistent with opencircuit potentials that were significantly higher than that of the untreated titanium, with respective values of approximately 400–470 mV (SCE) and approximately
y780 mV (SCE). These potentials were achieved after approximately 1 h immersion in the acid. Notably, the open-circuit potential of the foil treated by DPE using two pulses, approximately y290 mV (SCE), was somewhat lower than the potentials of other DPE- and PIDtreated foils. The reduced potential correlated with an unexpectedly low amount of deposited palladium, as indicated by ratios of signals from palladium and titanium determined by EDX for the variously treated foils. SEM examinations were carried out on DPE- and PID-treated specimens that revealed the greatest loss of mass (Fig. 7). The surface of the former specimen, treated with two pulses, revealed a relatively compact morphology. However, immersion in the acid resulted in the development of numerous crack-like features or
Fig. 7. Secondary electron scanning electron micrographs of: (a) DPE-treated (2 pulses) titanium foil before immersion in acid; (b) DPE-treated (2 pulses) titanium foil after immersion for 50 h in 0.1 M H2SO4 solution at 80 8C; (c) PID-treated (3 pulses) titanium foil before immersion in acid; (d) PID-treated (3 pulses) titanium foil after immersion for 100 h in 0.1 M H2SO4 solution at 80 8C.
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possibly intergranular penetrations. In contrast, the PIDtreated surface, using three pulses, had an initial finetextured morphology, which appeared to be transformed to a smoother morphology following exposure to the acid. Significantly for this surface, and also for surfaces treated by PID with two pulses, there was loss of coating material in relatively large patches. Such loss was not observed for any DPE-treated specimens. The surfaces treated by PID with a single pulse appeared relatively uniform by visual examination, compared with surfaces treated with either two or three pulses. Whether this observation relates to changed adhesion of the coating material during exposure to the acid requires further study. For specimens revealing detachment, the exposed regions were still protected against corrosion, associated with the high open-circuit potentials sustained by the still attached material at adjacent regions and any palladium remaining on the exposed material. At such potentials, the titanium is in the passive state. In contrast, untreated titanium undergoes relatively rapid general corrosion and intergranular corrosion. Due to the detachment of coating material, the measured changes in mass represent upper limits on the corrosion rate for PIDtreated foil. Further microstructural examination of the coating is needed to understand the transformations of the surface regions and the detachment of material during exposure to the acid, and the precise influence on degradation behaviour of the number of pulses in the DPE and PID processes. 6. Conclusions The presented results can be summarised as follows: ● 3 nitrogen plasma pulses of 3.5 Jycm2 energy density and 1 ms duration applied to a sample with 120 mgy cm2 of evaporated palladium are sufficient to drive palladium to a depth of 0.6 mm with the maximum palladium concentration in alloy at approximately 70 at.% at the surface. ● 3-pulse DPE process produces a palladium profile extending to approximately 0.4 mm but the concentration at the surface is lower and does not exceed 5 at.%. ● Palladium profiles in DPE layers seem to be in agreement with the limited surface source diffusion model with replenishment of the source after each pulse and with depth-dependent diffusion time taken into account. Such model predicts a shape close to Gaussian distribution for small number of pulses. ● Palladium profiles in evaporated and alloyed-in layers show complicated behaviour dependent on the relation between the melt depth and diffusion range. ● Both PID- and DPE-treated surfaces provide good protection against corrosion of the titanium by 0.1 M H2SO4 solution at 80 8C.
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● Surfaces treated by PID can undergo some detachment of coating material which was not encountered with DPE-treated foils. Acknowledgments This work was partially supported by Inco-Copernicus EBOGEM project under Contract No. ICA2-CT2000.1000S. Prof. A. Barcz of the Institute of Physics, PAS is gratefully acknowledged for making his SIMS equipment available for the present analyses. References w1x M. Stern, H. Wissenberg, J. Electrochem. Soc. 106 (1959) 759. w2x A.G. Chmielewski, E. Iller, Z. Zimek, J. Licki, Radiat. Phys. Chem. 40 (1992) 321. w3x A.G. Chmielewski, E. Iller, B. Tyminski, ´ Z. Zimek, J. Licki, Modern Power Syst. (2001) 53–54. w4x M. Stern, H. Wissenberg, J. Electrochem. Soc. 106 (1959) 759. w5x N.D. Tomashov, R.M. Altovsky, G.P. Chernova, J. Electrochem. Soc. 108 (1961) 113. w6x (a) N.D. Tomashov, G.P. Chernova, T.A. Fedoseeva, Prot. Met. (USSR) 13 (1977) 134 (b) N.D. Tomashov, G.P. Chernova, T.A. Fedoseeva, Corrosion 36 (1980) 201. w7x (a) E. McCafferty, G.K. Hubler, J. Electrochem. Soc. 125 (1978) 1892 (b) E. McCafferty, G.K. Hubler, Corrosion Sci. 20 (1980) 103. w8x C.W. Draper, L.S. Meyer, D.C. Jacobson, L. Buene, J.M. Poate, Thin Solid Films 75 (1981) 237. w9x P. Munn, G.K. Wolf, Nucl. Instr. Meth. B7y8 (1985) 205. w10x INCO Copernicus Contract No. IC15-CT97-0711 Final Report (1999). w11x S.D. Barson, P. Skeldon, G.E. Thomson, A. Kolitsch, E. Richter, E. Wieser, J. Piekoszewski, A.G. Chmielewski, Z. Werner, Surf. Coat. Technol. 127 (2000) 179. w12x S.D. Barson, P. Skeldon, G.E. Thomson, J. Piekoszewski, A.G. ¨ Chmielewski, Z. Werner, R. Grotzschel, E. Wieser, Corrosion Sci. 42 (2000) 1213. w13x Z. Werner, J. Piekoszewski, A. Barcz, R. Grotzschel, ¨ F. Pokert, J. Stanislawski, W. Szymczyk, Nucl. Instr. Meth. B175–177 { (2001) 767. w14x J. Piekoszewski, J. Langner, L. Walis, ´ Z. Werner, Surf. Coat. Technol. 93 (1997) 209. w15x J. Piekoszewski, R. Grotzschel, ¨ E. Wieser, J. Stanislawski, Z. { Werner, W. Szymczyk, J. Langner, Surf. Coat. Technol. 128– 129 (2000) 394. w16x J. Piekoszewski, Z. Werner, E. Wieser, J. Langner, R. Grotz¨ schel, H. Reuther, J. Jagielski, Nukleonika 44 (1999) 239. w17x J.M. Poate, G. Foti, D.C. Jacobson (Eds.), Surface Modification and Alloying by Laser, Ion, and Electron Beams, Plenum Press, New York, London, 1983, p. 86. w18x M. von Almen, Laser Beam Interactions with Materials, Physical Principles and Applications, Springer, Berlin, Heidelberg, New York, London, Paris, Tokyo, 1987, p. 118. w19x W. Szyszko, Odzialywanie wiazek jonowych i laserowych z { tarczami cial{ stalych { (Interaction of ion and laser beams with solid targets, in Polish), DSci Thesis, UMCS University, Lublin (1997).