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Oxide formation on aluminium alloys in boiling deionised water and NaCl, CeCl3 and CrCl3 solutions
Corrosion Science 45 (2003) 1103–1124 www.elsevier.com/locate/corsci
Oxide formation on aluminium alloys in boiling deionised water and NaCl, CeCl3 and CrCl3 solutions J.D. Gorman a
a,1
, A.E. Hughes b,*, D. Jamieson c, P.J.K. Paterson a
Department of Applied Physics, RMIT University, 124 La Trobe St., Melbourne 3000, Australia b CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, Clayton South MDC, Clayton, Vic. 3169, Australia c School of Physics, University of Melbourne, Parkville 3002, Australia Received 6 October 1999; accepted 21 October 2002
Abstract Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy have been employed to provide depth, surface chemical and morphological information respectively for a range of Al-alloys treated for 2 h at 90–100 °C in either deionised water (DIW), 30 mM NaCl, 10 mM CeCl3 or 10 mM CrCl3 solutions. Alloys included Al sheet 1100-O, 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6. In DIW all alloys developed the same oxide thickness with a boehmite-like appearance. In NaCl and CeCl3 solutions the oxide thickness varied with alloy composition but was considerably thicker than the deionised case and maintained a boehmite-like appearance. Ce was distributed throughout the oxide but at low levels. In all cases some CeIV was detected on the surface of the oxide, the remainder being CeIII . In CrCl3 solution, the oxide thickness was extremely thin with the appearance of a dense oxide rather than a boehmite-type structure. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Boehmite; Rutherford backscattering spectroscopy; Oxide growth and rare earth salts; Cerium; Chromium; X-ray photoelectron spectroscopy; B. Scanning electron microscopy
*
Corresponding author. Tel.: +61-3-9545-2777/2075; fax: +61-3-9544-1128. E-mail address: [email protected] (A.E. Hughes). 1 Tel.: +61-3-9925-2136.
0010-938X/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 2 ) 0 0 2 0 9 - 3
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1. Introduction Corrosion protection of aluminium often involves chemical pretreatment, incorporating, as the final step, either conversion coating or anodising, prior to the application of a paint system [1]. Anodising and conversion coating processes, historically, have involved the use of chromium (VI) which, for a number of years, has been considered a serious health risk for workers during processing (manufacture or repair) and for the environment in terms of discharge and disposal [2]. Despite these risks, chromate conversion coatings and chromic acid anodising continue to be used for applications with high performance requirements, for example, in the aerospace industry. For these reasons, there has been considerable interest in finding alternatives particularly to chromate conversion coatings as used in the aerospace industry. Alternatives may be broadly divided into a number of categories according to the type of chemistry involved in processing and/or the processing techniques [3]. First, there are inorganic coatings which are loosely based on the use of metal anion complexes or inorganic oxidising species which undergo a redox reaction with the aluminium surface and deposit an insoluble compound during the reaction [4–7]. Some of the best examples of these systems are based on Mo and Ce. Second, are coatings which rely on growing the hydrated aluminium oxide on the metal surface at elevated temperatures (up to, or even above, 100 °C). Examples of this include processes based on Li, Co, Ce, Mn or Ni salt solutions, or growth in water followed by some type of sealing [8–14]. The third category combines all coatings that do not fit into the first two categories. This category would include polymer coatings and surface chemical (e.g. self assembling monolayers, silanes) and physical (e.g. laser alloy treatment) modification. The focus of this paper is on the growth of hydrated aluminium oxides at temperatures close to 100 °C, in water and chloride solutions containing either Naþ , Cr3þ or Ce3þ cations in the presence of Cl anions. The growth of hydrated Al oxides was studied extensively during the 1960s and 1970s [15–19]. Most of this work was confined to relatively pure aluminium. Vedder and Vermilyea [16] identified a three stage growth mechanism including (i) formation of amorphous surface oxide, (ii) dissolution of the surface oxide which at low pH is rate limited by the rate of surface hydration, and (iii) precipitation of hydrated aluminium oxide (Al2 O3 nH2 O). Alwitt [19] later refined this model, concluding that steps (i) and (ii) were probably rate limited by the diffusion of H2 O (probably as OH and Hþ ) into the oxide surface. Surface oxides developed under these conditions typically have varying degrees of hydration ranging from Al2 O3 (n ¼ 0) to aluminium hydroxide Al(OH)3 (n ¼ 3). The growth of anodic oxide films in the presence of cations or anions has not been so extensively studied. Vermilyea and Vedder [18] demonstrated that the presence of cations in solution resulted in a range of effects from no inhibition to extremely strong inhibition. The strong inhibitors ‘‘keyed’’ onto aluminium oxide or hydroxide structure thus preventing boehmite growth. B€ ohni and Uhlig observed similar effects through studying movement in the pitting potential as a function of chloride addition and subsequently in the presence of various cations [20]. Attack on anodic films in the presence of various anions and cations has been studied extensively [20–23].
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Lorking and Mayne [22] and Sotoudah et al. [23] concluded that attack of anodically grown films was more dependent on the nature of the anions in solution, which modify the rate of hydrolysis of the surface film, than on pH, although at low pH (<3) and high pH (>10) attack due to either acidity or alkalinity had a pronounced effect and the weight loss was greater than in between these two values. F attack on the surface was more vigorous than Cl attack over a broad pH range and at pH 3–6 the major form of attack in the presence of Cl ions was pitting [22]. In this paper Rutherford backscattering spectroscopy (RBS) has been employed to estimate the thickness and cation distributions for oxides developed on various alloys of aluminium after treatment in a range of solutions. These included deionised water (DIW), DIW þ 30 mM NaCl, DIW þ 10 mM CeCl3 and DIW þ 10 mM CrCl3 . The treatment conditions were 1 h at 95 °C which is well into the region where the maximum thickness of oxide is developed. Scanning electron microscopy (SEM) was used to complement the RBS studies through characterisation of the surface oxide after treatment. Finally, X-ray photoelectron spectroscopy (XPS) was employed to study the surface composition of the oxides and the chemical state of some of the major elements present. 2. Experimental 2.1. Aluminium alloys The aluminium alloys were obtained from a variety of sources. All alloys were rolled sheet of varying thicknesses. Details of the alloy temper are given, along with ICP analyses of the alloy composition in Table 1. For the coating experiments the sheet was cut into coupons approximately 5 cm 2 cm in size. 2.2. Oxide coatings Each coupon was wiped with acetone prior to being threaded onto 1000 series Al wires and suspended from Teflon rods. Pretreatment included cleaning in an agitated Table 1 Composition in weight percent of various alloys as determined by ICP Alloy
1100-O
2024-T3
3004-H19
5005-O-O
6061-T6
7075-T6
Al Cu Zn Fe Mg Si Ti Zr Cr Mn
99.6 0.09 <0.01 0.17 <0.01 0.08 0.01 <0.01 <0.01 <0.01
92.9 4.61 0.09 0.15 1.47 0.12 0.05 <0.01 <0.01 0.61
97 0.23 <0.01 0.43 1.04 0.11 0.02 <0.01 0.01 1.11
98.7 <0.01 0.02 0.16 0.98 0.05 <0.01 <0.01 <0.01 0.03
97 0.4 0.04 0.68 0.96 0.62 0.04 <0.01 0.25 0.05
90.3 1.52 5.14 0.28 2.32 0.08 0.05 <0.01 0.21 0.07
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aqueous degreaser (Brulin 815GD) for 10 min at 60 °C followed by a 5 min tap water rinse, immersion in an agitated non-etch alkaline cleaner (Henkel, Ridoline 53) for 4 min at 65–68 °C, again followed by a 5 min tap water rinse and finally deoxidised in a chromate/HF/HNO3 solution (Henkel, Deoxidiser #4) for 10 min at 21 °C, (no agitation) followed by a 5 min tap water rinse. A series of coupons were retained after deoxidation for analyses, otherwise all other coupons were treated in one of the following solutions at 95–100 °C (hereafter nominated as 95 °C): (i) DIW, (ii) 30 mM NaCl, (iii) 10 mM CeCl3 7H2 O solution and (iv) 10 mM CrCl3 6H2 O. 2.3. RBS All RBS experiments were performed on a General Ionex Tandem Accelerator using a 2 MeV He2þ ion beam at normal incidence to the sample surface. Backscattered He particles were collected at an angle of 170° to the incident beam for a total charge accumulation of 50 lC. The beam spot size was set at approximately 1.5 mm in diameter. The energy axis of each RBS spectrum was calibrated using deposited samples of Al2 O3 , CeO2 , and a polished sample of high purity Cu. The oxide thicknesses were determined using RUMP simulations [24]. The absolute amount of Ce or Cr incorporated into the surface oxide is proportional to the number of Ce or Cr counts detected. 2.4. XPS XPS was performed on either a Fisons Microlab 310F or Kratos AXIS Hsi instrument. On the Fisons Microlab 310F photoelectron spectra were excited using a Al anode unmonochromated X-ray source operated at a power of 300 W and 15 kV excitation voltage. The energy of the Al Ka lines was taken to be 1486.6 eV. For the Kratos Axis––HSi X-ray Photoelectron Spectrometer, the operating pressure was <1:3 108 Pa. Measurements were obtained at normal incidence. Photoelectrons were excited using the monochromated Al source (1486.6 eV). Oxide thicknesses for the deoxidised samples were determined as described previously [25]. Both spectrometers were calibrated with a sputtered copper (99.999% pure) sample and deposited gold on silicon sample giving Cu 2p3=2 , Cu (KLL), Au 4f7=2 binding energies of 932.60 eV, 334.8 eV (KE) and 84.06 eV respectively. The C 1s (285.0 eV) or the metallic component of the Al 2p (72.7 eV) photoelectron line was used for internal calibration of the binding energies. All samples were mounted on aluminium stubs using double sided adhesive conducting carbon tape. The degree of surface hydration was determined from O 1s spectra by curvefitting. Gauss/Lorentz product functions (30% Lorentzian lineshape) were fitted to spectra using fitting techniques available on the instrument. For the deoxidised surface a single component O 1s peak adequately described the peak shape. This peak was in the range 531.4–531.8 eV when using 75.0 eV as the reference for Al3þ 2p levels and assumed to be oxygen anions. On the coupons with boehmite coatings two peaks were used for fitting: The lower binding energy peak (531.2–531.4 eV) accounted for
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oxygen anions (O2 ) while the higher binding energy peak (532.2–532.8 eV) accounted hydrated species (OH ). 2.5. SEM SEM was performed on a Leica Thermal Field Emission SEM (360FE SEM) on uncoated specimens mounted using conducting, double-sided adhesive, carbon tape. Specimens were imaged at an accelerating voltage of 20 keV. X-ray emission was detected using a Pentafet detector and EDXS spectra were obtained using an accelerating voltage of 20 keV on a Link X2-II system.
