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Sealing of anodic films obtained in oxalic acid baths
Surface and Coatings Technology 124 (2000) 76–84 www.elsevier.nl/locate/surfcoat
Sealing of anodic films obtained in oxalic acid baths V. Lo´pez, E. Otero, A. Bautista *, J.A. Gonza´lez Centro Nacional de Investigaciones Metalu´rgicas (CSIC), Avda. Gregorio del Amo No 8, 28040-Madrid, Spain Received 5 August 1999; accepted in revised form 16 October 1999
Abstract The response to sealing quality control tests of anodic films obtained in oxalic and sulphuric acid baths has been compared. Unsealed films obtained in oxalic acid remain virtually unaltered in humid atmospheres, contrary to unsealed films obtained in sulphuric acid which tend to autoseal. The former also appear to take a shorter time to reach the quality thresholds of all the usual control tests on traditional sealing in boiling deionized water. However, after sealing, both types of films age in a qualitatively identical way. Transmission electron microscopy revealed the presence of a complex structure of hexagonal cells, comprising three distinct zones in the films formed in oxalic acid, the pores in which were found to be about five times larger in diameter than those in the films obtained in sulphuric acid. This considerably facilitates microstructural examination and elucidation of the underlying sealing mechanism. Furthermore, the transformation of the film morphology under the electron beam is slower in films obtained in oxalic acid. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Anodic films; Film morphology; Sealing quality control tests; Transmission electron microscopy
1. Introduction The need to anodize aluminium and its alloys is imposed by aesthetic and durability considerations. The process is usually conducted in a phosphoric, oxalic or sulphuric acid bath. All these electrolytes dissolve the alumina layers formed in the process, to some extent, thereby giving rise to coatings consisting of two different regions: namely, an inner, compact layer a few nanometres (or tens of nanometres) thick called the barrier layer; and an outer, porous layer consisting of hexagonal cells in a honeycomb arrangement where each cell has a central pore normal to the metal substrate. The porous layer is usually about 103–104 times thicker than the barrier layer [1]. Anodizing in a sulphuric acid bath is the procedure which is used in widest industrial for this purpose; alternative treatments are reserved for specific applications [1]. For example, anodizing in oxalic acid baths is primarily used to obtain relatively hard coatings of an appealing colour (a soft bronze or golden tone, depending on the particular alloy). The porous nature of these layers requires subsequent sealing, which is usually done in boiling deionized water; * Corresponding author. Tel.: +34-91-553-8900; fax: +34-91-534-7425.
the water shuts pores through partial conversion of anhydrous alumina into boehmite, which is more voluminous [2]. The sealing operation is of such a high technical significance that it is subjected to universally accepted and applied quality control tests [3]. Notwithstanding the more extensive use of sulphuric acid anodizing, many fundamental studies on the anodizing and sealing of aluminium and its alloys have been performed on layers obtained in oxalic or phosphoric acid, which produce much larger pores, thus facilitating the detection of microstructural changes caused by any influential factor [4–12]. In this situation, we believed it of interest to compare the response to sealing quality control tests of anodic films obtained in oxalic and sulphuric acids. Only a similarity between the two types of film would justify extrapolating the mechanisms derived from studies of the former to porous anodic films produced by sulphuric acid baths.
