Excimer laser surface alloying of titanium with nickel and palladium for increased corrosion resistance

Corrosion Science 47 (2005) 1251–1269 www.elsevier.com/locate/corsci

Excimer laser surface alloying of titanium with nickel and palladium for increased corrosion resistance C. Blanco-Pinzon a, Z. Liu a, K. Voisey a, F.A. Bonilla a, P. Skeldon a,*, G.E. Thompson a, J. Piekoszewski b,c, A.G. Chmielewski b a

Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK b Institute of Nuclear Chemistry and Technology, Dorodna 16, Warzawa 03-195, Poland c The Andrzej Soltan Institute for Nuclear Studies, Otwock-Swierk 05-400, Poland Received 23 March 2004; accepted 24 June 2004 Available online 21 September 2004

Abstract In the electron beam treatment of flue gases, titanium foil is employed as an electron-transparent window. Due to its degradation in the flue gas environment and eventual failure, extension of the life of the window is being sought. Previous studies have indicated significant improvements of corrosion resistance from surface alloying with nickel or palladium, using high intensity pulsed plasma beams, but restricted size of vacuum systems prevents treatment of large surfaces. In the present work, an excimer laser was employed to surface alloy titanium foil with nickel or palladium, using fluences in the range 0.4–1.1 J cm 2 and either nitrogen or argon as the cover gas. The resultant surfaces provided high resistance to corrosion in 0.1 M H2SO4 solution at 80
C that simulates, under accelerated conditions, the degradation of titanium by the flue gas. The improved behaviour is associated with the corrosion potential being shifted to the region of passivity. Treatments at increased fluences reduced losses of nickel and increased alloying of palladium during processing of the foils. Palladium was largely retained during the subsequent immersion tests, contrasting with the depletion of nickel by corrosion *

Corresponding author. Tel.: +44 161 200 4872; fax: +44 161 200 4865. E-mail address: [email protected] (P. Skeldon).

0010-938X/$ - see front matter  2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.06.030

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that limits the durability of the treated foils. The corrosion rates of the optimum palladiumalloyed surfaces were about two orders of magnitude lower than that of untreated titanium.  2004 Elsevier Ltd. All rights reserved. Keywords: Titanium; Palladium; Nickel; Laser alloying; Excimer; Corrosion

1. Introduction Titanium is highly resistant to corrosion in many media, but its corrosion resistance is low in hot sulphuric acid. However, alloying with palladium [1–3], nickel [4–11] or molybdenum [4–9,11,12] can substantially improve the resistance. Such additions shift the corrosion potential into the range of passivity of titanium due to the reduced overpotential for hydrogen evolution in the presence of the alloying elements, although molybdenum possibly influences more the anodic reaction. Surface coating and alloying with these elements have been of recent interest for improvement of the life of titanium foils that are used as electron transparent windows in the removal of pollutants, including SO2, NOx and volatile organic compounds, from flue gases by electron beams [13–16]. The foils, 50 lm thick, undergo corrosion damage in service in a complex mixture of ions, radicals and excited species generated in the flue gas by the electron beam [17–20]. Similar damage results from exposure to sulphuric acid, a product of irradiation, which was used for accelerated laboratory tests at the operational temperature of the window, 80
C [13,14]. In previous work, the surface coating and alloying were accomplished by ion beam mixing, ion-beam-assisted deposition and plasma source ion beam assisted deposition (PSIAD) [14,15,21,22]. Alloying was limited to a narrow interface region between the coating layer and the substrate. However, with the use of high intensity pulsed plasma beams (HIPPB), surface melting and more extensive alloying can be achieved [13]. Such alloying, to depths of below 1 lm, may benefit retention of surface layers, which can detach during exposure to the hot acid when using the previous surface treatments. However, HIPPB and the other approaches are vacuumbased with attendant restrictions on the size of the treated article. Thus, treatment of full-size windows, with typical dimensions 275 · 35 cm, presents practical difficulty. For this reason, laser alloying of titanium foil is investigated here, since the approach has potential for treatment of large areas. Previous laser alloying of titanium for increased corrosion resistance has been confined to palladium, with pre-deposited thicknesses from 15 to 500 nm, and nitrogen additions, using Q-switched and standard Nd–YAG lasers, operating in the infra-red range at a wavelength of 1.06 lm, with respective pulse widths of 25 ns and 1.5 ms [23,24]. Improved behaviours were reported in boiling, concentrated hydrochloric acid [23] and in simulated physiological solution at 37
C [24]. Palladium enriches at the surface in the former environment, which also occurs for other types of palladium treatment, including ion implantation [25,26]. The surfaces tested in the latter environment appeared to be alloyed to depth of the order one hundred microns, although there was no quantification of palladium distributions. Neither

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study examined the detailed composition and morphology of treated surfaces on macroscopic and microscopic scales. In the present work, surface alloying of titanium foil with palladium and nickel is carried out using a KrF excimer laser, with a pulse width of 20 ns. The use of short pulses of ultra-violet irradiation minimizes the thermal penetration depth, which is important in the treatment of foil. The treated surfaces are characterized by a combination of analytical scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA), before and after corrosion tests.