3. Results 3.1. RBS Fig. 1 shows the RBS spectra for a variety of alloys treated at 95 °C for 1 h in DIW. The elemental positions (vertical lines) indicate where the step associated with each element would occur if the backscattering event was at the surface, thus some of the Al and the O signal occurs at the surface indicating oxide formation. The level of alloying can be ascertained from the height of the step below the Zn position, which, given the limitations of mass resolution for heavy elements, represents the total of alloying transition metals. 7075-T6 alloy has the highest level of alloying, followed by 2024-T3 and then the other three alloys (3004-H19, 6061-T6 and 5005-O) which have similar alloying levels. There was no significant difference in the average oxide
Fig. 1. RBS spectra of Al-alloys 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 after treatment in DIW for 1 h at 95 °C. The level of alloying can be ascertained from the height of the step below the Zn position, which, given the limitations of mass resolution for heavy elements, represents the total of alloying transition metals. 7075-T6 alloy has the highest level of alloying, followed by 2024-T3 and then the other three alloys (3004-H19, 6061-T6 and 5005-O) which have similar alloying levels.
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Table 2 Oxide thicknessesa (nm) on aluminium alloys after treatment at 95 °C in various solutions Alloy
1100-O
2024-T3
3004-H19
5005-O
6061-T6
7075-T6
Deoxidised surfaceb Deionised H2 O 30 mM NaCl 10 mM CeCl3 10 mM CrCl3
4.2 300 450 400 –
5.6 300 420 1000 60
6.5 300 580 700 200
4.0 300 500 300 40
5.4 300 500 400 60
5.0 300 500 800 20
a b
Determined by RUMP simulations of RBS spectra. Determined by XPS as described in [25].
thickness (Table 2) formed over the alloys as judged from the width of the O peak (channel 150) or Al step (channel 230). These oxide thicknesses were estimated by RUMP [24] simulations to be approximately 300 nm. The ratio of O to Al of the oxide was approximately 2:1, suggesting boehmite (Al2 O3 H2 O), which is normally formed under these treatment conditions [16]. Indeed, the appearance of the oxide (SEM), was similar to the appearance of boehmite reported elsewhere [16,26]. The ‘‘flatness’’ of the Al step below channel 230 implies that there was little variation in average oxide stoichiometry or thickness across the part of the surface analysed by the ion beam on all of the samples. Fig. 2 shows the RBS spectra for a variety of alloys treated at 95 °C for 1 h in DIW containing 30 mM NaCl. The detection of surface Al and O indicates oxide formation. The similarity of shape of the RBS spectra indicates similar oxide thickness and compositions for all alloys. The oxide thickness (Table 2), again estimated by RUMP [24] simulations of the O and Al peaks, were in the range 420–580 nm. The
Fig. 2. RBS spectra of Al-alloys 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 after treatment in 30 mM NaCl solution for 1 h at 95 °C. The level of alloying can be ascertained from the height of the step below the Zn position, which, given the limitations of mass resolution for heavy elements, represents the total of alloying transition metals. 7075-T6 alloy has the highest level of alloying, followed by 2024-T3 and then the other three alloys (3004-H19, 6061-T6 and 5005-O) which have similar alloying levels.
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Fig. 3. SEM of matrix after treatment in 30 mM NaCl solution for 1 h at 95 °C. Al-alloys (a) 1100-O, (b) 2024-T3, (c) 3004-H19, (d) 5005-O, (e) 6061-T6, (f) 7075-T6. Images taken at 50,000 times.
O:Al ratio was approximately 2:1, suggesting boehmite. For the NaCl treatment, the appearance of the oxide was similar to boehmite (Fig. 3). Bayerite crystals have been observed at lower treatment temperatures [26] but none was observed here, thus O/Al ratios higher than 2 (see Section 4 later) represent waters of hydration of the boehmite structure Al2 O3 n H2 O normally attributed to pseudoboehmite rather than phase change of boehmite to bayerite. The flatness of the Al step below channel 230 implies, as in the DI water case, that there was little variation in average oxide stoichiometry or thickness across that part of the surface analysed by the ion beam. Unlike the DIW treatment, treatment in NaCl solution resulted in localised attack leading to pitting of all alloy surfaces with the less pure alloys having the greatest
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Fig. 4. SEM of pitting on a typical alloy surface. Note the buildup of aluminium oxide (dark region) surrounding the pit. (a) and (c) 6061-T6, (b) and (d) 2024-T3.
degree of pitting. There was no detectable difference in the amount of pitting on the surface of the alloys after NaCl or CeCl3 treatment. Typical examples of pitting on 6061-T6 and 2024-T3 are shown in Fig. 4(a) and (b) respectively after treatment in NaCl solution. Features on either surface include intermetallic etchout and pitting events associated with intermetallics remnants. The pitting appeared more severe on 2024-T3 than on the 6061-T6 alloy. These pits had little influence on the oxide thickness reported here as explained in the Discussion section. The RBS spectra of the same alloys treated in 10 mM CeCl3 solution for 1 h revealed widely varying oxide thicknesses (Table 2) which can be seen in the Al-metal step (Fig. 5(a)) and Ce distributions (Fig. 5(b)). The oxide thicknesses varied from 300 nm for 1100-O alloy to 1000 nm for 2024-T3; the later was three time the thickness developed in DIW and twice the thickness developed in NaCl solution. Despite these differences the appearance of the oxide was similar to that observed for both the DIW and NaCl cases (Fig. 6). Furthermore, regardless of the differences in thicknesses, the O to Al ratio of the oxide treated in CeCl3 was consistently around 2:1, which was the same as that after treatment in DI water at 95 °C. From Table 3, it can be seen that the absolute amount of Ce incorporated into the surface oxide did not correlate with the oxide thickness. While Ce ions in solution do react with the intermetallic particles, the surface area of these particles is approximately 4–5% of
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Fig. 5. RBS spectra of (a) Al and O regions and (b) the Ce-regions for Al-alloys 2024-T3, 3004-H19, 5005O, 6061-T6 and 7075-T6 after treatment in 10 mM CeCl3 solution for 1 h at 95 °C. The step between the Al and O steps is the Al metal signal and the width from the marked Al line to the Al metal step represents the oxide thickness.
the total area, thus the cerium detected in the RBS spectra will be dominated by cerium ions incorporated into the oxide formed over the metal matrix. This is confirmed in Appendix A, which shows the Ce signal collected from the matrix of 2024-T3 after treatment in CeCl3 solution. With this in mind it can be inferred from the profiles that Ce was distributed evenly through the oxide formed on the alloys 2024-T3, 5005-O and 6061-T6. The 7075-T6 alloy showed a sub-surface peak in Ce concentration whilst 3004-H19 displayed a peak at the surface of the oxide. Fig. 7 shows an overlay of the RBS spectra taken from 6061-T6 alloy samples treated in either CeCl3 or CrCl3 . It is apparent from the oxygen region that the oxide developed in the CeCl3 solution was considerably thicker than that developed in the CrCl3 solution. Generally, the oxide thicknesses, developed in the CrCl3 solution, varied from 20 to 200 nm which was considerably less than those developed in the CeCl3 solution and even less than those developed in DIW but indicated some
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Fig. 6. SEM of matrix after treatment in 10 mM CeCl3 solution for 1 h at 95 °C. Al-alloys (a) 1100-O, (b) 2024-T3, (c) 3004-H19, (d) 5005-O, (e) 6061-T6, (f) 7075-T6. Images taken at 50,000 times.
Table 3 Atomic percenta; b of active cation incorporated into the oxide after treatment at 95 °C Alloy
1100-O
2024-T3
3004-H19
5005-O
6061-T6
7075-T6
10 mM CeCl3 10 mM CrCl3
0.2 –
0.9 0.03
0.6, 1.4 2
0.5 2
0.5 2
0.8, 1.4 0.03
a b
Determined by RUMP simulations of RBS spectra. Duplicate values agreed to within 5% except where otherwise stated.
thickening compared to the deoxidised surfaces (Table 2). RUMP simulations yielded lower O to Al ratios than the other treatments typically around 1.5:1 which
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Fig. 7. Overlay of RBS spectra of Al-alloy 6061-T6 after treatment in 10 mM CeCl3 and 10 mM CrCl3 solutions for 1 h at 95 °C. Note the lack of oxide features and buildup of Cu for the CrCl3 treatment.
implied that the oxide may have been Al2 O3 indicating a lower level of hydration than that found after CeCl3 immersion. The spectrum for the CrCl3 treated 6061-T6 alloy also showed considerable surface and sub-surface enrichment of Cu (channels 325–330). Cu-enrichment was also observed on 7075-T6, 2024-T3 and 3004-H19; very little was observed on 5005-O and none on 1100-O (Fig. 8). In the case of 7075T6 alloy, there was also surface enrichment of Zn (Fig. 8). While the overall Cr content in the surface oxide was small (Table 8), there was considerable variation in Cr content with respect to alloy type. In terms of ranking, the lowest levels of Cr were observed for the least pure alloys (2024-T3 and 7075-T6), opposite to the
Fig. 8. RBS spectra of the Cr–Cu regions for Al-alloys 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 after treatment in 10 mM CrCl3 solution for 1 h at 95 °C.
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trend for the Ce-treated samples, and the highest levels of Cr were incorporated into the most pure alloys, again roughly opposite to the Ce case. This suggests that the mechanism of interaction with the surface was different for the two cations.
