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Hydrated oxide film growth on aluminium alloys immersed in warm water
Surface & Coatings Technology 192 (2005) 199 – 207 www.elsevier.com/locate/surfcoat
Hydrated oxide film growth on aluminium alloys immersed in warm water P.R. Underhilla,*, A.N. Riderb a
b
Royal Military College of Canada Kingston, Ontario, Canada K7K 7B4 DSTO, Melbourne, 506 Lorimer St., Fishermans Bend, Victoria, 3207, Australia Received 22 October 2003; accepted in revised form 5 October 2004 Available online 26 November 2004
Abstract The growth of hydrated oxide films on 2024 bare and 7075 clad aluminium alloys immersed in deionized water at temperatures of 40 and 50 8C for periods up to a couple of hours was studied using grazing angle FTIR, weight gain measurements, high resolution SEM and AFM. The results show that a porous oxide structure, likely to be very suitable for adhesive bonding, develops. The films formed at 50 8C are much thicker than those formed at 40 8C and contain significantly more pseudoboehmite; however, the porosity of the films appears to be comparable at both temperatures. In contrast with film growth studies reported for pure aluminium, the alloy systems do not appear to show an incubation period prior to hydrated oxide growth. D 2004 Elsevier B.V. All rights reserved. Keywords: [D] Aluminium alloy; [B] Interfaces; [B] Roughness
1. Introduction The adhesive bonding of aluminium requires careful surface preparation to produce bonds which have good environmental stability under wet conditions. Traditional methods of surface preparation have used either anodization, typically phosphoric acid anodization [1], or surface etches, such as the Forest Products Laboratory (FPL) etch to prepare the surface and then chromate (VI)-containing primers [2] to protect it. Environmental concerns, however, have created an interest in finding alternative approaches, which do not use highly corrosive materials and which eliminate the use of chromate (VI)-containing compounds. One of the most promising approaches has been the use of silane coupling agents to improve the hydrolytic stability of the adhesive/substrate interface. Unfortunately, when silane coupling agents are used with simple preparations involving surface cleaning and grit-blasting and without chromate (VI)-containing primers, the hydrolytic stability of the adhesive joints as measured by the wedge test (ASTM
* Corresponding author. Tel.: +1 613 541 6000; fax: +1 613 542 8612. E-mail address: [email protected] (P.R. Underhill). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.011
D3762) has not performed as well as the traditional treatments. Such joints have always shown some crack growth resulting from adhesive failure. Recently, Rider and Arnot [3] have shown that a boiling water treatment can produce results equivalent to those achieved by the more traditional methods on both 2024 and 7075 aluminium alloys, which are important structural alloys in the aircraft industry. They demonstrated that this treatment produces a highly porous oxide layer of pseudoboehmite, which allows for interlocking of the adhesive and the substrate. The primary problem with this approach is the high temperature required for processing. Aluminium has a very high thermal conductivity, and achieving this temperature uniformly over the area to be processed can be problematic in repair scenarios. Alwitt [4] reviewed the behaviour of pure aluminium in water over the full range of temperatures. Pseudoboehmite oxide films could be grown on aluminium at temperatures between 100 and 50 8C, and the kinetics of the process were very similar throughout this range. Hydrated oxide film growth was characterized by an induction period followed by a period of rapid growth and a final stage of slower steady growth. The aluminium–water film growth mechanism suggests that results similar to those achieved by Rider and Arnot can be achieved at signifi-
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Table 1 Aluminium alloy compositions Alloy
2024 7075 7072
Composition Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Other total maximum
0.50 0.40 0.75 total
0.50 0.50
3.8–4.9 1.2–2.0 0.10
0.3–0.9 0.3 0.10
1.2–1.8 2.1–2.9 0.10
0.10 0.18–0.28
0.25 5.1–6.1 0.8–1.3
0.15 0.2
0.15 0.15 0.15
cantly lower temperatures. In this paper, the reaction of the unclad 2024 T3 and clad 7075 T6 aluminium alloys with water in the temperature range 40 to 50 8C is examined to determine the kinetics of the reaction, the chemical nature of the product and the morphology of the hydrated oxide film produced.
2. Experimental The compositions of the 2024 and 7075 alloys are shown in Table 1 along with that of 7072, a common cladding material for the 7075 alloy. Infrared spectra showing the evolution of the hydrated oxide growth were obtained from samples of 2024 T3 unclad and 7075 T6 clad aluminium alloy sheet, which had been cut into 57.5-cm samples. Prior to hot water treatment, the alloy sample was pretreated by abrasion with a Scotchbrite pad in deionized water and dried using an air gun with no heat. The sample was then placed in a grazing angle bench on a Nicolet 610P FTIR, and a background spectrum was taken using 128 scans at 4cm1 resolution. The sample was then removed and placed in deionized water (resistance N4 MVcm1) held at either 40F1 or 50F1 8C with a temperature-controlled hot plate. After an initial period (typically 1 min), the sample was
removed, dried as before and placed back in the spectrometer to record a spectrum. The sample was then placed back into the water bath and left for a period of time that resulted in a doubling of the cumulative exposure time. This process of exposing, drying and accumulating a spectrum was then repeated typically until cumulative times in excess of 90 min had been obtained. The displayed spectra have been corrected for background curvature where necessary and had residual water vapour peaks subtracted for clarity. Samples of both alloys with approximately 100 cm2 total area were grit-blasted and thoroughly rinsed with deionized water to remove any adherent grit particles and dried with a warm air gun. The samples were then weighed. Samples that were to be treated at 50 8C were weighed on a balance with an accuracy of 0.1 mg, while samples that were to be treated at 40 8C were weighed on a balance with a resolution of 1 Ag. The samples were then exposed to water at either 40 or 50 8C for times between 2 and 120 min followed by drying and reweighing. The treatment, drying and reweighing process was repeated several times. Film thickness measurements were carried out on aluminium alloy that had been ultramilled using a procedure previously reported [5] prior to being immersed in deionized water at 40 or 50 8C for times between 10 and 60 min. Apiezon wax was deposited across the surface, and the
Fig. 1. IR spectra obtained from 7075 T6 clad alloy for cumulative immersion times of 4, 8, 16, 32, 64 and 128 min in 40 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 4 min and the top to 128 min.
