Analyzing the Influence of Alloying Elements and Impurities on the

Feb 2, 2008 - Scanning electrochemical microscopy (SECM) was used to study the ... element analyses using the platform of COMSOL multiphysics software...
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Anal. Chem. 2008, 80, 1437-1447

Analyzing the Influence of Alloying Elements and Impurities on the Localized Reactivity of Titanium Grade-7 by Scanning Electrochemical Microscopy Renkang Zhu, Ziqiang Qin, James J. Noe 1 l, David W. Shoesmith,* and Zhifeng Ding*

Department of Chemistry, The University of Western Ontario, London, ON N6A 5B7, Canada

Scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) was applied to investigate the grain boundaries on ASTM grade-7 titanium (Ti-7) with a freshly polished surface, and the results showed that the alloying element, Pd, and the impurity, Fe, cosegregated to grain boundaries. Scanning electrochemical microscopy (SECM) was used to study the variations in reactivity on Ti-7 exposed to an aerated neutral solution of 0.1 M NaCl. Locations that possessed an enhanced reactivity compared to the oxide-covered (TiO2) surface of the grains on SECM images were proposed to be the boundaries. These areas were further activated by the application of a cathodic bias, and interconnection of the active locations allowed the construction of “grain boundary maps”. Variations in surface reactivity were quantitatively analyzed by fitting probe approach curves (PACs) to curves simulated with a model based on finite element analyses using the platform of COMSOL multiphysics software. The difference in reactivity between active grain boundaries and oxide-covered grains was up to a factor of 100 on freshly polished surfaces. This difference decreased to a factor of 10-15 after longer-term exposure of the Ti-7 to the aerated solution, indicating partial passivation of the Pd/Fe-stabilized β-phase in the grain boundaries. PAC analyses of oxide-covered grains showed that the reactivity increased logarithmically as the bias potential to the Ti-7 was decreased, consistent with reduction of the insulating TiO2 layer to a more conductive TiOOH layer. Due to their proven corrosion resistance,1,2 titanium and its alloys are being considered as candidate materials for the fabrication of high-level nuclear waste containers3-10 and associated corrosion-resistant structures. To enhance corrosion resis* To whom correspondence should be addressed. (D.W.S.) Phone: 519-6612111, ext. 86366. Fax: 519-661-3022. E-mail: [email protected]. (Z.D.) Phone: 519-661-2111, ext. 86161. Fax: 519-661-3022. E-mail: [email protected]. (1) Schutz, R. W.; Thomas, D. E. Corrosion of Titanium and Titanium Alloys. Metals Handbook, 9th ed.; ASM: Materials Park, OH, 1987; Vol. 13, pp 670706. (2) Been, J.; Grauman, J. S. In Uhlig’s Corrosion Handbook, 2nd ed.; Revie, R. W., Ed.; John Wiley and Sons, Inc.: New York, 2000; p 863. (3) Shoesmith, D. W.; Ikeda, B. M.; LeNeveu, D. M. Corrosion 1997, 53, 820829. (4) Shoesmith, D. W.; Noel, J. J.; Hardie, D.; Ikeda, B. M. Corros. Rev. 2000, 18, 331-359. (5) Nakayama, G.; Nakamura, N.; Fukaya, Y.; Akashi, M.; Ueda, H. Eur. Fed. Corros. Publ. 2003, 36, 373-394. 10.1021/ac701796u CCC: $40.75 Published on Web 02/02/2008

© 2008 American Chemical Society

tance, alloying elements such as Mo and Ni (Ti-12), Pd (Ti-16 and Ti-7), and Ru (Ti-26 and Ti-27) are added.1,2,11-13 In particular, the Ti-7 alloy is being considered by the United States Department of Energy as the primary candidate for the fabrication of drip shields to protect waste packages from seepage drips in the proposed high-level waste repository at Yucca Mountain, Nevada.7,9 The ability of the Pt group metals (PGM) to suppress the corrosion of Ti alloys under low-pH conditions has been attributed to their ability to cathodically modify the material, i.e., to catalyze proton reduction leading to cathodic currents in excess of the critical anodic current for metal dissolution, thereby forcing the alloy to adopt a potential in the passive region when the surface is covered by a thin, chemically inert TiO2 oxide. This mechanism is characterized in acidic solutions by the occurrence of cathodic currents at lower potentials with smaller Tafel slopes on PGMcontaining materials than on commercially available R-Ti (Ti-2).14,15 Evidence also exists to show the accumulation of Pd on Pd-Ti alloy surfaces during corrosion in acidic solutions,16,17 a process that would also catalyze proton reduction. The PGMs exhibit a low solubility in R-Ti (,0.5 wt %) and can lead to the formation of β-phase or the precipitation of intermetallic compounds.11 At the levels used in commercial Pdcontaining alloys, phases such as Ti2Pd could precipitate in the R-matrix.11 The ubiquitous impurity, Fe, can also stabilize β-phase (6) Nakayama, G.; Futraya, Y.; Akashi, M.; Sawa, S.; Kanno, T.; Owada, H.; Otsaki, A.; Asano, H. Hydrogen-Induced Stress Corrosion Crack Initiation and Propagation in Titanium Alloys in Deep Underground Environments. Prediction of Long Term Corrosion Behaviour in Nuclear Waste Systems; Andra: Chatenary-Malabry, France, 2004, p 35. (7) BSC Hydrogen-Induced Cracking of the Drip Shield; ANL-EBS-MD-000006 rev 02; Bechtel SAIC Company: Las Vegas, NV, 2004. (8) Hua, F.; Mon, K.; Pasupathi, P.; Gordon, G.; Shoesmith, D. Corrosion 2005, 61, 987-1003. (9) Hua, F.; Mon, K.; Pasupathi, P.; Gordon, G.; Shoesmith, D. JOM 2005, 57, 20-26. (10) Shoesmith, D. W. Corrosion 2006, 62, 703-722. (11) Schutz, R. W. Corrosion 2003, 59, 1043-1057. (12) Van der Lingen, E.; Sandenbergh, R. F. Corros. Sci. 2001, 43, 577-590. (13) Schutz, R. W. Platinum Met. Rev. 1996, 40, 54-61. (14) Schutz, R. W.; Xiao, M. Corros. Control Low-Cost Reliab., Proc.sInt. Corros. Congr., 12th 1993, 3A, 1213-1225. (15) Fukuzuka, T.; Shimogori, K.; Satoh, H. Role of Palladium in Hydrogen Adsorption of Ti-Pd Alloy. In Titanium’80, Science and Technology, Proceedings of the Fourth International Conference on Titanium, Metallurgical Society of AIME: Warrendale, PA, 1980. (16) Hubler, G. K.; McCafferty, E. Corros. Sci. 1980, 20, 103-116. (17) Armstrong, R. D.; Firman, R. E.; Thirsk, H. R. Corros. Sci. 1973, 13, 409420.

