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Formation of zirconium-based conversion coatings on aluminium and Al–Cu alloys
Corrosion Science 65 (2012) 231–237
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Formation of zirconium-based conversion coatings on aluminium and Al–Cu alloys F.O. George, P. Skeldon ⇑, G.E. Thompson Corrosion and Protection Centre, School of Materials, The University of Manchester, Manchester M13 9PL, UK
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Article history: Received 20 June 2012 Accepted 10 August 2012 Available online 19 August 2012 Keywords: A. Aluminium A. Copper A. Zirconium B. TEM B. RBS C. Oxide coatings
a b s t r a c t The influence of copper addition to aluminium on the formation of a zirconium-based conversion coating is investigated using sputtering-deposited substrates. Coatings formed on aluminium are 1.5 times the thickness of the aluminium consumed by oxidation, with an O:Zr atomic ratio of 2.5. Copper additions reduce the coating growth rate, especially when added in amounts above a few at.%. In contrast, the copper has relatively little effect on the oxidation rate of the substrate. Copper also promotes the formation of a layer of corrosion product beneath the coating and appears to influence the adherence of the coating to the substrate. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Conversion treatments of aluminium alloys for corrosion protection have frequently involved the use of chromate species. In particular, chromate–fluoride baths have been used extensively due to their ability to provide excellent corrosion resistance. The resultant coatings are suited to the application of paints and other organic layers and possess a corrosion inhibiting property due to the presence of chromate ions [1]. However, the use and disposal of chromium compounds are receiving regulatory attention because of the toxicity and carcinogenicity of chromium (VI) species. Consequently, much effort has been given to the development of alternative processes [2–14]. Such approaches have utilized, for example, sol–gel coatings, self-assembled monolayers and rare earth-, titanium- or zirconium-containing oxide/hydroxide coatings. The present study is concerned with zirconium-based conversion coatings, which are formed by a relatively brief immersion of the substrate in an acidic solution, containing zirconium hexafluoride. In the study of a commercial aluminium alloy, it was shown that the zirconium oxide/hydroxide coating material deposited preferentially at second phase particles [11]. The preferential deposition occurs due to the increase of pH at favored locations of cathodic reactions. The work here focuses on the effect of copper, which is an important alloying element in commercial aerospace alloys, on the formation of the coatings. It employs substrates of aluminium and Al–Cu alloys deposited by magnetron sputtering, which enables deposition of metastable alloys containing a wide
⇑ Corresponding author. Tel.: +44 161 306 4872; fax: +44 161 306 4865. E-mail address: [email protected] (P. Skeldon). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.08.031
range of copper contents. Further, the deposited layers allow measurements of the thicknesses of the coatings and also of the layers of oxidized metal that are consumed by the conversion treatment. The study examines the kinetics of coating growth and the compositions and morphologies of the coatings, which were investigated using scanning and transmission electron microscopies (SEM and TEM), Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA) and glow discharge optical emission spectroscopy (GDOES). 2. Experimental Aluminium and Al–1 at.%Cu, Al–5 at.%Cu and Al–25 at.%Cu alloys were deposited onto electropolished aluminium foil in an Atom Tech Ltd. magnetron sputtering system using 99.99% Al and 99.99% Cu targets. The system was evacuated to 2 10 5 Pa, with sputtering carried out in 99.998% argon at 0.5 Pa. The specimens were then masked with lacquer to leave a region with dimensions of 3 2 cm exposed for formation of the conversion coating. The copper in the alloys is expected to be present mainly in metastable solid-solution. However, low angle X-ray diffraction indicated the presence of small amounts of CuAl2. The conversion coatings were formed by immersing the specimens for up to 600 s in baths containing 0.0014 mol/l H2ZrF6, 5 g/l H2BO3, 10 g/l KNO3 and 0.4 ml HNO3 (4 N). The temperature and pH of the baths were 55 and 2.6 °C respectively. The specimens were rinsed with deionised water after the conversion process, dried in a cool air stream and stored in a desiccator for subsequent analyses. Ultramicrotomed sections, nominally 10 nm thick, of the specimens were examined by TEM in a JOEL 2000 FX II instrument at an accelerating voltage of 120 kV. Specimens were also examined by
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SEM, using a Zeiss EVO 50 instrument at an accelerating voltage of 20 kV. Ion beam analyses employed charged particles supplied by the Van de Graaff accelerator of the University of Paris. RBS was carried out using 1.53 or 1.86 MeV He+ ions at normal incidence, with detection of scattered particles at 165°. Data were interpreted using the RUMP program [15]. NRA employed 0.87 MeV 2H+ ions to determine the oxygen contents of specimens using the 16 O(d,p1)17O reaction, as described elsewhere [16]. Data were simultaneously obtained for carbon, fluorine and nitrogen from the 12C(d,p0)13C, 19F(d,p11,12)20F and 14N(d,p5)15N reactions. The areas of analysis in RBS and NRA were 1 mm2. Elemental depth profiles were obtained by GDOES using a Jobin Yvon GD Profiler 2 RF instrument at an argon pressure of 700 Pa by applying an RF of 13.56 MHz and a power of 35 W. The region of analysis was of 4 mm diameter. The relevant wavelengths (in nm) were as follows: aluminium, 396.152; copper 324.754; oxygen, 130.217; zirconium, 339.198.
(a)
80 nm
3. Results 3.1. Coating morphology Fig. 1 shows transmission electron micrographs of aluminium specimens before and after conversion treatment. Fig. 1(a) reveals a columnar-grained aluminium layer that is 440 nm thick. A thin oxide separates the deposited aluminium and the electropolished aluminium foil. The coatings formed after treatments of 180 and 600 s are shown in Fig. 1(b,c) respectively. The coating material is relatively featureless, suggesting an amorphous structure. The dark contrast of the coating is consistent with the presence of zirconium. The thicker coating formed in 600 s contains numerous cracks and has detached from the aluminium, which supports a thin layer of alumina. The cracking and detachment occur due to stresses in the coating, probably induced by dehydration and shrinkage of the coating either in air following formation of the coating or in the vacuum of the microscope during SEM. They may also be exacerbated by cutting of the section. The micrographs of Fig. 1 show that the coating growth is accompanied by a relatively uniform thinning of the deposited aluminium. Transmission electron micrographs of the Al–25 at.%Cu alloy before and after conversion treatment are shown in Fig. 2. Fig. 2(a) reveals that the alloy is initially 340 nm thick and appears to be fine-grained. The section is slightly affected by corrosion, which occurs as the ultramicrotomy involves collection of sections that float on water. The corrosion results in fine regions of penetration of the alloy, with relatively light appearance, due to dissolution of metal and triangular features in the substrate at the base of the alloy. As evident from comparison of Figs. 1 and 2, the final conversion coating is much thinner on the alloy than on the aluminium, reaching only 50 nm after 600 s. The final thickness corresponds to an average rate of coating growth of 0.08 nm s 1. Further, a corrosion product, with a relatively light contrast, is evident between the coating and the alloy. The thickness of the corrosion product is relatively uniform. The corrosion product is also detected by RBS and GDOES. Hence, it is not produced by corrosion during ultramicrotomy. The corroded layer thickens with increasing time of immersion. The thickness of the corroded layer was 0.6 times the thickness of the coating after treatment of the alloy for 30 s, but 1.9 times thicker than the coating formed for 600 s. Enrichment of copper in the alloy is suggested by the thin dark band immediately beneath the corrosion product. Fig. 3 presents the dependence of the thicknesses of the coatings and the consumed metal layers on the time of the conversion treatment for aluminium, Al–1 at.%Cu alloy and Al–25 at.%Cu alloy. Conversion treatment of a Al–5 at.%Cu alloy produced coatings and corrosion layers with thicknesses very similar to those of the Al–
(b)
80 nm
(c)
80 nm
Fig. 1. Transmission electron micrographs of ultramicrotomed sections of sputtering-deposited aluminium treated in a zirconium-based conversion bath for (a) 0, (b) 180 and (c) 600 s. Arrows indicate the sputtering-deposited layer.