4. XPS The oxide left after deoxidation was thin with both Al3þ and Al0 peaks detected in XPS resulting in a range of oxide thickness from 4.2 to 6.5 nm (reproducibility from one area to another on the same coupon was generally within 5%). In addition, a number of other elements were detected on the surface including N (1 at.%), P (2.0–5.4 at.%), F (4.9–7.9 at.%) and Si (1.8–7.5 at.%). The O:Al3þ ratios were in the range 1.60–3.84, with a reproducibility between measurements on the same sample of 0.18. The variation in composition and oxide thickness from one alloy to the next is due to the amount of Si left on the surface (reducing the total Al at.%) and F also modifies the surface O/Al ratio since F ions displace OH groups on the surface. O and Al were the major elements detected on the surface after all DIW, NaCl, CeCl3 and CrCl3 treatments (Tables 4–7). For specimens treated in DIW the O:Al ratio was in the region 2.3–2.5 and these were the only elements detected on the surface (Table 5). The high O:Al ratio and absence of other elements suggests greater hydration of the surface of the oxide compared to boehmite (Al2 O3 H2 O) observed by RBS for the bulk of the oxide. Higher O/Al ratios e.g. Al2 O3 2H2 O are more
Table 4 Atomic percent determined by XPS after deoxidation Al-alloy
Al 2pa
Cu 2p3=2
O 1s
F 1s
Si 2p
Cr 2p3=2
O/(Al þ Si þ Cr)
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
19:7 1:3 21:7 0:2 16:9 0:5 15:9 0:1 27.9 22.4
0:42 0:03 0:52 0:03 0:25 0:02 0:08 0:01 0.46 0.4
50:7 0:4 56:0 0:6 64:9 3:8 54:2 0:1 44.7 53.1
7:9 0:2 6:6 0:1 4:9 0:1 5:8 0:1 7.2 7.7
2:3 2:3 2:8 0:3 7:5 0:1 6:8 0:6 3.1 3.3
4:6 0:5 3:3 0:3 1:6 0:4 4:7 0:3 3.3 4.7
1:75 0:15 2:00 0:07 2.42 2:00 0:01 1.3 1.74
a
Only the Al3þ intensity was included in the calculation.
Table 5 XPS analyses of the surface oxide after treatment in DIW 1 h at 95 °C Al-alloy
O
Al
O/Al
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
69.3 70 69.8 71.1 69.4 68.7
29.2 29.6 29.0 28.1 28.9 29.4
2.4 2.4 2.4 2.5 2.4 2.3
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Table 6 XPS analyses of the surface oxide after treatment in 30 mM NaCl solution for 1 h at 95 °C Al-alloy
O
Al
Cl
O/Al
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
72.1 72.3 68.8 71.4 72.9 71.2
27 27 30.3 28.1 26.5 28.3
0.9 0.4 0.4 0.5 0.5 0.4
2.7 2.7 2.4 2.5 2.8 2.5
Table 7 XPS analyses of the surface oxide after treatment in 10 mM CeCl3 solution for 1 h at 95 °C Al-alloy
O
Al
Cl
Ce
O/Al
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
73.3 74 73 76.4 75 70
25.9 24.7 26 23.2 24.5 27.9
0.3 0.3 0.4 0.2 0.3 0.5
0.0 0.6 0.6 0.3 0.2 1.0
2.8 3.0 2.8 3.3 3.1 2.5
likely due to pseudoboemite [16]. The average Al3þ binding energy was 73.9 eV (1.9 eV FWHM) which is consistent with the literature for boehmite (Note the data in this paper is charge referenced to C 1s at 285.0 eV) [27]. The O 1s peak was wider than expected (2.9 eV FWHM) and, at a binding energy of approximately 531.8 eV, suggested a variety of chemical states including O anions and hydroxyl groups [28]. After treatment in NaCl solution the O:Al ratios for the different alloys were in the range 2.4–2.8 (Table 6). This range is higher than the RBS O:Al ratio of 2:1 suggesting surface hydration and the presence of pseudoboehmite. The surface chloride concentration was generally in the range 0.4–0.5 at.% apart from the 1100-O alloy which was 0.9 at.%. The chloride binding energy of 199.0 eV suggested that chloride was adsorbed onto the oxide surface rather than penetrated into the surface oxide [29]. After treatment in the CeCl3 solution the O:Al ratio was in the range 2.5–3.3 (Table 7), again, generally larger than the ratio of 2:1 determined from RBS simulations and indicating the presence of pseudoboehmite on the external surface of the oxide. Furthermore, apart from 7075-T6 alloy, the ratio was higher in all cases than the NaCl treatment suggesting a larger degree of surface hydration after treatment in CeCl3 solution. It was also interesting to note that the chloride levels were, in all cases, less than those detected on the surface of the alloys after NaCl treatment. Ce was detected in a mixture of CeIII and CeIV on the surface of all alloys. The amount of CeIV was determined from the ratio of the peak labelled CeIV , positioned at around 917 eV (Fig. 9) [30]. Both the 3004-H19 and 5005-O series alloys showed Ce present as CeIV whilst all other alloys showed Ce present predominantly as CeIII .
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Fig. 9. Comparison of the XPS Ce 3d region for Al-alloys 2024-T3 and 3004-H19 after treatment for 1 h at 95 °C. Note the higher level of CeIV on the surface of 3004-H19 alloy.
Table 8 XPS analyses of the surface oxide after treatment in 10 mM CrCl3 solution for 1 h at 95 °C Alloy
O
Al
Cl
Cr
Cu
O/Al
1000 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
72.9 57.1 63.3 67.9 67.8 69.8
23.8 33.1 31.9 29.2 26.7 23.9
0.3 2.2 0.3 0.6 0.5 2.4
0.5 0.4 0.3 1.7 1.3 1.2
0.0 0.0 4.2 0.6 3.6 1.7
3.1 1.7 2.0 2.3 2.5 2.9
These two alloys had the highest percentage of Ce at the surface according to RBS Ce-depth profiles (Fig. 6), thus the high CeIV for 3004-H19 and 5005-O might result, posttreatment, by oxidation of surface CeIII . Cl was found at a binding energy of approximately 199.0 eV, again, suggesting adsorbed chloride [29]. No Ce was detected on the 1100-O series alloy. Small amounts of F were detected using XPS on two samples, this was presumed to have been carried through from the rinse water since Melbourne tap water is fluorinated. None of the alloying components were detected on any of the alloys within the detectability limits (0.1 at.%). XPS for the CrCl3 boiled samples were in good agreement with the RBS results showing a lower level of oxygen in the surface oxide than found on the CeCl3 boiled samples. The O:Al ratio was in the range 1.7–3.1 (Table 8), generally a little lower than for the other treatments but closer to the RBS ratio of 1.5:1 The Cr 2p3=2 peak was found at a binding energy of approximately 578.0 eV on all samples. CrIII generally fell between 577.1 and 577.5, whereas CrVI binding energies were around 2.3–2.8 eV higher [31,32]. CrVI compounds would be expected to exhibit a binding energy of approximately 579.0 eV.
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5. Discussion The most notable features between the DIW, NaCl, CeCl3 and CrCl3 treatments are the following: (i) similar oxide thickness for different alloy types treated in either DIW or NaCl solutions, (ii) the larger oxide thicknesses developed on some alloys treated in CeCl3 solution compared to either DIW or NaCl solutions, (iii) the wide variation of oxide thicknesses between alloys after treatment in CeCl3 solution, (iv) poorly developed oxides after treatment in CrCl3 solution, (v) bulk stoichiometry of O:Al of 2:1 from RBS, but higher ratios for the surface of the oxide from XPS.
5.1. Influence of pitting on oxide thickness The presence of pitting and oxide material in the pit caps might be expected to lead to increased average oxide thickness as determined by RBS, and it will certainly have some effect. Thus it is important to establish the influence of oxidic pit caps on the width of the oxide peak measured in RBS. Taking the 2024-T3 case into consideration, the aerial density of intermetallics on the surface is around 4–5% and pitting is generally associated with intermetallic phases, thus the total area of the pits is small relative to the matrix. (Indeed deoxidation (see Section 2) generally removes a substantial amount of the intermetallic material [26].) In Fig. 10, RUMP simulations of aluminium with 1000 nm of oxide in a homogenous film is compared to aluminium with a similar oxide but with 5% surface coverage of 10 lm of oxide to model pitting on the 2024-T3 alloy. The thick oxide from the pit cap has a very broad oxygen peak and effectively influences the spectrum by raising the overall intensity across the thickness for the matrix oxide peak much in the same way as a constant background would do. Thus the width of the oxide peak is essentially the same for both scenarios. Fig. 10(b) is a section through the 2024-T3 sample treated in CeCl3 solution; the thickness of the overlayer is around 1000 nm which is in good agreement with the RBS thickness indicating that thick, oxidic pit caps have negligible influence on the determination of the matrix oxide thickness as measured by RBS. Hence the RBS is strongly weighted towards the matrix. In addition to the simulations, it is should be noted that the degree of pitting for NaCl and CeCl3 were similar but there were considerable differences in oxide thickness between the two treatments for 2024-T3, 3004-H19 and 7075-T6 alloys indicating that the oxide thickness determined by RBS is largely unrelated to corrosion product developed as a result of pitting. The differences in oxide thickness therefore point towards some other mechanism which likely relates to changes in the interfacial chemistry having a major influence of the final oxide thickness.
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Fig. 10. (a) Two model RBS spectra representing an oxide of nominally 1 lm thickness with and without a 5% surface coverage of pit caps with 10 lm thickness. It can be seen that the 5% coverage of 10 lm oxide effectively adds a background to the O region. (b) Backscatter electron image of a section through the coating confirming that the thickness of the boehmite coating is around 1000 nm.