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and precipitate in intermetallic compounds in R-Ti alloys. For intermediate Fe contents, Fe-containing β-phase is stabilized along grain boundaries, and for sufficiently high Fe contents. Watanabe et al.18 demonstrated the formation of TixFe precipitates, especially at grain boundary triple points. That the presence of Fe destabilizes oxide films at grain boundary locations is suggested by the potentiostatic experiments of Curty and Virtanen19 in which small current transients associated with film breakdown/repair events are observed on Fe-containing Ti-2 but not on high-purity Ti (99.99%).20 These observations raise the possibility that the distribution of Pd could be influenced by its coprecipitation with Fe, as suspected by Ikeda and co-workers21,22 for Pd in Ti-16 and demonstrated by Van der Lingen and Sandenbergh12 for Ru in Ti-0.15 wt % Ru. These last authors clearly demonstrated that the β-phase present in their alloy contained up to 12% Ru and 5% Fe. The location of these phases and precipitates could then provide “windows” in the oxide at which proton reduction would be catalyzed leading to enhanced hydrogen absorption under cathodically polarized conditions. In recent years, scanning electrochemical microscopy (SECM)23-29 has been developed for a wide range of applications in the areas of chemical and biochemical kinetics,30-34 chemical activity imaging,28,35-43 and micrometer-scale structur(18) Watamabe, T.; Shindo, T.; Naito, H. Effect of Iron Content on the Breakdown Potential for Pitting of Titanium in NaCl Solutions. Presented at the 6th World Conference on Titanium, Cannes, France, 1988. (19) Curty, C.; Virtanen, S. Proc.sElectrochem. Soc. 1999, 99-27, 445-452. (20) He, X.; Noel, J. J.; Shoesmith, D. W. Corrosion 2004, 60, 378-386. (21) Ikeda, B. M.; Quinn, M. J. A Preliminary Examination of the Effects of Hydrogen on the Behaviour of Grade-16 Titanium at Room Temperature; Ontario Power Generation Report No: 06819-REP-01200-0078-R00.1998. (22) Ikeda, B. M.; Quinn, M. J.; Noel, J. J.; Shoesmith, D. W. The hydrogeninduced cracking and hydrogen absorption behaviour of grade-16 titanium. In Environmentally Induced Cracking of Metals; Proceedings of the International Symposium Elboujdaini, M., Ghali, E., Zheng, W., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, QC, Aug 20-23, 2000; pp 235-248. (23) Bard, A. J. In Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001; pp 1-17. (24) Amemiya, S.; Ding, Z.; Zhou, J.; Bard, A. J. J. Electroanal. Chem. 2000, 483, 7-17. (25) Basame, S. B.; White, H. S. Proc.sElectrochem. Soc. 1999, 99-5, 15-22. (26) Spaine, T. W.; Baur, J. E. Anal. Chem. 2001, 73, 930-938. (27) Turyan, I.; Orel, B.; Reisfeld, R.; Mandler, D. Phys. Chem. Chem. Phys. 2003, 5, 3212-3219. (28) Wittstock, G.; Burchardt, M.; Pust, S. E.; Shen, Y.; Zhao, C. Angew. Chem., Int. Ed. 2007, 46, 1584-1617. (29) Diakowski, P. M.; Ding, Z. Electrochem. Commun. 2007, 9, 2617-2621. (30) Ding, Z.; Quinn, B. M.; Bard, A. J. J. Phys. Chem. B 2001, 105, 6367-6374. (31) Fan, F.-R. F. In Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001; pp 111-143. (32) Sun, P.; Laforge, F. O.; Mirkin, M. V. Phys. Chem. Chem. Phys. 2007, 9, 802-823. (33) Mandler, D.; Unwin, P. R. J. Phys. Chem. B 2003, 107, 407-410. (34) Edwards, M. A.; Martin, S.; Whitworth, A. L.; Macpherson, J. V.; Unwin, P. R. Physiol. Meas. 2006, 27, R63-108. (35) Zhao, X.; Petersen, N. O.; Ding, Z. Can. J. Chem. 2007, 85, 175-183. (36) Zhu, R.; Macfie, S. M.; Ding, Z. J. Exp. Bot. 2005, 56, 2831-2838. (37) Zhu, R.; Ding, Z. Can. J. Chem. 2005, 83, 1779-1791. (38) Zhu, R.; Nowierski, C.; Ding, Z.; Noeel, J. J.; Shoesmith, D. W. Chem. Mater. 2007, 19, 2533-2543. (39) Lee, Y.; Ding, Z.; Bard, A. J. Anal. Chem. 2002, 74, 3634-3643. (40) Fan, F.-R. F.; Bard, A. J. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 1422214227. (41) Wipf, D. O.; Bard, A. J. Anal. Chem. 1992, 64, 1362-1367. (42) Zoski, C. G.; Simjee, N.; Guenat, O.; Koudelka-Hep, M. Anal. Chem. 2004, 76, 62-72. (43) Diakowski, P. M.; Ding, Z. Phys. Chem. Chem. Phys. 2007, 9, 5966-5974.