25 at.%Cu alloy and results for this alloy are therefore not shown. In the case of aluminium, the rates of coating growth and aluminium consumption decrease as the coating thickens up to a treatment time of 300 s and then proceed at relatively constant rates up to 600 s, when the coating is 170 nm thick. The coating thickness is roughly about 1.5 times the thickness of the consumed aluminium layer, and the average rate of coating growth over 600 s is 0.3 nm/s. The coating thickness was significantly reduced by the addition of copper to the aluminium, with the Al–25 at.%Cu alloy forming the thinnest coatings. The results of Fig. 3 indicate that all substrates were oxidized at similar rates, i.e. regardless of the copper content. The thinning of the Al–1 at.%Cu alloy appeared to be mainly associated with the growth of the coating, since no corrosion layer was resolved by TEM beneath the coating.
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(a)
80 nm
Coating thickness (nm)
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Time (s)
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Fig. 2. Transmission electron micrographs of ultramicrotomed sections of sputtering-deposited Al–25 at.%Cu alloy conversion treated in a zirconium-based conversion bath for (a) 0, (b) 180 and (c) 600 s. Arrows indicate the sputtering-deposited layer.
Fig. 4(a–c) show the surfaces of the aluminium and Al–1 at.%Cu and Al–25 at.%Cu alloys after coating for 180 s. The coating on aluminium exhibited ‘‘mud-cracking’’, due to shrinkage of the coating during drying. Such cracking is absent from the coatings on the alloys. However, local detachment of the coating from the Al– 1 at.%Cu alloy is evident. The detachment became extensive following a treatment for 300 s, as shown in Fig. 4(d), with most regions of the surface being covered by loose pieces of coating. Similar extensive detachment was observed on the Al–5 at.%Cu alloy, but at an earlier time of 180 s. 3.2. Coating composition Examples of RBS spectra are shown in Fig. 5 for aluminium before conversion treatment and after conversion treatment for
Fig. 3. (a) Dependence of the thickness of the conversion coating on time for sputtering-deposited aluminium and Al–1 at.%Cu and Al–25 at.%Cu alloys treated in the zirconium-based conversion bath. (b) Reduction of thickness of the deposited layers during growth of the coatings of (a).
180 s. The spectrum for the non-coated aluminium reveals scattering from aluminium in the sputtering-deposited layer and the electropolished substrate, with the respective yields separated by a trough due to the thin oxide on the latter. A large zirconium peak is evident after treatment for 180 s, accompanied by an oxygen peak, indicative of the presence of a zirconium oxide/hydroxide layer. The aluminium yield shows a small step, which is due to either aluminium exposed at locations where the conversion coating is absent or to the presence of aluminium species that are incorporated into the coating. The alumina film that is expected to be present beneath the zirconium-rich layer that was indicated by TEM is too thin to be resolved by RBS. The small yield due to hafnium is due to hafnium impurity in the fluorozirconate ions of the treatment solution. The peak at channel 330 is possibly due to chloride contamination of the coating surface. The spectra for longer times of treatment displayed an increased height of the aluminium step and a reduced height of the zirconium peak, probably due to the shrinkage and cracking of the coating that exposes the substrate. The thicker coatings revealed a yield from
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(a)
50 μm
(b)
(c)
50 μm
(d)
50 μm
50 μm
Fig. 4. Scanning electron micrographs after treatment for 180 s in the zirconium-based conversion bath: (a) aluminium; (b) Al–1 at.%Cu; (c) Al–25 at.%Cu; (d) Al–1 at.%Cu alloy treated for 300 s.
fluorine. The fluorine peak is not evident in the specimen treated for 180 s since it coincides with the depression in the aluminium yield due to the oxide film on the electropolished aluminium. The fitting of the data for the specimen treated for 180 s utilized the following atomic ratios: O:Zr = 2.5, Al:Zr = 0.35, F:O = 0.22, Hf:Zr = 0.01. This specimen displayed the lowest signal for aluminium and hence the fitted composition is considered to be closest to that of the conversion coating. Fig. 6 shows the dependence of the zirconium content of the conversion coating on the time of conversion treatment. The zirconium contents have an accuracy of 10%. The zirconium level rises continuously up to the longest time of treatment, namely 600 s, with the rate of increase slowing as the coating thickens. Examples of RBS spectra are shown in Fig. 7 for the Al–25 at.%Cu alloy before conversion treatment and after conversion treatment for 600 s. The spectrum for the alloy reveals aluminium and copper yields corresponding to an alloy of composition Al–25 at.%Cu. After conversion treatment, yields from zirconium and oxygen are evident. Further, the signal from copper shows a small peak corresponding to copper near the surface of the specimen. There are also significant yields from fluorine and oxygen. The spectra for the conversion-treated specimens were fitted using a conversion layer incorporating zirconium, oxygen, hafnium, copper and aluminium, with an underlying layer of corrosion product containing aluminium, copper, oxygen, fluorine and zirconium. The conversion coating formed after 600 s was simulated using two sub-regions, with the copper enriched in the outer region. The average atomic ratios for the whole coating were O:Zr = 2.77, Cu:Zr = 0.26; Al:Zr = 0.73. The presence of fluorine in the coating could not be identified, since if fluorine were present the signal would coincide with the aluminium edge for the aluminium/alloy interface. The corrosion product was simulated with the following atomic ratios: Cu:Al = 0.25, Zr:Al = 0.06, O:Al = 2 and F:O = 0.8. The thickness of the corrosion product increased with increase of the time of the conversion treatment. The dependence of zirconium content of the conversion coating on the time of treatment is shown in Fig. 6. The zirconium content is seen to change slowly after treatment for 100 s, mirroring the behavior indicated by observation of the coating thickness using TEM.