2. Material and methods 2.1. Material We used 5×10 cm aluminium specimens of commercial purity (99.5%). The plates were anodized under the
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recommended conditions of the Eloxal GX process using solutions containing 4% oxalic acid, d.c. densities of 1–2 A dm−2 and voltages of 40–60 V [1]. The initial temperature in the bath, 20°C, rose by 4–5°C during anodizing. The thickness of the oxide layers ranged from 8 to 45 mm and increased with increasing voltage (current density) and anodizing time; however, it tended to an asymptotic value as the latter variable was increased. Aluminium plates were also anodized in 18% sulphuric acid at 20°C, using d.c. densities of 1.5 A dm−2 for different times to obtain films of different thickness. Specimens of widely variable thickness obtained in oxalic acid were sealed in boiling deionized water for 60 min, which was deemed long enough to achieve a sealing quality meeting industrial requirements. A set of 8 mm thick specimens anodized in oxalic acid, and another set anodized in sulphuric acid, were sealed in water at boiling temperature for 1, 5, 10, 20, 45 or 60 min. 2.2. Test methods The standard acid dissolution in phosphochromic acid, admittance and dye spot tests were applied to specimens obtained under the different sealing conditions [3]. The results of these control tests were completed with impedance measurements made in an aerated 3% w/w K SO solution at 25±1°C. The frequencies 2 4 used ranged from 100 kHz to 1 mHz. The microstructure of each anodic film was examined by transmission electron microscopy ( TEM ) following stripping from the metal substrate by dissolution in mercuric chloride ˚ on an ion beam thinner. and thinning to 100–300 A
3. Results Based on Fig. 1, the quality thresholds imposed by industrial practice are more rapidly reached by anodic films obtained in an oxalic acid bath than in a sulphuric acid bath. For example, a spot strength below 2, the value set by the applicable standards [3], was obtained in the dye spot test after only 5 min of sealing [Fig. 1(a)]. The situation was similar for the phosphochromic acid dissolution [Fig. 1(b)] and the 1 kHz admittance test [Fig. 1(c)]. In the former, mass losses fell below the allowed threshold (30 mg/dm2) at 4 and 8 min of sealing in the oxalic and sulphuric bath, respectively; in the latter, the quality threshold was reached at admittances below 20 mS [3], and also within a shorter time in the oxalic acid bath. However, qualitative changes were identical with both types of film in all tests. The use of a much more sensitive technique, such as electrochemical impedance spectroscopy ( EIS), to derive information about the physico-chemical properties of the barrier and porous layers provided similar responses
Fig. 1. Variation of the sealing quality in 8 mm anodic films obtained in sulphuric and oxalic acid baths as evaluated by using the dye spot (a), acid dissolution (b) and admittance test (c).
with both types of anodic film. As can be seen from Fig. 2, the impedances obtained in the intermediate and high frequency regions for sealed and unsealed layers differed by two to three orders of magnitude. Note, however, that the layers produced by the oxalic acid baths gave Nyquist diagrams with wider semicircles [Fig. 2(a)] and exhibited a parallel shift to increasing impedances throughout the frequency field in the Bode plots [log Z vs. v, Fig. 2(b)]. Fig. 3 compares the resistance of the porous layer, R , as measured immediately on sealing, after 1 month p and after 2 years of atmospheric exposure, for films of variable thickness obtained by sealing in an oxalic acid bath. The sealing procedure was identical for all specimens and involved immersion in boiling deionized water for 60 min. R , which can be regarded as a quality index p for anodic films [2,13], always increased with aging, consistent with the results in sulphuric acid [14–16 ]. R values were rather dispersed and negligibly affected p by the anodic film thickness, contrary to other factors such as the aging or the sealing time (Fig. 4). As seen in the previous figures, sealed anodic films obtained in sulphuric and oxalic acid baths behave similarly in many respects. However, unsealed anodic
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Fig. 4. Resistance of the porous layer, R , in 8 mm anodic films p obtained in oxalic and sulphuric acid baths as a function of the duration of the hydrothermal sealing process.
Fig. 2. Impedance diagrams for 20 mm unsealed anodic films obtained in sulphuric acid (#) and oxalic acid baths (%) and for similar films following proper sealing, ($) and (&) respectively.
films behave rather differently during exposure to high relative humidity (RH ) environments (#95%), depending on the particular procedure used to obtain them. Thus, while films obtained in sulphuric acid exhibit a high absorbing capacity and grow in mass to the extent that all their pores are saturated within about two weeks
Fig. 3. Resistance of the porous layer, R , in anodic films obtained in p an oxalic acid bath and sealed in boiling deionized water for 60 min; as measured immediately after sealing, and after 1 month and 2 years of exposure to the atmosphere at the University Campus of Madrid.
( Fig. 5), those obtained in oxalic acid absorb virtually no moisture, so they preserve a nearly constant mass. This results in slow autosealing of the former and hence in impedance diagrams similar to those of sealed materials of industrial quality after about two months (compare Fig. 6 and Fig. 2). Fig. 6 itself shows that the diagrams for anodic films formed in oxalic acid baths change very little over a similar period. Occasionally, the Nyquist diagram exhibits an acceptably well defined semicircle at low frequencies (Fig. 7), which, however, is most often masked by the high dispersion in that zone. The semicircle diameter is at least 100 times greater than that defined by R at p intermediate frequencies (see inset at the top of the figure). The data correspond to a 37 mm thick anodic film obtained in an oxalic acid bath, following hydrothermal sealing for 60 min and atmospheric exposure for 1 month. Also, the micrographs of Figs. 8 and 9 show substantial microstructural differences between the two types of film, particularly as regards the size of the hexagonal cells and the diameter of the pores. Fig. 8(a), which corresponds to an unsealed anodic
Fig. 5. Absorption of water by 20 mm anodic films obtained in oxalic and sulphuric acid baths during atmospheric exposure in chambers at 95% of RH and 20°C.