2. Experimental 2.1. Specimen preparation Titanium foil of 99.6% purity (O 0.2%, Fe 0.15%, Al 0.03%, Si 0.03%, C 0.003%, Sn 0.02%, N 0.015%, Mn 0.01%) and 50 lm thickness was employed for laser treatment of specimens of dimensions 2.5 · 2.5 cm on one side only. Nickel and palladium layers, respectively about 40 and 310 nm thick, were first deposited by electron-gun evaporation and PSIAD respectively. PSIAD employed argon as the working gas, with a DC bias of 40 V and a pulsed bias of 250 V, with a duration of 0.5 ms and frequency of 100 Hz. Following cleaning in acetone, surface treatment was carried out using a KrF excimer laser (LPX 210i) of wavelength 248 nm and pulse width 25 ns. A variable number of Hoya plates were used to attenuate the beam, thus providing fluences in the range 0.4–1.1 J cm 2. Additional Hoya plates after the homogenizer reflected part of the beam to a photo-thermal converter, for monitoring the energy of each pulse, and to a CCD camera, for monitoring the uniformity of the laser spot. Specimens were mounted in a stainless steel cell, filled with nitrogen or argon, located on a translation stage. The laser treatment conditions are given in Table 1. 2.2. Corrosion tests and electrochemical measurements Immersion tests were carried out in 0.1 M H2SO4 solution at 80 ± 1
C, with weight changes and open-circuit potentials (OCP) determined for duplicated Table 1 Parameters for excimer laser treatment Wavelength Pulse energy Repetition frequency Pulse duration Laser focused spot Energy density Number of shots Overlap Atmosphere

248 nm 600 mJ 2 Hz 25 ns 5 · 5 mm 0.4–1.1 J cm 2 50 50% Ar or N2 at 2 Bar

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specimens. Potentials were measured against a saturated calomel electrode (SCE) and recorded continuously for the initial 12.5 h (4.5 · 104 s), by which time approximately steady values were achieved. The time of exposure for the immersion tests was usually 50 h. The concentration of dissolved Ti4+ ions was measured after immersion tests of selected specimens with an inductively coupled optical emission axial spectrophotometer (VARIAN Vista MPX) at three emission wavelengths of 334.941, 336.122 and 337.280 nm, with a power of 1 kW, plasma flow of 15 l/min and replica read time of 15 s. Calibration employed 5 and 50 ppm solutions prepared from a certified 1000 ppm solution. For each measurement, 15 ml specimens were nebulized through a concentric nozzle, with argon as a carrier gas, at a flow rate of 0.8 ml/min. Three analyses were carried out for each solution. 2.3. Specimen examination Specimens of treated and untreated titanium foil were analysed by RBS using 1.8 or 2.0 MeV He+ ions supplied by the Van de Graff accelerator of the University of Paris. The beam, of 1 mm diameter, was incident along the normal to the specimen surface, with particles detected at 165
to the direction of the incident beam. Data were interpreted using the RUMP program [27]. The amounts of nickel and palladium in specimens, determined from fitting of RBS data, were accurate to about 10–20%, with uncertainty arising particularly in estimating the depth of alloying at low levels when signals are difficult to resolve from that of the titanium. Oxygen, nitrogen and carbon contents of the near surface regions were determined to an accuracy of about 20% by nuclear reaction analysis (NRA) using 850 keV deuterons, employing the 16O(d, p1)O17, 14N(d, p5)15N and 12C(d, p0)C13 reactions [28–30]. Oxygen and nitrogen were quantified using reference specimens of anodized tantalum containing 7.07 · 1017 oxygen atoms cm 2 and ion-implanted titanium containing 4.0 · 1017 nitrogen atoms cm 2. Amounts of carbon were determined from the yields relative to that of oxygen using the ratios of the relevant cross-sections. Finally, surface morphologies of specimens were examined by scanning electron microscopy (SEM) in an Amray 1810 instrument equipped with energy-dispersive X-ray (EDX) analysis facilities.