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Fig. 2. IR spectra obtained from 7075 T6 clad alloy for cumulative immersion times of 2, 4, 8, 16, 32 and 64 min in 50 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 2 min and the top to 64 min.
hydrated oxide film was etched away using a 2% CrO3 and 5% H3PO4 solution at 60 8C for 30 min. AFM was then employed to measure the resulting step after the Apiezon wax was removed using toluene. The AFM measurements were conducted with a Park Scientific Auto Probe LS model AFM operating in constant force mode. The scanning probe was calibrated using a grid of known step height. All images had a background subtraction performed on them using a second-order polynomial to compensate for thermal variations, piezoelectric hysteresis and sample tilt. A silicon
nitride microlever tip of 3 Am height and 558 angle was used for all measurements. High-resolution SEM images of 2024 and 7075 aluminium alloy, which had been exposed for times of 10, 30 and 60 min at either 40 or 50 8C, were obtained using a Leo Field emission SEM model 1530VP (Variable Pressure) operating in normal vacuum mode using the in-lens detector. Aluminium samples were prepared using the ultramilling procedure to produce a flat clean surface prior to water treatment. No sample preparation other than
Fig. 3. IR spectra obtained from unclad 2024 T3 alloy for cumulative immersion times of 2, 4, 8, 16, 32, 64 and 130 min in 40 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 2 min and the top to 130 min.
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Fig. 4. IR spectra obtained from unclad 2024 T3 alloy for cumulative immersion times of 2, 4, 8, 16, 32, 64 and 128 min in 50 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 2 min and the top to 128 min.
standard mounting using conductive tape was used for the SEM imaging. Accelerating voltage and working distance are indicated on the images.
3. Results and discussion The IR spectra collected on the two alloy systems at 40 and 50 8C are shown in Figs. 1–4. At 50 8C, samples that had been left for long periods (60 min or longer) developed a brownish hue and a spectrum which is an excellent match to a reference pseudoboehmite film (Fig. 5).
While, in each series of spectra, there are small shifts in the individual peak positions, all of the spectra can be characterized by features at approximately 740, 970, 1090, 1405, 1522 and 1650 cm1 in addition to a broad peak at about 3500 cm1. The peak at 670 cm1 is a spectrometer artifact, and that at about 2300 cm1 is due to CO2. The peak at approximately 740 cm1 is associated with the Al–O stretch mode. It appears to show a slight systematic variation between the two alloys, occurring at 734 cm1 in the 2024 alloy and 753 cm1 in the 7075 alloy. The peak at about 970 cm1 also shows a slight alloy dependence occurring at 968 cm1 in the 2024 alloy and
Fig. 5. IR spectrum obtained from unclad 2024 T3 alloy after an immersion time of 152 min in 50 8C water.
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976 cm1 in the 7075. The peak has been attributed by Vedder and Vermilyea [6] and Maelund et al. [7] to a longitudinal Al–O stretch mode perpendicular to the surface which has been frequency-shifted by surface polarization. The corresponding mode parallel to the surface occurs at 660 cm1 (not seen in reflection mode). As the film thickens, this surface-shifted mode will drop in intensity until it vanishes at a thickness of about 2.2 Am [6]. In fact, this is observed in all the spectra, especially those obtained at 50 8C. The feature at 1090 cm1, which appears as just a shoulder on the 970 cm1 peak in many of the spectra, is due to the yOH mode and is characteristic of boehmite and pseudoboehmite. In the trihydrates, gibbsite and bayerite, the most intense adsorption band due to this mode has been shifted down to 1020 cm1. Careful analysis of the spectra, accounting for the slope of the side of the 970 cm1 peak, clearly indicates that the shoulder can be characterized as a single peak centered on 1090 cm1. There is no indication of a significant peak at 1020 cm1 in any of the spectra. The peaks in the range from 1300–1700 cm1 have been attributed by Alwitt and Stralin and Hjertberg [4,8] to the bending mode of interstitial water. Baker and Pearson [9] have argued that the water is not interstitial but rather bound to different surface sites on extremely small crystallites. In either case, a 235 cm1 shift from 1640 to approximately 1405 cm1 seems extremely large for a hydrogen-bonded structure. Dorsey [10] and Fin et al. [11] have attributed the lower two peaks to AljO moieties. The 1650 cm1 peak increases in relative intensity compared with the two peaks at lower frequency as the treatment temperature increases. This variation lends support to the suggestion that the peak at 1650 cm1 and the two lower frequency peaks arise from different sources. On the other hand, the ratio of these three peaks does not change as the oxide layer thickens, which suggests that they must share some commonality. The spectra taken at 40 8C for each of the two alloys show what appears to be a single broad adsorption band with a maximum at about 3500 cm1. The width of the band suggests that several peaks may be present in the frequency range that cannot be resolved spectroscopically. Close examination of the spectra obtained at 50 8C shows
Fig. 6. Growth in the area under the OH stretch peak for clad 7075 T6 as a function of exposure time in warm water.