V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
Fig. 6. Impedance diagrams for 20 mm unsealed anodic films obtained in sulphuric acid (#) and oxalic acid baths (%) recorded immediately upon exposure to a wet chamber, and after one month of standing in it, ($) and (&), respectively.
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film obtained in a sulphuric acid bath, exhibits a pore ˚ ; Fig. 8(b)–(d ) show the size in the region of 225 A appearance of a sealed film of the same type immediately on observation under TEM, and after 2 and 4 min under the action of the electron beam. Obviously, the beam energy induces very rapid textural changes in the anodic films, which seemingly start at the pore fillings but eventually affect the whole surface of the hexagonal cells. The micrographs of Fig. 9(a)–(d ) show the structures of the films obtained in an oxalic acid bath, following sealing in boiling deionized water for 60 min, as well as the changes caused by the electron beam. The initial structure is quite complex, with pores approximately ˚ in diameter and greyish walls 350–400 A ˚ thick 600 A bounded by a darker, thin band. The pores are filled with nanoparticles alternating with empty spaces of similar size, which presumably contain some unreacted water. Under the action of the electron beam, residual water appears to react with anhydrous alumina from cell walls, thereby leading to a homogeneous appearance after a fairly short exposure time. The homogenization starts at the pore and eventually reaches whole cells, except for small bands between adjacent cells which remain virtually unaltered throughout the process [Fig. 9(a)–(d)]. Figs. 8(a) and 9(a) allow one to estimate the pore diameter and cell size of the films formed in sulphuric and oxalic acid baths respectively, and to calculate the specific surface and the specific volume of the films, using the pore and cell sizes and the density of the alumina. This information, together with the electrochemical parameters of the films derived from Fig. 2, is shown in Table 1.
4. Discussion
Fig. 7. Impedance spectrum for 37 mm thick anodic film obtained in an oxalic acid bath, following hydrothermal sealing for 60 min and atmospheric exposure for 1 month. At intermediate frequencies, the Nyquist diagrams for sealed films exhibit a semicircle, the diameter of which coincides with R (upper inset); occasionally, another ill defined p semicircle is observed at low frequencies which allows the resistance of the barrier layer, R , to be estimated. b
The absorbing capacity [Fig. 1(a)], the surface reactivity [Fig. 1(b)] and the extent to which the sealing process takes place throughout the oxide film thickness — measured by the admittance test [Fig. 1(c)] — require shorter immersion times in the traditional hydrothermal sealing process to meet industrial quality standards if anodization is performed in oxalic acid rather than in sulphuric acid. This can significantly reduce sealing costs and partly offset the increased cost of the anodizing operation in oxalic acid. Because acetate is known to accelerate the process [17,18], oxalate can be assumed to have similar effects in an anodizing bath on account of the chemical similarity between both anions. Taking into account that the impedance of an ideal capacitor is inversely proportional to the frequency (Z= 1/Cv) and that the impedance of a pure resistance is independent of it, the straight portions of slope close to −1 [typical of the impedance diagrams for anodic films
V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
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(a)
(b)
(c)
(d)
Fig. 8. Cross-sectional views of anodic films obtained in a sulphuric acid bath at 15 V as seen under TEM: (a) unsealed specimens; (b), (c) and (d) sealed specimens exposed to the TEM electron beam for 0, 2 and 4 min.
sealed at low and high frequencies ( Fig. 2)] must be governed by the capacitances of the barrier and porous layer (C and C respectively), whereas the nearly horib p zontal segment which joins them, at intermediate frequencies, must be controlled by the resistance of the porous layer (R ). The location of such branches allows p one to estimate the electrical properties of the barrier and porous layers at each degree of sealing, and hence to derive valuable information about the effect of any factor significantly influencing the quality of anodic films. The graphs in Fig. 2 were used to calculate the R , C and C values summarized in Table 1 for anodic p p b films obtained in oxalic and sulphuric acid baths. From
C and the expression b C=
ee S 0 , d
(1)
where e =8.85×10−14 F cm−1 is the dielectric constant 0 in vacuo, e=10 is the relative constant for alumina, S the electrode surface and d the dielectric thickness; the thickness of the barrier layer obtained in oxalic acid ˚ , which is approximately was calculated to be 420 A three times greater than that of the films formed in sulphuric acid. This is quite consistent with the expected thicknesses for the operating conditions used (15 V in sulphuric acid and 40 V in oxalic acid) as the barrier
V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
(a)
(b)
(c)
(d)
81
Fig. 9. Cross-sectional views of anodic films obtained in an oxalic acid bath at 40 V, followed by hydrothermal sealing for 60 min, as seen under TEM after (a) 0, (b) 2, (c) 6 and (d) 10 min of exposure to the electron beam.