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ing23,28,44,45 at liquid/liquid, liquid/solid, and liquid/membrane interfaces. The most significant advantage offered by SECM is its capability to probe interfacial electron and ion transfer processes occurring nonuniformly at interfaces. The technique has also been used to study a wide range of different processes on Ti and its alloys. Smyrl, White and co-workers46-48 studied the precursor sites for pitting corrosion on pure Ti surfaces, and Basame and White49-51 investigated the spatially localized electrochemical reactions occurring on a pure Ti electrode covered by a thin oxide film in aqueous solution and measured current densities at microscopic redox-active sites on Ti.51 They also studied25,51 electron-transfer reactions at randomly distributed sites with microscopic dimensions of 2-50 µm on oxide-covered Ti and tantalum (Ta). Fusimi et al.52 studied oxygen evolution on a polycrystalline Ti sample in an extremely acidic solution (0.1 mol/L sulfuric acid, H2SO4), and Hassel et al.53 carried out local analyses of anodic oxide films on single-crystal Ti and found differences in the surface reactivity due to the differences in oxide thickness. Schulte et al.54 used pH microsensors as SECM tips to measure changes in local acidity/basicity and to visualize localized corrosion processes occurring at microscopic defects in the passive oxide film on NiTi shape memory alloys. In our previous paper,38 SECM was used to map variations in surface reactivity on Ti-2 surfaces by using ferrocenemethanol (Fc) as a redox mediator. The most reactive regions on the surface were found to be associated with the triple points and/or grain boundaries and were attributed to the location of the impurity Fe at these sites. On the basis of the SECM images, the grain boundary structures were built and the localized reactivity was studied. In this report, we employed SECM combined with scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) to analyze the reactivity of grain boundaries present in the Pd-containing Ti-7. The apparent reaction rates of ferroceniummethanol (Fc+) reduced to Fc at the localized area on the Ti-7 surface were determined from probe approach curves (PACs) fitted with simulated curves based on finite element analysis using COMSOL software. THEORY AND SIMULATION MODEL The theory for a heterogeneous electron-transfer (ET) process at an unbiased conductor developed using a finite element method was published recently55 and shown to yield good values for the heterogeneous rate constants for reversible and quasi-reversible reactions on a disk substrate surface at the open circuit potential (44) Bard, A. J.; Denuault, G.; Lee, C.; Mandler, D.; Wipf, D. O. Acc. Chem. Res. 1990, 23, 357-363. (45) Turyan, I.; Matsue, T.; Mandler, D. Anal. Chem. 2000, 72, 3431-3435. (46) Casillas, N.; Charlebois, S. J.; Smyrl, W. H.; White, H. S. J. Electrochem. Soc. 1993, 140, L142-L145. (47) Casillas, N.; Charlebois, S.; Smyrl, W. H.; White, H. S. J. Electrochem. Soc. 1994, 141, 636-642. (48) Garfias-Mesias, L. F.; Alodan, M.; James, P. I.; Smyrl, W. H. J. Electrochem. Soc. 1998, 145, 2005-2010. (49) Basame, S. B.; White, H. S. J. Phys. Chem. 1995, 99, 16430-16435. (50) Basame, S. B.; White, H. S. J. Phys. Chem. B 1998, 102, 9812-9819. (51) Basame, S. B.; White, H. S. Anal. Chem. 1999, 71, 3166-3170. (52) Fusimi, K.; Okawa, T.; Seo, M. Electrochemistry (Tokyo) 2000, 68, 950954. (53) Hassel, A. W.; Fushimi, K.; Okawa, T.; Seo, M. Proc.sElectrochem. Soc. 2000, 99-28, 166-174. (54) Schulte, A.; Belger, S.; Schuhmann, W. Mater. Sci. Forum 2002, 394395, 145-148. (55) Xiong, H.; Guo, J.; Amemiya, S. Anal. Chem. 2007, 79, 2735-2744.

(OCP). However, the results from refs 55-57 cannot be used to simulate experimental data in many specific cases because of different tip shapes and sizes of active spots on the substrates. These parameters can greatly affect the simulation results. Therefore, we have developed our own simulation model using the finite element analysis in COMSOL software. The tip radius is the real one used in the experiments. The active spot radius is estimated from SECM images. In our case, the substrate (Ti-7) was studied at the OCP and under negative potential bias. In the bulk solution, only the reduced species, Fc, existed. The mediator, Fc, is oxidized to Fc+ at the SECM probe, which is biased to a potential of 0.400 V (eq 1). The generated Fc+ diffuses to, and is reduced to Fc at, the substrate (eq 2). The reaction at active areas on the Ti-7 surface is assumed to be irreversible (Supporting Information Figure S1), and the apparent rate constant, kf, was simulated and fit to the experimental PACs at different potentials.

Fc f Fc+ + e-

(at the SECM probe)

(1)

(on substrate)

(2)

kf

Fc+ + e- 98 Fc

In this case, the electrochemical process at the probe tip is under diffusion control. The reaction rate of Fc at the tip is related to the diffusive flux by the following equation:

[

]

∂2c(r, z) 1 ∂c(r, z) ∂2c(r, z) ∂c(r, z) )D + + ) 0 (3) ∂t r ∂r ∂r2 ∂z2 where c(r, z) is the local Fc concentration and the initial condition is given by

c(r, z) ) c0, t ) 0

(4)

Under diffusion control the tip current can be calculated by the following equation:

iT,∞ ) 4nFDc0a

(5)

where n is the number of electrons transferred in the tip reaction (here, n ) 1), F is the Faraday constant, D is the diffusion coefficient of Fc, a is the tip radius, and c0 is the concentration of Fc in the bulk solution. For an irreversible reaction, the boundary condition at the active spot surface is expressed as

[

D

]

∂c(r, z) ∂z

z)-d

) kf[c0 - c(r, -d)] (0 < r < rs, z ) -d) (6)

where rs is the radius of the active spot. For a PAC with a fixed kf, the tip currents corresponding to more than 20 tip-to-substrate distances, d, were calculated (see the Supporting Information) according to

iT ) 2πnFD

∫ r[ a

0

]

∂c(r, 0) dr ∂z

(7)

Figure 1. Principle of, and instrumentation for, scanning electrochemical microscopy (SECM).