The GDOES elemental depth profiles for the Al–25 at.%Cu alloy conversion coated for 180 s, shown in Fig. 8, reveal an oxygen profile that extends well beyond depth of the zirconium-containing surface layer, due to the corrosion product beneath the coating. The corrosion product appears to contain aluminium and copper. Copper also appears to be present in the coating, while aluminium, if at all present, appears to be only within the inner part of the coating. Analyses were also carried out of nitrogen and boron in the coating, since boric acid and nitrate ions were present in the conversion bath. However, the signals from these elements were at the level of the background noise. NRA spectra of the aluminium and Al–25 at.%Cu alloy are shown in Fig. 9 for the as-deposited and conversion coated conditions. The conversion treatments were carried out for 300 and 600 s on the aluminium and Al–25 at.%Cu alloy respectively. Small peaks are evident from oxygen in the as-deposited layers. The oxygen arises from oxygen in the air-formed film at the specimen surfaces, the oxide on the electropolished aluminium substrates and oxygen contaminant of the sputtering-deposited layer. From the thickness of the deposited layers and assuming oxide films on the surface of the deposit and the substrate of 3 nm thickness, the data indicate that the concentration of contaminant oxygen is <1 at.%. Carbon is due to adsorbed organic contaminants. Following conversion treatment, the oxygen and carbon peaks increase. In the case of the aluminium substrate, the oxygen is added mainly due to formation of the zirconium-rich coating, whereas a portion of the oxygen on the alloy surface is contained in the corrosion product beneath the coating. Small peaks due to nitrogen on the treated specimens are probably the result of residues of nitrate ions of the conversion bath. Fluorine generated a peak adjacent to the oxygen peak of the Al–25 at.%Cu alloy. However, this peak was comparatively small for the coating formed on aluminium, appearing as a shoulder on the oxygen peak. The greater amount of fluorine for the former specimen suggests that a significant proportion of the fluorine is located in the corrosion product on the Al–25 at.%Cu alloy. The fluorine content was not quantified due to lack of crosssection data. The oxygen contents from NRA and the zirconium contents from RBS for the coatings formed on aluminium for 300 s indicated an atomic ratio of oxygen to zirconium of 3.3.
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Energy (MeV)
Energy (MeV) 0.6
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Channel Fig. 5. RBS spectra of aluminium (a) as-deposited and (b) after 180 s immersion in the zirconium-based conversion bath. Simulated data shown as solid line.
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Fig. 7. RBS spectra of the Al–25 at.%Cu alloy (a) as-deposited and (b) after 600 s immersion in the zirconium-based conversion bath. Simulated data shown as solid line.
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3 2.5 2 Aluminium
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Al 10
O Zr 0
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Sputtering time (s) 0.5 0 0
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Fig. 6. Dependence of zirconium content of the conversion coating on time for sputtering-deposited aluminium and Al–25 at.%Cu alloy treated in the zirconiumbased conversion bath.
Fig. 8. GDOES elemental depth profile of the Al–25 at.%Cu alloy treated in the zirconium-based conversion bath for 180 s. The dashed line indicates the corrosion product/alloy interface.
The atomic ratio of nitrogen to oxygen was 0.03. The atomic ratio of oxygen to zirconium for the Al–25 at.%Cu alloy treated for 600 s was 5.7. The much higher ratio for the alloy is due to the additional oxygen within the corrosion product.
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Energy (MeV) 0.5
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Channel Fig. 9. NRA spectra for (a) aluminium and (b) Al–25 at.%Cu alloy before (solid symbols) and after (open symbols) treatment in the zirconium-based conversion bath. The treatment times were 300 and 600 s in (a) and (b) respectively.
4. Discussion At the start of the conversion treatment, corrosion of the aluminium and Al–Cu alloys is expected to occur at the relatively low pH and elevated temperature of the solution. The corrosion process involves growth of an alumina film and its simultaneous dissolution. During the formation of the alumina, Al3+ and O2 ions migrate through the film. The dissolution of the film in the conversion bath is accelerated by the presence of fluoride ions. The growth and dissolution processes maintain an oxide sufficiently thin for tunnelling of electrons through the alumina that enables the anodic and cathodic reactions to be sustained. During the growth of oxide films on sputtering-deposited Al–Cu alloys, copper that is in metastable solid-solution enriches in an alloy layer of 2 nm thickness, which is located immediately beneath the alumina film [17]. The overlying alumina film is free of copper species as the concentration of copper builds in the alloy as the oxidation of the alloy continues. However, when a sufficient enrichment of copper is reached, which usually occurs when the accumulation of copper lies between 4 and 6 1015 copper atoms cm 2, the cop-
per enters the film either as nanoparticles of copper metal [18] or as oxidized copper species [19], depending upon the potential of the alloy. Thereafter, a relatively constant amount of copper is maintained in the enriched layer as the oxidation of the substrate continues. The average concentration of copper in the oxide, relative to its concentration in the alloy, depends upon the mobility of the incorporated copper relative to that of the Al3+ ions. Nanoparticles of copper metal are expected to be immobile, whereas copper ions are expected to migrate outward through the film at a faster rate than Al3+ ions [20]. The previous considerations suggest that, during formation of the conversion coating on the Al–Cu alloys, aluminium species are firstly oxidized, forming a largely copper-free, alumina film, followed by incorporation of both aluminium and copper species into the film once the alloy has been fully enriched in copper. Thus, initially, mainly aluminium species, and later both copper and aluminium species, are released from the dissolving alumina, which can then be incorporated into the conversion coating. The enrichment of copper, which may alter the open-circuit potential during the coating process, is achieved more rapidly for an alloy of high copper concentration. For the alloys containing 1 and 25 at.% Cu, the copper should be fully enriched after oxidation of thicknesses of 100 and 4 nm respectively, which from Fig. 3(b) would correspond to treatment times of 250 and 5 s respectively. Since the alloys also contain a small amount of CuAl2, copper may enter the oxide at earlier times at regions where the intermetallic phase is present at the alloy surface. In the case of conversion treatments of sputtering-deposited Al–Cu alloys in chromate solutions, copper enriches in the alloy, with the achievement of the steady enrichment of copper coinciding closely with the detachment of the conversion coating from the alloy surface [21,22]. For the present Al– 1 at.%Cu and Al–5 at.