Table 1 Electrochemical parameters (R , C and C ) derived from Fig. 2 for anodic films formed in oxalic and sulphuric acid baths; and structural features p p b inferred from Figs. 8(a) and 9(a) (pore diameter, w , cell diagonal, d , specific surface, S , and specific volume, V ) p c spec spec Film type
R (kV cm2) p
R (kV cm2) b
C (nF ) p
C (mF ) b
˚) w (A p
Unsealed sulphuric Sealed sulphuric Unsealed oxalic Sealed oxalic
– 550 – 583
– – – 1.6×105
– 6.2 – 2.5
0.54 0.50 0.21 0.25
200 600
˚ V−1 [2]. layer is estimated to thicken at a rate of 10 A In any case, these calculations were only approximate as no significant differences between the capacitances of the barrier layers, C , obtained in oxalic acid solutions b
˚) d (A c
S (m2/g) spec
V
520
13.2
0.066
1300
6.4
0.095
spec
(cm3/g)
were detected on changing the anodization voltage to 50 and 60 V — presumably because the effect of the increased thickness was offset by the increase in e resulting from the increased contamination of the alu-
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mina with anions as the potential was raised, as previously revealed by the energy dispersive X-ray ( EDX ) technique [10]. It should be noted that EIS allows one to determine physico-chemical properties of the porous and barrier layers and provides information about the sealing and aging processes of a higher precision than obtained with any other technique ( Table 1). Thus, despite the high dispersion of data at low frequencies, the idealized semicircle of Fig. 7 provides an R value of b 1.6×108 V cm2 ( Table 1), which in turns yields a resistivity r =3.8×1013 V cm, acceptably consistent with b the r value assigned to alumina (≥1014 V cm) [19]. On the other hand, the fact that, at similar thicknesses of the porous layer, C is smaller for anodic films formed p in oxalic acid, suggests that such films also have a lower permittivity, e, than those formed in sulphuric acid. This is consistent with reported data in that contamination of alumina by anions in the anodizing bath increases in the following acid sequence [6 ]: chromic
Such a different interaction of water with the two types of capillary systems can be explained in thermodynamic terms using the Kelvin equation [20], ln
p p
0
=−
2Vl rRT
,
(2)
where p is the vapour pressure; p , V and l are the 0 saturation pressure, molar volume and surface tension of the fluid at temperature T; R is the ideal gas constant; and r is the pore radius. Water vapour will condensate within the pores as soon as it reaches pressure p in the Kelvin equation. At each p/p ratio (or at each RH ), a 0 critical pore size r will exist below which pores will c remain saturated with water and above which water will not be absorbed (or will be evaporated). Substitution of the water values at 20°C yields r values of 102, 210 c ˚ at the RH values 90%, and 1075 A 95% and 99% respectively. One can therefore expect the small, closed vessels containing water in their bottoms, which act as wet chambers at 95% RH, to cause the anodic films formed in sulphuric acid to autoseal; this, however, is not to be expected from anodic films obtained in oxalic acid. The micrograph sequences of Fig. 8(b)–(d) and Fig. 9(a)–(d) reveal that exposure to the electron beam causes a series of microstructural and nanostructural changes in the anodic films which are much more rapid and marked in those obtained in sulphuric acid than in those formed in oxalic acid; this is probably a result of the widely acknowledged stronger contamination of the films with anions from the anodizing bath in the former case. Such changes have been ascribed to drying, sintering, agglomeration and crystallization by Thompson et al. [5]. Presumably, on a very small time scale they reproduce the changes caused by hydration reactions in pore walls and pore fillings during aging, effects which last over very long periods as shown by EIS studies [14–16 ]. A gel consisting of pseudo-boehmite and aluminium hydroxide might be formed within the pores which would precipitate as nanoparticles, alternating with empty spaces of similar size [Fig. 9(a)]. According to Wefers [4], such a gel can retain excess water at a rate of up to 40 g per 100 g of Al O . In any case, pore 2 3 fillings will be thermodynamically unstable and the water they contain, in conjunction with the acid anions retained on cell walls, will promote a series of transformations leading to gradual widening of the pores (dissolution step); boundaries with cell walls gradually becoming more diffuse [Fig. 9(b)–(d)]. A gradual homogenization takes place which appears to have little or no effect on the boundary between adjacent cells, which consists of virtually pure Al O [6 ]. 2 3 As can be inferred by comparing Figs. 9(a) and 10,
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encompasses the initial pores and the cell walls in sealed films [Fig. 11(b)]. In natural environments, the initial nanoprecipitates undergo an aging process which decreases the free energy through recrystallization and agglomeration [4], both of which can occur over long periods [11]. The changes in the resistance (Fig. 3) and capacitance of the porous layer measured by EIS after months, years or even decades of aging reflect the underlying aging process [14–16 ].