3. Results 3.1. Open-circuit potential and weight loss Fig. 1 illustrates typical potential–time behaviours for laser-alloyed specimens, during 12.5 h immersion in H2SO4 solution, sufficient for achievement of relatively steady potentials. Data are illustrated for laser treatments in argon, with similar behaviours being found for treatment in nitrogen. The respective steady potentials of about 265 and 500 mV (SCE) for Ni- and Pd-alloying in argon are about 495 and 1260 mV higher than for untreated titanium (Table 2). The potential for nickel

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100 ------ 0.5 [J cm-2]

40

------ 0.7 [J cm-2]

-20

------ 0.8 [J cm-2]

-80

------ 0.9 [J cm-2]

0.9

-140

------ 1.1 [J cm-2] ------ titanium

OCP (mV vs. SCE)

-200 -260 -320 -380 0.7

0.5

-440 -500 -560 -620

Ti

-680 -740 -800 0

5000

1000 0

15000

(a)

20000

25000

30000

35000

40000

45000

Time (s) 560 480 400 0.9

320 1.1

240

OCP (mV vs. SCE)

160 80

------ 0.8 [J cm-2 ]

0

------ 0.9 [J cm-2 ]

-80

------ 1.1 [J cm-2 ] ------

-160

titanium

-240 -320 -400 -480 -560 Ti

-640 -720 -800 0

(b)

5000

10000

15000

20000

25000

30000

35000

40000

45000

Time (s)

Fig. 1. Potential–time behaviours for (a) Ti–Ni and (b) Ti–Pd laser-alloyed specimens (using argon) during 12.5 h (4.5 · 104 s) immersion in 0.1 M H2SO4 solution at 80
C.

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Table 2 Open circuit potentials and weight loss rates of titanium and laser-treated specimens in 0.1 M H2SO4 at 80
C following respective immersion times of 15 and 50 h Specimen

Laser fluence (J cm 2)

Cover gas

Ti

Potential (mV SCE)

Wt loss rate (g m

760

45.1 ± 0.50

Ti–Ni Laser treated

1.1 1.0 0.8 0.7 0.5 1.1 1.0 0.8 0.7 0.5

Ar ’’ ’’ ’’ ’’ N2 ’’ ’’ ’’ ’’

270 279 252 320 327 – 267 338 311 638

0.74 ± 0.02 1.26 ± 0.08 0.67 ± 0.01 5.72 ± 0.02 12.29 ± 0.03 0.62 ± 0.02 0.93 ± 0.05 3.69 ± 0.01 10.1 ± 0.06 18.40 ± 0.6

Ti–Pd Laser treated

1.1 0.9 0.8 1.1 0.9 0.8

Ar ’’ ’’ N2 ’’ ’’

520 490 488 518 497 494

0.45 ± 0.01 0.42 ± 0.06 0.49 ± 0.04 n.d. n.d. n.d.

2

d 1)

n.d.: Not determined.

treatment applies to fluences above about 0.8 J cm 2, since potentials fell below 300 mV (SCE) at lower fluences (Table 2). For specimens treated in nitrogen at the lowest fluence, 0.5 J cm 2, the potential fell to that of titanium after about 9.2 h immersion due to loss of nickel. In contrast, the fluence had a comparatively small influence on potentials for palladium treatments, for which the highest potentials coincided with the highest fluence. Comparatively low weight loss rates for nickel-treated foils, usually less than 1 g m 2 d 1 for fluences above about 0.8 J cm 2, correlated with increased open-circuit potentials (Table 2). Errors in rates result from weighing and area measurements. Selected solution analyses, by ICP-OES, determined low quantities of titanium in the solutions, consistent with the previous low weight loss, with titanium levels corresponding to corrosion rates of <0.7 g m2 d 1. In contrast to these low rates, titanium corrodes at least 50 times faster in tests limited to 15 h, since by 50 h the foil has disintegrated through the action of general and localized corrosion. Relatively high weight loss rates for the nickel-treated foils were recorded at a fluence of about 0.5 J cm 2, with the highest rate being due to depletion of nickel during immersion. Rates were generally higher with nitrogen than with argon as the cover gas. Weight losses of palladium-treated foils, using argon, were lower than those of the nickel-treated foils, with values of about 0.5 g m 2 d 1 for all fluences, with similar titanium losses determined from ICP-OES. Weight losses were not determined for foil treated in nitrogen, but later EDX and RBS analyses indicate similar behaviour as the foils treated in argon.

