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Polarization Controlled Kinetics and Composition of Trivalent Chromium Coatings on Aluminum Sameh Dardona,* Lei Chen, Michael Kryzman, Daniel Goberman, and Mark Jaworowski Department of Physical Sciences, United Technologies Research Center, East Hartford, Connecticut 06108, United States ABSTRACT: Combined in situ spectroscopic ellipsometry and electrochemistry have been employed to monitor, in real-time, the formation of trivalent Cr conversion coatings on polished Al substrates at applied sample potentials. It is found that the formation kinetics and chemical composition of the film can be controlled by adjusting the anodic and cathodic reactions. The growth kinetics are accelerated at more positive anodic potentials or more negative cathodic potentials. At more negative potentials, the percentage of chromium in the coating is found to increase, while the zirconium percentage decreases.
C
hromate conversion coatings (CCC) are used to replace the native oxide film on metal surfaces with a thin oxide film of desirable and predictable properties. These coatings have been effectively used in aerospace aluminum alloys as they offer active corrosion protection to the alloys and promote adhesion of overlayer coatings.1 The presence of the carcinogen, Cr(VI), in these coatings renders their continued use problematic and drives research into less toxic coating alternatives.2 Recently, a new trivalent chromium based coating, known as the trivalent chrome process (TCP), has been shown to offer excellent corrosion protection.3,4 TCP coating is typically comprised of Zr, Cr, and O. Zr and O levels track one another and are relatively constant with depth into the coating. The atomic concentration of O relative to Zr is about 2:1 consistent with the empirical formula of dehydrated zirconia, ZrO2. Only a trace amount of F is usually detected at the surface of fresh TCP coating. Al is also incorporated into the coating in the interfacial region. The Cr level is comparatively low in the coating at about 45 atomic %.57 TCP is currently the leading replacement for CCCs. TCP conversion coatings have been widely used in the automotive and architectural industries. However, the extension of these coatings to the aerospace industry has been slowed by apparent base alloy and process sensitivities in the TCP coating process.8 Moreover, compared with the well-studied CCC, the formation mechanisms and kinetics of the TCP coatings are not well understood, which in turn impairs the fundamental understanding of their corrosion inhibition properties. Spectroscopic ellipsometry measures the change in the polarization state of linearly polarized light upon reflection from a surface at non-normal incidence as a function of photon energy.9 The measured ellipsometry parameters are the amplitude ratio (tan Ψ) and the phase shift difference (Δ) of the two orthogonally polarized components of the reflected wave (rs and rp). These parameters are defined through tan(Ψ) exp(iΔ) = rp/rs. Ellipsometry is an indirect method and a model analysis must be r 2011 American Chemical Society
performed. With the use of an iterative procedure, unknown optical constants and/or thickness parameters are varied, and tan(Ψ) and cos(Δ) values are calculated using the Fresnel equations. The calculated tan(Ψ) and cos(Δ) values which best match the experimental data provide the optical constants and thickness parameters of the sample. Studies of TCP coatings have been mainly focused on its chemical and physical properties as well as the corrosion protection they provide. Techniques used in those studies include electrochemical and ex situ vacuum characterization methods. Limited studies have been directed at developing an understanding of the growth kinetics and structure evolution through real time and in situ measurements. In addition, the influence of applied potentials on TCP growth kinetics and composition as a method of optimizing the coating protection characteristics has not been explored to date. The in situ spectroscopic ellipsometry technique enables detailed characterization of thickness and optical properties evolution during film growth.10 In recent work, in situ spectroscopic ellipsometry has been shown effective in studying growth kinetics of TCP coatings.11 Ellipsometry combined with electrochemical measurements has been successfully applied to various domains such as oxide film growth,12 corrosion processes,13 and surface treatment.14 The objective of this study is to determine the effect of sample electrode potential on TCP formation kinetics and chemical composition.
’ MATERIALS AND METHODS The specimens used were 1 mm thick pure Al (99.998%). These specimens were cleaned in acetone under ultrasonic agitation for 10 min. A Teflon coated wire was attached to the Received: May 16, 2011 Accepted: July 4, 2011 Published: July 04, 2011 6127
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Figure 1. Schematic diagram of the combined ellipsometry and electrochemistry method.
Figure 3. Comparison of the experimental data (—) for (a) tan(Ψ) and (b) cos(Δ) to the modeled data (O) at 4 min growth time for all different potentials.