5. Conclusions
Fig. 10. Cross-sectional views of an anodic film obtained in an oxalic acid bath at 40 V, followed by hydrothermal sealing for 20 min, as seen under TEM.
obtained after two different sealing times, pore size appears to increase with an increase in this variable. However, the dissolution of pore walls and the increase in pore diameter also take place during the subsequent aging, at a rapid pace under the action of the electron beam [Fig. 9(a)–(d)] and much more slowly under natural exposure conditions. The micrographs of Fig. 9(a)–(d) illustrate the extent to which the scheme of Fig. 11(a), proposed by Thompson et al. for anodic films, conforms to the facts, even as regards the suggested size for the nanocrystals (≤2.5 nm) [7]. With time, the nanocrystalline structure
1. Anodic films obtained in oxalic acid baths reach the quality thresholds imposed by industrial sealing control tests within a shorter time than do those formed in sulphuric acid baths. 2. Once sealed, both types of anodic film behave qualitatively identically in relation to aging. 3. Anodic films obtained in sulphuric acid undergo rapid absorption of water and gradual autosealing in highly moist atmospheres; by contrast, films obtained in oxalic acid remain virtually unaltered under such conditions. 4. Differential contamination by anions from the anodizing bath results in a structure consisting of three distinct zones: pores filled with nanocrystals of colloid precipitates alternating with empty spaces of similar sizes, cell walls and their boundaries, which are much darker and consist of pure or scarcely contaminated alumina. Acknowledgement This research was funded by the Comisio´n Interministerial de Ciencia y Tecnologı´a (CICyT ) of the Spanish Ministry of Education and Culture within the framework of Project MAT98-0797-C02-01.
References
Fig. 11. (a) Schematic depiction of a unsealed anodic film according to Thompson et al. [7]. The black boundary around each cell represents the region of relatively pure alumina. The crystallites are probably uneven and of size ≤2.5 nm. (b) Proposed scheme for a sealed film.
[1] S. Wernick, R. Pinner, P. Sheasby, The Surface Treatments of Aluminium and its Alloys, 5th Edition, ASM International, Metals Park, Ohio, 1987, pp. 444, 801–806. [2] T.P. Hoar, G.C. Wood, Electrochim. Acta 7 (1962) 333. [3] UNE Standards 38016, 3817 and 38018; or ISO 2143 and 2931. [4] K. Wefers, Aluminium 49 (1973) 553, 622. [5] G.E. Thompson, R.C. Furneaux, G.C. Wood, R. Hutchings, Practical Metallography 18 (1981) 594. [6 ] G.E. Thompson, R.C. Furneaux, G.C. Wood, Corros. Sci. 18 (1978) 481. [7] G.E. Thompson, R.C. Furneaux, G.C. Wood, J.A. Richardson, J.S. Goode, Nature 272 (1978) 433. [8] D. Ho¨nicke, Aluminium 65 (1989) 1154.
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[9] R.C. Furneaux, G.E. Thompson, G.C. Wood, Corros. Sci. 18 (1992) 853. [10] S. Ono, N. Masuko, Corros. Sci. 33 (1992) 503. [11] G.C. Wood, J.P. O’Sullivan, J. Electrochem. Soc. 116 (1969) 1351. [12] S. Ono, H. Ichinose, T. Kawaguchi, N. Masuko, Corros. Sci. 31 (1990) 249. [13] R. Lizarbe, W. Lo´pez, E. Otero, J.A. Gonza´lez, Rev. Metal. (Madrid ) 26 (1990) 359. [14] R. Lizarbe, J.A. Gonza´lez, E. Otero, V. Lo´pez, Aluminium 69 (1993) 548.