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3.2. Surface characterization Scanning electron micrographs of the untreated titanium before immersion revealed features due to rolling of the foil (Fig. 2(a)). Extensive intergranular and general corrosion occurred during immersion for 15 h in H2SO4 solution, (Fig. 2(b)). RBS spectra revealed nickel and palladium layers of relatively uniform thickness, 40 and 310 nm respectively, containing 3.6 · 1017 Ni atoms cm 2 and 2.0 · 1018 Pd atoms cm 2 (Fig. 3(a) and (b)) on PVD-coated foils. Further, rolling features remained evident, with the surfaces appearing similar to that of untreated foil by SEM. Laser treatments melted and smoothed the surface regions of the coated foil, with less prominent rolling lines indicating a shallow melting zone. For nickel-alloyed surfaces, treatments in argon that gave relatively low and high weight losses provide representative examples. Foil treated at a fluence of 0.8 J cm 2, with low weight loss, disclosed initially numerous depressions and raised, nodular, features both of dimensions of the order 1 lm (Fig. 4(a)). Approximately cube-shaped features within the melted surface were suggestive of second phase. From EDX spot analyses, the nickel content was significantly higher in the nodules than in the adjacent regions (Table 3). Occasional locations contained very low amounts of nickel. The EDX results are semi-quantitative due to the depth of the region of X-ray generation exceeding the thickness of the nickel-rich surface regions. RBS indicated alloying to a depth of

Fig. 2. Scanning electron micrographs of the untreated titanium foil (a) before immersion in sulphuric acid solution and (b) following immersion for 15 h in 0.1 M H2SO4 solution at 80
C.

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Energy (MeV) 2500

0.5

1.0

1.5

0.5

2.0

1.0

1.5

8000

Ni at surface underneath

6000

1500

Counts

Counts

2000

2.0

Pd at surface underneath

4000

1000

Buried Ti

2000

500

buried Ti 0 100

(a)

200

300

400

Channel

500

0 100

600

(b)

200

300

400

500

600

Channel

Fig. 3. Experimental and simulated (solid line) RBS spectra of the titanium foil following PVD coating (a) with nickel and (b) with palladium.

Fig. 4. Scanning electron micrographs of the titanium foil following laser alloying with nickel (using Ar) at 0.8 J cm 2 (a) before immersion in sulphuric acid solution and (b) following immersion for 50 h in 0.1 M H2SO4 solution at 80
C.

about 200 nm, with nickel present at total levels of 3.3 · 1017 atoms cm 2 (Fig. 5(a)). Thus, little of the original nickel was lost through laser treatment. Traces of depressions, but no nodular features, remained after the immersion test (Fig. 4(b)). Corrosion of the foil was minor, with none of the severe degradation associated with

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Table 3 Results of EDX analyses of the titanium foil following laser alloying with nickel (using argon) and palladium (using nitrogen) and later immersion for 50 h in 0.1 M H2SO4 solution at 80
C Fluence (J cm 2)

Location

Ni (at.%) (before imm.)

Ni (at.%) (after imm.)

0.8 0.8 0.8 0.5

General area Nodule Dark region General area

42.6 87.0 2.9 <0.9

7.4–32.4 – – Not detected

Pd (at.%) (before imm.)

Pd (at.%) (after imm.)

1.1 1.1 1.1

Light region Grey region Dark region

77.6 41.6 23.4

70.1 42.7 22.1

0.8 0.8 0.8

Light region Grey region Dark region

67.2 45.7 3.6

66.7 62.1 2.3

The concentrations do not include the presence of oxygen or other non-metallic species.

Energy (MeV) 4000

0.5

1.0

1.5

Energy (MeV) 2.0

4000

Counts

Counts

1.0

1.5

2.0

3000

3000

2000

0.5

buried Ti

Ti at surface 2000

Ni at surface 1000

1000

(a)

0 100

200

300

400

Channel

500

600

(b)

0 100

200

300

400

500

600

Channel

Fig. 5. Experimental and simulated (solid line) RBS spectra of the titanium foil following laser alloying with nickel (using Ar) at 0.8 J cm 2 (a) before immersion in sulphuric acid solution and (b) following immersion for 50 h in 0.1 M H2SO4 solution at 80
C.

untreated foil. By this stage, much nickel had been lost from the surface, with no more than about 6 · 1016 atoms cm 2 remaining that corresponds to about 17% of the initial amount (Fig. 5(b)). The reduced level of nickel was confirmed by findings of EDX analyses, which disclosed differing levels of nickel across the surface reflecting the non-uniformity of the initial surface condition (Table 3). Various fine, cavities, some with cube-shapes, decorated the surface, suggesting loss of second phase material. The foil was able to survive with little damage for a further 50 h in the H2SO4 solution. A foil treated with a fluence of 0.5 J cm 2, with high weight loss, disclosed traces of rolling lines, with few nodular features or depressions of the previous types. Further, immersion in the H2SO4 solution resulted in extensive intergranular

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0.5

1.0

Energy (MeV)

1.5

2.0

2500

2500

0.5

1.0

1.5

2.0

2000

Ti at surface

Counts

Counts

2000

1500

Ti at surface

1500

1000 1000

Ni at surface

500

500

0 100

(a)