5.2. Oxide growth over the matrix The differences in oxide thickness should be considered in terms of current models of oxide growth, i.e., (a) formation of an amorphous oxide, (b) hydration of the oxide, and (c) growth of the boehmite phase (dissolution and precipitation). At elevated temperatures, stages (a) and (b) happen quickly (1 m) [16,19], thus the oxide growth is well into the dissolution and reprecipitation processes. This final stage of growth ends up being rate limited by diffusion of species across the dense inner oxide, and the growth rate is generally negligible after 1 h treatment. Thus the oxides studied here are effectively at the maximum thickness for the treatment conditions. The DIW treatments form the baselines for these results. After 1 h treatment, the oxide is the same thickness for all the alloys studied, suggesting that there was no influence of the alloy surface composition on formation and growth of pseudoboehmite, despite the variation in composition of the deoxidised surface. Comparing these results with the NaCl treatment, it is apparent that the presence of chloride in
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the DIW enhances the oxide growth beyond the DI water case, although, again there appears to be little differentiation between alloys. The literature is not clear on the effects of chloride addition on growth of oxides in aqueous solutions at elevated temperatures. For example, Godard and Torrible found that, in the presence of Cl , the film weights for high purity aluminium were similar to or slightly lower than those for pure water [33]. On the other hand, the data of Vermilyea and Vedder [18] suggested that for treatment times less than 10 min, treatment in NaCl solution at 100 °C will produce thinner coatings than pure water, but above 10 min the situation may be reversed. Thus for the 1 h treatment in NaCl solution, the thicker coatings reported here appear consistent with previous reports. Chloride ions may accelerate oxide growth by facilitating the hydration and dissolution process of amorphous oxide. The role of chloride in the hydration of aluminium surfaces comes largely from studies of chloride attack on anodically grown oxide films [33,34]. Early work suggested that chloride ions are not only adsorbed onto the oxide surface but also penetrate the surface [34]. More recently Skeldon et al. [35] confirmed these results, showing that while chloride ions do, in fact, penetrate the surface, they do not accumulate at the metal oxide interface, since the diffusion rate of OH and O2 is greater than Cl . At the surface, chloride ions compete for adsorption sites with hydroxyl ions which probably leads to the formation of soluble surface species, such as AlCl3 or hydrated oxychlorides, and subsequently to Al3þ ions in solution. In the absence of chloride, Alwitt suggests that the dominant solution species are probably Al(OH)2þ and Al(OH)þ 2 [19]. Since the oxides are only sparingly soluble, then once the reaction proceeds to any great extent these ions precipitate onto the surface forming either bayerite (Al(OH)3 ) and pseudoboehmite, at low temperatures (<50 °C), or pseudoboehmite alone at more elevated temperatures (>80 °C) [19]. It is clear from the large XPS O/Al ratios observed for CeCl3 and NaCl treatments and SEM that the surfaces have highly hydrated forms of pseudoboehmite. An O/Al ratio of 3 implies there would be three waters of hydration, whereas a value of 2 implies only one water of hydration which is typical of boehmite. It should be noted, however, that the vacuum conditions in the XPS spectrometer can lead to surface dehydration of aluminium oxides. For example, Nylund and Olefjord [28] have pointed out that hydrated forms of alumina decompose in vacuum to produce a surface layer of Al2 O3 . They found this layer to be approximately 7 nm thick which gave a mixed Al2 O3 and hydrated alumina spectrum when analysed by XPS. It is therefore possible that the XPS O/Al ratios presented here may be underestimates of the true degree of hydration. 5.3. Role of cerium The difference in oxide thickness between the Al-alloys after treatment in CeCl3 solution suggests an active role for the cerium cations and the alloy surface microstructure and/or composition. Alwitt observed that at 100 °C the pseudoboehmite layer develops an impenetrable inner layer due to the high density of crystallisation, however, this layer does not develop at 40 °C where the overall pseudoboehmite
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layer is much thicker than 100 °C. On the basis of radio tracer measurements it was concluded that the rate limiting step at 100 °C for the third stage of growth is solid state diffusion of water across the impenetrable layer. If this is the case, then the presence of Ce may modify the structure of this impenetrable layer making it more porous and promoting oxide growth. The modification of pseudoboehmite due to Ce incorporation, is supported to some extent by RBS Ce depth profiles which indicate that Ce is incorporated into the oxide coating, albeit at low levels. Similar results were observed in a previous report on the treatment of 2024-T3 and 6061-T6 Alalloys in Ce solutions [36]: in that case, as here, there was interaction of cerium cations with both intermetallics and the oxide covering the matrix. For the present results, the degree of incorporation corresponds well with oxide thickness, i.e. the alloys with the largest incorporated Ce levels (2024-T3, 7075-T6 and 3004-H19) have the thickest oxides, whereas those with the least incorporated cerium have the thinnest oxides (Table 2). Given the difference in ion size between Ce3þ (or Ce4þ ) at (or 0.92 A ) and Al3þ 0.51 A then some strain would be developed in both the 1.034 A boehmite lattice and the impenetrable oxide possibly leading to a more open structure and greater diffusion of reacting species. The other area where cerium cations may have an influence is in complex formation in the solution immediately adjacent to the surface. For example, Rider et al. [10] found moderate levels of Ni on the surface (4.0–4.7 at.% Al/Ni 7) of boehmitelike structures developed after treatment in solutions containing NiCl2 and NaCl. They proposed that oxide formation involved Ni/Al complex formation in the solution adjacent to the developing oxide. While hydrolysis of cerium ions in solution is well known [37] and mixed Al/Ce complexes may form at the metal/solution interface, a greater concentration of cerium would probably be expected on the surface of the alloys treated in CeCl3 solution if Al/Ce solution complexes had a major role to play in the acceleration of oxide growth. The minimum Al/Ce ratio observed here was 28 compared to Al/Ni of 7 for Rider et al. [10] suggesting, that for the Ce case, surface complexation does not play a major role in oxide formation. Additionally, oxide growth acceleration by Ce-complexes at the solution/oxide interface is unlikely to involve a role for the different alloy compositions. 5.4. Role of the alloy composition The alloy microstructure has an important role to play since, in the presence of Ce, a broad range of oxide thickness was observed for the different alloys which was not the case for the NaCl treatment. Since the thicknesses do not correlate directly with the total alloying composition the dependence of the oxide thickness on individual and combinations of alloying components was examined. The best correlations were obtained with (Cu þ Mn) content and the (Cu þ Mn)/Si ratio (Fig. 11). Either of these parameters seems reasonable on a physical basis:Cu-[38,39] and Mnenrichments [40] have been detected in thin aluminium oxide layers after various treatments. Both elements are more noble than Al and may act as accelerators for Al dissolution. On the other hand, Si in solution (e.g. from glassware or water) has been reported to inhibit oxide growth [15,16] and Si from the alloy may play the same
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Fig. 11. Correlation between oxide thickness and (a) atomic percent of Cu þ Mn and (b) (Cu þ Mn)/Si.
role. Interestingly, the oxide thickness does not depend on the surface composition after deoxidation despite the high levels of inhibiting ions such as Si4þ and Cr3þ or Cr6þ . It is conceivable that these surface species are dissolved from the surface quickly and lost to the solution, whereas, the bulk alloying components are continually exposed as the Al metal is oxidised to Al3þ . 5.5. Effects of chromium In the case of the CrCl3 treatments, the oxides were too thin to provide meaningful RBS O:Al ratios it was not possible to compare the bulk to surface compositions.
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The absence of precipitation of hydrated aluminas suggests that the dissolution process has been blocked, which, given the thin oxide coating and the presence of Cr on the surface, appeared to be due to the formation of a Cr-containing passive layer on the surface. Previous studies of Cr-oxides in chromate conversion coatings has shown it to be resistant to chloride attack [41], thus the presence of Cr inhibits oxide growth by minimising chloride attack. 6. Conclusions Growth of aluminium oxides (pseudoboehmite) in DIW for the aluminium alloys 1100-O, 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 resulted in a similar thickness for all alloys (around 200 nm). The addition of 30 mM NaCl resulted in an increase of oxide thickness of around 50–70% compared to the DIW case, but there was little variation in the oxide thickness. The increase in oxide thickness was attributed to the presence of chloride ions in solution which assist oxide dissolution. Further increases in oxide thickness were observed for 2024-T3, 3004-H19 and 7075T6 alloys after treatment in 10 mM CeCl3 solution. In this case the increase of oxide thickness was thought to occur via the incorporation of low levels of Ce ions into the pseudoboehmite causing greater diffusion of reacting species due to strain introduced by the large difference in ion size of Ce compared to Al. Finally, no oxide growth was observed in 10 mM CrCl3 solutions. This was attributed to blocking of active sites by chromate ions.
Acknowledgements The authors would like to thank Dr K. Wittel of Chemetall GmbH (Frankfurt) and Ms K. Nelson of CSIRO for critical reading of the manuscript. The authors would also like to thank Mr P. Curtis for the ICP analyses of the alloys and Mr S. Glanville for the preparing the section of the 2024-T3 sample after treatment in CeCl3 solution.
Appendix A Fig. 12 is the Ce profile taken from the matrix of 2024-T3 sample after treatment in CeCl3 solution. This particular profile was collected in the Melbourne Nuclear Microprobe system [42] using 2 MeV He ions and backscattered ions collected at an angle of 145°. In this instance maps of an area were collected and analysed. These maps have RBS spectra for each pixel in the map. Intermetallic particles were clearly distinguishable from the matrix; the spectrum in Fig. 12 has been compiled from the RBS spectra from the matrix. It is clear from the Ce profile that (i) Ce is present in the coating on the matrix and (ii) was detected throughout the coating as observed in Fig. 5(b).
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Fig. 12. Ce-RBS spectrum of the oxide covering the matrix excluding intermetallics, after treatment in boiling CeCl3 solution, demonstrating that Ce is incorporated into the boehmite oxide.
References [1] S. Wernick, R. Pinner, P.G. Sheasby, The Surface Treatment and Finishing of Aluminium and Its Alloys, fifth ed., Finishing Publications Ltd., Teddington, 1987, pp. 18–23. [2] K. Brown, Anti-Corros. Meth. Mat. 44 (5) (1997) 323. [3] M. Kendig, M. Cunningham, L. Warren, S. Jeanjaquet, Minutes of the Fourth Annual Workshop on Chromate Replacements in Light Metal Finishing, Sandia Report SAND94-3010. UC 600, 1996, pp. 93–109. [4] L. Wilson, B.R.W. Hinton, AU-B-1,485,888. [5] D.R. Gabe, G.D. Gabe, M.E. Warwick, Corros. Rev. 6 (4) (1986) 327. [6] M.P. Schriever, US Patent No. 5,551,994. [7] P. Deck, D.W. Reichgott, Met. Finish. (September) (1992) 29. [8] R.G. Buchheit, M.D. Bode, G.E. Stoner, Corrosion 50 (3) (1994) 205. [9] M.P. Schriever, Nonchromate cobalt complex conversion coating and primer in painting, Eur. Pat. Appl. (1992) 27, EP. 488430. [10] A.N. Rider, D.R. Arnott, A.R. Wilson, I. Danilidis, P.J.K. Paterson, Surf. Int. Anal. 24 (1996) 293. [11] F. Mansfeld, Y. Wang, Brit. Corros. J. 29 (3) (1994) 194. [12] R. Rungta, US Patent No. 5,362,335. [13] K.J.H. Nelson, R.J. Taylor, A.E. Hughes, B.R.W. Hinton, M.J. Henderson, L. Wilson, S.A. Nugent, US Patent No. 6,022,425. [14] J. Bibber, US Patent No. 4,711,667. [15] D. Altenpohl, Corrosion 15 (1966) 143t. [16] W. Vedder, D.A. Vermilyea, Trans. Far. Soc. 65 (1969) 561. [17] W.J. Bernard, J.J. Randall Jr., J. Electrochem. Soc. 107 (6) (1960) 483. [18] W. Vedder, D.A. Vermilyea, Trans. Far. Soc. 66 (1970) 2644. [19] R.S. Alwitt, J. Electrochem. Soc. 121 (10) (1974) 1322. [20] H. B€ ohni, H.H. Uhlig, J. Electrochem. Soc. 116 (7) (1969) 906. [21] K.F. Lorking, J.E.O. Mayne, J. Appl. Chem. (May) (1961) 170.