%Cu alloys, extensive detachment of the coating is observed locally after treatment for 300 and 180 s respectively, which is possibly also associated with the enrichment of copper. The analyses of the conversion-treated specimens show that coatings are based on zirconium oxide, which is consistent with previous results obtained by X-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectrometry [11,12,23,24]. The present findings suggest that the coatings also contain copper, aluminium and fluorine species. Nitrogen and boron species are minor species. RBS is unable to detect hydrogen. However, hydration of the coating is suggested by the ‘mud-cracking’ of the coating on aluminium, which is due to shrinkage of the coating when water is lost from the coating on drying. The various coating species may be integrated into the amorphous network of the zirconia or may be present in channels in the coating that allows the access of the reactant and product species to and from the substrate. The formation of a coating on aluminium for 180 s is accompanied by oxidation of 70 nm of the substrate, which would have contained 4.2 1017 aluminium atoms cm 2. In comparison, the upper limit on the aluminium content of the coating at this time of immersion was 0.5 1017 aluminium atoms cm 2. Hence, the majority of the oxidized aluminium had passed through the coating to reach the solution. The inward transport of fluoride ions is suggested to be necessary to maintain the oxidation of the substrate by thinning of the alumina film at the base of the zirconium-containing layer. The sites of deposition of the coating material are uncertain, but inward transport of zirconium species possibly enables deposition of zirconia in response to local increases in pH above the alumina layer caused by the cathodic reactions. It is evident from the present results that copper addition to aluminium slows the coating growth significantly, especially at levels above a few at.%. However, the oxidation rate of the alloy is similar to that during conversion treatment of the aluminium,
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as shown by the similarity of the thickness loss of the various substrates following the conversion treatments. The similarity suggests that copper does not affect either the supply of fluoride ions to the coating base or the associated thinning of the alumina film on the substrate. Further, the rate of generation of hydroxide ions at cathodic sites should also be unaffected. The absence of a major influence of the copper on the oxidation rate of the substrate suggests that the coating kinetics may be controlled through by the transport of reactant and product species through the coating. The transport may be the affected by modification of the composition and structure of the coating. A range of species are potentially present in the coating, including zirconium, aluminium, copper, fluorine, nitrogen, boron and hydrogen, either as components of the coating material or as adsorbed species within pores; the present results cannot distinguish between the two locations. Transport studies do not appear to have been carried out for the present composition of coating. However, marker experiments, using gold nanoparticles, suggest that chromate conversion coatings, which have a similar morphology to the zirconia-based coatings studied here, form at the aluminium/coating interface [21]. Thus, the slowing of the coating growth may be due to copper species impeding the transport of cation species to the coating base. A further potential influence on the coating growth in the presence of copper is the development of the corrosion product layer at the base of the coating, which separates the coating material from the substrate. This may lead to an altered solution chemistry at the base of the coating, which affects the deposition process. A precise mechanistic understanding of the effect of copper requires more detailed studies of the coating growth and its interaction with the corrosion process, encompassing the composition and structure of the coatings, the properties of the coating, such as stability in the conversion bath and ion selectivity, and the transport of reactant and product species through the coating thickness. In the case of commercial alloys, deposition of coating material occurs preferentially at cathodic sites, which are usually located at second phase particles [11]. In the present substrates, which have finer-scale microstructures than commercial alloys, the cathodic sites are probably of a more transient in nature, changing their location in response to the thinning of the alloy. The results suggest that coatings may be thinner and less adherent above copper-rich second phase of commercial alloys. However, such alloys, and their second phase particles, contain several alloying elements and the effects of interactions of alloying elements on the coating formation require examination in future work.
5. Conclusions 1. Conversion coatings formed in the present solution are based on zirconia with incorporated aluminium, copper and fluorine species. The coating thickness on aluminium is approximately 1.5 times the thickness of the aluminium layer that is consumed by oxidation during formation of the coating. 2. The addition of copper to aluminium has a relatively small influence on the oxidation rate of the substrate, up to additions of 25 at.% Cu. 3. After an initial period of relatively high growth rate, the copper addition results in a slowing of the rate of coating formation, which is particularly large when the addition exceeds a few atomic percent. 4. The presence of a sufficient amount of copper promotes the formation of a layer of corrosion product beneath the conversion coating, which can exceed the thickness of the conversion coating after long times of immersion in the conversion bath.
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5. In the presence of copper, detachment of coatings from the substrate appears to be promoted by an enrichment of copper in the alloy due to the coating formation.
Acknowledgements The authors are grateful to the Engineering and Physical Sciences Research Council (U.K.) (Programme grant: LATEST2) and the European Community (as an Integrating Activity ‘Support of Public and Industrial Research Using Ion Beam Technology (SPIRIT)’ under EC contract no. 227012) for support of this work. References [1] S. Wernick, R. Pinner, P.G. Sheasby, The Surface Treatment and Finishing of Aluminium Alloys, vol. 1, Finishing Publications Ltd., Teddington, 1989. [2] S.M. Cohen, Review: replacements for chromium pretreatments on aluminium, Corros. Eng. 51 (1995) 71–80. [3] P. Campestrini, H. Terryn, A. Hovestad, J.W.H. de Wit, Formation of a ceriumbased conversion coating on AA2024: relationship with the microstructure, Surf. Coat. Technol. 176 (2004) 365–381. [4] M. Dabalà, L. Armelao, A. Buchberger, I. Calliari, Cerium-based conversion layers on layers on aluminium alloys, Appl. Surf. Sci. 172 (2001) 312–322. [5] I. Danilidis, J.M. Sykes, J.A. Hunter, G.M. Scamans, Manganese based conversion treatment, Surf. Eng. 15 (1999) 401–405. [6] P.D. Deck, M. Molly, R.J. Sujdak, Investigation of fluoacid based conversion coatings on aluminium, Prog. Org. Coat. 34 (1998) 39–48. [7] L. Fedrizzi, F. Deflorian, P.L. Bonora, Corrosion behaviour of fluotitanate pretreated and painted aluminium sheets, Electrochim. Acta 42 (1997) 969– 978. [8] J.D. Gorman, S.T. Johnson, P.N. Johnston, P.J.K. Paterson, A.E. Hughes, The characterisation of Ce–Mo-based conversion coatings on Al-alloys: Part II, Corros. Sci. 38 (1996) 1977–1990. [9] A.S. Hamdy, A.M. Beccaria, P. Traverso, Corrosion protection of aluminium metal-matrix composites by cerium conversion coatings, Surf. Interface Anal. 34 (2002) 171–175. [10] A.S. Hamdy, A.M. Beccaria, Chrome-free pretreatment for aluminium composites, Surf. Interface Anal. 34 (2002) 160–163. [11] J.H. Nordlien, J.C. Walmsley, H. Østerberg, K. Nisancioglu, Formation of a zirconium–titanium based conversion layer on AA 6060 aluminium, Surf. Coat. Technol. 153 (2002) 72–78. [12] T. Schram, G. Goeminne, H. Terryn, W. Vanhoolst, Study of the composition of zirconium based chromium free conversion layers on aluminium, Trans. Inst. Met. Finish. 