200

300

Channel

400

500

600

0 100

(b)

200

300

400

500

600

Channel

Fig. 6. Experimental and simulated (solid line) RBS spectra of the titanium foil following laser alloying with nickel (using Ar) at 0.5 J cm 2 (a) before immersion in sulphuric acid solution and (b) following immersion for 50 h in 0.1 M H2SO4 solution at 80
C.

corrosion and general corrosion, similar to that of the untreated foil. According to RBS and EDX analyses, laser-treatment removed most of the deposited nickel, with none being detectable after immersion in acid (Fig. 6(a) and (b), Table 3). The RBS spectrum for the foil prior to the immersion test revealed a small peak for nickel at the surface, corresponding to about 1.5 · 1016 atoms cm 2. There may be some additional buried nickel due to a low level of alloying that is not resolved. Foils laser-treated with palladium resulted in low weight loss. Morphologies and compositions are compared for foils treated at the highest and lowest fluences in argon, which are typical. With the highest fluence of 1.1 J cm 2, the initial surface was decorated by a lace-like pattern of palladium-rich regions, which appear light in scanning electron micrographs (Fig. 7(a)). These regions, typically of dimensions 20–40 lm but sometimes extending in length to more than 100 lm, were raised above the general level of the surface and contained high levels of palladium according to EDX analyses (Table 3). The pattern was coarser than that of the rolling features of the initial foil. Sub-micron dark spots suggested fine cavities. The remaining relatively flat areas, representing more than 50% of the surface, were dark, but with parts of slightly lighter contrast. EDX analyses disclosed increased levels of palladium in the latter parts, although all flat regions were palladium-lean relative to the raised material (Table 3). Following immersion in H2SO4 solution, the pattern of the palladium-rich light regions was still evident (Fig. 7(b)). However, the regions of intermediate contrast in the flat areas were now mainly of similar lightness as the palladium-rich regions. EDX analyses revealed little difference in the palladium content of surfaces before and after the immersion test (Table 3). The network of palladium-rich material contained cracks that were absent in the original laser-treated surface (Fig. 7(c)). There was no significant attack of the titanium foil at the areas most depleted in palladium that now formed islands in the final palladium-rich surface.

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Fig. 7. Scanning electron micrographs of the titanium foil following laser alloying with palladium (using N2) at 1.1 J cm 2 (a) before immersion in sulphuric acid solution and (b,c) following immersion for 50 h in 0.1 M H2SO4 solution at 80
C.

The redistribution of palladium following surface alloying, and the later enrichment of palladium at the surface following exposure to the H2SO4 solution, were further confirmed by RBS (Fig. 8). Non-uniformity of the surface composition over the analysed area, which was evident by SEM, circumvents precise interpretation of the spectra. However, an edge due to titanium at the surface, readily apparent prior to immersion, decreased following exposure to the H2SO4 solution, while the signal for surface palladium increased. A significant tail on the palladium signal is probably due to a combination of alloying and redistribution of material during surface melting. The extent of the tail indicated palladium to depths of about 1 lm. The average concentrations of palladium on the initial and final surfaces were about 55 and

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40 00

0. 5

1. 0

1.5

Energy (MeV) 2.0

1.0

4000

1.5

2.0

buried Ti Pd at surface

30 00

20 00

10 00

0 1 00

(a)

Pd at surface

3000

Counts

Counts

buried Ti

2000

1000

20 0

30 0

40 0

Channel

500

600

0 100

(b)

Pt

200

300

400

500

600

Channel

Fig. 8. Experimental and simulated (solid line) RBS spectra of the titanium foil following laser alloying with palladium (using N2) at 1.1 J cm 2 (a) before immersion in sulphuric acid solution and (b) following immersion for 50 h in 0.1 M H2SO4 solution at 80
C.

