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Influence of surface composition on initial hydration of aluminium in boiling water
applied surface science ELSEVIER
Applied Surface Science 74 (1994) 263-275
Influence of surface composition on initial hydration of aluminium in boiling water Anders
Strfilin, Thomas
Hjertberg
*
Department of Polymer Technology, Chalmers University of Technology, 412 96 Gijteborg, Sweden
(Received 27 September 1993; accepted for publication 22 November 1993)
Abstract Eight aluminium samples were hydrated in boiling water for 5-60 s, which produces an adherent and porous pseudoboehmite (AlOOH) film on the surface. The progress of the hydration reaction was followed by IR reflection-absorption spectroscopy and a delay was observed owing to an induction period. The surface composition of the alloying elements was determined by XPS. AES depth profiling was used to analyse the sublayers of the hydrated oxide. Aluminium samples with Mg-enriched surface oxides were found to have the shortest induction periods, i.e. a shorter hydration time was required to obtain a certain thickness of the pseudoboehmite layer. The highest initial hydration rates were obtained in the interval lo%-20% Mg (determined by XPS) on the surface. XPS and AES analysis revealed that the hydrated oxide layer did not contain any Mg. By AES depth profiling Mg was
found to be present at the metal/hydrated oxide interface. This indicates that Mg (as MgO or MgAl,OJ in the surface oxide is not dissolved by boiling water. The higher reaction rate for the Mg-enriched surfaces is suggested to depend on the disrupted Al-Mg oxide morphology, which probably increases the rate of diffusion and transport of soluble species. The Al-Mg oxide is also suggested to increase the number of nucleation sites and the nucleation rate.
1. Introduction The reaction between aluminium and water can form many types of hydrous oxides, which has been reviewed extensively by Alwitt [l]. The structure and composition of the oxide films formed in water depends primarily on the reaction temperature. Between 20 and 90°C duplex films are formed, consisting of a pseudoboehmite layer (which is the initially formed product) next
* Corresponding
author.
0169-4332/94/$07.00 SD1
0 1994 Elsevier
0169-4332(93)E0309-A
Science
to the metal and a layer of bayerite in contact with water. At 90-lOo”C, an adherent layer of pseudoboehmite is formed, which is an oxyhydroxide that contains physically adsorbed water. From 100 to 374”C, pure boehmite is the only oxide phase formed. At or above the critical temperature, 374”, no hydrous oxide is formed, but formation of y-aluminium or corundum occurs. When aluminium is immersed in boiling water, the reaction can be followed by the evolution of hydrogen gas on the surface. This treatment topographically changes the surface to a rough
B.V. All rights reserved
264
A. St&n,
T. Hjertberg /Applied
porous surface, and chemically changes it to an oxyhydroxide according to the reaction below: Al+2H,O+4lOOH+;H,. The reaction is delayed by an induction period, owing to the presence of the protecting natural oxide film. The length of this induction period has been reported to vary with the water temperature and the thickness of the oxide film [2,3]. Bernard and Randall [3] suggested that the induction period is related to the water diffusion through the primary oxide film that is always present on the aluminium surface. Vedder and Vermilyea [4] suggested that the outer surface of the oxide is first hydrolysed and then dissolves to yield soluble species. These either remain in the solution or, at intermediate pH, precipitate as a porous oxide onto the surface. They also suggested that the rate of the hydration reaction was controlled by the dissolution of the primary oxide, diffusion of soluble products, and the disposition of the soluble products. Alwitt [2] suggested that the kinetics of the film growth is initially determined by nucleation and growth of hydrolysis sites on the oxide surface. He considered the rate-limiting step initially to be the diffusion of water into the surface oxide. As the oxide thickness increases, the rate control shifts to solid-state diffusion through the pseudoboehmite structure (probably of water diffusing inward as H+ and OH-). Baker and Balser [5] also studied the aluminium-water reaction and agreed with Alwitt’s mechanism for film formation on most points. However, they suggested that the reaction rate at longer hydration times is controlled by a denser, less soluble and less permeable barrier layer formed next to the metal. A study of the induction period by Zhuravlev and Zakharov [6] also emphasized the importance of the diffusion of water and dissolution of the oxide film in the initial stages of the hydration. Pseudoboehmite films produced on aluminium surfaces are important in a number of diverse applications, e.g.: sealing of anodic oxides [7-91; preventing corrosion of the aluminium surface [lo]; as a substrate for adhesive bonding or coating [l l-151; and in the manufacture of elec-
Surface Science 74 (1994) 263-275
trolytic capacitors [16]. In our previous work, improved adhesion was obtained in hot-pressed laminates with hydrated aluminium for LDPE, and for polar ethylene copolymers [13], as well as for ethylene copolymers containing methoxy silane groups [14]. The topographical change to a porous surface was suggested to improve the adhesion by increased surface area and mechanical keying. Strong interfacial interactions through hydrogen bonding and carboxylate formation have been shown to form for ester copolymers and acrylic acid copolymers [17], and also contribute to the improved adhesion after hydration. In another work, the adhesion between LDPE and hydrated aluminium in extrusion-coated laminates was shown to be influenced by the melt viscososity [151. When penetration into the pores and wetting of the surface was obtained in the extrusion-coated laminates, the adhesion between oxidized LDPE and hydrated aluminium was superior after ageing in acetic acid solutions as compared with untreated aluminium. This was explained to be the result of the adhesion mechanisms described above, and of the much better corrosion resistance of the hydrated aluminium surfaces. Different aluminium samples were used in the previous studies, and the length of the induction period was observed to vary. To produce a hydrated foil for extrusion coating in a continuous process, it is necessary that the induction period is short. Baker and Balser [5] reported that the alloy composition had very little effect on the hydration rate and film characteristics for NaOH etched samples with relatively long hydration times. In the present study, a significant relation to the surface composition was observed in the initial stage of the hydration reaction. The reaction rate was followed by IR reflection-absorption spectroscopy. The relation between the hydration rate and the bulk composition, as well as the surface composition determined by XPS, are discussed for eight samples. AES depth profiling was used to determine the thickness of the hydrated oxide and to estimate the composition at the metal/ hydrated oxide interface. The increased initial hydration rates obtained for Al-Mg oxides, and the relation to adhesion and durabil-
A. S&h,
ity of adhesive also discussed.
joints
2. Experimental
in humid
T Hjertberg /Applied Surface Science 74 (1994) 263-275
environments
are
details
2.1. Materials Eight aluminium samples from the 1000, 3000, 6000 and 8000 series were kindly supplied by Granges AB. Table 1 shows the bulk composition and the designation of the different materials. The samples were designated with A-H after the Mg concentration in the bulk. All materials have been inter-annealed and final-annealed. Samples A, B, C and E were final-annealed at the production plant and the others were final-annealed in a laboratory oven for 1 h at 300°C. The aluminium samples were hydrated by immersion in a Pyrex beaker containing de-ionized boiling water. 2.2. Bulk analysis The bulk composition of the elements were obtained from Granges Technology Centre. The Mg, Cu and Mn concentrations reported in ppm were analysed by ICP-AES on a Philips PV8050. Before the measurement, the samples were dissolved in hydrochloric acid with hydrogen peroxide. The content of Fe, Si and the other elements reported in % were determined by spark excitation optical emission spectrometry (OES) on a Spectrolab M5 (Spectra Analytical Instruments). 2.3. IR analysis The hydration spectroscopy on
reaction was followed a Perkin-Elmer 2000
by IR FT-IR
265
equipped with a liquid-nitrogen-cooled MCT detector. To reduce water vapour in the background, the spectrophotometer was purged with dry air. Samples of 2.5 X 5 cm were analysed on a reflection-absorption spectroscopy attachment (Spectratech FT-80) with a fixed angle of incidence of 80”. The spectral resolution was 4 cm-’ and the reported spectra represent the average of 50 scans. 2.4. SEM analysis A JEOL JSM-840 scanning electron microscope was used to study the topographical changes on the aluminium surface after hydration. 2.5. XPS analysis The surface composition of the samples before and after hydration was determined by XPS on a Perkin-Elmer PHI 5500. A Mg Ka X-ray source was used and the take-off angle was 45”. The analysed area had a diameter of 0.8 mm avd the information depth is approximately 50 A. To evaluate the surface composition, the PHI sensitivity factors given for the Mg Ka X-ray source in the instructors manual were used. 2.6. AES analysis The composition and thickness of the hydrated oxides were obtained by AES depth profiling with a Perkin-Elmer PHI 660. The depth profiles were obtained by using AES in conjunction with continuous ion etching by Art bombardment. The acceleration voltage of the ion gun was 2 kV and a rastered Ar+ beam was used to improve the accuracy of the ion etching. The analysed surface
Table 1 Composition in % of the bulk
Fe Mg Mn Si cu
A AA 1050
B AA 8079 +Fe
C AA 1200
D AA 3003
E AA 8016
F AA 6063
G AA 3004
H AA 3104
0.25 10 ppm 26 ppm 0.05 10 ppm
1.49 10 ppm 0.01 0.11 < 0.01
0.82 19 ppm 40 ppm 0.06 4 ppm
0.54 0.01 1.1 0.16 0.15
1.01 0.045 0.15 0.14 0.012
0.24 0.69 0.06 0.44 < 0.01
0.44 1.3 1.0 0.18 0.17
0.45 1.3 0.92 0.22 0.12
266
A. Str&n,
T. Hjertberg /Applied
area was approximately 5 X 5 pm and a 10 kV electron beam with a current density of 0.5 PA was used for excitation. In order to quantify the sputtered depth, it is important to know the sputter yield of the samples. For the equipment used, the etching rate was calibrated using a 1000 A thick tantalum oxide sample and was found to be 22 A/min. The etching rate of Al,O, has been reported to be 70% of that of Ta,O,Jl8]. In the present study, one etching rate (15.5 A/min) was assumed to be valid for all samples, despite the fact that the oxide structure is probably duplex and contains many different oxide types such as Al,O, (amorphous and crystalline), MgO, MgAl,O, (spine11 and AlOOH. For that reason, there is an uncertainty in the thicknesses reported.