73 (1995) 91–95. [13] D. Wang, G.P. Bierwagen, Sol–gel coatings on metals for corrosion protection, Prog. Org. Coat. 64 (2009) 327–328. [14] P.E. Hintze, L.M. Calle, Electrochemical properties and corrosion protection of organosilane self-assembled monolayer on aluminium 2024 T3, Electrochim. Acta 51 (2006) 1761–1766. [15] L.R. Doolittle, Algorithms for the rapid simulation of Rutherford backscattering spectra, Nucl. Instrum. Meth. B 9 (1985) 344–351. [16] G. Amsel, D. Samuel, Microanalysis of the stable isotopes of oxygen by means of nuclear reactions, Anal. Chem. 39 (1967) 1689–1698. [17] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Effects of alloying elements in anodizing of aluminium alloys, Trans. Inst. Met. Finish. 75 (1997) 18–23. [18] Y. Liu, M.A. Arenas, S.J. Garcia-Vergara, T. Hashimoto, P. Skeldon, G.E. Thompson, H. Habazaki, P. Bailey, T.C.Q. Noakes, Behaviour of copper during alkaline corrosion of Al–Cu alloys, Corros. Sci. 50 (2008) 1475–1480. [19] X. Zhou, G.E. Thompson, P. Skeldon, K. Shimizu, H. Habazaki, G.C. Wood, The valence state of copper in anodic films formed on Al–1 at.%Cu alloy, Corros. Sci. 47 (2005) 1299–1306. [20] H. Habazaki, X. Zhou, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, Mobility of copper ions in anodic alumina films, Electrochim. Acta 42 (1997) 2627–2635. [21] Y. Liu, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, Chromate conversion coatings on aluminium: influences of alloying, Corros. Sci. 46 (2004) 297–312. [22] Y. Liu, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, Chromate conversion coatings on aluminium–copper alloys, Corros. Sci. 47 (2005) 341–354. [23] S. Verdier, N. van der Laak, F. Dalard, J. Metson, S. Delalande, An electrochemical and SEM study of the mechanism of formation, morphology, and composition of titanium or zirconium fluoride-based coatings, Surf. Coat. Technol. 200 (2006) 2955–2964. [24] N.J. Newhard Jr., Conversion coatings – chromate and non-chromate types, in: H. Leidheiser (Ed.), Proceedings of Corrosion Control Coatings, Science Press, Princeton, 1979, p. 225.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Formation of zirconium-based conversion coatings on aluminium and Al–Cu alloys F.O. George, P. Skeldon ⇑, G.E. Thompson Corrosion and Protection Centre, School of Materials, The University of Manchester, Manchester M13 9PL, UK
a r t i c l e
i n f o
Article history: Received 20 June 2012 Accepted 10 August 2012 Available online 19 August 2012 Keywords: A. Aluminium A. Copper A. Zirconium B. TEM B. RBS C. Oxide coatings
a b s t r a c t The influence of copper addition to aluminium on the formation of a zirconium-based conversion coating is investigated using sputtering-deposited substrates. Coatings formed on aluminium are 1.5 times the thickness of the aluminium consumed by oxidation, with an O:Zr atomic ratio of 2.5. Copper additions reduce the coating growth rate, especially when added in amounts above a few at.%. In contrast, the copper has relatively little effect on the oxidation rate of the substrate. Copper also promotes the formation of a layer of corrosion product beneath the coating and appears to influence the adherence of the coating to the substrate. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Conversion treatments of aluminium alloys for corrosion protection have frequently involved the use of chromate species. In particular, chromate–fluoride baths have been used extensively due to their ability to provide excellent corrosion resistance. The resultant coatings are suited to the application of paints and other organic layers and possess a corrosion inhibiting property due to the presence of chromate ions [1]. However, the use and disposal of chromium compounds are receiving regulatory attention because of the toxicity and carcinogenicity of chromium (VI) species. Consequently, much effort has been given to the development of alternative processes [2–14]. Such approaches have utilized, for example, sol–gel coatings, self-assembled monolayers and rare earth-, titanium- or zirconium-containing oxide/hydroxide coatings. The present study is concerned with zirconium-based conversion coatings, which are formed by a relatively brief immersion of the substrate in an acidic solution, containing zirconium hexafluoride. In the study of a commercial aluminium alloy, it was shown that the zirconium oxide/hydroxide coating material deposited preferentially at second phase particles [11]. The preferential deposition occurs due to the increase of pH at favored locations of cathodic reactions. The work here focuses on the effect of copper, which is an important alloying element in commercial aerospace alloys, on the formation of the coatings. It employs substrates of aluminium and Al–Cu alloys deposited by magnetron sputtering, which enables deposition of metastable alloys containing a wide
⇑ Corresponding author. Tel.: +44 161 306 4872; fax: +44 161 306 4865. E-mail address: [email protected] (P. Skeldon). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.08.031
range of copper contents. Further, the deposited layers allow measurements of the thicknesses of the coatings and also of the layers of oxidized metal that are consumed by the conversion treatment. The study examines the kinetics of coating growth and the compositions and morphologies of the coatings, which were investigated using scanning and transmission electron microscopies (SEM and TEM), Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA) and glow discharge optical emission spectroscopy (GDOES). 2. Experimental Aluminium and Al–1 at.%Cu, Al–5 at.%Cu and Al–25 at.%Cu alloys were deposited onto electropolished aluminium foil in an Atom Tech Ltd. magnetron sputtering system using 99.99% Al and 99.99% Cu targets. The system was evacuated to 2 10 5 Pa, with sputtering carried out in 99.998% argon at 0.5 Pa. The specimens were then masked with lacquer to leave a region with dimensions of 3 2 cm exposed for formation of the conversion coating. The copper in the alloys is expected to be present mainly in metastable solid-solution. However, low angle X-ray diffraction indicated the presence of small amounts of CuAl2. The conversion coatings were formed by immersing the specimens for up to 600 s in baths containing 0.0014 mol/l H2ZrF6, 5 g/l H2BO3, 10 g/l KNO3 and 0.4 ml HNO3 (4 N). The temperature and pH of the baths were 55 and 2.6 °C respectively. The specimens were rinsed with deionised water after the conversion process, dried in a cool air stream and stored in a desiccator for subsequent analyses. Ultramicrotomed sections, nominally 10 nm thick, of the specimens were examined by TEM in a JOEL 2000 FX II instrument at an accelerating voltage of 120 kV. Specimens were also examined by
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SEM, using a Zeiss EVO 50 instrument at an accelerating voltage of 20 kV. Ion beam analyses employed charged particles supplied by the Van de Graaff accelerator of the University of Paris. RBS was carried out using 1.53 or 1.86 MeV He+ ions at normal incidence, with detection of scattered particles at 165°. Data were interpreted using the RUMP program [15]. NRA employed 0.87 MeV 2H+ ions to determine the oxygen contents of specimens using the 16 O(d,p1)17O reaction, as described elsewhere [16]. Data were simultaneously obtained for carbon, fluorine and nitrogen from the 12C(d,p0)13C, 19F(d,p11,12)20F and 14N(d,p5)15N reactions. The areas of analysis in RBS and NRA were 1 mm2. Elemental depth profiles were obtained by GDOES using a Jobin Yvon GD Profiler 2 RF instrument at an argon pressure of 700 Pa by applying an RF of 13.56 MHz and a power of 35 W. The region of analysis was of 4 mm diameter. The relevant wavelengths (in nm) were as follows: aluminium, 396.152; copper 324.754; oxygen, 130.217; zirconium, 339.198.