70 at.% respectively, considering only titanium as the other constituent. The amounts of palladium in the surface regions corresponded to 1.2 · 1018 and 0.9 · 1018 palladium atoms cm 2 for the laser-alloyed and immersion-tested specimens respectively. The difference is not significant considering the accuracy of the RBS analyses and variability in the initial amount of palladium between specimens. Further, EDX analyses indicated minor influence of the immersion in acid on levels of palladium (Table 3). The apparently reduced level of palladium relative to that of the initially coated foil, about 2.0 · 1018 palladium atoms cm 2, may be partly due to variations in the deposit thickness and difficulty in identifying the precise depth of the palladium layer following laser treatment. However, some loss of palladium may have occurred during alloying. Small enrichments of platinum, typically about 1 · 1015 Pt atoms cm 2, were often present at the surfaces of the foils before and after the immersion tests. These enrichments, arising from platinum impurity in the palladium source, are formed during deposition of the palladium by PSIAD [14]. In contrast to the greatly modified morphology and composition of the surface achieved by alloying at a fluence of 1.1 J cm 2, the influence of laser treatment at 0.8 J cm 2 on the distribution of palladium was less pronounced. Scanning electron micrographs revealed melting of the palladium coating, leading to elimination of much of the evidence of rolling and development of numerous fine cavities of size about 1 lm, with occasional islands at which the titanium substrate was exposed (Fig. 9(a)). RBS revealed a total palladium content of about 1.4 · 1018 palladium atoms cm 2 (Fig. 10(a)). Similar to the previous specimen, the amount of palladium was apparently reduced by laser treatment. Any alloying was limited to a depth of the order 100 nm as indicated by the tail on the palladium signal, although the tail may be largely an effect of non-uniform thickness of the palladium layer. The average composition at the surface was about 86 at.% palladium, with the decrease below 100% being mainly attributable to exposure of titanium at the base of cavities.

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Fig. 9. Scanning electron micrographs of the titanium foil following laser alloying with palladium (using N2) at 0.8 J cm 2 (a) before immersion in sulphuric acid solution and (b) following immersion for 50 h in 0.1 M H2SO4 solution at 80
C.

Energy (MeV) 1.0

5000

1.5

buried Ti

Pd at surface

Counts

3000

2000

2000

1000

1000

(a)

200

300

400

Channel

500

1.5

2.0

4000

3000

0 100

1.0

5000

4000

Counts

Energy (MeV) 2.0

0 100

600

(b)

Pd at surface buried Ti

200

300

400

500

600

Channel

Fig. 10. Experimental and simulated (solid line) RBS spectra of the titanium foil following laser alloying with palladium (using N2) at 0.8 J cm 2 (a) before immersion in sulphuric acid solution and (b) following immersion for 50 h in 0.1 M solution at 80
C.

Following exposure to the H2SO4 solution, the morphology and composition of the surface region were almost unchanged (Figs. 9(b) and 10(b)). The absence of significant loss of palladium was confirmed by results of EDX analyses (Table 3).

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3.3. Nuclear reaction analysis NRA spectra for the original titanium, PVD-coated titanium and laser-treated titanium revealed peaks associated with the 12C(d, p0)C13, 16O(d, p0)O17, 16 O(d, p1)O17 and 14N(d, p5)15N reactions for laser treatments in both nitrogen and argon; typical examples are shown for palladium-alloyed foils treated in argon (Fig. 13). The results of NRA are summarized in Table 4, which indicate the amounts of oxygen, carbon and nitrogen in the outer  500 nm of the foils. The levels of these species were negligibly dependent upon the conditions of laser treatment. The oxygen contents in the surface regions of the laser-treated foils ranged from 0.9 · 1017 to 2.0 · 1017 atoms cm 2, the levels decreasing following immersion in the H2SO4 solution by up to about 40%. The oxygen peaks were asymmetric, with tails toward low energies due to oxygen in the bulk foil, about 0.5 at.%. The carbon and nitrogen levels ranged from 1.3 · 1016 to 1.1 · 1017 atoms cm 2 and 1 · 1015 to 1.8 · 1016 atoms cm 2 respectively, with negligible dependence upon immersion in the H2SO4 solution. Oxygen, carbon and nitrogen levels in the untreated titanium and in the PVD-coated foil were within, or close to, the previous ranges. Carbon is mainly an adventitious contaminant from the atmosphere and cleaning of specimens either prior to or following immersion tests in H2SO4 solution. The amounts of oxygen and nitrogen in laser-treated specimens were similar to those detected previously in foil coated with palladium, using PSIAD, prior to laser treatment [14]. Thus, the near-surface oxygen and nitrogen contents of the present laser-treated foils are mainly due to pre-existing contaminants. Oxygen is probably associated with oxides formed on exposed surfaces or due to incorporation of residual oxygen species in the layers deposited by the PVD processes (Fig. 11).