3. Results
and discussion
3.1. IR analysis IR reflection-absorption spectroscopy (RAS) with large, almost grazing angles of incidence, is a quick, sensitive and well suited method for studying chemical conversions of the aluminium surface [19-211. Fig. 1 shows the IR spectra for sample E: (A) untreated and (B) hydrated for 40 s in boiling water. Similar spectra were obtained for all other samples. The assignments for the spectra are shown in Table 2. Only one absorbance appears for untreated aluminium, that is a bending mode of Al-O-Al in the thin, thermally oxidized surface layer. After hydration, many new absorbances arise, the most characteristic appearing at 1080 cm-‘, owing to bending deformations of the hydroxyl groups. Absorbances also arise at 1700-1300 cm-‘, owing to physically adsorbed and coordinatively bonded water molecules [22], and at 3700-3000 cm-‘, owing to O-H stretching in hydroxyl groups and in water. The absorbance of the characteristic Al-OH group at 1080 cm-’ (measured as the difference between the absorbance at 1080 cm-’ and the baseline around 1300 cm-‘) was used to follow
Surface Science 74 (1994) 263-275
1 -I
d
Wavenumber Fig. 1. IR spectra drated for 40 s.
of sample
(cm-‘)
E, (A) untreated
and (B) hy-
the initial stage of the hydration reaction. In Fig. 2, the absorbance of this group is shown as a function of hydration time in boiling water for all samples. In the beginning, no hydration was obtained owing to the induction period. The length of the induction period varied considerably among the samples, and they could be divided into four groups with decreasing induction times, as follows: A=B=C>D>E=F>G=H. When the length of the induction period is compared with the concentration of the elements in the bulk, there seems to be a good relation to Mg. After the induction period, the reaction is rapid for the first 30 s. Thereafter, the hydration rate decreases, because an increased thickness and density of the hydrated film probably decreases the rates of diffusion of water and transport of soluble products. The changes in the reaction rate could also be identified by visual observation of the evolution of the hydrogen gas on the surface.
Table 2 Assignment Position
(cm
960 1080 1700-1300 3700-3000 a Assignments
for IR spectra
‘)
shown in Fig. 1 ’
Assignment
Spectrum
Al-O-Al bend Al-OH bend H-O-H bend O-H stretch
A B B B
are based
on Refs. [1,22].
267
A. Str&n, T. Hjertberg /Applied Surface Science 74 (1994) 263-275
.
0 sample A l sample B v sample C v sample D
20
0
._
40
tXJ
0
Hydration time (s) Fig. 2. IR absorbance
at 1080 cm-’
40
20 Hydration
as a function
of hydration
3.2. SEA4 analysis
The surface topography changes simultaneously to the chemical conversion. This was studied with SEM, and the result for sample G is shown in Figs. 3 and 4. At 60 s, a fully developed hydrated oxide was obtained. This hydrated oxide is coarse and has cavities of an approximate diameter of 0.1 Frn. At 60 s hydration time, oxide structures similar to that shown in Fig. 3b were obtained for all samples. At short hydration times or in the initial stage of the hydration, a significant difference of the surface topography was observed between the samples with rapid and slow hydration rates. This is illustrated by SEM micrographs of samples C and G in Fig. 4. In the micrograph of sample C
Fig. 3. SEM micrographs
of sample
time for: (a) samples
A-D,
60
time (s)
(b) samples
E-H.
hydrated for 15 s (not shown), it was possible to observe traces of hydrated species sparsely distributed on the surface. After 17.5 s, the quantity of protruding hydrated oxide increased, but the structure was still relatively sparse, see Fig. 4a. For a longer hydration time, 20 s, a higher extent of the reaction led to a denser and thicker layer of hydrated oxide, as shown in Fig. 4b. The micrograph of sample G immersed in boiling water for only 1 s shows a thin but dense hydrated structure with a crackled appearance, see Fig. 4c. After 2 s hydration, the micrograph, Fig. 4d, shows a similar, but somewhat rougher and thicker oxide having more “depth”. The denser hydrated structure of sample G with a short induction period, indicates a larger quantity of nucleation sites on the surface. This may explain the
G: (a) untreated
and (b) hydrated
for 60 s.
A. Str&n,
268
Fig. 4. SEM micrographs
of sample
7: Hjertherg /Applied
C hydrated
for: (a) 17.5 s and (b) 20 s, and of sample
high reactivity for samples G and H, because the nucleation is very important in the initial stage of the hydration reaction. 3.3. XPS analysis Because the hydration reaction gested to start with the diffusion Table 3 Surface composition
Al (%I) Mg (%) 0 (%I
determined
has been of water
sugand
G hydrated
for: (c) 1 s and (d) 2 s.
dissolution of the primary oxide, the characteristics of the original surface oxide are important. The results of the XPS analysis of the original oxides are shown for all samples in Table 3. Of the elements in the bulk, only Mg was detectable within the information depth of XPS. The evaluation of the chemical composition in the oxide was obtained by XPS data of the Al2p, Mg2p and
by XPS
A
B
C
D
E
F
G
H
34.9 < 0.1 65.1
36.1 < 0.1 63.7
36.0 < 0.1 64.0
34.4 0.8 64.7
31.0 4.2 64.8
31.1 5.0 63.8
28.8 7.4 63.9
16.4 21.3 62.3
C-fraction I(AI’+)/I(Alm) “ Al”
Surface Science 74 (1994) 2636275
was not measurable.
0.068 14
0.059 7.7
0.089 12
0.094 17
0.13 7.1
0.084 13
0.13 7.9
0.11 a
A. Str&, 0.15
. 0
Hydrated Hydrated
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
method decreases and, for that reason, we have not reported the calculated oxide thicknesses. The results of the Z(A13’>/Z(Alm> ratio shown in Table 3 can, however, be used as a relative estimation of the oxide thickness. For the samples used in the present study, no significant correlation could be found between the Z(A13’>/Z(Al”> ratio and the hydration rate. A high C-fraction, as a consequence of remaining rolling oil or contamination left on the surface, may reduce the water wettability and delay the hydration reaction. The values of the C-fraction obtained, Table 3, are quite similar, and the differences observed do not seem to have any influence on the hydration rate. The length of the induction period must thus depend on the elemental composition, but only Mg (except Al) was found in detectable amounts on the surface. As for aluminium, the Mg2p signal of metal (50 eV) and oxide (+ 1.5 eV> are separated [23,241. The position (around 52 eV> and the shape of the Mg2p signals indicate that Mg is present on the surface only as oxide. The relation between the concentration of Mg on the surface and the length of the induction period or the absorbance of Al-OH at short hydration times seems to be good, as shown in Fig. 5. It is possible by thermal treatments to increase the Mg concentration in the surface oxide [25-311. For samples D, E and G, the surface concentration of Mg was changed by an additional heat treatment at 300°C for l-72 h. The increase in
40 seconds 20 seconds
iP
o.oo+
’ 10
5
’
’
15
20
Mg concentration
25
(%)
Fig. 5. IR absorbance at 1080 cm-’ as a function concentration on the surface for samples A-H.
269
of Mg
0 1s signals only. The fraction of the C 1s signal is presented separately. Bernard and Randall [3] reported that the length of the induction period is strongly dependent on the oxide thickness. Because XPS is sensitive to the chemical environment, the Al2p signal of the oxide and metal is separated by 2.8 + 0.1 eV. As shown by Olefjord et al. [181, the oxide thickness can be determined from XPS data, by means of the ZL4l”+)/Z(Al”) ratio. The samples used in the present study were oxidized thermally at elevated temperatures, which yielded relatively thick oxides, considering the XPS information depth. When Z(A13’>/Z(Al”) is 10 at a take-of: angle of 45”, it corresponds to a thickness of 3.5 A. For higher ratios, the accuracy of the
7
[
$$$$g I Oh
1
I/ 538
536
534
532
530
528
80
78
,, 76
74
,, 72
70
56
54
52
50
46
Binding energy (eV) Fig. 6. High resolution (normalized) XPS spectra of the 0 Is, Al 2p and Mg2p signals of sample at 300°C. The corresponding C 1s signals appeared at 287.1 f 0.1 eV (see also Table 4).
G heat-treated
for 0, 3, 24 and 72 h
A. St&in,
270
T. Hjertberg/Applied
the surface concentration of Mg is shown for sample G in Table 4. The long treatments of this sample led to a new appearance of the Al2p and 0 1s signals, as shown in Fig. 6. After 3 h, the oxide thickness increased, which is illustrated by the decrease in the Al” signal at 73.1 eV. When the sample was treated for 24 h, a shoulder appeared at a lower binding energy (at about 74.2 eV> on the Al2p signal and, after 72 h, the relative intensity of this new signal increased (to 44% of total Al2p). It has been suggested in many previous reports that Mg reduces Al,O, to Al metal according to the reactions below [30-331: 3 Mg + 4 Al,O,
(amorphous)
e 3 MgAl,O, 3 Mg + Al,O,
(amorphous)
Table 4 Surface composition
for sample
Treatment
C-fraction
010
r 0
0 A
. sample sample A sample
l
0 E G
l.
.
: ;
0.05
.
Lo
‘A . A
.
A . Filled hydrated .
.
.
A
:
20 s
A*
AA
A
.