(a)
80 nm
3. Results 3.1. Coating morphology Fig. 1 shows transmission electron micrographs of aluminium specimens before and after conversion treatment. Fig. 1(a) reveals a columnar-grained aluminium layer that is 440 nm thick. A thin oxide separates the deposited aluminium and the electropolished aluminium foil. The coatings formed after treatments of 180 and 600 s are shown in Fig. 1(b,c) respectively. The coating material is relatively featureless, suggesting an amorphous structure. The dark contrast of the coating is consistent with the presence of zirconium. The thicker coating formed in 600 s contains numerous cracks and has detached from the aluminium, which supports a thin layer of alumina. The cracking and detachment occur due to stresses in the coating, probably induced by dehydration and shrinkage of the coating either in air following formation of the coating or in the vacuum of the microscope during SEM. They may also be exacerbated by cutting of the section. The micrographs of Fig. 1 show that the coating growth is accompanied by a relatively uniform thinning of the deposited aluminium. Transmission electron micrographs of the Al–25 at.%Cu alloy before and after conversion treatment are shown in Fig. 2. Fig. 2(a) reveals that the alloy is initially 340 nm thick and appears to be fine-grained. The section is slightly affected by corrosion, which occurs as the ultramicrotomy involves collection of sections that float on water. The corrosion results in fine regions of penetration of the alloy, with relatively light appearance, due to dissolution of metal and triangular features in the substrate at the base of the alloy. As evident from comparison of Figs. 1 and 2, the final conversion coating is much thinner on the alloy than on the aluminium, reaching only 50 nm after 600 s. The final thickness corresponds to an average rate of coating growth of 0.08 nm s 1. Further, a corrosion product, with a relatively light contrast, is evident between the coating and the alloy. The thickness of the corrosion product is relatively uniform. The corrosion product is also detected by RBS and GDOES. Hence, it is not produced by corrosion during ultramicrotomy. The corroded layer thickens with increasing time of immersion. The thickness of the corroded layer was 0.6 times the thickness of the coating after treatment of the alloy for 30 s, but 1.9 times thicker than the coating formed for 600 s. Enrichment of copper in the alloy is suggested by the thin dark band immediately beneath the corrosion product. Fig. 3 presents the dependence of the thicknesses of the coatings and the consumed metal layers on the time of the conversion treatment for aluminium, Al–1 at.%Cu alloy and Al–25 at.%Cu alloy. Conversion treatment of a Al–5 at.%Cu alloy produced coatings and corrosion layers with thicknesses very similar to those of the Al–
(b)
80 nm
(c)
80 nm
Fig. 1. Transmission electron micrographs of ultramicrotomed sections of sputtering-deposited aluminium treated in a zirconium-based conversion bath for (a) 0, (b) 180 and (c) 600 s. Arrows indicate the sputtering-deposited layer.
25 at.%Cu alloy and results for this alloy are therefore not shown. In the case of aluminium, the rates of coating growth and aluminium consumption decrease as the coating thickens up to a treatment time of 300 s and then proceed at relatively constant rates up to 600 s, when the coating is 170 nm thick. The coating thickness is roughly about 1.5 times the thickness of the consumed aluminium layer, and the average rate of coating growth over 600 s is 0.3 nm/s. The coating thickness was significantly reduced by the addition of copper to the aluminium, with the Al–25 at.%Cu alloy forming the thinnest coatings. The results of Fig. 3 indicate that all substrates were oxidized at similar rates, i.e. regardless of the copper content. The thinning of the Al–1 at.%Cu alloy appeared to be mainly associated with the growth of the coating, since no corrosion layer was resolved by TEM beneath the coating.
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(a)
80 nm
Coating thickness (nm)
(a)
Time (s)
(b)
80 nm
Substrate consumed (nm)
(b)
Time (s)
(c)
80 nm
Fig. 2. Transmission electron micrographs of ultramicrotomed sections of sputtering-deposited Al–25 at.%Cu alloy conversion treated in a zirconium-based conversion bath for (a) 0, (b) 180 and (c) 600 s. Arrows indicate the sputtering-deposited layer.
Fig. 4(a–c) show the surfaces of the aluminium and Al–1 at.%Cu and Al–25 at.%Cu alloys after coating for 180 s. The coating on aluminium exhibited ‘‘mud-cracking’’, due to shrinkage of the coating during drying. Such cracking is absent from the coatings on the alloys. However, local detachment of the coating from the Al– 1 at.%Cu alloy is evident. The detachment became extensive following a treatment for 300 s, as shown in Fig. 4(d), with most regions of the surface being covered by loose pieces of coating. Similar extensive detachment was observed on the Al–5 at.%Cu alloy, but at an earlier time of 180 s. 3.2. Coating composition Examples of RBS spectra are shown in Fig. 5 for aluminium before conversion treatment and after conversion treatment for
Fig. 3. (a) Dependence of the thickness of the conversion coating on time for sputtering-deposited aluminium and Al–1 at.%Cu and Al–25 at.%Cu alloys treated in the zirconium-based conversion bath. (b) Reduction of thickness of the deposited layers during growth of the coatings of (a).