Table 4 Results of NRA analyses of foils before (b) and after (a) immersion for 50 h in 0.1 M H2SO4 at 80
C Specimen

Laser fluence (J cm 2)

Cover gas

Oxygen (atoms cm 2)

Nitrogen (atoms cm 2)

Carbon (atoms cm 2)

Untreated Ti

–

–

1.0 · 1017

1 · 1015

2.0 · 1016

17

PVD-coated Pd PVD-coated Ni

– –

– –

1.8 · 10 1.0 · 1017

4.4 · 10 2 · 1015

1.3 · 1016 8.0 · 1016

Ti–Ni laser treated

0.5 (b) 0.5 (a)

N2 ’’

1.0 · 1017 0.9 · 1017

5 · 1015 7 · 1015

3.8 · 1016 7.0 · 1016

1.1 1.1 0.8 0.8 1.1 1.1 0.8 0.8

Ar ’’ ’’ ’’ N2 ’’ ’’ ’’

1.8 · 1017 1.1 · 1017 2.0 · 1017 1.8 · 1017 1.6 · 1017 1.2 · 1017 2.0 · 1017 1.4 · 1017

1.0 · 1016 9 · 1015 1.6 · 1016 1.8 · 1016 7 · 1015 4 · 1015 4 · 1015 1 · 1015

7.9 · 1016 1.6 · 1016 3.0 · 1016 2.9 · 1016 8.1 · 1016 1.3 · 1016 6.2 · 1016 1.11 · 1017

Ti–Pd laser treated

(b) (a) (b) (a) (b) (a) (b) (a)

16

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500 16

O(d,p1)17O

Counts

400

12

C(d,po)13C

300 200 14

100

N(d,p5)15N 16 O(d,po)17O

0 0

100

200

300

400

Channel Fig. 11. NRA spectra for titanium foil following laser alloying with palladium (using Ar) at 1.1 J cm 2 before and after immersion for 50 h in 0.1 M H2SO4 solution at 80
C. The spectrum for the specimen before immersion reveals higher oxygen peaks and a lower carbon peak than that for the specimen following immersion.

4. Discussion 4.1. Weight loss rates in H2SO4 solution Excimer laser alloying of the titanium foil with palladium, in argon, resulted in weight loss rates that are about two orders of magnitude lower than that of untreated titanium. Further, the rates are similar to those achievable using high intensity pulsed plasma beams [13]. In the case of alloying with nitrogen, small weight gains were recorded. However, these were not related to pick-up of nitrogen, and their origin is uncertain. Weight loss rates were generally higher with nickel alloying than with palladium alloying, and the surfaces were depleted of nickel by exposure to the H2SO4 solution, such that by the end of a 50 h period the amount of nickel remaining on the surface was sometimes negligible. Thus, although under appropriate laser alloying conditions, low weight losses resulted, the durability of the treated surface in the H2SO4 solution was inferior to that produced using palladium. The loss of nickel from the surface of nickel-alloyed titanium in H2SO4 solutions has been reported previously for plasma-treated material and also for bulk alloy [10,31]. Eventual loss of all the nickel on the present specimens was followed by rapid corrosion of the underlying titanium. 4.2. Losses of deposited alloying element during laser treatments A drawback of the nickel alloying was the relatively large loss of the deposited nickel during the laser treatment at relatively low fluence, about 0.5 J cm 2, presumed to be due to vaporization. The melting points of titanium, palladium and nickel are 1850, 1549 and 1450
C respectively. Low fluences may therefore melt the nickel layer but not the underlying titanium to any significant extent. Redistribution of melted nickel according to its surface tension may increase the surface area in repeated

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melting cycles. Vaporization of nickel, without significant alloying, is suggested to result in reduced levels of nickel on the final surface. However, increased fluences, with more extensive melting of the titanium, promote alloying, and reduce nickel loss. The nodular features of nickel-alloyed foils possibly arise from redeposition of nickel. Palladium, on the other hand, is largely retained at low fluences, possibly due to a relatively low vapour pressure. Although there were no significant effects of overlapping on the corrosion behaviours of the foils, a fine band of material of slightly modified morphology was produced at the edges of the overlap steps, probably coinciding with the declining energy density at the edge of the beam. Consistent with the negligible effects on corrosion, analyses of surface features by EDX indicated levels of nickel broadly similar to those on the treated by the middle regions of the beam. 4.3. Mechanism of protection The mechanism of protection of titanium by nickel and palladium is usually attributed to the reduced overpotential for hydrogen generation that results in a shift in the open-circuit potential from the region of corrosion to the region of passivity [1–11]. The potential of the nickel-alloyed foil decreased with reduced alloying element in the original surface and further decreased as the nickel was lost by corrosion. In the present work, the absence of nodular features following exposure to the H2SO4 solution was the most readily evident indication of nickel loss. Eventually, the level of nickel was no longer sufficient to maintain the passive state and rapid corrosion ensued. In the case of palladium coatings, melting and redistribution of the palladium proceeded with limited alloying at low fluence. At increased fluences, alloying was achieved to significant depth, although the palladium was distributed non-uniformly on the final surface. Some regions were covered by a significant thickness of relatively pure palladium, while other regions, of increased alloying, revealed comparatively high levels of titanium. The alloyed material may also lie beneath the palladium-rich regions. Following immersion in the H2SO4 solution, palladium enriches at the surface of the alloyed regions, as observed previously for laser-melted Ti–Pd alloy [23]. The palladium-treated alloys exhibited surface cracking following exposure to the acid, possibly due to an environmentally-assisted mechanism assisted by high tensile stress following melting and solidification of the surface material. The process may be facilitated by absorption of hydrogen by the palladium. The weight loss rates achieved with palladium alloying were about a factor of 10 lower than that found in previous work for boiling, concentrated hydrochloric acid solution [23], which is reasonable given the differences in test conditions. 4.4. Compositions of surface regions The nickel and palladium alloying is confined to layers of depth less than about 1 lm. Both nickel and palladium have relatively low solubility in titanium, with the probability of generation of intermetallics, such as Ti2Ni and Ti2Pd [32]. Further, the composition of the alloyed layer varies over the melt depth, with influence on sta-