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K.F. Lorking, J.E.O. Mayne, Brit. Corros. J. 1 (1966) 181. K. Sotoudeh, T.H. Nguyen, R.T. Foley, B.F. Brown, Corrosion 37 (6) (1981) 358. L.R. Doolittle, Computer code RUMP, Nucl. Instr. Meth. B 15 (1986) 227. A.E. Hughes, R.J. Taylor, K.J.H. Nelson, B.R.W. Hinton, L. Wilson, Mat. Sci. Technol. 12 (1996) 928. R.K. Hart, Trans. Far. Soc. 53 (1957) 1020. C.D. Wagner, D.E. Passoja, H.F. Hillery, T.G. Kinisky, H.A. Six, W.T. Jansen, J. Vac. Sci. Technol. 21 (1982) 933. A. Nylund, I. Olefjord, Surf. Int. Anal. 21 (1994) 283. P.M. Natishan, E. McCafferty, W.E.O. Grady, in: R. Baer, G.D. Davis, C.R. Clayton (Eds.), Applications of Surface Analysis Methods to Environmental/Materials Interactions, Electrochemical Society, Pennington, US, 1991. M. Hoang, A.E. Hughes, T.W. Turney, Appl. Surf. Sci. 72 (1993) 55. J.A. Treverton, N.C. Davies, Surf. Int. Anal. 3 (5) (1981) 194. A.E. Hughes, R.J. Taylor, B.R.W. Hinton, Surf. Int. Anal. 25 (1997) 223. H.P. Goddard, E.G. Torrible, Corros. Sci. 10 (1970) 135. H. Kaesche, Localised Corrosion, in: International Corrosion Conference Series (Williamsburg Virginia USA, December 6–10, 1971), NACE, Houston, 1974, p. 516. M. Skeldon, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, Corros. Sci. 37 (9) (1995) 1473. J.D. Gorman, S.T. Johnson, P.N. Johnston, P.J.K. Paterson, A.E. Hughes, Corros. Sci. 38 (1997) 1977. F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fifth ed., John Wiley and Sons, New York, 1988 (p. 966 ff). H. Habazaki, K. Shimizu, M.A. Paez, P. Skeldon, G.E. Thompson, G.C. Wood, X. Xhou, Surf. Int. Anal. 23 (1995) 892. A.E. Hughes, J.D. Gorman, P.R. Miller, B.A. Sexton, P.J.K. Paterson, Surf. Int. Anal., in preparation. O. Lunder, K. Nisancioglu, Corros. Sci. 44 (1988) 414. M.A. Heine, M.J. Pryor, J. Electrochem. Soc. 114 (1967) 1001. D.N. Jamieson, Nucl. Instr. Meth. B 136 (1998) 1.
Oxide formation on aluminium alloys in boiling deionised water and NaCl, CeCl3 and CrCl3 solutions J.D. Gorman a
a,1
, A.E. Hughes b,*, D. Jamieson c, P.J.K. Paterson a
Department of Applied Physics, RMIT University, 124 La Trobe St., Melbourne 3000, Australia b CSIRO Manufacturing and Infrastructure Technology, Private Bag 33, Clayton South MDC, Clayton, Vic. 3169, Australia c School of Physics, University of Melbourne, Parkville 3002, Australia Received 6 October 1999; accepted 21 October 2002
Abstract Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy have been employed to provide depth, surface chemical and morphological information respectively for a range of Al-alloys treated for 2 h at 90–100 °C in either deionised water (DIW), 30 mM NaCl, 10 mM CeCl3 or 10 mM CrCl3 solutions. Alloys included Al sheet 1100-O, 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6. In DIW all alloys developed the same oxide thickness with a boehmite-like appearance. In NaCl and CeCl3 solutions the oxide thickness varied with alloy composition but was considerably thicker than the deionised case and maintained a boehmite-like appearance. Ce was distributed throughout the oxide but at low levels. In all cases some CeIV was detected on the surface of the oxide, the remainder being CeIII . In CrCl3 solution, the oxide thickness was extremely thin with the appearance of a dense oxide rather than a boehmite-type structure. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Boehmite; Rutherford backscattering spectroscopy; Oxide growth and rare earth salts; Cerium; Chromium; X-ray photoelectron spectroscopy; B. Scanning electron microscopy
*
Corresponding author. Tel.: +61-3-9545-2777/2075; fax: +61-3-9544-1128. E-mail address: [email protected] (A.E. Hughes). 1 Tel.: +61-3-9925-2136.
0010-938X/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 2 ) 0 0 2 0 9 - 3
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1. Introduction Corrosion protection of aluminium often involves chemical pretreatment, incorporating, as the final step, either conversion coating or anodising, prior to the application of a paint system [1]. Anodising and conversion coating processes, historically, have involved the use of chromium (VI) which, for a number of years, has been considered a serious health risk for workers during processing (manufacture or repair) and for the environment in terms of discharge and disposal [2]. Despite these risks, chromate conversion coatings and chromic acid anodising continue to be used for applications with high performance requirements, for example, in the aerospace industry. For these reasons, there has been considerable interest in finding alternatives particularly to chromate conversion coatings as used in the aerospace industry. Alternatives may be broadly divided into a number of categories according to the type of chemistry involved in processing and/or the processing techniques [3]. First, there are inorganic coatings which are loosely based on the use of metal anion complexes or inorganic oxidising species which undergo a redox reaction with the aluminium surface and deposit an insoluble compound during the reaction [4–7]. Some of the best examples of these systems are based on Mo and Ce. Second, are coatings which rely on growing the hydrated aluminium oxide on the metal surface at elevated temperatures (up to, or even above, 100 °C). Examples of this include processes based on Li, Co, Ce, Mn or Ni salt solutions, or growth in water followed by some type of sealing [8–14]. The third category combines all coatings that do not fit into the first two categories. This category would include polymer coatings and surface chemical (e.g. self assembling monolayers, silanes) and physical (e.g. laser alloy treatment) modification. The focus of this paper is on the growth of hydrated aluminium oxides at temperatures close to 100 °C, in water and chloride solutions containing either Naþ , Cr3þ or Ce3þ cations in the presence of Cl anions. The growth of hydrated Al oxides was studied extensively during the 1960s and 1970s [15–19]. Most of this work was confined to relatively pure aluminium. Vedder and Vermilyea [16] identified a three stage growth mechanism including (i) formation of amorphous surface oxide, (ii) dissolution of the surface oxide which at low pH is rate limited by the rate of surface hydration, and (iii) precipitation of hydrated aluminium oxide (Al2 O3 nH2 O). Alwitt [19] later refined this model, concluding that steps (i) and (ii) were probably rate limited by the diffusion of H2 O (probably as OH and Hþ ) into the oxide surface. Surface oxides developed under these conditions typically have varying degrees of hydration ranging from Al2 O3 (n ¼ 0) to aluminium hydroxide Al(OH)3 (n ¼ 3). The growth of anodic oxide films in the presence of cations or anions has not been so extensively studied. Vermilyea and Vedder [18] demonstrated that the presence of cations in solution resulted in a range of effects from no inhibition to extremely strong inhibition. The strong inhibitors ‘‘keyed’’ onto aluminium oxide or hydroxide structure thus preventing boehmite growth. B€ ohni and Uhlig observed similar effects through studying movement in the pitting potential as a function of chloride addition and subsequently in the presence of various cations [20]. Attack on anodic films in the presence of various anions and cations has been studied extensively [20–23].
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Lorking and Mayne [22] and Sotoudah et al. [23] concluded that attack of anodically grown films was more dependent on the nature of the anions in solution, which modify the rate of hydrolysis of the surface film, than on pH, although at low pH (<3) and high pH (>10) attack due to either acidity or alkalinity had a pronounced effect and the weight loss was greater than in between these two values. F attack on the surface was more vigorous than Cl attack over a broad pH range and at pH 3–6 the major form of attack in the presence of Cl ions was pitting [22]. In this paper Rutherford backscattering spectroscopy (RBS) has been employed to estimate the thickness and cation distributions for oxides developed on various alloys of aluminium after treatment in a range of solutions. These included deionised water (DIW), DIW þ 30 mM NaCl, DIW þ 10 mM CeCl3 and DIW þ 10 mM CrCl3 . The treatment conditions were 1 h at 95 °C which is well into the region where the maximum thickness of oxide is developed. Scanning electron microscopy (SEM) was used to complement the RBS studies through characterisation of the surface oxide after treatment. Finally, X-ray photoelectron spectroscopy (XPS) was employed to study the surface composition of the oxides and the chemical state of some of the major elements present. 2. Experimental 2.1. Aluminium alloys The aluminium alloys were obtained from a variety of sources. All alloys were rolled sheet of varying thicknesses. Details of the alloy temper are given, along with ICP analyses of the alloy composition in Table 1. For the coating experiments the sheet was cut into coupons approximately 5 cm 2 cm in size. 2.2. Oxide coatings Each coupon was wiped with acetone prior to being threaded onto 1000 series Al wires and suspended from Teflon rods. Pretreatment included cleaning in an agitated Table 1 Composition in weight percent of various alloys as determined by ICP Alloy
1100-O
2024-T3
3004-H19
5005-O-O
6061-T6
7075-T6
Al Cu Zn Fe Mg Si Ti Zr Cr Mn
99.6 0.09 <0.01 0.17 <0.01 0.08 0.01 <0.01 <0.01 <0.01
92.9 4.61 0.09 0.15 1.47 0.12 0.05 <0.01 <0.01 0.61
97 0.23 <0.01 0.43 1.04 0.11 0.02 <0.01 0.01 1.11
98.7 <0.01 0.02 0.16 0.98 0.05 <0.01 <0.01 <0.01 0.03
97 0.4 0.04 0.68 0.96 0.62 0.04 <0.01 0.25 0.05
90.3 1.52 5.14 0.28 2.32 0.08 0.05 <0.01 0.21 0.07
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aqueous degreaser (Brulin 815GD) for 10 min at 60 °C followed by a 5 min tap water rinse, immersion in an agitated non-etch alkaline cleaner (Henkel, Ridoline 53) for 4 min at 65–68 °C, again followed by a 5 min tap water rinse and finally deoxidised in a chromate/HF/HNO3 solution (Henkel, Deoxidiser #4) for 10 min at 21 °C, (no agitation) followed by a 5 min tap water rinse. A series of coupons were retained after deoxidation for analyses, otherwise all other coupons were treated in one of the following solutions at 95–100 °C (hereafter nominated as 95 °C): (i) DIW, (ii) 30 mM NaCl, (iii) 10 mM CeCl3 7H2 O solution and (iv) 10 mM CrCl3 6H2 O. 2.3. RBS All RBS experiments were performed on a General Ionex Tandem Accelerator using a 2 MeV He2þ ion beam at normal incidence to the sample surface. Backscattered He particles were collected at an angle of 170° to the incident beam for a total charge accumulation of 50 lC. The beam spot size was set at approximately 1.5 mm in diameter. The energy axis of each RBS spectrum was calibrated using deposited samples of Al2 O3 , CeO2 , and a polished sample of high purity Cu. The oxide thicknesses were determined using RUMP simulations [24]. The absolute amount of Ce or Cr incorporated into the surface oxide is proportional to the number of Ce or Cr counts detected. 2.4. XPS XPS was performed on either a Fisons Microlab 310F or Kratos AXIS Hsi instrument. On the Fisons Microlab 310F photoelectron spectra were excited using a Al anode unmonochromated X-ray source operated at a power of 300 W and 15 kV excitation voltage. The energy of the Al Ka lines was taken to be 1486.6 eV. For the Kratos Axis––HSi X-ray Photoelectron Spectrometer, the operating pressure was <1:3 108 Pa. Measurements were obtained at normal incidence. Photoelectrons were excited using the monochromated Al source (1486.6 eV). Oxide thicknesses for the deoxidised samples were determined as described previously [25]. Both spectrometers were calibrated with a sputtered copper (99.999% pure) sample and deposited gold on silicon sample giving Cu 2p3=2 , Cu (KLL), Au 4f7=2 binding energies of 932.60 eV, 334.8 eV (KE) and 84.06 eV respectively. The C 1s (285.0 eV) or the metallic component of the Al 2p (72.7 eV) photoelectron line was used for internal calibration of the binding energies. All samples were mounted on aluminium stubs using double sided adhesive conducting carbon tape. The degree of surface hydration was determined from O 1s spectra by curvefitting. Gauss/Lorentz product functions (30% Lorentzian lineshape) were fitted to spectra using fitting techniques available on the instrument. For the deoxidised surface a single component O 1s peak adequately described the peak shape. This peak was in the range 531.4–531.8 eV when using 75.0 eV as the reference for Al3þ 2p levels and assumed to be oxygen anions. On the coupons with boehmite coatings two peaks were used for fitting: The lower binding energy peak (531.2–531.4 eV) accounted for
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oxygen anions (O2 ) while the higher binding energy peak (532.2–532.8 eV) accounted hydrated species (OH ). 2.5. SEM SEM was performed on a Leica Thermal Field Emission SEM (360FE SEM) on uncoated specimens mounted using conducting, double-sided adhesive, carbon tape. Specimens were imaged at an accelerating voltage of 20 keV. X-ray emission was detected using a Pentafet detector and EDXS spectra were obtained using an accelerating voltage of 20 keV on a Link X2-II system.