The PAC was obtained by plotting the tip current versus the tipto-substrate distance. A group of PACs with various kf values were superimposed on to the experimental PAC, and a kf value was determined from the simulated PAC that overlapped the experimental one. Other detailed parameters, simulation geometry, and the related theory can be found in the Supporting Information. EXPERIMENTAL SECTION Chemicals. The electrolyte solution contained 0.9 mM Fc (97%, Aldrich) as the mediator and 0.1 M sodium chloride (NaCl, ACS reagent, >99%, Aldrich) as the supporting electrolyte. Deionized water (Milli-Q, Millipore, 18.2 MΩ resistivity) was used to prepare all aqueous solutions. Platinum Ultramicroelectrode (Pt-UME) Fabrication. The fabrication of the Pt-UME has been previously reported,58,59 and our modified procedure for making Pt-UMEs described elsewhere.36,37 Characterization of Pt-UMEs for SECM Probes. Normally, three methods were used to characterize the Pt-UMEs: microscopic videography, cyclic voltammetry (CV), and PACs to a glass substrate.36-38 SECM Instrumentation. Our custom-built SECM instrument has been described in detail elsewhere.36,37 As illustrated in Figure 1, it consists of three major components: the electrochemical system including the electrochemical cell and bipotentiostat (electrochemical analyzer, CHI-832A, CH Instruments, Austin, TX), the closed-loop positioning system (FREEDOM 1500-3 Nano robot system, EXFO Burleigh Products Group Inc., Canada), and the active data acquisition system consisting of a computer loaded with homemade virtual instruments (VIs) programmed in LabVIEW (version 7, National Instruments, Austin, TX), a general purpose interface bus (GPIB) board (PCI-GPIB, National Instruments) to communicate with the 8200 controller, and a 16-bit DAQ card (PCI-6052E, National Instruments) with a connection board (BNC-2090, National Instruments). The electrochemical and positioning systems were built together in a Faraday cage to (56) Shao, Y.; Mirkin, M. V. J. Phys. Chem. B 1998, 102, 9915-9921. (57) Amphlett, J. L.; Denuault, G. J. Phys. Chem. B 1998, 102, 9946-9951. (58) Wightman, R. M. Science 1988, 240, 415-420. (59) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1988; Vol. 15, p 267.

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Table 1. Pd and Fe Contents at Different Positions on the Ti-7 Surfacea position

Fe (wt %)

Pd (wt %)

1 2 3 4 5 6 7 8 9 10 range (wt %) at boundary range (wt %) at grain

6.9 2.6 ND 4.2 0.1 5.5 3.8 4.4 4.9 ND 2.6 to ∼6.9

0.8 0.5 0.4 0.5 0.20 0.5 0.3 0.6 0.5 ND 0.3 to ∼0.8

ND to ∼0.1

ND to ∼0.2

ratio (Fe/Pd) 8.6 5.2 8.4 0.5 11 13 7.3 9.8

location boundary boundary “boundary” boundary grain boundary boundary boundary boundary grain

a The Ti content was balanced during the measurement. ND means not detected. Boundary means the grain boundary (the visible boundaries in SEM images), and grain means the surface beyond the grain boundaries. Ratio of Fe/Pd means the ratio of Fe wt % to Pd wt %.

Figure 2. SEM images of a freshly polished Ti-7 surface. Image aslow magnification; image bshigh magnification.

isolate external electrical noise. The constant-height mode was used for imaging where the Pt-UME current was recorded versus lateral coordinates at a fixed height in the vicinity of the Ti-7 sample. All the experiments were performed at room temperature. SEM/EDX Analysis. SEM imaging, EDX analysis, and EDX mapping experiments were run on a Leo 1540FIB/SEM with CrossBeam (Zeiss Nano Technology Systems Division, Germany). The EDX system connected with a 1540 FIB/SEM CrossBeam was an Oxford Instruments INCA.38 Ti-7 Sample Preparation and SECM Experiments. The Ti-7 sample (supplied as 6 mm thick plates, Heat S3504, Timet, Henderson, NV) contained 0.16 wt % Pd and 0.18 wt % Fe. The full composition can be found in ref 60. Prior to each experiment, the Ti-7 top surface was mechanically polished with wet silicon carbide paper (Buehler Ltd., IL), in the sequence of 320, 500, 800, 1000, 1200 grit, and then rinsed copiously with deionized water. The surface was then polished with 1.0, 0.3, and 0.005 µm alumina powder suspensions, respectively, and rinsed with excess amounts of deionized water. This sample was then immersed in deionized water and ultrasonicated for 10 min to remove any attached (60) He, X. Effects of Temperature, Impurities, and Alloying Elements on the Crevice Corrosion of Alpha Titanium Alloys. Ph.D. Thesis, The University of Western Ontario, London ON, 2003.

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Figure 3. SEM image (a) and EDX maps for Ti KR1 (b), Fe KR1 (c), and Pd LR1 (d). The numbers labeled on image a indicate the points selected for EDX analyses of Pd and Fe.

polishing particulate residue, rinsed again with copious water, and left in air to dry. Thus, the surface of the Ti-7 sample was covered with a thin air-formed layer of titanium oxide.61 This sample was clamped between two pieces of Teflon in the electrochemical cell designed for our homemade SECM, and only ∼0.50 cm2 in area of Ti-7 contacted the electrolyte solution.37 The whole cell was mounted on the stage of the SECM instrument, and the Fc solution was added. A KCl-saturated Ag/AgCl electrode was used as the reference electrode (RE) and a Pt wire as the counter electrode (CE). All potentials quoted in this paper are referred to this RE. Potentials could be applied to both the Pt-UME and the Ti-7 sample (Figure 1) using the electrochemical analyzer (CHI-832A). In order to obtain a thin layer of oxide film on Ti-7, the experiments normally started less than 2 h after polishing. RESULTS AND DISCUSSION SEM Imaging, EDX Analysis, and EDX Mapping. Figure 2 shows SEM micrographs of the polished surfaces. The low(61) Andreeva, V. V. Corrosion 1964, 20, 35t-46t.