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Fig. 7. Growth in the area under the OH stretch peak for unclad 2024 T3 as a function of exposure time in warm water.
the evolution of some structure in this region, especially a peak at about 3100 cm1 and a third peak on the high wave number side. These can be clearly seen in Fig. 5 and appears to be associated with the development of pseudoboehmite. The intensity of the peaks in the range 500–1200 cm1 cannot be used to quantitatively measure the amount of oxide present because of the difficulties in fitting an accurate baseline to the spectra in this region. However, qualitatively, they suggest that the rate of oxide formation on the two alloys at both temperatures is comparable. To obtain a quantitative indication of the rate of growth of the two oxides, the rate of growth of the OH stretch peak was plotted as a function of time. The results for the 7075 alloy are shown in Fig. 6 and for the 2024 alloy in Fig. 7. The results show that the growth rate can be characterized with a power law. The exponent and 95% confidence interval for the 2024 alloy is 0.51F0.07 at 40 8C and 0.59F0.06 at 50 8C. For the 7075 alloy, the exponents are 0.73F0.06 and 0.71F0.30 for 40 and 50 8C, respectively. Assuming that there is a correlation between the OH stretch intensity and the amount of oxide present, these results are rather surprising. While there are significant differences in the
Fig. 8. Weight gain as a function of time of immersion for unclad 2024 alloy. Solid symbols are for samples processed at 40 8C, open symbols for those processed at 50 8C and the crosses are for Alwitt’s data [4] taken at 50 8C. At a given treatment temperature, different symbol shapes represent different samples.
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Fig. 9. Comparison of weight gain as a function of time of immersion in 50 8C water for 7075 clad samples (solid symbols) and unclad 2024 samples (open symbols).
rate of growth of the area of the OH stretch peak between the two alloys, there is no significant difference on each alloy at 40 and 50 8C at least over the range of times examined here. In pure aluminium, Alwitt [4] found completely different mechanisms operated at these two temperatures and that the growth at 40 8C was considerably slower. The second significant difference is the absence of an induction time as shown by the linearity of the graphs down to the smallest times measured. The 40 8C data in Fig. 7 only goes to 10 min (the start of that data run); however, it is clear from Fig. 3 that there is growth even at 2 min. At 50 8C, Alwitt found an induction time in excess of 10 min, and, at 40 8C, the induction period was on the order of several hundred minutes. It should be noted that Alwitt measured weight gain, while, here, it is the growth of the OH moiety that is being measured. To understand the infrared results further, weight gain experiments were conducted. The results, shown in Fig. 8,
show a significant difference between the weight gain at 40 and 50 8C, consistent with the Alwitt data. The weight gain at 50 8C is less than that observed by Alwitt on pure aluminium, particularly if it is considered that the relative surface area of the grit-blasted aluminium was approximately 1.3–1.5 [12]. The log–log plot of the film weight gain also contrasts with the Alwitt data as there is no indication of an induction period. Fig. 9 compares the weight gain at 50 8C between the 2024 and 7075 samples. It is seen that the two are very comparable. No data were gathered on the 7075 sample at 40 8C due to difficulties in producing a sufficiently thin specimen with an intact cladding layer. The weight gain measurements were complemented with thickness measurements determined by AFM. Two AFM images used for the film thickness measurements are presented in 3-D format in Fig. 10. The Z-scale has been magnified to highlight the step and film topography. The overall thickness values for the 50 8C treatments are slightly greater for the 7075 than the 2024 alloy and are markedly lower at 40 8C. In comparison with the thickness of films produced on 2024 in boiling water [5], the 50 8C films are notably thinner. The correlation between the thickness and the weight gain measurements is shown in Fig. 11. The graph incorporates data from both alloys at both 40 and 50 8C. The correlation is excellent and yields a slope of 5.2F0.3 nm/(Ag/cm2) or 0.52 cm3/g. The linearity of the results indicates that the density of the film remains constant as it grows. If it were assumed that the film formed according to reaction (1) and that the density of pseudoboehmite was 2.4 g/cm3 [13], then, for the gritblasted surfaces with a relative surface area of 1.4, the films formed at 50 8C would have a porosity of approximately 10–15%. This compares with values of between 10% and 30% for films produced on pure aluminium immersed in
Fig. 10. 3-D AFM images indicating the film topography and step produced by the wax deposition and CrO3/H3PO4 etch process.
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porosity does not change significantly during the times examined in this work. 2Al þ 5:1H2 OYAl2 O3 2:1H2 O þ 3H2
Fig. 11. Correlation between weight gain per unit area and film thickness. Data points are taken from measurements on both alloys at 40 (open symbols) and 50 8C (2024 E, 7075 S ). The straight line is fitted to all the data points.
boiling water for periods between 10 and 30 min reported by Alwitt [4]. Clearly, further work is required to validate these porosity estimates; however, the similar trend in weight gain and film thickness would suggest that the film
ð1Þ
It would appear that the OH stretch intensities obtained from the FTIR work do not correlate with the hydrated oxide film thickness and weight gain measurements. It is believed that this is an artifact of the grazing angle and the roughness of the oxide surface, which apparently results in only the outer film surface being sampled. Despite the difference in infrared and weight gain measurements, the infrared data still provides an accurate characterization of the outer film chemistry. Fig. 12 shows the micrographs obtained after immersion in 40 8C water for the two alloys. On the 2024 alloy, after 30 min, there is no real sign of a porous microstructure, although there appears to be small nodules all over the surface. After 60 min, a porous structure, similar to that observed at 50 8C, only finer, is obtained. On the 7075 alloy, there is no indication of any structure after 30 min. At 60 min, the structure seems comparable to that obtained on the 2024 sample after 30 min. Fig. 13 shows the topography of the surface of each alloy after exposure to water at 50 8C. On the 2024 sample, at 50
Fig. 12. SEM micrographs of 2024 unclad aluminium immersed in 40 8C water for (A) 30 and (B) 60 min and 7075 clad aluminium immersed in 40 8C water for (C) 30 and (D) 60 min. Micrographs were taken using (A) 0.75-, (B) 1-, (C) 5- and (D) 5-kV accelerating voltages and 3-mm working distances.