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bility of phases developed under the rapid cooling conditions of the laser treatment. Thus, alpha, stabilized beta, and martensite phases may co-exist with intermetallics leading to a high hardness of the surface layer. Nitrogen and oxygen are also present either in solid solution or as nitride and oxide. However, neither species appears to be related to laser alloying to any significant extent, since results of analyses were independent of the alloying conditions. Evidently, the absence of significant nitriding of the surface during laser treatments with nitrogen may relate to either the short timescale for reaction or a barrier property of nickel and palladium, or intermetallic reaction products. In contrast, nitridation has been reported during laser surface melting of titanium in nitrogen at higher fluence than used in the present work [33]. The oxygen detected in the foils after laser alloying and immersion in the H2SO4 solution probably arises from oxide films on the titanium foil, with rolling resulting in a high surface area and incorporation of film material, and any oxygen associated with deposited palladium and nickel. Notably, the rolling lines in the foil surface are of a depth similar to that of the oxygen detected by NRA. Melting of the surface may disrupt oxide films and incorporate oxide particles into the molten metal, with their reduction to sub-oxide or solid solution species in repeated melting cycles. The solubility of oxygen in titanium is relatively high and gives rise to embrittlement of surface regions [34]. Melting and redistribution of nickel or palladium on the surface of the titanium, through either surface energy effects or vaporization and deposition, can also affect the apparent depth of the oxygen as a non-uniform thickness of the alloy is generated. The corrosion of initially exposed titanium, which occurs in the palladium-lean areas of the laser-treated surface, leads to a surface enrichment of palladium at these sites. Loss of titanium will be accompanied by loss of any oxygen contained within the corroded layer, and hence a decrease in the total amount of oxygen in the foil (Table 4). The enrichment of palladium at the surface of the foils during immersion in the H2SO4 solution possibly proceeds by a de-alloying process, with formation of nano-particles of palladium [3], which, in unpublished work of the authors, were found to detach as layers from HIPPB-treated foils. However, detachment may also be assisted by the presence of incorporated oxide. Thus, further studies are needed of foils laser-alloyed following initial removal of deformed surface layers of higher oxygen content, for example using chemical polishing or mechanical polishing. From a practical point of view, the laser treatments offer similar levels of corrosion resistance as achieved by HIPPB treatments, with the added capability of the possibility of treatment of full-size windows for flue gas, clean-up systems. In order to further the technology, laser-treated windows require evaluation at the pilot-scale in flue gas environments under electron irradiation and in the presence of operational stresses.

5. Conclusions 1. Excimer laser alloying of titanium foil with palladium and nickel results in major improvement of the corrosion resistance in 0.1 M H2SO4 solution at 80
C. The improved behaviour is associated with shifting of the potential to the region of passivity of titanium.

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2. Nickel is lost from the surface during immersion in the H2SO4 solution, while losses of palladium, which is enriched in the surface regions of the foil, are negligible. Thus, palladium alloying results in extended life of the foils. 3. Low fluences during laser treatment are associated with relatively large losses of the pre-deposited nickel and reduced alloying of palladium. 4. Laser-treated surfaces retain relatively high concentrations of oxygen originating in the rolled surface of the initial titanium foil and the deposited palladium and nickel layers, with some loss of oxygen occurring during subsequent immersion in the H2SO4 solution. Nitriding of the surfaces during treatment in nitrogen gas was negligible.

Acknowledgments The authors are grateful to the Engineering and Physical Sciences Research Council for use of the excimer laser at the Rutherford Appleton Laboratory. They also wish to thank Dr. C. Ortega of the Groupe de Physique des Solides, Universite´s Paris 7 et 6, for assistance with RBS measurements (work partially supported by the Centre National de la Recherche Scientifique (GDR86)). The project was supported by the European Community through the International Scientific Co-operation Projects scheme (Contract No. ICA2-CT-2000-10005). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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