Figure 2. Open circuit potential recorded for pure Al substrate during the TCP coating process for 20% TCP solution.
back of specimens using highly conductive epoxy Silver-402 from Ellsworth Adhesive. The specimens were epoxy mounted, and their surface (1 cm 1 cm) was prepared using Struers grinding/ polishing TegraPol-31 equipment under method 1477.15 These substrates were cleaned using ethanol and dried in air after being polished. The roughness of the resulting surface was on the order of 3 nm as determined from AFM studies. The TCP solution was 20 vol % SurTec 650, a product of Sur Tec Corp. The solution is comprised of potassium fluorozirconate and Cr sulfate, adjusted to pH 3.8 with KOH/deionized water. A schematic diagram of the combined ellipsometry and electrochemistry method is shown in Figure 1. Ellipsometry measurements were performed using a Sopra model GES5E rotating analyzer with a variable angle of incidence spectroscopic ellipsometer. A custom electrochemical cell that enables measurements at any angle of incidence with a photon transmission window in the range of 1.3 to 4.0 eV was used. Electrochemical studies were conducted in a three electrode arrangement. The prepared specimen was used as the working electrode, and platinum foil was used as the counter electrode. The working electrode potential was controlled relative to a miniature flexible Ag/AgCl reference electrode model ET073 from eDAQ Pty Ltd. A Solartron 1287A potentiostat with CorrWare software was used to control the potential of the working electrode. The chemical composition of coatings was analyzed using X-ray photoelectron spectroscopy (XPS). The analysis chamber was maintained at a pressure lower than 1.2 106 Pa during analysis. The XPS analysis was performed using a monochromated aluminum probe that was 100 μm in diameter. The active,
as-received surface was analyzed (no ion-beam cleaning was used prior to the data acquisition). A survey scan was performed on each sample in order to confirm the elemental composition of the coating surface. Then high-resolution scans of the main photoelectron peak for each element were acquired. It was from these scans that the elemental composition was calculated using sensitivity factors appropriate for our XPS system. These highresolution scans result in data with very low noise levels compared to the signal.
’ RESULTS AND DISCUSSION The evolution of the open circuit potential (OCP) of a pure Al substrate in 20% TCP solution is shown in Figure 2. The electrode potential decreased with time and stabilized at ∼1.4 V after roughly 600 s. The initial rapid decease of the OCP may be due to the dissolution of native oxide assisted by hydrofluoric acid in the solution.11 In contrast to the CCC coating, the OCP transient resulting from the TCP treatment is much longer.16 The long transient suggests the chemistry of the Al surface as well as the local solution might have undergone prolonged changes during the TCP treatment compared with the chromate treatment. This may be explained by the absence of the strong oxidizing reagent Cr(VI) and the less aggressive solution. Although the exact composition of SurTech 650 solution is unknown, it likely consists of ZrF62, Cr3+, SO42, and F. According to the E-pH diagram of aluminum, Al and aluminum oxide dissolution and predominating hydrogen evolution are expected during the TCP treatment without electrode polarization.17 The nonfaradaic aluminum oxide dissolution reaction, the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) are expected to result in an increase in local pH. Increased pH promotes the hydrolysis of metal cations present, e.g., Al(III), Cr (III), and Zr(IV), leading to subsequent precipitation of metal hydroxides and hydrated zirconia on the surface.18 In addition, 6128
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Figure 4. Current-evolution during TCP coating at applied sample potentials for pure Al substrate in 20% TCP solution (a). The corresponding TCP film thickness as extracted from fitting of in situ ellipsometry data (b).
the dissolution of aluminum or aluminum oxide will result in the formation of aluminum fluoride complexes while decreasing the local fluoride ion activity in solution to further promote the hydrolysis of the zirconium fluoride complex. Consumption of hydroxide ions will balance the predominant cathodic reaction (HER), leading to the opposite effect on pH. Specifically, the hydrolysis of fluoride zirconium likely proceeds as follows ZrF2 þ 4OH f ZrO2 -2H2 O þ 6F