[15] J.A. Gonza´lez, V. Lo´pez, E. Otero, A. Bautista, R. Lizarbe, C. Barba, J.L. Baldonedo, Corros. Sci. 39 (1997) 1109. [16 ] R. Lizarbe, V. Lo´pez, J.A. Gonza´lez, Pinturas y Acabados Industriales 35 (207) (1993) 70. [17] R. Lizarbe, Rev. Metal. (Madrid) 22 (1986) 235. [18] R. Lizarbe, Rev. Metal. (Madrid) 24 (1988) 402. [19] Goodfellow Catalogue (1996/97) 409. [20] S.J. Gregg, K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, London, 1967, p. 162.
Sealing of anodic films obtained in oxalic acid baths V. Lo´pez, E. Otero, A. Bautista *, J.A. Gonza´lez Centro Nacional de Investigaciones Metalu´rgicas (CSIC), Avda. Gregorio del Amo No 8, 28040-Madrid, Spain Received 5 August 1999; accepted in revised form 16 October 1999
Abstract The response to sealing quality control tests of anodic films obtained in oxalic and sulphuric acid baths has been compared. Unsealed films obtained in oxalic acid remain virtually unaltered in humid atmospheres, contrary to unsealed films obtained in sulphuric acid which tend to autoseal. The former also appear to take a shorter time to reach the quality thresholds of all the usual control tests on traditional sealing in boiling deionized water. However, after sealing, both types of films age in a qualitatively identical way. Transmission electron microscopy revealed the presence of a complex structure of hexagonal cells, comprising three distinct zones in the films formed in oxalic acid, the pores in which were found to be about five times larger in diameter than those in the films obtained in sulphuric acid. This considerably facilitates microstructural examination and elucidation of the underlying sealing mechanism. Furthermore, the transformation of the film morphology under the electron beam is slower in films obtained in oxalic acid. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Anodic films; Film morphology; Sealing quality control tests; Transmission electron microscopy
1. Introduction The need to anodize aluminium and its alloys is imposed by aesthetic and durability considerations. The process is usually conducted in a phosphoric, oxalic or sulphuric acid bath. All these electrolytes dissolve the alumina layers formed in the process, to some extent, thereby giving rise to coatings consisting of two different regions: namely, an inner, compact layer a few nanometres (or tens of nanometres) thick called the barrier layer; and an outer, porous layer consisting of hexagonal cells in a honeycomb arrangement where each cell has a central pore normal to the metal substrate. The porous layer is usually about 103–104 times thicker than the barrier layer [1]. Anodizing in a sulphuric acid bath is the procedure which is used in widest industrial for this purpose; alternative treatments are reserved for specific applications [1]. For example, anodizing in oxalic acid baths is primarily used to obtain relatively hard coatings of an appealing colour (a soft bronze or golden tone, depending on the particular alloy). The porous nature of these layers requires subsequent sealing, which is usually done in boiling deionized water; * Corresponding author. Tel.: +34-91-553-8900; fax: +34-91-534-7425.
the water shuts pores through partial conversion of anhydrous alumina into boehmite, which is more voluminous [2]. The sealing operation is of such a high technical significance that it is subjected to universally accepted and applied quality control tests [3]. Notwithstanding the more extensive use of sulphuric acid anodizing, many fundamental studies on the anodizing and sealing of aluminium and its alloys have been performed on layers obtained in oxalic or phosphoric acid, which produce much larger pores, thus facilitating the detection of microstructural changes caused by any influential factor [4–12]. In this situation, we believed it of interest to compare the response to sealing quality control tests of anodic films obtained in oxalic and sulphuric acids. Only a similarity between the two types of film would justify extrapolating the mechanisms derived from studies of the former to porous anodic films produced by sulphuric acid baths.