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Fig. 13. SEM micrographs of 2024 unclad aluminium immersed in 50 8C water for (A) 10, (B) 30 and (C) 60 min and 7075 clad aluminium immersed in 50 8C water for (D) 10, (E) 30 and (F) 60 min. Micrographs were taken using 5-kV accelerating voltage and 3-mm working distance.
8C, a fine highly porous structure can be seen after 10 min. This structure coarsens somewhat over the next 50 min, but otherwise seems to be morphologically very similar. At 10 min, there is only a hint of hydrated oxide structure on the 7075 sample. By 30 min, however, a porous microstructure is clearly visible. As in the case of the 2024 sample, this coarsens over the next 30 min of exposure. Fig. 14 shows a cross-section of the oxide film on the 2024 alloy after 60 min of exposure. The sample is tilted so that the top half of the picture is the top surface of the film. The image clearly shows that the porosity of the film is uniform throughout its thickness.
It is interesting to note that the film structures observed, particularly for the 30- and 60-min treatments of unclad 2024 and clad 7075 samples immersed in 50 8C water, bear a good resemblance to those obtained from clad 2024 alloy after 30 s of exposure to boiling water [3]. However, the hydrated oxide film does not go on to develop the platelet structure observed after immersion of aluminium in boiling water for times greater than 10 min. The reduced thickness of the 50 8C, and particularly the 40 8C, films relative to the boiling water films may contribute to the absence of the well-defined topography.
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4. Conclusions
Fig. 14. 2024 T3 aluminium alloy immersed for 60 min in 50 8C water. Micrograph of the hydrated oxide indicating the film cross-sectional structure.
At first glance, there is no obvious correlation between the level of porosity and the type of hydrated oxide present on the surface nor is there an obvious correlation between porosity and weight gain and film thickness. The shape of the IR spectrum at 32 min on the 2024 sample heated at 40 8C is not different from that at 64 min, yet the SEM images show a marked increase in film porosity. On the other hand, as the films produced at 50 8C thicken and become more porous, there is a clear transition to a pseudoboehmite spectrum. One explanation for the evolution of the structure and chemical composition of the film is that it arises from a competition between dissolution and precipitation of the hydrated aluminium oxide with the preferred precipitation product being pseudoboehmite [4]. As the temperature was lowered, the rate of dissolution would be decreased and precipitation increased so that the oxide becomes more compact. The situation would be further complicated for alloys because the cationic reaction (H2O+e YOH+1/2H2) could be spatially separated to constituent particles. This would result in the diffusion-controlled spatial distribution of the pH. Since the solubility of aluminium and its hydrated oxides are extremely sensitive to pH, this could be expected to affect the dissolution–precipitation equilibrium on a highly local scale. In this model, as the temperature increased, the film structure would be expected to coarsen. This would explain why the films created in boiling water show a much coarser pattern of pseudoboehmite platelets. The continuous dissolution and precipitation throughout the films formed at 40 and 50 8C would also explain why the film appears to coarsen as it grows and yet still has a uniform porosity in cross-section after 60 min instead of a gradient of porosity.
The reaction of 2024 unclad and 7075 clad aluminium alloys with water in the temperature range from 40 to 50 8C has been studied. The results show that the principal reaction products over time frames of approximately 2 h appear to be hydrated aluminium oxide and pseudoboehmite. At 40 8C, the chemical nature of the hydrated oxide does not appear to change over the time frame of these experiments, but, at 50 8C, there is a clear increase in the pseudoboehmite content of the film. Unlike pure aluminium, the water reaction with 2024 unclad and 7075 clad alloys does not possess an incubation period. Films formed at 50 8C developed a highly porous nature over a 1-h period. As expected, this porous film appeared faster on the less corrosion-resistant unclad 2024 alloy than on the clad 7075 alloy. At 40 8C, a porous film developed in 1 h on only the 2024 alloy. It is suggested that the porous film arises from the dissolution and precipitation of the initial hydrated oxide as a pseudoboehmite. This process is facilitated by higher temperatures and may also be affected by pH gradients arising from the presence of constituent particles on the surface of the aluminium. The highly porous hydrated oxides should make excellent substrates for bonding to organic films.
Acknowledgements The authors would like to acknowledge the assistance of Mr John Russell, Maritime Platforms Division, DSTO, Melbourne, Australia and Mr Colin MacRae of Mineral Products, CSIRO for assistance with the SEM imaging and AFM measurements.