2
1 .
0.00
0
goOBy 5
10 Mg
A
Hollow hydrated
15
concentration
20
10 s
4
25
(%)
Fig. 7. IR absorbance at 1080 cm-’ as a function of Mg concentration on the surface for samples D, E and G. Mg content on the surface was increased by thermal treatment at 300°C.
+ 2 Al, w 3 MgO + 2 Al.
On the basis of free-energy calculations, it has been proposed that the formation of spine1 MgAl,O, is most favourable [32], but high Mg : Al ratios in the oxide should favour the formation of MgO. The high Mg: Al ratio obtained for the oxides on sample G, heat-treated for 24 and 72 h (as well as for sample H), indicates that MgO is present on the surface. Using XPS, Sun [30] et al. showed that the signal of Al metal increased by heat treatments in vacuum of an Al-2.7%Mg2.3%Zn aIloy. The shoulder that appeared on the Al2p signal at about 74.2 eV indicates that Al,O, is reduced. We have not found any reported XPS data that could explain the A12p signal at 74.2 eV. According to the limited XPS data found for MgAl,O,, we suggest that the new signal can be related to this oxide type. The binding energy of the 0 1s signal in MgO has been reported to
Al (%I Mg (%) 0 (%)
Surface Science 74 (1994) 263-275
G, heat-treated
at 300°C
time (h)
0
3
24
72
28.8 7.4 63.9
25.7 10.0 64.3
16.5 19.7 63.8
14.8 24.7 60.5
0.13
0.14
0.14
0.11
appear at 531.0 eV [34] and that of Al,O, at 532.7 + 0.3 eV [181. The broadening and gradual change of the 0 Is signal can be explained by the gradual increase in the MgO and/or MgAl,O, concentration in the outermost oxide. For the Mg2p signal, only a minor increase of the fullwidth at half-maximum (from 1.85 to 2.0 eV> and a displacement of the maximum signal (from 52.3 to 52.0 eV> were observed after heat treatment for 72 h at 300°C. The XPS spectrum of sample H with an original surface concentration of 21.4% Mg had a similar appearance as the 72 h XPS spectrum of G shown in Fig. 6, which indicates a similar oxide structure present on its surface. After thermal treatment of samples D, E and G, and determination of the Mg concentration on the surface by XPS, hydration was performed in boiling water for 10 and 20 s. In Fig. 7, the absorbance of the hydroxyl group for these samples is plotted against the Mg concentration on the surface before hydration. Up to a surface concentration of 7%-10% Mg, the hydration level increases as the Mg concentration increases. The highest values of the IR absorbance were obtained at about lo%-20%, and the hydration rate in this interval was almost constant. At higher surface concentrations of Mg, the hydration rate seems to decrease. This can probably be explained by the fact that thicker oxides are formed for the longest thermal treatments (60 and 72 h at
A. Stra”lin, T. Hjertberg /Applied Table 5 Surface composition for samples with high Mg contents hydration for 60 s in boiling water, determined by XPS
Al (%) Mg (%I 0 (o/o) C-fraction
E
F
G
H
29.0 < 0.1 71.0
30.3 < 0.1 69.6
29.4 < 0.1 70.5
29.2
0.031
0.037
0.038
after
explain the fact that no Mg was detected on the hydrated surface. However, the solubility of oxidized Mg in water is very low (0.0086 g MgO/e in hot water) [37]. AES depth profiling of sample H was performed to study the composition of the MgO/MgAl,O, enriched oxide after hydration. 3.4. AES analysis
0.029
3OO”C), which prevents a continued rapid reaction. The surface composition of the samples with the highest Mg concentration (E-H) was determined by XPS after hydration for 60 s. A broadening and increase of the 0 1s signal was observed, owing to the introduction of hydroxyl[351. As shown in Table 5, no detectable amount of Mg was present at the outermost surface. The solubility of Mg in water at neutral pH is much higher than that of Al [36]. Mg species that have diffused to the surface by the hydration may therefore go into the water solution and not precipitate onto the surface, as Al does. This may
The high-energy Al, Mg and 0 KL,L, Auger lines were used in the high-resolution spectra to the depth profiles shown in Fig. 8. As expected, the concentration of Mg in the original oxide was highest at the outermost surface, as shown in Fig. 8a. The slow decline of Mg, the relatively low Al concentration and the slow increase in the Al concentration in the beginning of the depth profile indicate a duplex MgO/Al,O, oxide. The total thickness of t,he original oxide was estimated t,o be about 90 A, using an etch rate of 15.5 A/min. Using AJZS (Figs. 8b-8d) and XPS (Table 61, the Mg concentration in the outermost surface was observed to decrease after immersion in boil-
I
I 0
271
Surface Science 74 (1994) 263-275
5
IO
15
10
Etching time (min)
20
40
60
Etching time (min)
Fig. 8. AES depth profiles
20
Etching time (min)
0
30
60
Etching time (min)
for sample
H hydrated
for: (a) 0 s, (b) 10 s, (c) 20 s and (d) 40 s.
A. S&h,
272
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
Table 6 Results from AES depth profiles ’ and Mg concentration determined by XPS of hydrated aluminium, sample H Hydration time
Oxide thickness
Mg,,,
(s)
(A,
(A,
0 5 10 20 40 60
90 130 270 450 750 1340
0 60 170 300 510 1150
Area of Mg peak ’ (arbitrary units)
Mg content (XPS %)
0.52 0.58 0.49 0.52 0.65 0.56
22.2 4.7 1.8 < 0.1 < 0.1 < 0.1
* The AES results represent the mean from two measurements. ’ The average value and standard deviation of all area measurements is 0.55 kO.17.
ing water. However, the AES profiles revealed that Mg remains in the sublayer of the hydrated oxides. At the same depth as the Mg signal began to increase, the intensity of the Al signal decreased, as shown in Figs. 8b-8d for samples hydrated for 10, 20 and 40 s. A broadening of the Mg peak occurred for the longest hydration times. This broadening may be the result of many factors, such as sputtering-induced effects [38,391, variations of the oxide thickness, a real effect of magnesium diffusion or dilution with hydrated Al species that probably diffuse through the Mg-rich layer. Table 6 summarizes the estimated oxide thickness, position of Mg maximum, area of Mg peak and the Mg concentration, determined by XPS, for sample H hydrated for 5-60 s. In Fig. 9, the IR absorbance of the Al-OH group at 1080 cm-’ is plotted as a function of the growth of the hydrated oxide. In accordance with the LambertBeer law, a relatively good linear relation was obtained between the hydrated oxide thickness and the IR absorbance. The plot shown in Fig. 9 can be used as a calibration line to determine the oxide thickness for the other samples by using the absorbance values shown in Fig. 2. The results shown in Table 6 demonstrate that the Mg maximum approximately appears at a thickness corresponding to that of the hydrated layer. The areas of the Mg peaks (obtained by cutting and weighing) is a measure of the total Mg present, and
indicate that the total amount of Mg did not decrease upon hydration. The reported values of the areas are the mean from two measurements and, as shown in the footnote to Table 6, a large scatter was obtained in these data. The results shown in Table 6 indicate that MgO (and/or MgAl,O,) is not dissolved by the boiling water. Therefore, dissolution of MgO cannot explain the higher hydration rate for the samples with high Mg concentrations. 3.5. Morphology and hydration of Al-MS oxides Wefers [251 and Lea and Ball 1261 studied and reviewed the oxidation mechanisms for Mg-containing Al alloys. The oxide surface on pure aluminium is mainly amorphous [321 but, when heated above about 350°C it attains a cubic crystalline gamma form [40]. For Mg-containing Al alloys, the oxide films formed at high temperatures become Mg-rich because the activation energy for diffusion of Mg is lower than Al in both the metal [41,42] and the oxide film 1431. When Mg is present in the oxide, it facilitates nucleation of y-crystallites [44], most probably through the formation of short-range spine1 domains. When these crystallites are formed in the amorphous matrix, the continuity of the film is disrupted, which opens channels to the underlying metal. Mg can then diffuse to the free surface, by _
,
0.15
o.oo
L
0
500 Thickness
1000
1500
(A)
Fig. 9. Relation between the IR absorbance at 1080 cm the thickness of the pseudoboehmite layer determined AES for sample H.
’ and by
A. Strh,
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
I. Amorphous oxide on pure Al or cold rolled AI-MS alloy.
II. Crystalline oxide formed in the amorphous matrix, open pathways in the AI-MS oxide.
Al
M9
III. mick layer of secondary crystalline oxide (MgO, MgAl204) which slows down the metal/oxygen reaction.
Fig. 10. Morphologies for Al-Mg oxides at different stages, according to Wefers [251.