Figure 4. SECM images recorded on a freshly polished Ti-7 sample at the open circuit potential (OCP) in 0.9 mM Fc solution with 0.1 M NaCl as the supporting electrolyte. The gap distance between the tip and Ti-7 sample surface was at 2.6 µm. The tip, with a radius of 1.0 µm and RG of 6, was polarized at 0.400 V. The total scan area is 600 µm × 600 µm, which is divided into nine 200 µm × 200 µm images. All images are on the same current scale. One magnified image (70 µm × 70 µm with a tip-to-substrate distance of 0.9 µm) is shown in the top right panel, and the current scale is indicated beneath it.

magnification micrograph in Figure 2a demonstrates a network of light strips suggesting the presence of β-phase, most likely along the R-grain boundaries. These strips are nonuniformly distributed across the surface. Figure 2b shows a magnified image of an area of the surface containing a number of these light areas. This image shows these strips can be up to several micrometers in length and up to ∼0.4 µm in width. The possible grains can be seen on SEM images; the apparent grain size varies from 6 to 28 µm. The distribution of the alloying element, Pd, and the impurity, Fe, were examined by EDX analyses. Ten spots on two different Ti-7 surfaces were investigated, and the results are listed in Table 1. Figure 3a shows six spots analyzed by EDX on an SEM image. The other four spots, and their EDX spectra, are displayed in Figure S2 in the Supporting Information. The rectangle in Figure 3a indicates the area mapped with EDX, and the maps for Ti (KR1), Fe (KR1), and Pd (LR1) are shown in Figure 3b-d. The Fe content at the visible boundaries (spots 4-6 and spots 7 and 9 in Supporting Information Figure S2) ranges from 2.6 to ∼6.9 wt %, with an average value of ∼4.6 wt %, which is about 25 times higher than the general Fe content of 0.18%. For the darker regions (spots 3 and 5 in Figure 3 and spot 10 in Supporting Information Figure S2), the Fe content is in the range of 0 (not detected, ND) to 0.1 wt %; that is, less than the general content of 0.18 wt %. These results indicate that Fe segregates to stabilize β-phase, most probably at grain boundaries and triple points, as observed previously for Ti-2.38 This segregation is clearly visible in the EDX map for Fe, Figure 3c. Previously, similar analyses for Ti-238 showed the Fe content to be in the range of 5-17% on the “visible grain boundaries”; i.e., the Fe content of the grain boundaries was higher than observed here for Ti-7, despite the fact that the average Fe content of that material (0.078 wt %) was considerably lower than the 0.18

wt % in our Ti-7. Since Fe in Ti inhibits grain growth during alloy formation, this lower Fe content may reflect the smaller grain size in the Ti-7; i.e., the segregated Fe is distributed more dilutely on the larger area of available grain boundaries. The Pd content in the visible boundaries is in the range of 0.3-0.8 wt %, with an average value of about 0.5%. This is 3 times higher than the general alloy content of 0.16%. For the nonvisible regions assumed to be associated with the grains, the Pd content varies from ND to 0.2 wt %, which is not significantly different from the average value of 0.16%. Although not clearly apparent in the EDX map in Figure 3d, these analyses confirm that Pd cosegregates to some degree with Fe. No specific spots exhibited a high enough Pd content to indicate the formation of TixPd in intermetallic precipitates. Since the solubility of Pd in Ti is about 1 wt %,62 which is greater than the maximum localized Pd concentration (spot 1 in Table 1), precipitation would not be expected. In addition, the ratio of Fe wt % to Pd wt % was investigated, and the results are listed in Table 1. The ratio is in the range of 5.2-13 on the visible boundary structures, indicating that Fe cosegregates with Pd, but the cosegregation level varies in different boundaries. SECM Survey on the Ti-7 Surface at the OCP. The SECM tip was positioned above the Ti-7 sample at a tip-substrate distance of 2.60 µm and scanned over the Ti-7 sample surface using a closed-loop system.38 The tip currents versus the lateral tip positions were recorded to obtain SECM images. An area of 600 µm × 600 µm was surveyed in our experiments to obtain the localized surface reactivity at the OCP. In order to avoid damaging the tip and/or sample surface via tip-substrate crashes, the whole (62) Murray, J. L.; Bennet, L. H.; Baker, H. In Binary Alloy Phase Diagrams Massalski, T. B., Murray, J. L., Bennett, L. H., Baker, H., Eds.; American Society for Metals: Metals Park, OH, 1986.

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Figure 5. Probe approach curves (PACs) toward different regions on a freshly polished Ti-7 surface at the open circuit potential (OCP). PAC a: toward a nonactive spot (black triangles) with a rate constant of 6.0 × 10-5 cm/s, which is not distinguishable with that of an insulator (black line). PAC b: toward a partially active spot with a diameter of 8 µm (blue diamonds) with a rate constant of 6.7 × 10-4 cm/s. PAC c: toward a partially active spot with a diameter of 8 µm (green circles) with a rate constant of 3.0 × 10-3 cm/s. PAC d: a partially active spot with a diameter of 8 µm (red stars) and a rate constant of 5.3 × 10-2 cm/s. The PACs expected for an insulator (black line) and a conductor (gray line) are also plotted for comparison. The tip approach speed was at 1.0 µm/s. The corresponding simulated PACs (red) are superposed to the experimental ones. Other experimental conditions were the same as those described in Figure 4.