180 s. The spectrum for the non-coated aluminium reveals scattering from aluminium in the sputtering-deposited layer and the electropolished substrate, with the respective yields separated by a trough due to the thin oxide on the latter. A large zirconium peak is evident after treatment for 180 s, accompanied by an oxygen peak, indicative of the presence of a zirconium oxide/hydroxide layer. The aluminium yield shows a small step, which is due to either aluminium exposed at locations where the conversion coating is absent or to the presence of aluminium species that are incorporated into the coating. The alumina film that is expected to be present beneath the zirconium-rich layer that was indicated by TEM is too thin to be resolved by RBS. The small yield due to hafnium is due to hafnium impurity in the fluorozirconate ions of the treatment solution. The peak at channel 330 is possibly due to chloride contamination of the coating surface. The spectra for longer times of treatment displayed an increased height of the aluminium step and a reduced height of the zirconium peak, probably due to the shrinkage and cracking of the coating that exposes the substrate. The thicker coatings revealed a yield from
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(a)
50 μm
(b)
(c)
50 μm
(d)
50 μm
50 μm
Fig. 4. Scanning electron micrographs after treatment for 180 s in the zirconium-based conversion bath: (a) aluminium; (b) Al–1 at.%Cu; (c) Al–25 at.%Cu; (d) Al–1 at.%Cu alloy treated for 300 s.
fluorine. The fluorine peak is not evident in the specimen treated for 180 s since it coincides with the depression in the aluminium yield due to the oxide film on the electropolished aluminium. The fitting of the data for the specimen treated for 180 s utilized the following atomic ratios: O:Zr = 2.5, Al:Zr = 0.35, F:O = 0.22, Hf:Zr = 0.01. This specimen displayed the lowest signal for aluminium and hence the fitted composition is considered to be closest to that of the conversion coating. Fig. 6 shows the dependence of the zirconium content of the conversion coating on the time of conversion treatment. The zirconium contents have an accuracy of 10%. The zirconium level rises continuously up to the longest time of treatment, namely 600 s, with the rate of increase slowing as the coating thickens. Examples of RBS spectra are shown in Fig. 7 for the Al–25 at.%Cu alloy before conversion treatment and after conversion treatment for 600 s. The spectrum for the alloy reveals aluminium and copper yields corresponding to an alloy of composition Al–25 at.%Cu. After conversion treatment, yields from zirconium and oxygen are evident. Further, the signal from copper shows a small peak corresponding to copper near the surface of the specimen. There are also significant yields from fluorine and oxygen. The spectra for the conversion-treated specimens were fitted using a conversion layer incorporating zirconium, oxygen, hafnium, copper and aluminium, with an underlying layer of corrosion product containing aluminium, copper, oxygen, fluorine and zirconium. The conversion coating formed after 600 s was simulated using two sub-regions, with the copper enriched in the outer region. The average atomic ratios for the whole coating were O:Zr = 2.77, Cu:Zr = 0.26; Al:Zr = 0.73. The presence of fluorine in the coating could not be identified, since if fluorine were present the signal would coincide with the aluminium edge for the aluminium/alloy interface. The corrosion product was simulated with the following atomic ratios: Cu:Al = 0.25, Zr:Al = 0.06, O:Al = 2 and F:O = 0.8. The thickness of the corrosion product increased with increase of the time of the conversion treatment. The dependence of zirconium content of the conversion coating on the time of treatment is shown in Fig. 6. The zirconium content is seen to change slowly after treatment for 100 s, mirroring the behavior indicated by observation of the coating thickness using TEM.
The GDOES elemental depth profiles for the Al–25 at.%Cu alloy conversion coated for 180 s, shown in Fig. 8, reveal an oxygen profile that extends well beyond depth of the zirconium-containing surface layer, due to the corrosion product beneath the coating. The corrosion product appears to contain aluminium and copper. Copper also appears to be present in the coating, while aluminium, if at all present, appears to be only within the inner part of the coating. Analyses were also carried out of nitrogen and boron in the coating, since boric acid and nitrate ions were present in the conversion bath. However, the signals from these elements were at the level of the background noise. NRA spectra of the aluminium and Al–25 at.%Cu alloy are shown in Fig. 9 for the as-deposited and conversion coated conditions. The conversion treatments were carried out for 300 and 600 s on the aluminium and Al–25 at.%Cu alloy respectively. Small peaks are evident from oxygen in the as-deposited layers. The oxygen arises from oxygen in the air-formed film at the specimen surfaces, the oxide on the electropolished aluminium substrates and oxygen contaminant of the sputtering-deposited layer. From the thickness of the deposited layers and assuming oxide films on the surface of the deposit and the substrate of 3 nm thickness, the data indicate that the concentration of contaminant oxygen is <1 at.%. Carbon is due to adsorbed organic contaminants. Following conversion treatment, the oxygen and carbon peaks increase. In the case of the aluminium substrate, the oxygen is added mainly due to formation of the zirconium-rich coating, whereas a portion of the oxygen on the alloy surface is contained in the corrosion product beneath the coating. Small peaks due to nitrogen on the treated specimens are probably the result of residues of nitrate ions of the conversion bath. Fluorine generated a peak adjacent to the oxygen peak of the Al–25 at.%Cu alloy. However, this peak was comparatively small for the coating formed on aluminium, appearing as a shoulder on the oxygen peak. The greater amount of fluorine for the former specimen suggests that a significant proportion of the fluorine is located in the corrosion product on the Al–25 at.%Cu alloy. The fluorine content was not quantified due to lack of crosssection data. The oxygen contents from NRA and the zirconium contents from RBS for the coatings formed on aluminium for 300 s indicated an atomic ratio of oxygen to zirconium of 3.3.
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Energy (MeV)
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Channel Fig. 5. RBS spectra of aluminium (a) as-deposited and (b) after 180 s immersion in the zirconium-based conversion bath. Simulated data shown as solid line.
500
Fig. 7. RBS spectra of the Al–25 at.%Cu alloy (a) as-deposited and (b) after 600 s immersion in the zirconium-based conversion bath. Simulated data shown as solid line.
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3 2.5 2 Aluminium
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Fig. 6. Dependence of zirconium content of the conversion coating on time for sputtering-deposited aluminium and Al–25 at.%Cu alloy treated in the zirconiumbased conversion bath.
Fig. 8. GDOES elemental depth profile of the Al–25 at.%Cu alloy treated in the zirconium-based conversion bath for 180 s. The dashed line indicates the corrosion product/alloy interface.
The atomic ratio of nitrogen to oxygen was 0.03. The atomic ratio of oxygen to zirconium for the Al–25 at.%Cu alloy treated for 600 s was 5.7. The much higher ratio for the alloy is due to the additional oxygen within the corrosion product.
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Channel Fig. 9. NRA spectra for (a) aluminium and (b) Al–25 at.%Cu alloy before (solid symbols) and after (open symbols) treatment in the zirconium-based conversion bath. The treatment times were 300 and 600 s in (a) and (b) respectively.