oxidation
diffusion along the interface boundaries between the crystalline and amorphous AI,O,. This process will produce a fine network structure of crystalline MgO in the oxide [45]. An MgO layer may also be enriched and cover the outer surface partially or entirely, depending on the original Mg concentration and the thermal history (time and temperature). A model of morphologies of Al-Mg oxides at three different oxidation stages has been proposed by Wefers [25], as shown in Fig. 10. The AES profile of sample H and the XPS data of the other samples indicate that the morphologies of the oxide films, when Mg is detected on the surface, are like structure II, between II and III, or possibly III, as shown in Fig. 10. The precipitation of the hydrated Al species on top of the original oxide, as shown by AES, and the fact that no dissolution of MgO was observed, indicate that diffusion of Al species takes place through the oxide. At hydration, the initial diffusion of Al species through the disrupted Al-Mg oxide is probably facilitated by the channels formed after the gamma crystallization. In the channels, the diffusion of Al species probably takes place at the interface boundaries between MgO and Al,O,. At the surface, these bound-
273
aries may increase the number of nucleation sites and nucleation rate, which probably is the most important factor influencing the initial hydration. This proposal is supported by the SEM micrographs in Fig. 4. A structure similar to III, as shown in Fig. 10, is possible for the highest Mg concentrations on the surface (H and heat-treated G). The total thickness of these oxides is greater, which leads to longer diffusion paths for Al through the oxide. The outermost MgO layer of these films may also delay the nucleation and the diffusion through the oxide. This may explain the existence of an optimal Mg concentration, leading to the highest hydration rates. 3.6. Relation to adhesion In our previous reports on adhesion between ethylene copolymers and hydrated aluminium, samples C [14,151 and E [13] were used. To remove the rolling oil and contamination, the foils were annealed 16 h at 300°C before hydration or lamination. By this thermal treatment, < 0.1% and 6.8% Mg on the surface was obtained for C and E, respectively. For laminates with hydrated Al, the quicker hydration rate of sample E led to improved adhesion at shorter hydration times than for sample C. After hydration a cohesive failure in the polymer was obtained for both Al samples with LDPE as well as for an ethylenebutyl acrylate copolymer. This indicates that the strength of the hydrated oxide is sufficient, at least for coating and paint applications. An Mgenriched oxide consequently increases the rate of hydration, which might be useful in a process for hydrating aluminium on an industrial scale. The environmental durability of adhesivealuminium joints is very low, especially for Al-Mg alloys, unless the surface has been pretreated [46-481. Many mechanisms have been used to explain the degradation of the bond strength. The most probable seems to be that a gelatinous boehmite layer is formed on the aluminium surface. This hydrated oxide (formed at lower temperatures) is mechanically weak, adheres poorly to the substrate and may cause a volume increase at the interface, which can lead to interfacial stresses and cracks. The heat used for curing
274
A. St&n,
T Hjertherg /Applied
Swface Science 74 (1994) 263-275
many adhesives, enriches Mg to the surface. The mechanism for hydration at different temperatures probably proceeds in a similar way initially. The proposed mechanism for the increased hydration rate of Mg-enriched oxides, may explain the poor durability in humid environments for joints with untreated Al-Mg alloys. Hydration for one minute has, however, been shown to be a useful method for preventing the environmental attack in extrusion-coated laminates [15].
probably the most important factor influencing the length of the induction period. The suggested mechanism for rapid hydration of Mg-enriched Al oxides in boiling water is probably also true for hydration at lower temperatures. This may explain the low durability in humid environments of adhesive joints with untreated Al-Mg alloys.
4. Conclusions
The authors would like to thank Lars Gstensson at Granges Technology Centre (SEMI, Anne Wendel (XPS), and Thomas Tunberg at the Department of Engineering Metals V&S) for their assistance with the analysis instruments. The financial support provided by the National Swedish Board for Industrial and Technical Development, Granges AB, Neste Polyethylene AB, and Tetra Pak AB is also gratefully acknowledged.
The initial hydration rate of aluminium in boiling water increases as the Mg concentration in the surface oxide increases. By the enrichment of Mg to the surface by thermal treatments at 300°C a relation between the hydration rate and the Mg concentration on the surface was obtained. For the highest Mg concentrations, thick duplex oxides lead to longer diffusion paths, and the outermost layer of MgO may decrease the rates for diffusion of water and delay nucleation of the hydration reaction. For that reason, an optimal Mg concentration yields the highest reaction rates. For a sample with 1.3% Mg in the bulk, the highest reaction rates were obtained at a relatively wide interval of about lo%-20% Mg (determined by XPS) on the surface. A suitably Mg-enriched Al surface can therefore be used to increase the initial hydration rate, which might be useful in producing aluminium substrates for adhesive bonding. After hydration Mg, enriched at the surface, remains at the metal/ hydrated oxide interface. Thus dissolution of MgO into the boiling water does not occur. High Mg concentrations in the oxide induce gamma crystallization at elevated temperatures, which opens channels to the underlying metal and forms a network structure of MgO and MgAI,O,. This disrupted oxide morphology is suggested to increase the initial hydration rate by an increased initial diffusion rate of Al species through the oxide. At the outermost surface, the interface boundaries between MgO and Al,O, are suggested to increase the number of nucleation sites and the nucleation rate. This is
5. Acknowledgements
6. References 111R.S. Alwitt, in: Oxides and Oxide Films, Eds. J.W. Diggle and A.K. Vijh (Dekker, New York, 1976) pp. 169-254. [2] R.S. Alwitt, J. Electrochem. Sot. 121 (1974) 1322. [3] W.J. Bernard and J.J. Randall, J. Electrochem. Sot. 107 (1960) 483. [4] W. Vedder and D.A. Vermilyea, Trans. Faraday Sot. 65 (1969) 561. [5] B.R. Baker and J.D. Balser, Aluminium 52 (1976) 197. [6] V.A. Zhuravlev and P.A. Zakharov, translated from Dokl. Akad. Nauk SSSR 254 (1980) 1155. [7] B.R. Baker, J.D. Balser, R.M. Pearson and E.O. Strahl, Aluminium 52 (1976) 241. [8] K. Wefers, Aluminium 49 (1973) 532, 622. [9] S. Wernick, R. Pinner and P.G. Sheasby, The Surface Treatment and Finishing of Aluminium and its Alloys, 5th ed., Vol. 2 (Finishing, Teddington, 1987) pp. 773-855. [lo] D.G. Altenpohl, Corrosion 18 (1962) 143t. [ll] T. Uchiyama, E. Isoyama, H. Takenara and Y. Kato, Keikinzoku 35 (1985) 29. [12] R. Bainbridge, P. Lewis and J.M. Sykes, Int. J. Adhes. Adhes. 2 11982) 175. [13] A. Strflin and T. Hjertberg, J. Appl. Polym. Sci. 49 (1993) 511. [14] A. St&in and T. Hjertberg, J. Adhes. 41 (1993) 51. [15] A. St&in and T. Hjertberg, J. Adhes. Sci. Technol., in press. [16] R.S. Alwitt, J. Electrochem. 114 (1967) 843.
A. Str&,
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
[17] A. S&lin and T. Hjertberg, J. Adhes. Sci. Technol. 6 (1992) 1233. [18] I. Olefjord, H.J. Mathieu and P. Marcus, Surf. Interf. Anal. 15 (1990) 681. [19] H. Ishida, Rubber Chem. Technol. 60 (1987) 497. 1201 M.D. Porter, Anal. Chem. 60 (1988) 1143A. [21] F.J. Boerio and C.A. Gosselin, in: Advances in Chemistry Series, Vol. 203, Ed. C.D. Craver (American Chemical Society, Washington, DC, 1983) pp. 541-558. [22] L. Vlaev, D. Damyanov and M.M. Mohamed, Colloids Surf. 36 (1989) 427. [23] N.C. Halder, J. Alonso, Jr. and W.E. Swartz, Jr., Z. Naturforsch. 30a (1975) 1485. [24] C.D. Wagner, in: Practical Surface Analysis, Eds. D. Briggs and M.P. Seah (Wiley, Chichester, 1983) p. 486. [25] K. Wefers, Aluminium 57 (1981) 722. [26] C. Lea and J. Ball, Appl. Surf. Sci. 17 (1984) 344. [27] R.K. Wisvanadham, T.S. Sun and J.A.S. Green, Corrosion 36 (1980) 275. 1281 T.S. Sun, J.M. Chen and J.D. Venables, Appl. Surf. Sci. 1 (1978) 202. [29] I. Olefjord and A. Karlsson, in: Aluminium Technology 86, Ed. T. Sheppard (The Institute of Metals, London, 19861 pp. 383-391. [30] T.S. Sun, J.M. Chen, R.K. Wisvanadham and J.A.S. Green, J. Vat. Sci. Technol. 16 (1979) 668. [31] B. Goldstein and J. Dresner, Surf. Sci. 71 (1978) 15.
275
[32] I.M. Ritchie, J.V. Sanders and P.L. Wecikhardt, Oxid. Met. 3 (1971) 91. [33] G.M. Scamans and E.P. Butler, Met. Trans. 6A (1975) 2055. [34] J.C. Fuggle, L.M. Watson, D.J. Fabian and S. Affrosman, Surf. Sci. 49 (1975) 61. [35] N.A. Thorne, P. Thuery, A. Frichet, P. Gimenez and A. Sartre, Surf. Interf. Anal. 16 (1990) 236. [36] M. Pourbaix, Atlas of Electrochemical Equilibria in Acqeous Solutions (National Association of Corrosion Engineers, Houston, TX, 1974)‘~~. 139, 168. [37] R.C. Weast, Ed., Handbook of Chemistry and Physics (CRC Press, Boca Raton, FL, 1982). 1381 M.P. Sheah and C. Lea, Thin Solid Films 81 (1981) 257. [39] J.W. Coburn, J. Vat. Sci. Technol. 13 (1976) 1037. [40] R.K. Hart and J.K. Maurin, Surf. Sci. 20 (1970) 285. [41] T. Federighi, Phil. Mag. 4 (1959) 502. [42] M. Bishop and K.E. Fletcher, Int. Met. Rev. 17 (1972) 203. [43] C. Lea and C. Molinari, J. Mater. Sci. 19 (19841 2336. [44] M.J. Dignam, J. Electrochem. Sot. 109 (1962) 184. [45] A.J. Brock and M.A. Heine, J. Electrochem. Sot. 119 (1972) 1124. [46] A.J. Kinloch, Ed., Durability of Structural Adhesives (Applied Science, London, 1983). [47] A.J. Kinloch, J. Adhes. 14 (1982) 105. [48] J.D. Venables, J. Mater. Sci. 19 (19841 2431.