scan range was divided into nine smaller images,38 each of which had a scan area of 200 µm × 200 µm. And a tip-substrate distance of 2.60 µm was kept for each individual image. Typical images of a freshly polished Ti-7 sample are illustrated in Figure 4. All these images are on the same current scale, which is shown at the bottom right side of the figure. The maximum current difference is about 0.03 nA (the tip current in bulk solution is about 0.27 nA), indicating that the tip is a little far away from the sample surface. However, the contrast between the regions of higher and lower reactivity is quite good. The blue spots with the highest current are the active spots, as indicated by the partial positive feedback observed in PACs (Figure 5, curves c and d).38 These active spots are not distributed uniformly on the Ti-7 surface, and the scattering of active spots is similar to the distribution observed on the SEM images. On the basis of the SEM/EDX behavior described above, we might expect regions of higher reactivity at those locations where the Fe and Pd contents of the surface are enhanced. These locations appear to be associated with Fe/Pd-stabilized β-phase at R-grain boundaries and triple points. This could lead to the distribution in tip currents for reaction 2 from region to region, as observed in Figure 4. Figure 4 also shows a higher resolution image of one section of the top right-hand image, which was obtained when the tip-tosubstrate distance was decreased to 0.90 µm. Several active spots are found within this expanded image of 70 µm × 70 µm. The 1442 Analytical Chemistry, Vol. 80, No. 5, March 1, 2008

area with the highest tip current is oval in shape with a longer axis of up to 10 µm. The other three areas of enhanced tip current are much weaker, with the weakest spot located to the left of the active oval area. These observations confirm that the surface reactivity varies from region to region. The oval shape of this reactive spot seems consistent with its location at a triple point because of the size and shape determined from the image on the top right hand. It may be associated with one of the light strips observed in SEM images (Figure 2), which have a long axis of a similar length. This coincidence suggests these regions of higher activity are associated with the areas of cosegregation of Fe and Pd thought to be locations of stabilized β-phase. The short axis of the reactive spots is about 4 µm, which is larger than the dimensions of the light strips observed by SEM. This might be caused by the tip size49 (the tip diameter is 2.0 µm) which could be larger than the width of the areas observed by SEM. In our previous paper the effect of Fe segregation to grain boundaries and triple points on local reactivity was clearly demonstrated.38 Here, the observed cosegregation of Pd with Fe yields a similar enhancement in local reactivity. SECM PACs to the Ti-7 Surface at the OCP. The SECM tip (radius, 1.0 µm; RG, 8.0) was biased at 0.400 V, and the diffusion-controlled current for reaction 1 was measured during the approach of the tip to the spots under investigation. This biased tip was driven to the Ti-7 surface at a speed of 1.0 µm/s. The feedback (reaction 2) on the Ti-7 was monitored by recording the tip current change. Figure 5 shows several PACs to different spots recorded at the OCP on the Ti-7 surface immersed in the 0.9 mM Fc solution with 0.1 M NaCl as the supporting electrolyte. Superimposed on these plots are the simulated PACs obtained using the model described above. For curve a in Figure 5, the tip approached a grain on the freshly polished Ti-7 sample and the experimental data (black triangles) overlapped with the simulation PAC to an insulator (black line), indicating that the grain behaves like an insulator, as expected for a TiO2-covered location, and as previously observed for Ti-2 at the OCP.37 Note that a rate constant of 6.0 × 10-5 cm/s was determined by fitting, which is not distinguishable from that above an insulator. This kind of PAC can be used to determine the distance between the tip and substrate. For some areas of the surface the measured PAC showed a larger deviation from the theoretical PAC for an insulator, curve b in Figure 5 (blue diamonds, with an apparent rate constant of 6.7 × 10-4 cm/s). It is worth emphasizing how well the simulated PAC fits the experimental plot in this figure and that these two areas, corresponding to curves a and b in Figure 5, were indistinguishable in SECM images. Curves c and d in Figure 5 show two PACs to partially active spots. The rate constant for curve c (green circles) in Figure 5 was determined to be 3.0 × 10-3 cm/s, which is ∼5 times higher than that obtained from curve b, while the experimental PAC, curve d (red stars), overlapped well with the simulated PAC with a kf of 5.3 × 10-2 cm/s. This was the highest rate constant found in our experiments on a Ti-7 at the OCP. In comparison with the simulated PAC to a conductor displayed in Figure 5 (gray line), this spot is still only partially conductive. However, the rate constant is about 100 times higher than that observed for the area

Figure 6. SECM images of the same area of a Ti-7 surface biased at different potentials: (a) the open circuit potential (OCP), (b) 0.000, (c) -0.100, (d) -0.200, (e) -0.250, (f) -0.300, (g) -0.350, (h) -0.400, and (i) -0.500 V. The tip current range is shown at the right side for each image. The tip-to-substrate distance was 1.3 µm. Other experimental parameters were the same as those in Figure 4. The black arrows with labels in image h point out the spots selected for the further analysis presented in Figure 7. The possible grain boundaries are shown on image i.

probed in curve b Figure 5. The two spots corresponding to curves c and d in Figure 5 can be distinguished on SECM images (green or blue regions corresponding to higher tip currents). This analysis demonstrates that a much higher sensitivity is achieved by the PAC approach than by imaging. As discussed above, in neutral aqueous solutions Ti-7 is always covered by a thin oxide layer which is normally nonconductive,63,64 since TiO2 film has a Ti4+ (3d0) electron configuration, which makes it an insulator (curve a in Figure 5). However, deviations from stoichiometry (TiO2-x) give the thin oxide n-type semiconductor characteristics, due to the presence of a combination of oxygen vacancies and interstitial Ti3+ ions, which lead to the trapping of electrons in a band just below the conduction band edge.65 Thus, the coupling of reactions 8 and 9 to reaction 2 as the tip is rastered over the electrode surface might account for the differences in apparent rate constant for the areas corresponding to PACs a and b in Figure 5.