2. Material and methods 2.1. Material We used 5×10 cm aluminium specimens of commercial purity (99.5%). The plates were anodized under the
0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 62 6 - X
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recommended conditions of the Eloxal GX process using solutions containing 4% oxalic acid, d.c. densities of 1–2 A dm−2 and voltages of 40–60 V [1]. The initial temperature in the bath, 20°C, rose by 4–5°C during anodizing. The thickness of the oxide layers ranged from 8 to 45 mm and increased with increasing voltage (current density) and anodizing time; however, it tended to an asymptotic value as the latter variable was increased. Aluminium plates were also anodized in 18% sulphuric acid at 20°C, using d.c. densities of 1.5 A dm−2 for different times to obtain films of different thickness. Specimens of widely variable thickness obtained in oxalic acid were sealed in boiling deionized water for 60 min, which was deemed long enough to achieve a sealing quality meeting industrial requirements. A set of 8 mm thick specimens anodized in oxalic acid, and another set anodized in sulphuric acid, were sealed in water at boiling temperature for 1, 5, 10, 20, 45 or 60 min. 2.2. Test methods The standard acid dissolution in phosphochromic acid, admittance and dye spot tests were applied to specimens obtained under the different sealing conditions [3]. The results of these control tests were completed with impedance measurements made in an aerated 3% w/w K SO solution at 25±1°C. The frequencies 2 4 used ranged from 100 kHz to 1 mHz. The microstructure of each anodic film was examined by transmission electron microscopy ( TEM ) following stripping from the metal substrate by dissolution in mercuric chloride ˚ on an ion beam thinner. and thinning to 100–300 A
3. Results Based on Fig. 1, the quality thresholds imposed by industrial practice are more rapidly reached by anodic films obtained in an oxalic acid bath than in a sulphuric acid bath. For example, a spot strength below 2, the value set by the applicable standards [3], was obtained in the dye spot test after only 5 min of sealing [Fig. 1(a)]. The situation was similar for the phosphochromic acid dissolution [Fig. 1(b)] and the 1 kHz admittance test [Fig. 1(c)]. In the former, mass losses fell below the allowed threshold (30 mg/dm2) at 4 and 8 min of sealing in the oxalic and sulphuric bath, respectively; in the latter, the quality threshold was reached at admittances below 20 mS [3], and also within a shorter time in the oxalic acid bath. However, qualitative changes were identical with both types of film in all tests. The use of a much more sensitive technique, such as electrochemical impedance spectroscopy ( EIS), to derive information about the physico-chemical properties of the barrier and porous layers provided similar responses
Fig. 1. Variation of the sealing quality in 8 mm anodic films obtained in sulphuric and oxalic acid baths as evaluated by using the dye spot (a), acid dissolution (b) and admittance test (c).
with both types of anodic film. As can be seen from Fig. 2, the impedances obtained in the intermediate and high frequency regions for sealed and unsealed layers differed by two to three orders of magnitude. Note, however, that the layers produced by the oxalic acid baths gave Nyquist diagrams with wider semicircles [Fig. 2(a)] and exhibited a parallel shift to increasing impedances throughout the frequency field in the Bode plots [log Z vs. v, Fig. 2(b)]. Fig. 3 compares the resistance of the porous layer, R , as measured immediately on sealing, after 1 month p and after 2 years of atmospheric exposure, for films of variable thickness obtained by sealing in an oxalic acid bath. The sealing procedure was identical for all specimens and involved immersion in boiling deionized water for 60 min. R , which can be regarded as a quality index p for anodic films [2,13], always increased with aging, consistent with the results in sulphuric acid [14–16 ]. R values were rather dispersed and negligibly affected p by the anodic film thickness, contrary to other factors such as the aging or the sealing time (Fig. 4). As seen in the previous figures, sealed anodic films obtained in sulphuric and oxalic acid baths behave similarly in many respects. However, unsealed anodic
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V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
Fig. 4. Resistance of the porous layer, R , in 8 mm anodic films p obtained in oxalic and sulphuric acid baths as a function of the duration of the hydrothermal sealing process.
Fig. 2. Impedance diagrams for 20 mm unsealed anodic films obtained in sulphuric acid (#) and oxalic acid baths (%) and for similar films following proper sealing, ($) and (&) respectively.
films behave rather differently during exposure to high relative humidity (RH ) environments (#95%), depending on the particular procedure used to obtain them. Thus, while films obtained in sulphuric acid exhibit a high absorbing capacity and grow in mass to the extent that all their pores are saturated within about two weeks
Fig. 3. Resistance of the porous layer, R , in anodic films obtained in p an oxalic acid bath and sealed in boiling deionized water for 60 min; as measured immediately after sealing, and after 1 month and 2 years of exposure to the atmosphere at the University Campus of Madrid.