Applied Surface Science 74 (1994) 263-275
Influence of surface composition on initial hydration of aluminium in boiling water Anders
Strfilin, Thomas
Hjertberg
*
Department of Polymer Technology, Chalmers University of Technology, 412 96 Gijteborg, Sweden
(Received 27 September 1993; accepted for publication 22 November 1993)
Abstract Eight aluminium samples were hydrated in boiling water for 5-60 s, which produces an adherent and porous pseudoboehmite (AlOOH) film on the surface. The progress of the hydration reaction was followed by IR reflection-absorption spectroscopy and a delay was observed owing to an induction period. The surface composition of the alloying elements was determined by XPS. AES depth profiling was used to analyse the sublayers of the hydrated oxide. Aluminium samples with Mg-enriched surface oxides were found to have the shortest induction periods, i.e. a shorter hydration time was required to obtain a certain thickness of the pseudoboehmite layer. The highest initial hydration rates were obtained in the interval lo%-20% Mg (determined by XPS) on the surface. XPS and AES analysis revealed that the hydrated oxide layer did not contain any Mg. By AES depth profiling Mg was
found to be present at the metal/hydrated oxide interface. This indicates that Mg (as MgO or MgAl,OJ in the surface oxide is not dissolved by boiling water. The higher reaction rate for the Mg-enriched surfaces is suggested to depend on the disrupted Al-Mg oxide morphology, which probably increases the rate of diffusion and transport of soluble species. The Al-Mg oxide is also suggested to increase the number of nucleation sites and the nucleation rate.
1. Introduction The reaction between aluminium and water can form many types of hydrous oxides, which has been reviewed extensively by Alwitt [l]. The structure and composition of the oxide films formed in water depends primarily on the reaction temperature. Between 20 and 90°C duplex films are formed, consisting of a pseudoboehmite layer (which is the initially formed product) next
* Corresponding
author.
0169-4332/94/$07.00 SD1
0 1994 Elsevier
0169-4332(93)E0309-A
Science
to the metal and a layer of bayerite in contact with water. At 90-lOo”C, an adherent layer of pseudoboehmite is formed, which is an oxyhydroxide that contains physically adsorbed water. From 100 to 374”C, pure boehmite is the only oxide phase formed. At or above the critical temperature, 374”, no hydrous oxide is formed, but formation of y-aluminium or corundum occurs. When aluminium is immersed in boiling water, the reaction can be followed by the evolution of hydrogen gas on the surface. This treatment topographically changes the surface to a rough
B.V. All rights reserved
264
A. St&n,
T. Hjertberg /Applied
porous surface, and chemically changes it to an oxyhydroxide according to the reaction below: Al+2H,O+4lOOH+;H,. The reaction is delayed by an induction period, owing to the presence of the protecting natural oxide film. The length of this induction period has been reported to vary with the water temperature and the thickness of the oxide film [2,3]. Bernard and Randall [3] suggested that the induction period is related to the water diffusion through the primary oxide film that is always present on the aluminium surface. Vedder and Vermilyea [4] suggested that the outer surface of the oxide is first hydrolysed and then dissolves to yield soluble species. These either remain in the solution or, at intermediate pH, precipitate as a porous oxide onto the surface. They also suggested that the rate of the hydration reaction was controlled by the dissolution of the primary oxide, diffusion of soluble products, and the disposition of the soluble products. Alwitt [2] suggested that the kinetics of the film growth is initially determined by nucleation and growth of hydrolysis sites on the oxide surface. He considered the rate-limiting step initially to be the diffusion of water into the surface oxide. As the oxide thickness increases, the rate control shifts to solid-state diffusion through the pseudoboehmite structure (probably of water diffusing inward as H+ and OH-). Baker and Balser [5] also studied the aluminium-water reaction and agreed with Alwitt’s mechanism for film formation on most points. However, they suggested that the reaction rate at longer hydration times is controlled by a denser, less soluble and less permeable barrier layer formed next to the metal. A study of the induction period by Zhuravlev and Zakharov [6] also emphasized the importance of the diffusion of water and dissolution of the oxide film in the initial stages of the hydration. Pseudoboehmite films produced on aluminium surfaces are important in a number of diverse applications, e.g.: sealing of anodic oxides [7-91; preventing corrosion of the aluminium surface [lo]; as a substrate for adhesive bonding or coating [l l-151; and in the manufacture of elec-
Surface Science 74 (1994) 263-275
trolytic capacitors [16]. In our previous work, improved adhesion was obtained in hot-pressed laminates with hydrated aluminium for LDPE, and for polar ethylene copolymers [13], as well as for ethylene copolymers containing methoxy silane groups [14]. The topographical change to a porous surface was suggested to improve the adhesion by increased surface area and mechanical keying. Strong interfacial interactions through hydrogen bonding and carboxylate formation have been shown to form for ester copolymers and acrylic acid copolymers [17], and also contribute to the improved adhesion after hydration. In another work, the adhesion between LDPE and hydrated aluminium in extrusion-coated laminates was shown to be influenced by the melt viscososity [151. When penetration into the pores and wetting of the surface was obtained in the extrusion-coated laminates, the adhesion between oxidized LDPE and hydrated aluminium was superior after ageing in acetic acid solutions as compared with untreated aluminium. This was explained to be the result of the adhesion mechanisms described above, and of the much better corrosion resistance of the hydrated aluminium surfaces. Different aluminium samples were used in the previous studies, and the length of the induction period was observed to vary. To produce a hydrated foil for extrusion coating in a continuous process, it is necessary that the induction period is short. Baker and Balser [5] reported that the alloy composition had very little effect on the hydration rate and film characteristics for NaOH etched samples with relatively long hydration times. In the present study, a significant relation to the surface composition was observed in the initial stage of the hydration reaction. The reaction rate was followed by IR reflection-absorption spectroscopy. The relation between the hydration rate and the bulk composition, as well as the surface composition determined by XPS, are discussed for eight samples. AES depth profiling was used to determine the thickness of the hydrated oxide and to estimate the composition at the metal/ hydrated oxide interface. The increased initial hydration rates obtained for Al-Mg oxides, and the relation to adhesion and durabil-
A. S&h,
ity of adhesive also discussed.
joints
2. Experimental
in humid
T Hjertberg /Applied Surface Science 74 (1994) 263-275
environments
are
details
2.1. Materials Eight aluminium samples from the 1000, 3000, 6000 and 8000 series were kindly supplied by Granges AB. Table 1 shows the bulk composition and the designation of the different materials. The samples were designated with A-H after the Mg concentration in the bulk. All materials have been inter-annealed and final-annealed. Samples A, B, C and E were final-annealed at the production plant and the others were final-annealed in a laboratory oven for 1 h at 300°C. The aluminium samples were hydrated by immersion in a Pyrex beaker containing de-ionized boiling water. 2.2. Bulk analysis The bulk composition of the elements were obtained from Granges Technology Centre. The Mg, Cu and Mn concentrations reported in ppm were analysed by ICP-AES on a Philips PV8050. Before the measurement, the samples were dissolved in hydrochloric acid with hydrogen peroxide. The content of Fe, Si and the other elements reported in % were determined by spark excitation optical emission spectrometry (OES) on a Spectrolab M5 (Spectra Analytical Instruments). 2.3. IR analysis The hydration spectroscopy on
reaction was followed a Perkin-Elmer 2000
by IR FT-IR
265
equipped with a liquid-nitrogen-cooled MCT detector. To reduce water vapour in the background, the spectrophotometer was purged with dry air. Samples of 2.5 X 5 cm were analysed on a reflection-absorption spectroscopy attachment (Spectratech FT-80) with a fixed angle of incidence of 80”. The spectral resolution was 4 cm-’ and the reported spectra represent the average of 50 scans. 2.4. SEM analysis A JEOL JSM-840 scanning electron microscope was used to study the topographical changes on the aluminium surface after hydration. 2.5. XPS analysis The surface composition of the samples before and after hydration was determined by XPS on a Perkin-Elmer PHI 5500. A Mg Ka X-ray source was used and the take-off angle was 45”. The analysed area had a diameter of 0.8 mm avd the information depth is approximately 50 A. To evaluate the surface composition, the PHI sensitivity factors given for the Mg Ka X-ray source in the instructors manual were used. 2.6. AES analysis The composition and thickness of the hydrated oxides were obtained by AES depth profiling with a Perkin-Elmer PHI 660. The depth profiles were obtained by using AES in conjunction with continuous ion etching by Art bombardment. The acceleration voltage of the ion gun was 2 kV and a rastered Ar+ beam was used to improve the accuracy of the ion etching. The analysed surface
Table 1 Composition in % of the bulk
Fe Mg Mn Si cu
A AA 1050
B AA 8079 +Fe
C AA 1200
D AA 3003
E AA 8016
F AA 6063
G AA 3004
H AA 3104
0.25 10 ppm 26 ppm 0.05 10 ppm
1.49 10 ppm 0.01 0.11 < 0.01
0.82 19 ppm 40 ppm 0.06 4 ppm
0.54 0.01 1.1 0.16 0.15
1.01 0.045 0.15 0.14 0.012
0.24 0.69 0.06 0.44 < 0.01
0.44 1.3 1.0 0.18 0.17
0.45 1.3 0.92 0.22 0.12
266
A. Str&n,
T. Hjertberg /Applied
area was approximately 5 X 5 pm and a 10 kV electron beam with a current density of 0.5 PA was used for excitation. In order to quantify the sputtered depth, it is important to know the sputter yield of the samples. For the equipment used, the etching rate was calibrated using a 1000 A thick tantalum oxide sample and was found to be 22 A/min. The etching rate of Al,O, has been reported to be 70% of that of Ta,O,Jl8]. In the present study, one etching rate (15.5 A/min) was assumed to be valid for all samples, despite the fact that the oxide structure is probably duplex and contains many different oxide types such as Al,O, (amorphous and crystalline), MgO, MgAl,O, (spine11 and AlOOH. For that reason, there is an uncertainty in the thicknesses reported.