suppress hydrogen evolution, whereas cathodic polarization (more active potential) will result in the opposite effects. Figure 4 shows current densities and thickness evolution for TCP films at different anodic and cathodic polarizations (from 1.0 to 1.6 V) during TCP treatment. The TCP film thickness evolution, at different applied potentials, as determined from the aforementioned optical models is shown in Figure 4b. It is seen from Figure 4b that the film growth underwent three stages under anodic polarizations (1.0 and 1.3 V): an initial induction period representing negligible or very slow film growth, a rapid linear film growth, and a final stage of steady but slower film growth. In contrast to the anodic polarization, TCP film growth under cathodic polarization exhibited a slower growth rate at the intermediate stage followed by a more rapid late stage growth as evidenced by the film growth behavior under 1.6 V. However, the film growth kinetics at a slightly more positive potential 1.5 V was quite different from that at 1.6 V. This difference appears to be consistent with the amount of charge transferred under the two potentials, where more charge was transferred at 1.6 V as revealed by the higher cathodic current density shown in Figure 4a. As the electrode was polarized at 1.0 and 1.3 V, cathodic current was observed in the initial growth stages, followed by anodic current throughout the treatment (Figure 4a). The current reversal associated with 1.3 V approximately corresponds to the transition from the induction period to normal growth of the coatings. The subsequent rapid film growth under anodic polarization (at 1.0 V) may be explained by the enhanced rate of Al dissolution. Because the HER was in fact suppressed under applied anodic potential, the near surface layer of TCP solution is relatively acidic and the observed rapid film growth is likely due to fluoride ion sequestration by Al rather than alkaline hydrolysis. The sequestration reaction liberates more zirconate ions, which are prone to hydrolysis and precipitation. The sequestration process can be described by the reaction below Al3þ þ ZrF6 2 f AlF6 3 þ Zr4þ
ð1Þ
In order to clearly distinguish the substrate from the TCP film in the ellipsometry data modeling, the optical response of the substrate was measured. The optical constant of the nascent Al substrate was determined by eliminating the contribution of the native oxide layer using a measured thickness and previously determined optical constants of the oxide layer.19 The optical constant dispersion for the TCP solution was measured using a focused light beam. A standard Cauchy relationship was used to describe the TCP film dispersion of the refractive index. The in situ measurements were fitted using a single-layer optical model with the TCP solution as the ambient layer. The model parameters and thickness of the TCP film are best optimized for films that have bulk like material properties. The parameters determined are used as starting conditions to model the data obtained at earlier stages of the growth. Figure 3 illustrates the agreement between experimental and fitted spectroscopic spectra of tan(Ψ) and cos(Δ) at 4 min growth time for all applied potentials. Electrode polarization was tested as a way to adjust the rates of cathodic and anodic reactions, hence the film formation kinetics. Specifically, anodic polarization (more noble potential) during TCP treatment will concurrently promote Al dissolution and
ð2Þ
It is seen in Figure 4a that the anodic current went through a maxima and stabilized at a small positive current for both 1.0 and 1.3 V. The decreasing current transients under the anodic polarization suggest that either the HER was promoted or the Al dissolution was inhibited or both situations occurred as a result of the TCP film formation. Over time, the current densities associated with 1.0 and 1.3 V bias settled to approximately the same value, which suggests the dominant electrochemical reactions had evolved to be diffusion limited. The HER is expected to be affected primarily by diffusion in the boundary layer of the solution, whereas Al dissolution was increasingly retarded by the formed film. Therefore, the diffusion controlled kinetics is likely attributed predominantly to Al dissolution. Although cathodic polarization can more effectively increase the local pH, TCP film thickness at 1.6 V did not exceed that at 1.0 V for times less than 400 s. The subsequent film growth rate increased dramatically after 400 s at 1.6 V. Concomitantly, the current density also increased as the cathodic polarization TCP treatment continued from about 400 to 600 s (Figure 4a). When an Al electrode is not biased, dissolution of Al is balanced predominantly by proton reduction. A rise in pH triggers the precipitation of Cr3+ and Zr4+. Al dissolution may also dislodge native oxide to accelerate local pH rise, resulting in an increased 6129
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’ CONCLUSIONS In conclusion, in situ spectroscopic ellipsometry combined with electrochemical measurements are effective in studying the growth kinetics of TCP films on Al at applied potentials. Sample polarization is found to affect the growth rate and chemical composition of the films. It is therefore proposed that the TCP film growth on Al depends on hydrolysis reactions governed both by fluoride competition with Al and the near-surface pH value, resulting in compositional and deposition rate variations with applied potential. TCP films, created using potential control, may provide versatility in improving and optimizing conversion coatings for enhanced corrosion protection. Figure 5. Atomic percentage of chromium and zirconium in TCP films as a function of applied sample potentials as determined from XPS analysis.