4. Discussion At the start of the conversion treatment, corrosion of the aluminium and Al–Cu alloys is expected to occur at the relatively low pH and elevated temperature of the solution. The corrosion process involves growth of an alumina film and its simultaneous dissolution. During the formation of the alumina, Al3+ and O2 ions migrate through the film. The dissolution of the film in the conversion bath is accelerated by the presence of fluoride ions. The growth and dissolution processes maintain an oxide sufficiently thin for tunnelling of electrons through the alumina that enables the anodic and cathodic reactions to be sustained. During the growth of oxide films on sputtering-deposited Al–Cu alloys, copper that is in metastable solid-solution enriches in an alloy layer of 2 nm thickness, which is located immediately beneath the alumina film [17]. The overlying alumina film is free of copper species as the concentration of copper builds in the alloy as the oxidation of the alloy continues. However, when a sufficient enrichment of copper is reached, which usually occurs when the accumulation of copper lies between 4 and 6 1015 copper atoms cm 2, the cop-
per enters the film either as nanoparticles of copper metal [18] or as oxidized copper species [19], depending upon the potential of the alloy. Thereafter, a relatively constant amount of copper is maintained in the enriched layer as the oxidation of the substrate continues. The average concentration of copper in the oxide, relative to its concentration in the alloy, depends upon the mobility of the incorporated copper relative to that of the Al3+ ions. Nanoparticles of copper metal are expected to be immobile, whereas copper ions are expected to migrate outward through the film at a faster rate than Al3+ ions [20]. The previous considerations suggest that, during formation of the conversion coating on the Al–Cu alloys, aluminium species are firstly oxidized, forming a largely copper-free, alumina film, followed by incorporation of both aluminium and copper species into the film once the alloy has been fully enriched in copper. Thus, initially, mainly aluminium species, and later both copper and aluminium species, are released from the dissolving alumina, which can then be incorporated into the conversion coating. The enrichment of copper, which may alter the open-circuit potential during the coating process, is achieved more rapidly for an alloy of high copper concentration. For the alloys containing 1 and 25 at.% Cu, the copper should be fully enriched after oxidation of thicknesses of 100 and 4 nm respectively, which from Fig. 3(b) would correspond to treatment times of 250 and 5 s respectively. Since the alloys also contain a small amount of CuAl2, copper may enter the oxide at earlier times at regions where the intermetallic phase is present at the alloy surface. In the case of conversion treatments of sputtering-deposited Al–Cu alloys in chromate solutions, copper enriches in the alloy, with the achievement of the steady enrichment of copper coinciding closely with the detachment of the conversion coating from the alloy surface [21,22]. For the present Al– 1 at.%Cu and Al–5 at.%Cu alloys, extensive detachment of the coating is observed locally after treatment for 300 and 180 s respectively, which is possibly also associated with the enrichment of copper. The analyses of the conversion-treated specimens show that coatings are based on zirconium oxide, which is consistent with previous results obtained by X-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectrometry [11,12,23,24]. The present findings suggest that the coatings also contain copper, aluminium and fluorine species. Nitrogen and boron species are minor species. RBS is unable to detect hydrogen. However, hydration of the coating is suggested by the ‘mud-cracking’ of the coating on aluminium, which is due to shrinkage of the coating when water is lost from the coating on drying. The various coating species may be integrated into the amorphous network of the zirconia or may be present in channels in the coating that allows the access of the reactant and product species to and from the substrate. The formation of a coating on aluminium for 180 s is accompanied by oxidation of 70 nm of the substrate, which would have contained 4.2 1017 aluminium atoms cm 2. In comparison, the upper limit on the aluminium content of the coating at this time of immersion was 0.5 1017 aluminium atoms cm 2. Hence, the majority of the oxidized aluminium had passed through the coating to reach the solution. The inward transport of fluoride ions is suggested to be necessary to maintain the oxidation of the substrate by thinning of the alumina film at the base of the zirconium-containing layer. The sites of deposition of the coating material are uncertain, but inward transport of zirconium species possibly enables deposition of zirconia in response to local increases in pH above the alumina layer caused by the cathodic reactions. It is evident from the present results that copper addition to aluminium slows the coating growth significantly, especially at levels above a few at.%. However, the oxidation rate of the alloy is similar to that during conversion treatment of the aluminium,
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as shown by the similarity of the thickness loss of the various substrates following the conversion treatments. The similarity suggests that copper does not affect either the supply of fluoride ions to the coating base or the associated thinning of the alumina film on the substrate. Further, the rate of generation of hydroxide ions at cathodic sites should also be unaffected. The absence of a major influence of the copper on the oxidation rate of the substrate suggests that the coating kinetics may be controlled through by the transport of reactant and product species through the coating. The transport may be the affected by modification of the composition and structure of the coating. A range of species are potentially present in the coating, including zirconium, aluminium, copper, fluorine, nitrogen, boron and hydrogen, either as components of the coating material or as adsorbed species within pores; the present results cannot distinguish between the two locations. Transport studies do not appear to have been carried out for the present composition of coating. However, marker experiments, using gold nanoparticles, suggest that chromate conversion coatings, which have a similar morphology to the zirconia-based coatings studied here, form at the aluminium/coating interface [21]. Thus, the slowing of the coating growth may be due to copper species impeding the transport of cation species to the coating base. A further potential influence on the coating growth in the presence of copper is the development of the corrosion product layer at the base of the coating, which separates the coating material from the substrate. This may lead to an altered solution chemistry at the base of the coating, which affects the deposition process. A precise mechanistic understanding of the effect of copper requires more detailed studies of the coating growth and its interaction with the corrosion process, encompassing the composition and structure of the coatings, the properties of the coating, such as stability in the conversion bath and ion selectivity, and the transport of reactant and product species through the coating thickness. In the case of commercial alloys, deposition of coating material occurs preferentially at cathodic sites, which are usually located at second phase particles [11]. In the present substrates, which have finer-scale microstructures than commercial alloys, the cathodic sites are probably of a more transient in nature, changing their location in response to the thinning of the alloy. The results suggest that coatings may be thinner and less adherent above copper-rich second phase of commercial alloys. However, such alloys, and their second phase particles, contain several alloying elements and the effects of interactions of alloying elements on the coating formation require examination in future work.
5. Conclusions 1. Conversion coatings formed in the present solution are based on zirconia with incorporated aluminium, copper and fluorine species. The coating thickness on aluminium is approximately 1.5 times the thickness of the aluminium layer that is consumed by oxidation during formation of the coating. 2. The addition of copper to aluminium has a relatively small influence on the oxidation rate of the substrate, up to additions of 25 at.% Cu. 3. After an initial period of relatively high growth rate, the copper addition results in a slowing of the rate of coating formation, which is particularly large when the addition exceeds a few atomic percent. 4. The presence of a sufficient amount of copper promotes the formation of a layer of corrosion product beneath the conversion coating, which can exceed the thickness of the conversion coating after long times of immersion in the conversion bath.
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5. In the presence of copper, detachment of coatings from the substrate appears to be promoted by an enrichment of copper in the alloy due to the coating formation.
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