3. Results
and discussion
3.1. IR analysis IR reflection-absorption spectroscopy (RAS) with large, almost grazing angles of incidence, is a quick, sensitive and well suited method for studying chemical conversions of the aluminium surface [19-211. Fig. 1 shows the IR spectra for sample E: (A) untreated and (B) hydrated for 40 s in boiling water. Similar spectra were obtained for all other samples. The assignments for the spectra are shown in Table 2. Only one absorbance appears for untreated aluminium, that is a bending mode of Al-O-Al in the thin, thermally oxidized surface layer. After hydration, many new absorbances arise, the most characteristic appearing at 1080 cm-‘, owing to bending deformations of the hydroxyl groups. Absorbances also arise at 1700-1300 cm-‘, owing to physically adsorbed and coordinatively bonded water molecules [22], and at 3700-3000 cm-‘, owing to O-H stretching in hydroxyl groups and in water. The absorbance of the characteristic Al-OH group at 1080 cm-’ (measured as the difference between the absorbance at 1080 cm-’ and the baseline around 1300 cm-‘) was used to follow
Surface Science 74 (1994) 263-275
1 -I
d
Wavenumber Fig. 1. IR spectra drated for 40 s.
of sample
(cm-‘)
E, (A) untreated
and (B) hy-
the initial stage of the hydration reaction. In Fig. 2, the absorbance of this group is shown as a function of hydration time in boiling water for all samples. In the beginning, no hydration was obtained owing to the induction period. The length of the induction period varied considerably among the samples, and they could be divided into four groups with decreasing induction times, as follows: A=B=C>D>E=F>G=H. When the length of the induction period is compared with the concentration of the elements in the bulk, there seems to be a good relation to Mg. After the induction period, the reaction is rapid for the first 30 s. Thereafter, the hydration rate decreases, because an increased thickness and density of the hydrated film probably decreases the rates of diffusion of water and transport of soluble products. The changes in the reaction rate could also be identified by visual observation of the evolution of the hydrogen gas on the surface.
Table 2 Assignment Position
(cm
960 1080 1700-1300 3700-3000 a Assignments
for IR spectra
‘)
shown in Fig. 1 ’
Assignment
Spectrum
Al-O-Al bend Al-OH bend H-O-H bend O-H stretch
A B B B
are based
on Refs. [1,22].
267
A. Str&n, T. Hjertberg /Applied Surface Science 74 (1994) 263-275
.
0 sample A l sample B v sample C v sample D
20
0
._
40
tXJ
0
Hydration time (s) Fig. 2. IR absorbance
at 1080 cm-’
40
20 Hydration
as a function
of hydration
3.2. SEA4 analysis
The surface topography changes simultaneously to the chemical conversion. This was studied with SEM, and the result for sample G is shown in Figs. 3 and 4. At 60 s, a fully developed hydrated oxide was obtained. This hydrated oxide is coarse and has cavities of an approximate diameter of 0.1 Frn. At 60 s hydration time, oxide structures similar to that shown in Fig. 3b were obtained for all samples. At short hydration times or in the initial stage of the hydration, a significant difference of the surface topography was observed between the samples with rapid and slow hydration rates. This is illustrated by SEM micrographs of samples C and G in Fig. 4. In the micrograph of sample C
Fig. 3. SEM micrographs
of sample
time for: (a) samples
A-D,
60
time (s)
(b) samples
E-H.
hydrated for 15 s (not shown), it was possible to observe traces of hydrated species sparsely distributed on the surface. After 17.5 s, the quantity of protruding hydrated oxide increased, but the structure was still relatively sparse, see Fig. 4a. For a longer hydration time, 20 s, a higher extent of the reaction led to a denser and thicker layer of hydrated oxide, as shown in Fig. 4b. The micrograph of sample G immersed in boiling water for only 1 s shows a thin but dense hydrated structure with a crackled appearance, see Fig. 4c. After 2 s hydration, the micrograph, Fig. 4d, shows a similar, but somewhat rougher and thicker oxide having more “depth”. The denser hydrated structure of sample G with a short induction period, indicates a larger quantity of nucleation sites on the surface. This may explain the
G: (a) untreated
and (b) hydrated
for 60 s.
A. Str&n,
268
Fig. 4. SEM micrographs
of sample
7: Hjertherg /Applied
C hydrated
for: (a) 17.5 s and (b) 20 s, and of sample
high reactivity for samples G and H, because the nucleation is very important in the initial stage of the hydration reaction. 3.3. XPS analysis Because the hydration reaction gested to start with the diffusion Table 3 Surface composition
Al (%I) Mg (%) 0 (%I
determined
has been of water
sugand
G hydrated
for: (c) 1 s and (d) 2 s.
dissolution of the primary oxide, the characteristics of the original surface oxide are important. The results of the XPS analysis of the original oxides are shown for all samples in Table 3. Of the elements in the bulk, only Mg was detectable within the information depth of XPS. The evaluation of the chemical composition in the oxide was obtained by XPS data of the Al2p, Mg2p and
by XPS
A
B
C
D
E
F
G
H
34.9 < 0.1 65.1
36.1 < 0.1 63.7
36.0 < 0.1 64.0
34.4 0.8 64.7
31.0 4.2 64.8
31.1 5.0 63.8
28.8 7.4 63.9
16.4 21.3 62.3
C-fraction I(AI’+)/I(Alm) “ Al”
Surface Science 74 (1994) 2636275
was not measurable.
0.068 14
0.059 7.7
0.089 12
0.094 17
0.13 7.1
0.084 13
0.13 7.9
0.11 a
A. Str&, 0.15
. 0
Hydrated Hydrated
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
method decreases and, for that reason, we have not reported the calculated oxide thicknesses. The results of the Z(A13’>/Z(Alm> ratio shown in Table 3 can, however, be used as a relative estimation of the oxide thickness. For the samples used in the present study, no significant correlation could be found between the Z(A13’>/Z(Al”> ratio and the hydration rate. A high C-fraction, as a consequence of remaining rolling oil or contamination left on the surface, may reduce the water wettability and delay the hydration reaction. The values of the C-fraction obtained, Table 3, are quite similar, and the differences observed do not seem to have any influence on the hydration rate. The length of the induction period must thus depend on the elemental composition, but only Mg (except Al) was found in detectable amounts on the surface. As for aluminium, the Mg2p signal of metal (50 eV) and oxide (+ 1.5 eV> are separated [23,241. The position (around 52 eV> and the shape of the Mg2p signals indicate that Mg is present on the surface only as oxide. The relation between the concentration of Mg on the surface and the length of the induction period or the absorbance of Al-OH at short hydration times seems to be good, as shown in Fig. 5. It is possible by thermal treatments to increase the Mg concentration in the surface oxide [25-311. For samples D, E and G, the surface concentration of Mg was changed by an additional heat treatment at 300°C for l-72 h. The increase in
40 seconds 20 seconds
iP
o.oo+
’ 10
5
’
’
15
20
Mg concentration
25
(%)
Fig. 5. IR absorbance at 1080 cm-’ as a function concentration on the surface for samples A-H.
269
of Mg
0 1s signals only. The fraction of the C 1s signal is presented separately. Bernard and Randall [3] reported that the length of the induction period is strongly dependent on the oxide thickness. Because XPS is sensitive to the chemical environment, the Al2p signal of the oxide and metal is separated by 2.8 + 0.1 eV. As shown by Olefjord et al. [181, the oxide thickness can be determined from XPS data, by means of the ZL4l”+)/Z(Al”) ratio. The samples used in the present study were oxidized thermally at elevated temperatures, which yielded relatively thick oxides, considering the XPS information depth. When Z(A13’>/Z(Al”) is 10 at a take-of: angle of 45”, it corresponds to a thickness of 3.5 A. For higher ratios, the accuracy of the
7
[
$$$$g I Oh
1
I/ 538
536
534
532
530
528
80
78
,, 76
74
,, 72
70
56
54
52
50
46
Binding energy (eV) Fig. 6. High resolution (normalized) XPS spectra of the 0 Is, Al 2p and Mg2p signals of sample at 300°C. The corresponding C 1s signals appeared at 287.1 f 0.1 eV (see also Table 4).
G heat-treated
for 0, 3, 24 and 72 h
A. St&in,
270
T. Hjertberg/Applied
the surface concentration of Mg is shown for sample G in Table 4. The long treatments of this sample led to a new appearance of the Al2p and 0 1s signals, as shown in Fig. 6. After 3 h, the oxide thickness increased, which is illustrated by the decrease in the Al” signal at 73.1 eV. When the sample was treated for 24 h, a shoulder appeared at a lower binding energy (at about 74.2 eV> on the Al2p signal and, after 72 h, the relative intensity of this new signal increased (to 44% of total Al2p). It has been suggested in many previous reports that Mg reduces Al,O, to Al metal according to the reactions below [30-331: 3 Mg + 4 Al,O,
(amorphous)
e 3 MgAl,O, 3 Mg + Al,O,
(amorphous)
Table 4 Surface composition
for sample
Treatment
C-fraction
010
r 0
0 A
. sample sample A sample
l
0 E G
l.
.
: ;
0.05
.
Lo
‘A . A
.
A . Filled hydrated .
.
.
A
:
20 s
A*
AA
A
.
2
1 .