References [1] A.H. Landrock, Adhesives Technology Handbook, Noyes Publications, Park Ridge, NJ, 1985, p. 66. [2] H.M. Clearfield, D.K. McNamara, G.D. Davis, Engineered Materials Handbook, vol. 3, Adhesives and Sealants, H.F. Brinson (technical chairman), ASM International, 1990, p. 254. [3] A.N. Rider, D.R. Arnot, Int. J. Adhes. Adhes. 20 (2000) 209. [4] R.S. Alwitt, in: J.W. Dingle, K.V. Ashok (Eds.), Oxides and Oxide Films, vol. 4, Marcel Dekker, New York, 1976. [5] A.N. Rider, J. Adhes. Sci. Technol. 15 (2001) 395. [6] W. Vedder, D.A. Vermilyea, Trans. Faraday Soc. 65 (1969) 561. [7] A.J. Maelund, R. Rittenhouse, W. Lahar, P.V. Romano, Thin Solid Films 21 (1974) 67. [8] A. Stralin, T. Hjertberg, Appl. Surf. Sci. 74 (1994) 263. [9] B.R. Baker, R.M. Pearson, J. Catal. 33 (1974) 265. [10] G.A. Dorsey, J. Electrochem. Soc. 113 (1966) 169. [11] N. Fin, H. Dodiuk, A.E. Yaniv, L. Drori, Appl. Surf. Sci. 28 (1987) 11. [12] A.N. Rider, Int. J. Adhes. Adhes. (in print). [13] W.J. Bernhard, J.J. Randall, J. Electrochem. Soc. 107 (1960) 483.
Hydrated oxide film growth on aluminium alloys immersed in warm water P.R. Underhilla,*, A.N. Riderb a
b
Royal Military College of Canada Kingston, Ontario, Canada K7K 7B4 DSTO, Melbourne, 506 Lorimer St., Fishermans Bend, Victoria, 3207, Australia Received 22 October 2003; accepted in revised form 5 October 2004 Available online 26 November 2004
Abstract The growth of hydrated oxide films on 2024 bare and 7075 clad aluminium alloys immersed in deionized water at temperatures of 40 and 50 8C for periods up to a couple of hours was studied using grazing angle FTIR, weight gain measurements, high resolution SEM and AFM. The results show that a porous oxide structure, likely to be very suitable for adhesive bonding, develops. The films formed at 50 8C are much thicker than those formed at 40 8C and contain significantly more pseudoboehmite; however, the porosity of the films appears to be comparable at both temperatures. In contrast with film growth studies reported for pure aluminium, the alloy systems do not appear to show an incubation period prior to hydrated oxide growth. D 2004 Elsevier B.V. All rights reserved. Keywords: [D] Aluminium alloy; [B] Interfaces; [B] Roughness
1. Introduction The adhesive bonding of aluminium requires careful surface preparation to produce bonds which have good environmental stability under wet conditions. Traditional methods of surface preparation have used either anodization, typically phosphoric acid anodization [1], or surface etches, such as the Forest Products Laboratory (FPL) etch to prepare the surface and then chromate (VI)-containing primers [2] to protect it. Environmental concerns, however, have created an interest in finding alternative approaches, which do not use highly corrosive materials and which eliminate the use of chromate (VI)-containing compounds. One of the most promising approaches has been the use of silane coupling agents to improve the hydrolytic stability of the adhesive/substrate interface. Unfortunately, when silane coupling agents are used with simple preparations involving surface cleaning and grit-blasting and without chromate (VI)-containing primers, the hydrolytic stability of the adhesive joints as measured by the wedge test (ASTM
* Corresponding author. Tel.: +1 613 541 6000; fax: +1 613 542 8612. E-mail address: [email protected] (P.R. Underhill). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.011
D3762) has not performed as well as the traditional treatments. Such joints have always shown some crack growth resulting from adhesive failure. Recently, Rider and Arnot [3] have shown that a boiling water treatment can produce results equivalent to those achieved by the more traditional methods on both 2024 and 7075 aluminium alloys, which are important structural alloys in the aircraft industry. They demonstrated that this treatment produces a highly porous oxide layer of pseudoboehmite, which allows for interlocking of the adhesive and the substrate. The primary problem with this approach is the high temperature required for processing. Aluminium has a very high thermal conductivity, and achieving this temperature uniformly over the area to be processed can be problematic in repair scenarios. Alwitt [4] reviewed the behaviour of pure aluminium in water over the full range of temperatures. Pseudoboehmite oxide films could be grown on aluminium at temperatures between 100 and 50 8C, and the kinetics of the process were very similar throughout this range. Hydrated oxide film growth was characterized by an induction period followed by a period of rapid growth and a final stage of slower steady growth. The aluminium–water film growth mechanism suggests that results similar to those achieved by Rider and Arnot can be achieved at signifi-
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Table 1 Aluminium alloy compositions Alloy
2024 7075 7072
Composition Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Other total maximum
0.50 0.40 0.75 total
0.50 0.50
3.8–4.9 1.2–2.0 0.10
0.3–0.9 0.3 0.10
1.2–1.8 2.1–2.9 0.10
0.10 0.18–0.28
0.25 5.1–6.1 0.8–1.3
0.15 0.2
0.15 0.15 0.15
cantly lower temperatures. In this paper, the reaction of the unclad 2024 T3 and clad 7075 T6 aluminium alloys with water in the temperature range 40 to 50 8C is examined to determine the kinetics of the reaction, the chemical nature of the product and the morphology of the hydrated oxide film produced.