film formation rate. When the Al electrode is polarized cathodically, the cathodic reaction (proton reduction) is accelerated, whereas the anodic reaction (Al dissolution) is retarded. It is likely that the kinetics associated with (non-Faradic) alkaline dissolution of the native oxide is slow initially due to a stronger field repelling F needed for Al dissolution.18 In contrast to the 1.6 V, polarization at 1.5 V did not exhibit the same acceleration in film formation. It is seen in Figure 4a that the cathodic current associated with 1.5 V is smaller, which suggests the surface catalytic activity, in particular for HER, was inhibited. Under anodic polarization, the surface is acidified and the hydrolysis reactions are disfavored. However, aluminum oxidation is enhanced under these conditions, and the excess anodic charge transferred between 0 and 400 s at 1.0 V compared to 1.3 V is faradaically equivalent to 45 nm of excess ZrO2 coating, under the assumption that ZrO2 deposition is triggered by the loss of a single fluoride ion from the ZrF62 complex by aluminum abstraction. Under extreme cathodic polarization at 1.6 V, hydrolysis-based deposition reactions are favored due to the generation of surface alkalinity and the relatively low rate of aluminum oxidation. The hydrolysis and ultimate precipitation of trivalent chromium is known to proceed by an olation process that is strongly dependent on both time and pH.20 The sudden film growth event at 500 s under extreme cathodic polarization is interpreted as the rapid precipitation of olated Cr3+ under conditions of increasing pH. The effect of sample polarization on the film chemical composition is shown in Figure 5. It is noted that Cr concentration in the film increased under more negative polarization, while the concentration of Zr decreased. For the case of the Zr atomic concentration, a ∼0.2 atomic % standard deviation was calculated as a measure of data quality. This standard deviation indicates that the 1% change in composition is significant. The aforementioned fluoride ion sequestration by Al ions, resulting in hydrolysis of more Zr ions freed, may have contributed to the higher Zr composition under anodic polarization. The elevated Cr concentration arises primarily from the decrease in F and Zr content. The higher Cr concentration may be more desirable because it is suggested that Cr(III) hydroxide of a TCP film provides the protection to the substrate through cathodic inhibition.21,22 Additional efforts continue to be focused on coating morphology, cross-plane composition, and corrosion resistance.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Funding by the United Technologies Research Center (UTRC) is acknowledged. Discussions with Professors Gerald Frankel of Ohio State University and Greg Swain of Michigan State University are appreciated. ’ REFERENCES (1) Iyer, A.; Willis, W.; Frueh, S.; Nickerson, W.; Fowler, A.; Barnes, J.; Hagos, L.; Escarsega, J.; La Scala, J.; Suib, S. L. Plat. Surf. Finish. 2010, 5, 32–41. (2) Friberg, L.; Nordberg, G. F.; Vouk, V. B. Handbook of Toxicology of Metals; Elsevier: Amsterdam, The Netherlands, 1986. (3) La Scala, J. SERDP Report 2009, 68. (4) Bhatt, H.; Manavbasi, A.; Rosenquist, D. Metal Finish. 2009, 107, 39–47. (5) Matzdorf, C.; Kane, M.; Green, J. Corrosion resistant coatings for aluminum and aluminum alloys. U.S. Patent 6,375,726, April 23, 2002. (6) Unpublished research and personal communication with Bill Nickerson (NAVAIR) and Greg M. Swain (Michigan State University). (7) Iyer, A.; Willis, W.; Frueh, S.; Nickerson, W.; Fowler, A.; Barnes, J.; Hagos, L.; Escarsega, J.; Scala, L.; Suib, S. L. Plat. Surf. Finish. 2010, 97, 32–41. (8) Legg, K. O. Technology Roadmap Summary: Coatings for the Canadian Aerospace Industry; Industry Canada: Ottawa, Ontario, Canada, March 2010. (9) Fujiwara, H. Spectroscopic Ellipsometry: Principles and Applications, John Wiley & Sons: Chichester, U.K., 2007. (10) Johs, B.; Hale, J.; Ianno, J. N.; Herzinger, C.; Tiwald, T.; Woollam, J. A. SPIE Proc. 2001, 4449, 41–57. (11) Dardona, S.; Jaworowski, M. Appl. Phys. Lett. 2010, 97, 181908-3. (12) Stein, N.; Rommelfangen, M.; Hody, V.; Johann, L. Electrochim. Acta 2002, 47, 1811–1817. (13) Stein, N.; Johann, L.; Rapin, C.; Lecuire, J. Electrochim. Acta 1998, 43, 3227–3234. (14) Van Gils, S.; Melendres, C. A.; Terryn, H.; Stijns, E. Thin Solid Films 2004, 455, 742–746. (15) Struers preparation methods database: http://www.struers. com/modules/emetalog. (16) Campestrini, P.; Goeminne, G.; Terryn, H.; Vereecken, J.; de Wit, J. H. W. J. Electrochem. Soc. 2004, 151, 59–70. (17) Roberge, R. P., Corrosion Engineering: Principles and Practice; McGraw-Hill: New York, 2008. (18) Dong, X.; Wang, P.; Argekar, S.; Schaefer, D. W. Langmuir 2010, 26, 10833–10841. 6130
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(19) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: New York, 1985. (20) Drljaca, A.; Spiccia, L. Polyhedron 1995, 14, 1653–1660. (21) Chen, W.; Bai, C.; Liu, C.; Lin, C.; Ger, M. Appl. Surf. Sci. 2010, 256, 4924–4929. (22) Yu, H.; Chen, B.; Shi, X; Sun, X.; Li, B. Mater. Lett. 2008, 62, 2828–2831.
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