0.00
0
goOBy 5
10 Mg
A
Hollow hydrated
15
concentration
20
10 s
4
25
(%)
Fig. 7. IR absorbance at 1080 cm-’ as a function of Mg concentration on the surface for samples D, E and G. Mg content on the surface was increased by thermal treatment at 300°C.
+ 2 Al, w 3 MgO + 2 Al.
On the basis of free-energy calculations, it has been proposed that the formation of spine1 MgAl,O, is most favourable [32], but high Mg : Al ratios in the oxide should favour the formation of MgO. The high Mg: Al ratio obtained for the oxides on sample G, heat-treated for 24 and 72 h (as well as for sample H), indicates that MgO is present on the surface. Using XPS, Sun [30] et al. showed that the signal of Al metal increased by heat treatments in vacuum of an Al-2.7%Mg2.3%Zn aIloy. The shoulder that appeared on the Al2p signal at about 74.2 eV indicates that Al,O, is reduced. We have not found any reported XPS data that could explain the A12p signal at 74.2 eV. According to the limited XPS data found for MgAl,O,, we suggest that the new signal can be related to this oxide type. The binding energy of the 0 1s signal in MgO has been reported to
Al (%I Mg (%) 0 (%)
Surface Science 74 (1994) 263-275
G, heat-treated
at 300°C
time (h)
0
3
24
72
28.8 7.4 63.9
25.7 10.0 64.3
16.5 19.7 63.8
14.8 24.7 60.5
0.13
0.14
0.14
0.11
appear at 531.0 eV [34] and that of Al,O, at 532.7 + 0.3 eV [181. The broadening and gradual change of the 0 Is signal can be explained by the gradual increase in the MgO and/or MgAl,O, concentration in the outermost oxide. For the Mg2p signal, only a minor increase of the fullwidth at half-maximum (from 1.85 to 2.0 eV> and a displacement of the maximum signal (from 52.3 to 52.0 eV> were observed after heat treatment for 72 h at 300°C. The XPS spectrum of sample H with an original surface concentration of 21.4% Mg had a similar appearance as the 72 h XPS spectrum of G shown in Fig. 6, which indicates a similar oxide structure present on its surface. After thermal treatment of samples D, E and G, and determination of the Mg concentration on the surface by XPS, hydration was performed in boiling water for 10 and 20 s. In Fig. 7, the absorbance of the hydroxyl group for these samples is plotted against the Mg concentration on the surface before hydration. Up to a surface concentration of 7%-10% Mg, the hydration level increases as the Mg concentration increases. The highest values of the IR absorbance were obtained at about lo%-20%, and the hydration rate in this interval was almost constant. At higher surface concentrations of Mg, the hydration rate seems to decrease. This can probably be explained by the fact that thicker oxides are formed for the longest thermal treatments (60 and 72 h at
A. Stra”lin, T. Hjertberg /Applied Table 5 Surface composition for samples with high Mg contents hydration for 60 s in boiling water, determined by XPS
Al (%) Mg (%I 0 (o/o) C-fraction
E
F
G
H
29.0 < 0.1 71.0
30.3 < 0.1 69.6
29.4 < 0.1 70.5
29.2
0.031
0.037
0.038
after
explain the fact that no Mg was detected on the hydrated surface. However, the solubility of oxidized Mg in water is very low (0.0086 g MgO/e in hot water) [37]. AES depth profiling of sample H was performed to study the composition of the MgO/MgAl,O, enriched oxide after hydration. 3.4. AES analysis
0.029
3OO”C), which prevents a continued rapid reaction. The surface composition of the samples with the highest Mg concentration (E-H) was determined by XPS after hydration for 60 s. A broadening and increase of the 0 1s signal was observed, owing to the introduction of hydroxyl[351. As shown in Table 5, no detectable amount of Mg was present at the outermost surface. The solubility of Mg in water at neutral pH is much higher than that of Al [36]. Mg species that have diffused to the surface by the hydration may therefore go into the water solution and not precipitate onto the surface, as Al does. This may
The high-energy Al, Mg and 0 KL,L, Auger lines were used in the high-resolution spectra to the depth profiles shown in Fig. 8. As expected, the concentration of Mg in the original oxide was highest at the outermost surface, as shown in Fig. 8a. The slow decline of Mg, the relatively low Al concentration and the slow increase in the Al concentration in the beginning of the depth profile indicate a duplex MgO/Al,O, oxide. The total thickness of t,he original oxide was estimated t,o be about 90 A, using an etch rate of 15.5 A/min. Using AJZS (Figs. 8b-8d) and XPS (Table 61, the Mg concentration in the outermost surface was observed to decrease after immersion in boil-
I
I 0
271
Surface Science 74 (1994) 263-275
5
IO
15
10
Etching time (min)
20
40
60
Etching time (min)
Fig. 8. AES depth profiles
20
Etching time (min)
0
30
60
Etching time (min)
for sample
H hydrated
for: (a) 0 s, (b) 10 s, (c) 20 s and (d) 40 s.
A. S&h,
272
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
Table 6 Results from AES depth profiles ’ and Mg concentration determined by XPS of hydrated aluminium, sample H Hydration time
Oxide thickness
Mg,,,
(s)
(A,
(A,
0 5 10 20 40 60
90 130 270 450 750 1340
0 60 170 300 510 1150
Area of Mg peak ’ (arbitrary units)
Mg content (XPS %)
0.52 0.58 0.49 0.52 0.65 0.56
22.2 4.7 1.8 < 0.1 < 0.1 < 0.1
* The AES results represent the mean from two measurements. ’ The average value and standard deviation of all area measurements is 0.55 kO.17.
ing water. However, the AES profiles revealed that Mg remains in the sublayer of the hydrated oxides. At the same depth as the Mg signal began to increase, the intensity of the Al signal decreased, as shown in Figs. 8b-8d for samples hydrated for 10, 20 and 40 s. A broadening of the Mg peak occurred for the longest hydration times. This broadening may be the result of many factors, such as sputtering-induced effects [38,391, variations of the oxide thickness, a real effect of magnesium diffusion or dilution with hydrated Al species that probably diffuse through the Mg-rich layer. Table 6 summarizes the estimated oxide thickness, position of Mg maximum, area of Mg peak and the Mg concentration, determined by XPS, for sample H hydrated for 5-60 s. In Fig. 9, the IR absorbance of the Al-OH group at 1080 cm-’ is plotted as a function of the growth of the hydrated oxide. In accordance with the LambertBeer law, a relatively good linear relation was obtained between the hydrated oxide thickness and the IR absorbance. The plot shown in Fig. 9 can be used as a calibration line to determine the oxide thickness for the other samples by using the absorbance values shown in Fig. 2. The results shown in Table 6 demonstrate that the Mg maximum approximately appears at a thickness corresponding to that of the hydrated layer. The areas of the Mg peaks (obtained by cutting and weighing) is a measure of the total Mg present, and
indicate that the total amount of Mg did not decrease upon hydration. The reported values of the areas are the mean from two measurements and, as shown in the footnote to Table 6, a large scatter was obtained in these data. The results shown in Table 6 indicate that MgO (and/or MgAl,O,) is not dissolved by the boiling water. Therefore, dissolution of MgO cannot explain the higher hydration rate for the samples with high Mg concentrations. 3.5. Morphology and hydration of Al-MS oxides Wefers [251 and Lea and Ball 1261 studied and reviewed the oxidation mechanisms for Mg-containing Al alloys. The oxide surface on pure aluminium is mainly amorphous [321 but, when heated above about 350°C it attains a cubic crystalline gamma form [40]. For Mg-containing Al alloys, the oxide films formed at high temperatures become Mg-rich because the activation energy for diffusion of Mg is lower than Al in both the metal [41,42] and the oxide film 1431. When Mg is present in the oxide, it facilitates nucleation of y-crystallites [44], most probably through the formation of short-range spine1 domains. When these crystallites are formed in the amorphous matrix, the continuity of the film is disrupted, which opens channels to the underlying metal. Mg can then diffuse to the free surface, by _
,
0.15
o.oo
L
0
500 Thickness
1000
1500
(A)
Fig. 9. Relation between the IR absorbance at 1080 cm the thickness of the pseudoboehmite layer determined AES for sample H.
’ and by
A. Strh,
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
I. Amorphous oxide on pure Al or cold rolled AI-MS alloy.
II. Crystalline oxide formed in the amorphous matrix, open pathways in the AI-MS oxide.
Al
M9
III. mick layer of secondary crystalline oxide (MgO, MgAl204) which slows down the metal/oxygen reaction.
Fig. 10. Morphologies for Al-Mg oxides at different stages, according to Wefers [251.