Ti3+ f Ti4+ + eTi f Ti4+ + 4e-

(in the oxide film)

(8)

(at the Ti/oxide film interface) (9)

Since these two reactions involve defect annealing (reaction 8) and further film growth (reaction 9), we would expect the oxide(63) McAleer, J. F.; Peter, L. M. Faraday Discuss. 1980, 70, 67. (64) Ohtsuka, T.; Masuda, M.; Sato, N. J. Electrochem. Soc. 1987, 134, 24062410.

covered grain surfaces to become less reactive with continued exposure to solution and, hence, for these differences to disappear. SECM Images at Different Bias Potentials on Ti-7. To further investigate the boundary structures, one area on the Ti-7 surface was randomly selected to run further SECM experiments with a negative potential biased to the substrate. The SECM tip, polarized at 0.400 V, was placed above the substrate at a tip-substrate distance of 1.3 µm and scanned across an area 60 µm × 60 µm to obtain images. Since our custom-built SECM has a closed-loop positioning system with a resolution of 20 nm,36-38 the same area was imaged while the applied potential varied from the OCP to -0.450 V. The images obtained are shown in Figure 6. At the OCP, Figure 6a, the tip current was slightly higher in the circled region (designated the original active region) than its surroundings, indicating partial positive feedback in this area. When the potential of 0.000 V was applied (Figure 6b), the tip current above this active spot remained the same but the contrast with the surrounding area was lost. Very little change in the tip current occurs for potentials down to -0.200 V (Figure 6d). For polarization to -0.250 V, Figure 6e, distinct reactive spots begin to appear within, or close to, the original active region, and further decreases in polarization lead to increasing tip currents and the appearance of a number of reactive spots, Figure 6g-i. (65) Jarjebski, Z. M. Oxide Semiconductors; Pergamon Press: Oxford, New York, 1973.

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Figure 7. Change in tip current with the bias potential on Ti-7 for the four spots (ia-id), labeled in Figure 6. Spot R (blue line in plots a-d) is the reference current on a nonactivated area of the surface. The current difference (∆i) between each of the four spots and the reference spot (iR) is plotted in black in each panel.

No additional reactive sites were uncovered for applied potentials < -0.450 V, suggesting all available sites were activated by this polarization. Interconnection of the nearest uncovered reactive sites yields the pattern shown in Figure 6i. Since it is our hypothesis that these reactive sites are the locations of Fe/Pd-stabilized β-phase within the grain boundaries, the interconnecting lines yield a pattern of the grain structure of the material. It is possible that this “grain boundary map” is incomplete since not all grain boundaries are necessarily decorated with Fe/Pd-stabilized β-phase. The map in Figure 6i indicates at least seven complete grains existing within the analyzed area varying in size from 10 to 30 µm. This is very similar to the grain size distribution indicated by SEM (Figures 2 and 3a) and in close agreement with the grain size distribution for this material previously determined by transmission electron microscopy.60 This distribution of grain sizes is confirmed by drawing a similar pattern on an image recorded over a larger scan area with an applied potential of -0.450 V, as illustrated in Figure S3 in the Supporting Information. These observations add considerable credibility to our claim that we have mapped the possible grain boundary structure of this material and that enhanced reactivity exists in the grain boundaries most likely at the locations of Fe/Pdstabilized β-phase. The number of grains indicated in the grain boundary map in Figure 6i is considerably greater than the number observed within an area of similar size for the non-Pd-containing Ti-2.38 This is not surprising since the grain size distribution in the Ti-2 is considerably larger (20-100 µm)60 than in the Ti-7 used here. This comparison shows indirectly the reasonableness of mapping grain boundary structures using SECM. 1444

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Influence of Substrate Potential on Tip Currents at Various Locations. The five spots (R, and a-d) in Figure 6h were selected to examine the influence of the potential applied to the substrate on the tip current. The spot R is located in an apparently nonreactive (red) area associated with the surface of a grain and can be used as a reference point against which the reactivity of other locations can be gauged. Location a is close to a reactive spot, whereas locations b-d are reactive spots which appeared at different potentials. The tip current values extracted from the images in Figure 6 are plotted against the Ti-7 substrate potentials in Figure 7. The plot for the reference point R is plotted in each panel of Figure 7 (blue lines) to facilitate the comparison of reactivities. The tip current response above spot a, Figure 7a (green line), is effectively identical to the reference current, confirming that this location is nonreactive. The difference in currents between the two locations (black line) is extremely small and potential-independent, although the tip current above both locations increases as the applied potential is made more negative. A similar increase in tip current was observed for Ti-2.38 For locations b, c, and d, the tip currents are similar to that for the reference location, R, in the potential range between 0.000 and -0.150 V but increase much more rapidly as the substrate potential is made more negative. This is clearly demonstrated by the current difference curves in Figure 7b-d. However, the current difference curves show that these three locations are activated at different applied substrate potentials. Point b begins to activate at an applied potential around -0.150 V and becomes fully activated (i.e., the difference current becomes independent of potential) at a potential less than -0.250 V. In contrast, points

Figure 8. Probe approach curves (PACs) to a nonactive spot (X ) 25.51 µm, Y ) 231.65 µm in Figure 5) on a Ti-7 surface at different potentials: (a) 0.000, (b) -0.100, (c) -0.150, (d) -0.200, (e) -0.250, (f) -0.300, (g) -0.350, and (h) -0.400 V. Simulated PACs (red) are superposed with the experimental ones (blue circles). log(kf) vs the applied potential is plotted in (i). Other experimental parameters are the same as those in Figure 4.

c and d do not fully activate until the potential is < -0.350 and -0.450 V, respectively. By comparison with similar experiments performed on Ti-2,38 the activation of points c and d requires a considerably more negative substrate potential. This difference between Ti-2 and Ti-7 could suggest that the presence of Pd in the stabilized grain boundary phases in Ti-7 inhibits the activation of these sites. The differences in activation potential for locations b, c, and d could then be attributed to the different Pd contents of individual sites. The EDX analyses in Table 1 confirm that significant differences in Pd content, as well as the Pd/Fe ratio, occur at different grain boundary locations. Since Pd is a noble metal and Fe a base metal, it is possible that the grain boundary Pd/Fe-stabilized β-phase is more difficult to activate than the purely Fe-stabilized β-phase and intermetallic precipitates. However, this claim requires confirmation, since it is possible that the application of a potential of 0.000 V and/or extended exposure at the OCP at the beginning of these experiments could have caused a small degree of anodic film growth that would be expected to subsequently retard activation of the surface. In fact, this phenomenon is observed in the experiments, which will be discussed in detail in another paper. Quantitative Analyses of Localized Surface Reactivity at Different Potentials on Ti-7. Figure 8 shows a series of PACs (blue circles) recorded on a nonreactive grain surface as the potential of the substrate is changed from 0.000 V, Figure 8a, to -0.400 V, Figure 8h. The black curve in these figures is the theoretical PAC for the approach of the tip to an insulating surface,

and the red curves show the numerical simulation curves generated by the model to fit the experimental curves. As the potential is made more negative, the experimental curves gradually separate from the theoretical curve for approach to an insulator, indicating an increase in reactivity of the surface. The relationship between the apparent rate constant (kf) and the substrate potential is shown to be logarithmic, Figure 8i, as expected from the currents shown in Figure 7a. This increase in kf can be attributed to the electrochemical reduction of the passive TiO2 film on the surface of the grain, a process which leads to the coadsorption of hydrogen,64