2. Experimental The compositions of the 2024 and 7075 alloys are shown in Table 1 along with that of 7072, a common cladding material for the 7075 alloy. Infrared spectra showing the evolution of the hydrated oxide growth were obtained from samples of 2024 T3 unclad and 7075 T6 clad aluminium alloy sheet, which had been cut into 57.5-cm samples. Prior to hot water treatment, the alloy sample was pretreated by abrasion with a Scotchbrite pad in deionized water and dried using an air gun with no heat. The sample was then placed in a grazing angle bench on a Nicolet 610P FTIR, and a background spectrum was taken using 128 scans at 4cm1 resolution. The sample was then removed and placed in deionized water (resistance N4 MVcm1) held at either 40F1 or 50F1 8C with a temperature-controlled hot plate. After an initial period (typically 1 min), the sample was
removed, dried as before and placed back in the spectrometer to record a spectrum. The sample was then placed back into the water bath and left for a period of time that resulted in a doubling of the cumulative exposure time. This process of exposing, drying and accumulating a spectrum was then repeated typically until cumulative times in excess of 90 min had been obtained. The displayed spectra have been corrected for background curvature where necessary and had residual water vapour peaks subtracted for clarity. Samples of both alloys with approximately 100 cm2 total area were grit-blasted and thoroughly rinsed with deionized water to remove any adherent grit particles and dried with a warm air gun. The samples were then weighed. Samples that were to be treated at 50 8C were weighed on a balance with an accuracy of 0.1 mg, while samples that were to be treated at 40 8C were weighed on a balance with a resolution of 1 Ag. The samples were then exposed to water at either 40 or 50 8C for times between 2 and 120 min followed by drying and reweighing. The treatment, drying and reweighing process was repeated several times. Film thickness measurements were carried out on aluminium alloy that had been ultramilled using a procedure previously reported [5] prior to being immersed in deionized water at 40 or 50 8C for times between 10 and 60 min. Apiezon wax was deposited across the surface, and the
Fig. 1. IR spectra obtained from 7075 T6 clad alloy for cumulative immersion times of 4, 8, 16, 32, 64 and 128 min in 40 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 4 min and the top to 128 min.
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Fig. 2. IR spectra obtained from 7075 T6 clad alloy for cumulative immersion times of 2, 4, 8, 16, 32 and 64 min in 50 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 2 min and the top to 64 min.
hydrated oxide film was etched away using a 2% CrO3 and 5% H3PO4 solution at 60 8C for 30 min. AFM was then employed to measure the resulting step after the Apiezon wax was removed using toluene. The AFM measurements were conducted with a Park Scientific Auto Probe LS model AFM operating in constant force mode. The scanning probe was calibrated using a grid of known step height. All images had a background subtraction performed on them using a second-order polynomial to compensate for thermal variations, piezoelectric hysteresis and sample tilt. A silicon
nitride microlever tip of 3 Am height and 558 angle was used for all measurements. High-resolution SEM images of 2024 and 7075 aluminium alloy, which had been exposed for times of 10, 30 and 60 min at either 40 or 50 8C, were obtained using a Leo Field emission SEM model 1530VP (Variable Pressure) operating in normal vacuum mode using the in-lens detector. Aluminium samples were prepared using the ultramilling procedure to produce a flat clean surface prior to water treatment. No sample preparation other than
Fig. 3. IR spectra obtained from unclad 2024 T3 alloy for cumulative immersion times of 2, 4, 8, 16, 32, 64 and 130 min in 40 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 2 min and the top to 130 min.
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Fig. 4. IR spectra obtained from unclad 2024 T3 alloy for cumulative immersion times of 2, 4, 8, 16, 32, 64 and 128 min in 50 8C water. Spectra increase in intensity monotonically with immersion time. The bottom spectrum corresponds to 2 min and the top to 128 min.
standard mounting using conductive tape was used for the SEM imaging. Accelerating voltage and working distance are indicated on the images.
3. Results and discussion The IR spectra collected on the two alloy systems at 40 and 50 8C are shown in Figs. 1–4. At 50 8C, samples that had been left for long periods (60 min or longer) developed a brownish hue and a spectrum which is an excellent match to a reference pseudoboehmite film (Fig. 5).
While, in each series of spectra, there are small shifts in the individual peak positions, all of the spectra can be characterized by features at approximately 740, 970, 1090, 1405, 1522 and 1650 cm1 in addition to a broad peak at about 3500 cm1. The peak at 670 cm1 is a spectrometer artifact, and that at about 2300 cm1 is due to CO2. The peak at approximately 740 cm1 is associated with the Al–O stretch mode. It appears to show a slight systematic variation between the two alloys, occurring at 734 cm1 in the 2024 alloy and 753 cm1 in the 7075 alloy. The peak at about 970 cm1 also shows a slight alloy dependence occurring at 968 cm1 in the 2024 alloy and
Fig. 5. IR spectrum obtained from unclad 2024 T3 alloy after an immersion time of 152 min in 50 8C water.
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976 cm1 in the 7075. The peak has been attributed by Vedder and Vermilyea [6] and Maelund et al. [7] to a longitudinal Al–O stretch mode perpendicular to the surface which has been frequency-shifted by surface polarization. The corresponding mode parallel to the surface occurs at 660 cm1 (not seen in reflection mode). As the film thickens, this surface-shifted mode will drop in intensity until it vanishes at a thickness of about 2.2 Am [6]. In fact, this is observed in all the spectra, especially those obtained at 50 8C. The feature at 1090 cm1, which appears as just a shoulder on the 970 cm1 peak in many of the spectra, is due to the yOH mode and is characteristic of boehmite and pseudoboehmite. In the trihydrates, gibbsite and bayerite, the most intense adsorption band due to this mode has been shifted down to 1020 cm1. Careful analysis of the spectra, accounting for the slope of the side of the 970 cm1 peak, clearly indicates that the shoulder can be characterized as a single peak centered on 1090 cm1. There is no indication of a significant peak at 1020 cm1 in any of the spectra. The peaks in the range from 1300–1700 cm1 have been attributed by Alwitt and Stralin and Hjertberg [4,8] to the bending mode of interstitial water. Baker and Pearson [9] have argued that the water is not interstitial but rather bound to different surface sites on extremely small crystallites. In either case, a 235 cm1 shift from 1640 to approximately 1405 cm1 seems extremely large for a hydrogen-bonded structure. Dorsey [10] and Fin et al. [11] have attributed the lower two peaks to AljO moieties. The 1650 cm1 peak increases in relative intensity compared with the two peaks at lower frequency as the treatment temperature increases. This variation lends support to the suggestion that the peak at 1650 cm1 and the two lower frequency peaks arise from different sources. On the other hand, the ratio of these three peaks does not change as the oxide layer thickens, which suggests that they must share some commonality. The spectra taken at 40 8C for each of the two alloys show what appears to be a single broad adsorption band with a maximum at about 3500 cm1. The width of the band suggests that several peaks may be present in the frequency range that cannot be resolved spectroscopically. Close examination of the spectra obtained at 50 8C shows
Fig. 6. Growth in the area under the OH stretch peak for clad 7075 T6 as a function of exposure time in warm water.