3. Results 3.1. RBS Fig. 1 shows the RBS spectra for a variety of alloys treated at 95 °C for 1 h in DIW. The elemental positions (vertical lines) indicate where the step associated with each element would occur if the backscattering event was at the surface, thus some of the Al and the O signal occurs at the surface indicating oxide formation. The level of alloying can be ascertained from the height of the step below the Zn position, which, given the limitations of mass resolution for heavy elements, represents the total of alloying transition metals. 7075-T6 alloy has the highest level of alloying, followed by 2024-T3 and then the other three alloys (3004-H19, 6061-T6 and 5005-O) which have similar alloying levels. There was no significant difference in the average oxide
Fig. 1. RBS spectra of Al-alloys 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 after treatment in DIW for 1 h at 95 °C. The level of alloying can be ascertained from the height of the step below the Zn position, which, given the limitations of mass resolution for heavy elements, represents the total of alloying transition metals. 7075-T6 alloy has the highest level of alloying, followed by 2024-T3 and then the other three alloys (3004-H19, 6061-T6 and 5005-O) which have similar alloying levels.
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Table 2 Oxide thicknessesa (nm) on aluminium alloys after treatment at 95 °C in various solutions Alloy
1100-O
2024-T3
3004-H19
5005-O
6061-T6
7075-T6
Deoxidised surfaceb Deionised H2 O 30 mM NaCl 10 mM CeCl3 10 mM CrCl3
4.2 300 450 400 –
5.6 300 420 1000 60
6.5 300 580 700 200
4.0 300 500 300 40
5.4 300 500 400 60
5.0 300 500 800 20
a b
Determined by RUMP simulations of RBS spectra. Determined by XPS as described in [25].
thickness (Table 2) formed over the alloys as judged from the width of the O peak (channel 150) or Al step (channel 230). These oxide thicknesses were estimated by RUMP [24] simulations to be approximately 300 nm. The ratio of O to Al of the oxide was approximately 2:1, suggesting boehmite (Al2 O3 H2 O), which is normally formed under these treatment conditions [16]. Indeed, the appearance of the oxide (SEM), was similar to the appearance of boehmite reported elsewhere [16,26]. The ‘‘flatness’’ of the Al step below channel 230 implies that there was little variation in average oxide stoichiometry or thickness across the part of the surface analysed by the ion beam on all of the samples. Fig. 2 shows the RBS spectra for a variety of alloys treated at 95 °C for 1 h in DIW containing 30 mM NaCl. The detection of surface Al and O indicates oxide formation. The similarity of shape of the RBS spectra indicates similar oxide thickness and compositions for all alloys. The oxide thickness (Table 2), again estimated by RUMP [24] simulations of the O and Al peaks, were in the range 420–580 nm. The
Fig. 2. RBS spectra of Al-alloys 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 after treatment in 30 mM NaCl solution for 1 h at 95 °C. The level of alloying can be ascertained from the height of the step below the Zn position, which, given the limitations of mass resolution for heavy elements, represents the total of alloying transition metals. 7075-T6 alloy has the highest level of alloying, followed by 2024-T3 and then the other three alloys (3004-H19, 6061-T6 and 5005-O) which have similar alloying levels.
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Fig. 3. SEM of matrix after treatment in 30 mM NaCl solution for 1 h at 95 °C. Al-alloys (a) 1100-O, (b) 2024-T3, (c) 3004-H19, (d) 5005-O, (e) 6061-T6, (f) 7075-T6. Images taken at 50,000 times.
O:Al ratio was approximately 2:1, suggesting boehmite. For the NaCl treatment, the appearance of the oxide was similar to boehmite (Fig. 3). Bayerite crystals have been observed at lower treatment temperatures [26] but none was observed here, thus O/Al ratios higher than 2 (see Section 4 later) represent waters of hydration of the boehmite structure Al2 O3 n H2 O normally attributed to pseudoboehmite rather than phase change of boehmite to bayerite. The flatness of the Al step below channel 230 implies, as in the DI water case, that there was little variation in average oxide stoichiometry or thickness across that part of the surface analysed by the ion beam. Unlike the DIW treatment, treatment in NaCl solution resulted in localised attack leading to pitting of all alloy surfaces with the less pure alloys having the greatest
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Fig. 4. SEM of pitting on a typical alloy surface. Note the buildup of aluminium oxide (dark region) surrounding the pit. (a) and (c) 6061-T6, (b) and (d) 2024-T3.
degree of pitting. There was no detectable difference in the amount of pitting on the surface of the alloys after NaCl or CeCl3 treatment. Typical examples of pitting on 6061-T6 and 2024-T3 are shown in Fig. 4(a) and (b) respectively after treatment in NaCl solution. Features on either surface include intermetallic etchout and pitting events associated with intermetallics remnants. The pitting appeared more severe on 2024-T3 than on the 6061-T6 alloy. These pits had little influence on the oxide thickness reported here as explained in the Discussion section. The RBS spectra of the same alloys treated in 10 mM CeCl3 solution for 1 h revealed widely varying oxide thicknesses (Table 2) which can be seen in the Al-metal step (Fig. 5(a)) and Ce distributions (Fig. 5(b)). The oxide thicknesses varied from 300 nm for 1100-O alloy to 1000 nm for 2024-T3; the later was three time the thickness developed in DIW and twice the thickness developed in NaCl solution. Despite these differences the appearance of the oxide was similar to that observed for both the DIW and NaCl cases (Fig. 6). Furthermore, regardless of the differences in thicknesses, the O to Al ratio of the oxide treated in CeCl3 was consistently around 2:1, which was the same as that after treatment in DI water at 95 °C. From Table 3, it can be seen that the absolute amount of Ce incorporated into the surface oxide did not correlate with the oxide thickness. While Ce ions in solution do react with the intermetallic particles, the surface area of these particles is approximately 4–5% of
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Fig. 5. RBS spectra of (a) Al and O regions and (b) the Ce-regions for Al-alloys 2024-T3, 3004-H19, 5005O, 6061-T6 and 7075-T6 after treatment in 10 mM CeCl3 solution for 1 h at 95 °C. The step between the Al and O steps is the Al metal signal and the width from the marked Al line to the Al metal step represents the oxide thickness.
the total area, thus the cerium detected in the RBS spectra will be dominated by cerium ions incorporated into the oxide formed over the metal matrix. This is confirmed in Appendix A, which shows the Ce signal collected from the matrix of 2024-T3 after treatment in CeCl3 solution. With this in mind it can be inferred from the profiles that Ce was distributed evenly through the oxide formed on the alloys 2024-T3, 5005-O and 6061-T6. The 7075-T6 alloy showed a sub-surface peak in Ce concentration whilst 3004-H19 displayed a peak at the surface of the oxide. Fig. 7 shows an overlay of the RBS spectra taken from 6061-T6 alloy samples treated in either CeCl3 or CrCl3 . It is apparent from the oxygen region that the oxide developed in the CeCl3 solution was considerably thicker than that developed in the CrCl3 solution. Generally, the oxide thicknesses, developed in the CrCl3 solution, varied from 20 to 200 nm which was considerably less than those developed in the CeCl3 solution and even less than those developed in DIW but indicated some
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Fig. 6. SEM of matrix after treatment in 10 mM CeCl3 solution for 1 h at 95 °C. Al-alloys (a) 1100-O, (b) 2024-T3, (c) 3004-H19, (d) 5005-O, (e) 6061-T6, (f) 7075-T6. Images taken at 50,000 times.