TiO2 + xH+ + xe- f TiO2-x(OH)x

(10)

The hydrogen in the film acts as an electron donor (Habs f H+ + e-) thereby introducing a new absorption band in the band gap region of the oxide leading to an increase in film conductivity and electron transfer to Fc+ (reaction 2). The experimental PACs (blue circles) to an active spot at various applied potentials are shown in Figure 9. The simulation curves (red line) are plotted as well. The simulation PACs to an insulator or a conductor (black lines) are plotted for comparison. The apparent rate constant at the substrate potential of -0.250 V is 6.0 × 10-3 cm/s, which is 15 times higher than that above a nonactive spot at the same potential (4.0 × 10-4 cm/s in Figure 8e). The apparent rate constant dropped to 7.0 × 10-4 and Analytical Chemistry, Vol. 80, No. 5, March 1, 2008

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Figure 9. Probe approach curves (PACs) to an active spot (X ) 213.39 µm, Y ) 321.39 µm in Figure 6) on a Ti-7 surface at different potentials biased on Ti-7: (a) -0.250, (b) -0.300, (c) -0.350, and (d) -0.450 V. Other experimental parameters are the same as those in Figure 4.

1.0 × 10-3 cm/s when the potential decreased to -0.300 V (Figure 9b) and -0.350 V (Figure 9c), respectively, which agrees well with the tip current-substrate voltage curves shown in Figure 7. When the potential decreased to -0.450 V (Figure 9d), the apparent rate constant was about 0.012 cm/s, which is ∼10 times higher than the value above nonreactive regions (0.0012 cm/s). The reactivity behavior of this spot is similar to that of spot d found in Figure 6h. In comparison with the factor of 100 times difference in reactivity observed between nonactive and active locations observed after brief immersion on open circuit, Figure 6, this difference of 10-15 is considerably smaller. This may reflect the partial passivation of these sites by the application of a bias potential of 0.000 V, as discussed above. It remains to be conclusively determined whether these Pd/Fe-containing grain boundary phases can act as preferential hydrogen absorption sites in neutral solutions. CONCLUSIONS SECM and SEM/EDX were used to study variations in reactivity on grade-7 (Ti-7) titanium exposed to an aerated neutral solution of 0.1 M NaCl and changes in element compositions. SEM/EDX showed that the alloying element, Pd, and the impurity, Fe, cosegregate to grain boundary locations. Since both Fe and Pd are β-phase stabilizers, it is likely that the visible areas in SEM images are strips of Fe/Pd-stabilized β-phase. SECM shows that, on freshly polished surfaces, the surface reactivity varies from region to region. The application of a sufficient cathodic bias leads to the enhanced activation of specific regions of the surface. The interconnection of active locations on the surface yielded struc1446 Analytical Chemistry, Vol. 80, No. 5, March 1, 2008

tures with very similar dimensions to those observed in SEM, indicating that the active locations may coincide with the Pd/Festabilized β-phase in the grain boundaries; i.e., these SECM images provide an outline and measure of reactivities of grain boundaries on the alloy surface. The localized surface reactivity was quantitatively analyzed by the fitting of experimental PACs to simulated ones using a model based on a finite element analysis developed with COMSOL multiphysics software. For the freshly polished surface on open circuit, the difference in reactivity between passive (TiO2-covered) grain surfaces and Fe/Pd-containing grain boundaries was up to a factor of 100. These nonpassivated locations might act as preferential hydrogen absorption sites, and further investigation is under way. The reactivity on nonreactive locations (the surface of grains) was determined from PACs performed as a function of substrate bias potential. The reactivity increased logarithmically with decreasing potential, consistent with the electrochemical reduction of the TiO2 film to yield a more conductive TiOOH layer. The activation of reactive locations was also investigated as a function of bias potential. Activation of individual locations occurred at different potentials, suggesting that the Pd/Fe content of the grain boundaries (which varies widely from location to location) influenced the passivity of the site and, hence, its activation. Determination of the reactivity of these locations from PACs indicated only an enhanced reactivity of 10-15 compared to the oxide-covered grain surface. This is considerably less than the factor of 100 observed on a freshly polished surface, supporting the claim that these sites may have been partially passivated after long exposure to the aerated solution.

ACKNOWLEDGMENT We appreciate the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC, New Discovery and Equipment Grants), Ontario Photonics Consortium (OPC), Canada Foundation for Innovation (CFI), Ontario Innovation Trust (OIT), the Premier’s Research Excellence Award (PREA), and the University of Western Ontario (Academic Development Fund (ADF) and a Start-up Fund). We thank Dr. J. Grauman (Timet, Henderson, NV) for donating the Ti grade-7 plates. We are grateful to Drs. Xihua He, Ian Mitchell, and Mr. Yimin Zeng for their kind help and discussion. John Vanstone, Jon Aukema, Barakat Misk, Sherrie McPhee, Mary Lou Hart, and Marty Scheiring are gratefully acknowledged for their technical support. Thanks go to Dr. Todd Simpson and Ms. Nancy Bell of the Nanofabrication Laboratory at UWO for their assistance in

SEM/EDX experiments. Dr. Shigeru Amemiya and Ms. Hui Xiong are thanked for sending us ref 55 and the COMSOL simulation example before its publication. We thank Ms. Catherine Nowierski for proofreading of this paper. SUPPORTING INFORMATION AVAILABLE Detailed COMSOL simulation, EDX spectra, additional SEM images, additional SECM images of a 200 µm × 200 µm area at various applied potential. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review August 25, 2007. Accepted December 9, 2007. AC701796U

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