oxidation
diffusion along the interface boundaries between the crystalline and amorphous AI,O,. This process will produce a fine network structure of crystalline MgO in the oxide [45]. An MgO layer may also be enriched and cover the outer surface partially or entirely, depending on the original Mg concentration and the thermal history (time and temperature). A model of morphologies of Al-Mg oxides at three different oxidation stages has been proposed by Wefers [25], as shown in Fig. 10. The AES profile of sample H and the XPS data of the other samples indicate that the morphologies of the oxide films, when Mg is detected on the surface, are like structure II, between II and III, or possibly III, as shown in Fig. 10. The precipitation of the hydrated Al species on top of the original oxide, as shown by AES, and the fact that no dissolution of MgO was observed, indicate that diffusion of Al species takes place through the oxide. At hydration, the initial diffusion of Al species through the disrupted Al-Mg oxide is probably facilitated by the channels formed after the gamma crystallization. In the channels, the diffusion of Al species probably takes place at the interface boundaries between MgO and Al,O,. At the surface, these bound-
273
aries may increase the number of nucleation sites and nucleation rate, which probably is the most important factor influencing the initial hydration. This proposal is supported by the SEM micrographs in Fig. 4. A structure similar to III, as shown in Fig. 10, is possible for the highest Mg concentrations on the surface (H and heat-treated G). The total thickness of these oxides is greater, which leads to longer diffusion paths for Al through the oxide. The outermost MgO layer of these films may also delay the nucleation and the diffusion through the oxide. This may explain the existence of an optimal Mg concentration, leading to the highest hydration rates. 3.6. Relation to adhesion In our previous reports on adhesion between ethylene copolymers and hydrated aluminium, samples C [14,151 and E [13] were used. To remove the rolling oil and contamination, the foils were annealed 16 h at 300°C before hydration or lamination. By this thermal treatment, < 0.1% and 6.8% Mg on the surface was obtained for C and E, respectively. For laminates with hydrated Al, the quicker hydration rate of sample E led to improved adhesion at shorter hydration times than for sample C. After hydration a cohesive failure in the polymer was obtained for both Al samples with LDPE as well as for an ethylenebutyl acrylate copolymer. This indicates that the strength of the hydrated oxide is sufficient, at least for coating and paint applications. An Mgenriched oxide consequently increases the rate of hydration, which might be useful in a process for hydrating aluminium on an industrial scale. The environmental durability of adhesivealuminium joints is very low, especially for Al-Mg alloys, unless the surface has been pretreated [46-481. Many mechanisms have been used to explain the degradation of the bond strength. The most probable seems to be that a gelatinous boehmite layer is formed on the aluminium surface. This hydrated oxide (formed at lower temperatures) is mechanically weak, adheres poorly to the substrate and may cause a volume increase at the interface, which can lead to interfacial stresses and cracks. The heat used for curing
274
A. St&n,
T Hjertherg /Applied
Swface Science 74 (1994) 263-275
many adhesives, enriches Mg to the surface. The mechanism for hydration at different temperatures probably proceeds in a similar way initially. The proposed mechanism for the increased hydration rate of Mg-enriched oxides, may explain the poor durability in humid environments for joints with untreated Al-Mg alloys. Hydration for one minute has, however, been shown to be a useful method for preventing the environmental attack in extrusion-coated laminates [15].
probably the most important factor influencing the length of the induction period. The suggested mechanism for rapid hydration of Mg-enriched Al oxides in boiling water is probably also true for hydration at lower temperatures. This may explain the low durability in humid environments of adhesive joints with untreated Al-Mg alloys.
4. Conclusions
The authors would like to thank Lars Gstensson at Granges Technology Centre (SEMI, Anne Wendel (XPS), and Thomas Tunberg at the Department of Engineering Metals V&S) for their assistance with the analysis instruments. The financial support provided by the National Swedish Board for Industrial and Technical Development, Granges AB, Neste Polyethylene AB, and Tetra Pak AB is also gratefully acknowledged.
The initial hydration rate of aluminium in boiling water increases as the Mg concentration in the surface oxide increases. By the enrichment of Mg to the surface by thermal treatments at 300°C a relation between the hydration rate and the Mg concentration on the surface was obtained. For the highest Mg concentrations, thick duplex oxides lead to longer diffusion paths, and the outermost layer of MgO may decrease the rates for diffusion of water and delay nucleation of the hydration reaction. For that reason, an optimal Mg concentration yields the highest reaction rates. For a sample with 1.3% Mg in the bulk, the highest reaction rates were obtained at a relatively wide interval of about lo%-20% Mg (determined by XPS) on the surface. A suitably Mg-enriched Al surface can therefore be used to increase the initial hydration rate, which might be useful in producing aluminium substrates for adhesive bonding. After hydration Mg, enriched at the surface, remains at the metal/ hydrated oxide interface. Thus dissolution of MgO into the boiling water does not occur. High Mg concentrations in the oxide induce gamma crystallization at elevated temperatures, which opens channels to the underlying metal and forms a network structure of MgO and MgAI,O,. This disrupted oxide morphology is suggested to increase the initial hydration rate by an increased initial diffusion rate of Al species through the oxide. At the outermost surface, the interface boundaries between MgO and Al,O, are suggested to increase the number of nucleation sites and the nucleation rate. This is
5. Acknowledgements
6. References 111R.S. Alwitt, in: Oxides and Oxide Films, Eds. J.W. Diggle and A.K. Vijh (Dekker, New York, 1976) pp. 169-254. [2] R.S. Alwitt, J. Electrochem. Sot. 121 (1974) 1322. [3] W.J. Bernard and J.J. Randall, J. Electrochem. Sot. 107 (1960) 483. [4] W. Vedder and D.A. Vermilyea, Trans. Faraday Sot. 65 (1969) 561. [5] B.R. Baker and J.D. Balser, Aluminium 52 (1976) 197. [6] V.A. Zhuravlev and P.A. Zakharov, translated from Dokl. Akad. Nauk SSSR 254 (1980) 1155. [7] B.R. Baker, J.D. Balser, R.M. Pearson and E.O. Strahl, Aluminium 52 (1976) 241. [8] K. Wefers, Aluminium 49 (1973) 532, 622. [9] S. Wernick, R. Pinner and P.G. Sheasby, The Surface Treatment and Finishing of Aluminium and its Alloys, 5th ed., Vol. 2 (Finishing, Teddington, 1987) pp. 773-855. [lo] D.G. Altenpohl, Corrosion 18 (1962) 143t. [ll] T. Uchiyama, E. Isoyama, H. Takenara and Y. Kato, Keikinzoku 35 (1985) 29. [12] R. Bainbridge, P. Lewis and J.M. Sykes, Int. J. Adhes. Adhes. 2 11982) 175. [13] A. Strflin and T. Hjertberg, J. Appl. Polym. Sci. 49 (1993) 511. [14] A. St&in and T. Hjertberg, J. Adhes. 41 (1993) 51. [15] A. St&in and T. Hjertberg, J. Adhes. Sci. Technol., in press. [16] R.S. Alwitt, J. Electrochem. 114 (1967) 843.
A. Str&,
T. Hjertberg /Applied Surface Science 74 (1994) 263-275
[17] A. S&lin and T. Hjertberg, J. Adhes. Sci. Technol. 6 (1992) 1233. [18] I. Olefjord, H.J. Mathieu and P. Marcus, Surf. Interf. Anal. 15 (1990) 681. [19] H. Ishida, Rubber Chem. Technol. 60 (1987) 497. 1201 M.D. Porter, Anal. Chem. 60 (1988) 1143A. [21] F.J. Boerio and C.A. Gosselin, in: Advances in Chemistry Series, Vol. 203, Ed. C.D. Craver (American Chemical Society, Washington, DC, 1983) pp. 541-558. [22] L. Vlaev, D. Damyanov and M.M. Mohamed, Colloids Surf. 36 (1989) 427. [23] N.C. Halder, J. Alonso, Jr. and W.E. Swartz, Jr., Z. Naturforsch. 30a (1975) 1485. [24] C.D. Wagner, in: Practical Surface Analysis, Eds. D. Briggs and M.P. Seah (Wiley, Chichester, 1983) p. 486. [25] K. Wefers, Aluminium 57 (1981) 722. [26] C. Lea and J. Ball, Appl. Surf. Sci. 17 (1984) 344. [27] R.K. Wisvanadham, T.S. Sun and J.A.S. Green, Corrosion 36 (1980) 275. 1281 T.S. Sun, J.M. Chen and J.D. Venables, Appl. Surf. Sci. 1 (1978) 202. [29] I. Olefjord and A. Karlsson, in: Aluminium Technology 86, Ed. T. Sheppard (The Institute of Metals, London, 19861 pp. 383-391. [30] T.S. Sun, J.M. Chen, R.K. Wisvanadham and J.A.S. Green, J. Vat. Sci. Technol. 16 (1979) 668. [31] B. Goldstein and J. Dresner, Surf. Sci. 71 (1978) 15.
275
[32] I.M. Ritchie, J.V. Sanders and P.L. Wecikhardt, Oxid. Met. 3 (1971) 91. [33] G.M. Scamans and E.P. Butler, Met. Trans. 6A (1975) 2055. [34] J.C. Fuggle, L.M. Watson, D.J. Fabian and S. Affrosman, Surf. Sci. 49 (1975) 61. [35] N.A. Thorne, P. Thuery, A. Frichet, P. Gimenez and A. Sartre, Surf. Interf. Anal. 16 (1990) 236. [36] M. Pourbaix, Atlas of Electrochemical Equilibria in Acqeous Solutions (National Association of Corrosion Engineers, Houston, TX, 1974)‘~~. 139, 168. [37] R.C. Weast, Ed., Handbook of Chemistry and Physics (CRC Press, Boca Raton, FL, 1982). 1381 M.P. Sheah and C. Lea, Thin Solid Films 81 (1981) 257. [39] J.W. Coburn, J. Vat. Sci. Technol. 13 (1976) 1037. [40] R.K. Hart and J.K. Maurin, Surf. Sci. 20 (1970) 285. [41] T. Federighi, Phil. Mag. 4 (1959) 502. [42] M. Bishop and K.E. Fletcher, Int. Met. Rev. 17 (1972) 203. [43] C. Lea and C. Molinari, J. Mater. Sci. 19 (19841 2336. [44] M.J. Dignam, J. Electrochem. Sot. 109 (1962) 184. [45] A.J. Brock and M.A. Heine, J. Electrochem. Sot. 119 (1972) 1124. [46] A.J. Kinloch, Ed., Durability of Structural Adhesives (Applied Science, London, 1983). [47] A.J. Kinloch, J. Adhes. 14 (1982) 105. [48] J.D. Venables, J. Mater. Sci. 19 (19841 2431.