Table 3 Atomic percenta; b of active cation incorporated into the oxide after treatment at 95 °C Alloy
1100-O
2024-T3
3004-H19
5005-O
6061-T6
7075-T6
10 mM CeCl3 10 mM CrCl3
0.2 –
0.9 0.03
0.6, 1.4 2
0.5 2
0.5 2
0.8, 1.4 0.03
a b
Determined by RUMP simulations of RBS spectra. Duplicate values agreed to within 5% except where otherwise stated.
thickening compared to the deoxidised surfaces (Table 2). RUMP simulations yielded lower O to Al ratios than the other treatments typically around 1.5:1 which
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Fig. 7. Overlay of RBS spectra of Al-alloy 6061-T6 after treatment in 10 mM CeCl3 and 10 mM CrCl3 solutions for 1 h at 95 °C. Note the lack of oxide features and buildup of Cu for the CrCl3 treatment.
implied that the oxide may have been Al2 O3 indicating a lower level of hydration than that found after CeCl3 immersion. The spectrum for the CrCl3 treated 6061-T6 alloy also showed considerable surface and sub-surface enrichment of Cu (channels 325–330). Cu-enrichment was also observed on 7075-T6, 2024-T3 and 3004-H19; very little was observed on 5005-O and none on 1100-O (Fig. 8). In the case of 7075T6 alloy, there was also surface enrichment of Zn (Fig. 8). While the overall Cr content in the surface oxide was small (Table 8), there was considerable variation in Cr content with respect to alloy type. In terms of ranking, the lowest levels of Cr were observed for the least pure alloys (2024-T3 and 7075-T6), opposite to the
Fig. 8. RBS spectra of the Cr–Cu regions for Al-alloys 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 after treatment in 10 mM CrCl3 solution for 1 h at 95 °C.
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trend for the Ce-treated samples, and the highest levels of Cr were incorporated into the most pure alloys, again roughly opposite to the Ce case. This suggests that the mechanism of interaction with the surface was different for the two cations.
4. XPS The oxide left after deoxidation was thin with both Al3þ and Al0 peaks detected in XPS resulting in a range of oxide thickness from 4.2 to 6.5 nm (reproducibility from one area to another on the same coupon was generally within 5%). In addition, a number of other elements were detected on the surface including N (1 at.%), P (2.0–5.4 at.%), F (4.9–7.9 at.%) and Si (1.8–7.5 at.%). The O:Al3þ ratios were in the range 1.60–3.84, with a reproducibility between measurements on the same sample of 0.18. The variation in composition and oxide thickness from one alloy to the next is due to the amount of Si left on the surface (reducing the total Al at.%) and F also modifies the surface O/Al ratio since F ions displace OH groups on the surface. O and Al were the major elements detected on the surface after all DIW, NaCl, CeCl3 and CrCl3 treatments (Tables 4–7). For specimens treated in DIW the O:Al ratio was in the region 2.3–2.5 and these were the only elements detected on the surface (Table 5). The high O:Al ratio and absence of other elements suggests greater hydration of the surface of the oxide compared to boehmite (Al2 O3 H2 O) observed by RBS for the bulk of the oxide. Higher O/Al ratios e.g. Al2 O3 2H2 O are more
Table 4 Atomic percent determined by XPS after deoxidation Al-alloy
Al 2pa
Cu 2p3=2
O 1s
F 1s
Si 2p
Cr 2p3=2
O/(Al þ Si þ Cr)
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
19:7 1:3 21:7 0:2 16:9 0:5 15:9 0:1 27.9 22.4
0:42 0:03 0:52 0:03 0:25 0:02 0:08 0:01 0.46 0.4
50:7 0:4 56:0 0:6 64:9 3:8 54:2 0:1 44.7 53.1
7:9 0:2 6:6 0:1 4:9 0:1 5:8 0:1 7.2 7.7
2:3 2:3 2:8 0:3 7:5 0:1 6:8 0:6 3.1 3.3
4:6 0:5 3:3 0:3 1:6 0:4 4:7 0:3 3.3 4.7
1:75 0:15 2:00 0:07 2.42 2:00 0:01 1.3 1.74
a
Only the Al3þ intensity was included in the calculation.
Table 5 XPS analyses of the surface oxide after treatment in DIW 1 h at 95 °C Al-alloy
O
Al
O/Al
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
69.3 70 69.8 71.1 69.4 68.7
29.2 29.6 29.0 28.1 28.9 29.4
2.4 2.4 2.4 2.5 2.4 2.3
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Table 6 XPS analyses of the surface oxide after treatment in 30 mM NaCl solution for 1 h at 95 °C Al-alloy
O
Al
Cl
O/Al
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
72.1 72.3 68.8 71.4 72.9 71.2
27 27 30.3 28.1 26.5 28.3
0.9 0.4 0.4 0.5 0.5 0.4
2.7 2.7 2.4 2.5 2.8 2.5
Table 7 XPS analyses of the surface oxide after treatment in 10 mM CeCl3 solution for 1 h at 95 °C Al-alloy
O
Al
Cl
Ce
O/Al
1100-O 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
73.3 74 73 76.4 75 70
25.9 24.7 26 23.2 24.5 27.9
0.3 0.3 0.4 0.2 0.3 0.5
0.0 0.6 0.6 0.3 0.2 1.0
2.8 3.0 2.8 3.3 3.1 2.5
likely due to pseudoboemite [16]. The average Al3þ binding energy was 73.9 eV (1.9 eV FWHM) which is consistent with the literature for boehmite (Note the data in this paper is charge referenced to C 1s at 285.0 eV) [27]. The O 1s peak was wider than expected (2.9 eV FWHM) and, at a binding energy of approximately 531.8 eV, suggested a variety of chemical states including O anions and hydroxyl groups [28]. After treatment in NaCl solution the O:Al ratios for the different alloys were in the range 2.4–2.8 (Table 6). This range is higher than the RBS O:Al ratio of 2:1 suggesting surface hydration and the presence of pseudoboehmite. The surface chloride concentration was generally in the range 0.4–0.5 at.% apart from the 1100-O alloy which was 0.9 at.%. The chloride binding energy of 199.0 eV suggested that chloride was adsorbed onto the oxide surface rather than penetrated into the surface oxide [29]. After treatment in the CeCl3 solution the O:Al ratio was in the range 2.5–3.3 (Table 7), again, generally larger than the ratio of 2:1 determined from RBS simulations and indicating the presence of pseudoboehmite on the external surface of the oxide. Furthermore, apart from 7075-T6 alloy, the ratio was higher in all cases than the NaCl treatment suggesting a larger degree of surface hydration after treatment in CeCl3 solution. It was also interesting to note that the chloride levels were, in all cases, less than those detected on the surface of the alloys after NaCl treatment. Ce was detected in a mixture of CeIII and CeIV on the surface of all alloys. The amount of CeIV was determined from the ratio of the peak labelled CeIV , positioned at around 917 eV (Fig. 9) [30]. Both the 3004-H19 and 5005-O series alloys showed Ce present as CeIV whilst all other alloys showed Ce present predominantly as CeIII .
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Fig. 9. Comparison of the XPS Ce 3d region for Al-alloys 2024-T3 and 3004-H19 after treatment for 1 h at 95 °C. Note the higher level of CeIV on the surface of 3004-H19 alloy.
Table 8 XPS analyses of the surface oxide after treatment in 10 mM CrCl3 solution for 1 h at 95 °C Alloy
O
Al
Cl
Cr
Cu
O/Al
1000 2024-T3 3004-H19 5005-O 6061-T6 7075-T6
72.9 57.1 63.3 67.9 67.8 69.8
23.8 33.1 31.9 29.2 26.7 23.9
0.3 2.2 0.3 0.6 0.5 2.4
0.5 0.4 0.3 1.7 1.3 1.2
0.0 0.0 4.2 0.6 3.6 1.7
3.1 1.7 2.0 2.3 2.5 2.9
These two alloys had the highest percentage of Ce at the surface according to RBS Ce-depth profiles (Fig. 6), thus the high CeIV for 3004-H19 and 5005-O might result, posttreatment, by oxidation of surface CeIII . Cl was found at a binding energy of approximately 199.0 eV, again, suggesting adsorbed chloride [29]. No Ce was detected on the 1100-O series alloy. Small amounts of F were detected using XPS on two samples, this was presumed to have been carried through from the rinse water since Melbourne tap water is fluorinated. None of the alloying components were detected on any of the alloys within the detectability limits (0.1 at.%). XPS for the CrCl3 boiled samples were in good agreement with the RBS results showing a lower level of oxygen in the surface oxide than found on the CeCl3 boiled samples. The O:Al ratio was in the range 1.7–3.1 (Table 8), generally a little lower than for the other treatments but closer to the RBS ratio of 1.5:1 The Cr 2p3=2 peak was found at a binding energy of approximately 578.0 eV on all samples. CrIII generally fell between 577.1 and 577.5, whereas CrVI binding energies were around 2.3–2.8 eV higher [31,32]. CrVI compounds would be expected to exhibit a binding energy of approximately 579.0 eV.
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5. Discussion The most notable features between the DIW, NaCl, CeCl3 and CrCl3 treatments are the following: (i) similar oxide thickness for different alloy types treated in either DIW or NaCl solutions, (ii) the larger oxide thicknesses developed on some alloys treated in CeCl3 solution compared to either DIW or NaCl solutions, (iii) the wide variation of oxide thicknesses between alloys after treatment in CeCl3 solution, (iv) poorly developed oxides after treatment in CrCl3 solution, (v) bulk stoichiometry of O:Al of 2:1 from RBS, but higher ratios for the surface of the oxide from XPS.
5.1. Influence of pitting on oxide thickness The presence of pitting and oxide material in the pit caps might be expected to lead to increased average oxide thickness as determined by RBS, and it will certainly have some effect. Thus it is important to establish the influence of oxidic pit caps on the width of the oxide peak measured in RBS. Taking the 2024-T3 case into consideration, the aerial density of intermetallics on the surface is around 4–5% and pitting is generally associated with intermetallic phases, thus the total area of the pits is small relative to the matrix. (Indeed deoxidation (see Section 2) generally removes a substantial amount of the intermetallic material [26].) In Fig. 10, RUMP simulations of aluminium with 1000 nm of oxide in a homogenous film is compared to aluminium with a similar oxide but with 5% surface coverage of 10 lm of oxide to model pitting on the 2024-T3 alloy. The thick oxide from the pit cap has a very broad oxygen peak and effectively influences the spectrum by raising the overall intensity across the thickness for the matrix oxide peak much in the same way as a constant background would do. Thus the width of the oxide peak is essentially the same for both scenarios. Fig. 10(b) is a section through the 2024-T3 sample treated in CeCl3 solution; the thickness of the overlayer is around 1000 nm which is in good agreement with the RBS thickness indicating that thick, oxidic pit caps have negligible influence on the determination of the matrix oxide thickness as measured by RBS. Hence the RBS is strongly weighted towards the matrix. In addition to the simulations, it is should be noted that the degree of pitting for NaCl and CeCl3 were similar but there were considerable differences in oxide thickness between the two treatments for 2024-T3, 3004-H19 and 7075-T6 alloys indicating that the oxide thickness determined by RBS is largely unrelated to corrosion product developed as a result of pitting. The differences in oxide thickness therefore point towards some other mechanism which likely relates to changes in the interfacial chemistry having a major influence of the final oxide thickness.