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Fig. 7. Growth in the area under the OH stretch peak for unclad 2024 T3 as a function of exposure time in warm water.
the evolution of some structure in this region, especially a peak at about 3100 cm1 and a third peak on the high wave number side. These can be clearly seen in Fig. 5 and appears to be associated with the development of pseudoboehmite. The intensity of the peaks in the range 500–1200 cm1 cannot be used to quantitatively measure the amount of oxide present because of the difficulties in fitting an accurate baseline to the spectra in this region. However, qualitatively, they suggest that the rate of oxide formation on the two alloys at both temperatures is comparable. To obtain a quantitative indication of the rate of growth of the two oxides, the rate of growth of the OH stretch peak was plotted as a function of time. The results for the 7075 alloy are shown in Fig. 6 and for the 2024 alloy in Fig. 7. The results show that the growth rate can be characterized with a power law. The exponent and 95% confidence interval for the 2024 alloy is 0.51F0.07 at 40 8C and 0.59F0.06 at 50 8C. For the 7075 alloy, the exponents are 0.73F0.06 and 0.71F0.30 for 40 and 50 8C, respectively. Assuming that there is a correlation between the OH stretch intensity and the amount of oxide present, these results are rather surprising. While there are significant differences in the
Fig. 8. Weight gain as a function of time of immersion for unclad 2024 alloy. Solid symbols are for samples processed at 40 8C, open symbols for those processed at 50 8C and the crosses are for Alwitt’s data [4] taken at 50 8C. At a given treatment temperature, different symbol shapes represent different samples.
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Fig. 9. Comparison of weight gain as a function of time of immersion in 50 8C water for 7075 clad samples (solid symbols) and unclad 2024 samples (open symbols).
rate of growth of the area of the OH stretch peak between the two alloys, there is no significant difference on each alloy at 40 and 50 8C at least over the range of times examined here. In pure aluminium, Alwitt [4] found completely different mechanisms operated at these two temperatures and that the growth at 40 8C was considerably slower. The second significant difference is the absence of an induction time as shown by the linearity of the graphs down to the smallest times measured. The 40 8C data in Fig. 7 only goes to 10 min (the start of that data run); however, it is clear from Fig. 3 that there is growth even at 2 min. At 50 8C, Alwitt found an induction time in excess of 10 min, and, at 40 8C, the induction period was on the order of several hundred minutes. It should be noted that Alwitt measured weight gain, while, here, it is the growth of the OH moiety that is being measured. To understand the infrared results further, weight gain experiments were conducted. The results, shown in Fig. 8,
show a significant difference between the weight gain at 40 and 50 8C, consistent with the Alwitt data. The weight gain at 50 8C is less than that observed by Alwitt on pure aluminium, particularly if it is considered that the relative surface area of the grit-blasted aluminium was approximately 1.3–1.5 [12]. The log–log plot of the film weight gain also contrasts with the Alwitt data as there is no indication of an induction period. Fig. 9 compares the weight gain at 50 8C between the 2024 and 7075 samples. It is seen that the two are very comparable. No data were gathered on the 7075 sample at 40 8C due to difficulties in producing a sufficiently thin specimen with an intact cladding layer. The weight gain measurements were complemented with thickness measurements determined by AFM. Two AFM images used for the film thickness measurements are presented in 3-D format in Fig. 10. The Z-scale has been magnified to highlight the step and film topography. The overall thickness values for the 50 8C treatments are slightly greater for the 7075 than the 2024 alloy and are markedly lower at 40 8C. In comparison with the thickness of films produced on 2024 in boiling water [5], the 50 8C films are notably thinner. The correlation between the thickness and the weight gain measurements is shown in Fig. 11. The graph incorporates data from both alloys at both 40 and 50 8C. The correlation is excellent and yields a slope of 5.2F0.3 nm/(Ag/cm2) or 0.52 cm3/g. The linearity of the results indicates that the density of the film remains constant as it grows. If it were assumed that the film formed according to reaction (1) and that the density of pseudoboehmite was 2.4 g/cm3 [13], then, for the gritblasted surfaces with a relative surface area of 1.4, the films formed at 50 8C would have a porosity of approximately 10–15%. This compares with values of between 10% and 30% for films produced on pure aluminium immersed in
Fig. 10. 3-D AFM images indicating the film topography and step produced by the wax deposition and CrO3/H3PO4 etch process.
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porosity does not change significantly during the times examined in this work. 2Al þ 5:1H2 OYAl2 O3 2:1H2 O þ 3H2
Fig. 11. Correlation between weight gain per unit area and film thickness. Data points are taken from measurements on both alloys at 40 (open symbols) and 50 8C (2024 E, 7075 S ). The straight line is fitted to all the data points.
boiling water for periods between 10 and 30 min reported by Alwitt [4]. Clearly, further work is required to validate these porosity estimates; however, the similar trend in weight gain and film thickness would suggest that the film
ð1Þ
It would appear that the OH stretch intensities obtained from the FTIR work do not correlate with the hydrated oxide film thickness and weight gain measurements. It is believed that this is an artifact of the grazing angle and the roughness of the oxide surface, which apparently results in only the outer film surface being sampled. Despite the difference in infrared and weight gain measurements, the infrared data still provides an accurate characterization of the outer film chemistry. Fig. 12 shows the micrographs obtained after immersion in 40 8C water for the two alloys. On the 2024 alloy, after 30 min, there is no real sign of a porous microstructure, although there appears to be small nodules all over the surface. After 60 min, a porous structure, similar to that observed at 50 8C, only finer, is obtained. On the 7075 alloy, there is no indication of any structure after 30 min. At 60 min, the structure seems comparable to that obtained on the 2024 sample after 30 min. Fig. 13 shows the topography of the surface of each alloy after exposure to water at 50 8C. On the 2024 sample, at 50
Fig. 12. SEM micrographs of 2024 unclad aluminium immersed in 40 8C water for (A) 30 and (B) 60 min and 7075 clad aluminium immersed in 40 8C water for (C) 30 and (D) 60 min. Micrographs were taken using (A) 0.75-, (B) 1-, (C) 5- and (D) 5-kV accelerating voltages and 3-mm working distances.
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Fig. 13. SEM micrographs of 2024 unclad aluminium immersed in 50 8C water for (A) 10, (B) 30 and (C) 60 min and 7075 clad aluminium immersed in 50 8C water for (D) 10, (E) 30 and (F) 60 min. Micrographs were taken using 5-kV accelerating voltage and 3-mm working distance.
8C, a fine highly porous structure can be seen after 10 min. This structure coarsens somewhat over the next 50 min, but otherwise seems to be morphologically very similar. At 10 min, there is only a hint of hydrated oxide structure on the 7075 sample. By 30 min, however, a porous microstructure is clearly visible. As in the case of the 2024 sample, this coarsens over the next 30 min of exposure. Fig. 14 shows a cross-section of the oxide film on the 2024 alloy after 60 min of exposure. The sample is tilted so that the top half of the picture is the top surface of the film. The image clearly shows that the porosity of the film is uniform throughout its thickness.
It is interesting to note that the film structures observed, particularly for the 30- and 60-min treatments of unclad 2024 and clad 7075 samples immersed in 50 8C water, bear a good resemblance to those obtained from clad 2024 alloy after 30 s of exposure to boiling water [3]. However, the hydrated oxide film does not go on to develop the platelet structure observed after immersion of aluminium in boiling water for times greater than 10 min. The reduced thickness of the 50 8C, and particularly the 40 8C, films relative to the boiling water films may contribute to the absence of the well-defined topography.
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4. Conclusions
Fig. 14. 2024 T3 aluminium alloy immersed for 60 min in 50 8C water. Micrograph of the hydrated oxide indicating the film cross-sectional structure.
At first glance, there is no obvious correlation between the level of porosity and the type of hydrated oxide present on the surface nor is there an obvious correlation between porosity and weight gain and film thickness. The shape of the IR spectrum at 32 min on the 2024 sample heated at 40 8C is not different from that at 64 min, yet the SEM images show a marked increase in film porosity. On the other hand, as the films produced at 50 8C thicken and become more porous, there is a clear transition to a pseudoboehmite spectrum. One explanation for the evolution of the structure and chemical composition of the film is that it arises from a competition between dissolution and precipitation of the hydrated aluminium oxide with the preferred precipitation product being pseudoboehmite [4]. As the temperature was lowered, the rate of dissolution would be decreased and precipitation increased so that the oxide becomes more compact. The situation would be further complicated for alloys because the cationic reaction (H2O+e YOH+1/2H2) could be spatially separated to constituent particles. This would result in the diffusion-controlled spatial distribution of the pH. Since the solubility of aluminium and its hydrated oxides are extremely sensitive to pH, this could be expected to affect the dissolution–precipitation equilibrium on a highly local scale. In this model, as the temperature increased, the film structure would be expected to coarsen. This would explain why the films created in boiling water show a much coarser pattern of pseudoboehmite platelets. The continuous dissolution and precipitation throughout the films formed at 40 and 50 8C would also explain why the film appears to coarsen as it grows and yet still has a uniform porosity in cross-section after 60 min instead of a gradient of porosity.
The reaction of 2024 unclad and 7075 clad aluminium alloys with water in the temperature range from 40 to 50 8C has been studied. The results show that the principal reaction products over time frames of approximately 2 h appear to be hydrated aluminium oxide and pseudoboehmite. At 40 8C, the chemical nature of the hydrated oxide does not appear to change over the time frame of these experiments, but, at 50 8C, there is a clear increase in the pseudoboehmite content of the film. Unlike pure aluminium, the water reaction with 2024 unclad and 7075 clad alloys does not possess an incubation period. Films formed at 50 8C developed a highly porous nature over a 1-h period. As expected, this porous film appeared faster on the less corrosion-resistant unclad 2024 alloy than on the clad 7075 alloy. At 40 8C, a porous film developed in 1 h on only the 2024 alloy. It is suggested that the porous film arises from the dissolution and precipitation of the initial hydrated oxide as a pseudoboehmite. This process is facilitated by higher temperatures and may also be affected by pH gradients arising from the presence of constituent particles on the surface of the aluminium. The highly porous hydrated oxides should make excellent substrates for bonding to organic films.
Acknowledgements The authors would like to acknowledge the assistance of Mr John Russell, Maritime Platforms Division, DSTO, Melbourne, Australia and Mr Colin MacRae of Mineral Products, CSIRO for assistance with the SEM imaging and AFM measurements.
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