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Fig. 10. (a) Two model RBS spectra representing an oxide of nominally 1 lm thickness with and without a 5% surface coverage of pit caps with 10 lm thickness. It can be seen that the 5% coverage of 10 lm oxide effectively adds a background to the O region. (b) Backscatter electron image of a section through the coating confirming that the thickness of the boehmite coating is around 1000 nm.
5.2. Oxide growth over the matrix The differences in oxide thickness should be considered in terms of current models of oxide growth, i.e., (a) formation of an amorphous oxide, (b) hydration of the oxide, and (c) growth of the boehmite phase (dissolution and precipitation). At elevated temperatures, stages (a) and (b) happen quickly (1 m) [16,19], thus the oxide growth is well into the dissolution and reprecipitation processes. This final stage of growth ends up being rate limited by diffusion of species across the dense inner oxide, and the growth rate is generally negligible after 1 h treatment. Thus the oxides studied here are effectively at the maximum thickness for the treatment conditions. The DIW treatments form the baselines for these results. After 1 h treatment, the oxide is the same thickness for all the alloys studied, suggesting that there was no influence of the alloy surface composition on formation and growth of pseudoboehmite, despite the variation in composition of the deoxidised surface. Comparing these results with the NaCl treatment, it is apparent that the presence of chloride in
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the DIW enhances the oxide growth beyond the DI water case, although, again there appears to be little differentiation between alloys. The literature is not clear on the effects of chloride addition on growth of oxides in aqueous solutions at elevated temperatures. For example, Godard and Torrible found that, in the presence of Cl , the film weights for high purity aluminium were similar to or slightly lower than those for pure water [33]. On the other hand, the data of Vermilyea and Vedder [18] suggested that for treatment times less than 10 min, treatment in NaCl solution at 100 °C will produce thinner coatings than pure water, but above 10 min the situation may be reversed. Thus for the 1 h treatment in NaCl solution, the thicker coatings reported here appear consistent with previous reports. Chloride ions may accelerate oxide growth by facilitating the hydration and dissolution process of amorphous oxide. The role of chloride in the hydration of aluminium surfaces comes largely from studies of chloride attack on anodically grown oxide films [33,34]. Early work suggested that chloride ions are not only adsorbed onto the oxide surface but also penetrate the surface [34]. More recently Skeldon et al. [35] confirmed these results, showing that while chloride ions do, in fact, penetrate the surface, they do not accumulate at the metal oxide interface, since the diffusion rate of OH and O2 is greater than Cl . At the surface, chloride ions compete for adsorption sites with hydroxyl ions which probably leads to the formation of soluble surface species, such as AlCl3 or hydrated oxychlorides, and subsequently to Al3þ ions in solution. In the absence of chloride, Alwitt suggests that the dominant solution species are probably Al(OH)2þ and Al(OH)þ 2 [19]. Since the oxides are only sparingly soluble, then once the reaction proceeds to any great extent these ions precipitate onto the surface forming either bayerite (Al(OH)3 ) and pseudoboehmite, at low temperatures (<50 °C), or pseudoboehmite alone at more elevated temperatures (>80 °C) [19]. It is clear from the large XPS O/Al ratios observed for CeCl3 and NaCl treatments and SEM that the surfaces have highly hydrated forms of pseudoboehmite. An O/Al ratio of 3 implies there would be three waters of hydration, whereas a value of 2 implies only one water of hydration which is typical of boehmite. It should be noted, however, that the vacuum conditions in the XPS spectrometer can lead to surface dehydration of aluminium oxides. For example, Nylund and Olefjord [28] have pointed out that hydrated forms of alumina decompose in vacuum to produce a surface layer of Al2 O3 . They found this layer to be approximately 7 nm thick which gave a mixed Al2 O3 and hydrated alumina spectrum when analysed by XPS. It is therefore possible that the XPS O/Al ratios presented here may be underestimates of the true degree of hydration. 5.3. Role of cerium The difference in oxide thickness between the Al-alloys after treatment in CeCl3 solution suggests an active role for the cerium cations and the alloy surface microstructure and/or composition. Alwitt observed that at 100 °C the pseudoboehmite layer develops an impenetrable inner layer due to the high density of crystallisation, however, this layer does not develop at 40 °C where the overall pseudoboehmite
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layer is much thicker than 100 °C. On the basis of radio tracer measurements it was concluded that the rate limiting step at 100 °C for the third stage of growth is solid state diffusion of water across the impenetrable layer. If this is the case, then the presence of Ce may modify the structure of this impenetrable layer making it more porous and promoting oxide growth. The modification of pseudoboehmite due to Ce incorporation, is supported to some extent by RBS Ce depth profiles which indicate that Ce is incorporated into the oxide coating, albeit at low levels. Similar results were observed in a previous report on the treatment of 2024-T3 and 6061-T6 Alalloys in Ce solutions [36]: in that case, as here, there was interaction of cerium cations with both intermetallics and the oxide covering the matrix. For the present results, the degree of incorporation corresponds well with oxide thickness, i.e. the alloys with the largest incorporated Ce levels (2024-T3, 7075-T6 and 3004-H19) have the thickest oxides, whereas those with the least incorporated cerium have the thinnest oxides (Table 2). Given the difference in ion size between Ce3þ (or Ce4þ ) at (or 0.92 A ) and Al3þ 0.51 A then some strain would be developed in both the 1.034 A boehmite lattice and the impenetrable oxide possibly leading to a more open structure and greater diffusion of reacting species. The other area where cerium cations may have an influence is in complex formation in the solution immediately adjacent to the surface. For example, Rider et al. [10] found moderate levels of Ni on the surface (4.0–4.7 at.% Al/Ni 7) of boehmitelike structures developed after treatment in solutions containing NiCl2 and NaCl. They proposed that oxide formation involved Ni/Al complex formation in the solution adjacent to the developing oxide. While hydrolysis of cerium ions in solution is well known [37] and mixed Al/Ce complexes may form at the metal/solution interface, a greater concentration of cerium would probably be expected on the surface of the alloys treated in CeCl3 solution if Al/Ce solution complexes had a major role to play in the acceleration of oxide growth. The minimum Al/Ce ratio observed here was 28 compared to Al/Ni of 7 for Rider et al. [10] suggesting, that for the Ce case, surface complexation does not play a major role in oxide formation. Additionally, oxide growth acceleration by Ce-complexes at the solution/oxide interface is unlikely to involve a role for the different alloy compositions. 5.4. Role of the alloy composition The alloy microstructure has an important role to play since, in the presence of Ce, a broad range of oxide thickness was observed for the different alloys which was not the case for the NaCl treatment. Since the thicknesses do not correlate directly with the total alloying composition the dependence of the oxide thickness on individual and combinations of alloying components was examined. The best correlations were obtained with (Cu þ Mn) content and the (Cu þ Mn)/Si ratio (Fig. 11). Either of these parameters seems reasonable on a physical basis:Cu-[38,39] and Mnenrichments [40] have been detected in thin aluminium oxide layers after various treatments. Both elements are more noble than Al and may act as accelerators for Al dissolution. On the other hand, Si in solution (e.g. from glassware or water) has been reported to inhibit oxide growth [15,16] and Si from the alloy may play the same
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Fig. 11. Correlation between oxide thickness and (a) atomic percent of Cu þ Mn and (b) (Cu þ Mn)/Si.
role. Interestingly, the oxide thickness does not depend on the surface composition after deoxidation despite the high levels of inhibiting ions such as Si4þ and Cr3þ or Cr6þ . It is conceivable that these surface species are dissolved from the surface quickly and lost to the solution, whereas, the bulk alloying components are continually exposed as the Al metal is oxidised to Al3þ . 5.5. Effects of chromium In the case of the CrCl3 treatments, the oxides were too thin to provide meaningful RBS O:Al ratios it was not possible to compare the bulk to surface compositions.
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The absence of precipitation of hydrated aluminas suggests that the dissolution process has been blocked, which, given the thin oxide coating and the presence of Cr on the surface, appeared to be due to the formation of a Cr-containing passive layer on the surface. Previous studies of Cr-oxides in chromate conversion coatings has shown it to be resistant to chloride attack [41], thus the presence of Cr inhibits oxide growth by minimising chloride attack. 6. Conclusions Growth of aluminium oxides (pseudoboehmite) in DIW for the aluminium alloys 1100-O, 2024-T3, 3004-H19, 5005-O, 6061-T6 and 7075-T6 resulted in a similar thickness for all alloys (around 200 nm). The addition of 30 mM NaCl resulted in an increase of oxide thickness of around 50–70% compared to the DIW case, but there was little variation in the oxide thickness. The increase in oxide thickness was attributed to the presence of chloride ions in solution which assist oxide dissolution. Further increases in oxide thickness were observed for 2024-T3, 3004-H19 and 7075T6 alloys after treatment in 10 mM CeCl3 solution. In this case the increase of oxide thickness was thought to occur via the incorporation of low levels of Ce ions into the pseudoboehmite causing greater diffusion of reacting species due to strain introduced by the large difference in ion size of Ce compared to Al. Finally, no oxide growth was observed in 10 mM CrCl3 solutions. This was attributed to blocking of active sites by chromate ions.
Acknowledgements The authors would like to thank Dr K. Wittel of Chemetall GmbH (Frankfurt) and Ms K. Nelson of CSIRO for critical reading of the manuscript. The authors would also like to thank Mr P. Curtis for the ICP analyses of the alloys and Mr S. Glanville for the preparing the section of the 2024-T3 sample after treatment in CeCl3 solution.
Appendix A Fig. 12 is the Ce profile taken from the matrix of 2024-T3 sample after treatment in CeCl3 solution. This particular profile was collected in the Melbourne Nuclear Microprobe system [42] using 2 MeV He ions and backscattered ions collected at an angle of 145°. In this instance maps of an area were collected and analysed. These maps have RBS spectra for each pixel in the map. Intermetallic particles were clearly distinguishable from the matrix; the spectrum in Fig. 12 has been compiled from the RBS spectra from the matrix. It is clear from the Ce profile that (i) Ce is present in the coating on the matrix and (ii) was detected throughout the coating as observed in Fig. 5(b).
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Fig. 12. Ce-RBS spectrum of the oxide covering the matrix excluding intermetallics, after treatment in boiling CeCl3 solution, demonstrating that Ce is incorporated into the boehmite oxide.
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