( Fig. 5), those obtained in oxalic acid absorb virtually no moisture, so they preserve a nearly constant mass. This results in slow autosealing of the former and hence in impedance diagrams similar to those of sealed materials of industrial quality after about two months (compare Fig. 6 and Fig. 2). Fig. 6 itself shows that the diagrams for anodic films formed in oxalic acid baths change very little over a similar period. Occasionally, the Nyquist diagram exhibits an acceptably well defined semicircle at low frequencies (Fig. 7), which, however, is most often masked by the high dispersion in that zone. The semicircle diameter is at least 100 times greater than that defined by R at p intermediate frequencies (see inset at the top of the figure). The data correspond to a 37 mm thick anodic film obtained in an oxalic acid bath, following hydrothermal sealing for 60 min and atmospheric exposure for 1 month. Also, the micrographs of Figs. 8 and 9 show substantial microstructural differences between the two types of film, particularly as regards the size of the hexagonal cells and the diameter of the pores. Fig. 8(a), which corresponds to an unsealed anodic
Fig. 5. Absorption of water by 20 mm anodic films obtained in oxalic and sulphuric acid baths during atmospheric exposure in chambers at 95% of RH and 20°C.
V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
Fig. 6. Impedance diagrams for 20 mm unsealed anodic films obtained in sulphuric acid (#) and oxalic acid baths (%) recorded immediately upon exposure to a wet chamber, and after one month of standing in it, ($) and (&), respectively.
79
film obtained in a sulphuric acid bath, exhibits a pore ˚ ; Fig. 8(b)–(d ) show the size in the region of 225 A appearance of a sealed film of the same type immediately on observation under TEM, and after 2 and 4 min under the action of the electron beam. Obviously, the beam energy induces very rapid textural changes in the anodic films, which seemingly start at the pore fillings but eventually affect the whole surface of the hexagonal cells. The micrographs of Fig. 9(a)–(d ) show the structures of the films obtained in an oxalic acid bath, following sealing in boiling deionized water for 60 min, as well as the changes caused by the electron beam. The initial structure is quite complex, with pores approximately ˚ in diameter and greyish walls 350–400 A ˚ thick 600 A bounded by a darker, thin band. The pores are filled with nanoparticles alternating with empty spaces of similar size, which presumably contain some unreacted water. Under the action of the electron beam, residual water appears to react with anhydrous alumina from cell walls, thereby leading to a homogeneous appearance after a fairly short exposure time. The homogenization starts at the pore and eventually reaches whole cells, except for small bands between adjacent cells which remain virtually unaltered throughout the process [Fig. 9(a)–(d)]. Figs. 8(a) and 9(a) allow one to estimate the pore diameter and cell size of the films formed in sulphuric and oxalic acid baths respectively, and to calculate the specific surface and the specific volume of the films, using the pore and cell sizes and the density of the alumina. This information, together with the electrochemical parameters of the films derived from Fig. 2, is shown in Table 1.
4. Discussion
Fig. 7. Impedance spectrum for 37 mm thick anodic film obtained in an oxalic acid bath, following hydrothermal sealing for 60 min and atmospheric exposure for 1 month. At intermediate frequencies, the Nyquist diagrams for sealed films exhibit a semicircle, the diameter of which coincides with R (upper inset); occasionally, another ill defined p semicircle is observed at low frequencies which allows the resistance of the barrier layer, R , to be estimated. b
The absorbing capacity [Fig. 1(a)], the surface reactivity [Fig. 1(b)] and the extent to which the sealing process takes place throughout the oxide film thickness — measured by the admittance test [Fig. 1(c)] — require shorter immersion times in the traditional hydrothermal sealing process to meet industrial quality standards if anodization is performed in oxalic acid rather than in sulphuric acid. This can significantly reduce sealing costs and partly offset the increased cost of the anodizing operation in oxalic acid. Because acetate is known to accelerate the process [17,18], oxalate can be assumed to have similar effects in an anodizing bath on account of the chemical similarity between both anions. Taking into account that the impedance of an ideal capacitor is inversely proportional to the frequency (Z= 1/Cv) and that the impedance of a pure resistance is independent of it, the straight portions of slope close to −1 [typical of the impedance diagrams for anodic films
V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
80
(a)
(b)
(c)
(d)
Fig. 8. Cross-sectional views of anodic films obtained in a sulphuric acid bath at 15 V as seen under TEM: (a) unsealed specimens; (b), (c) and (d) sealed specimens exposed to the TEM electron beam for 0, 2 and 4 min.
sealed at low and high frequencies ( Fig. 2)] must be governed by the capacitances of the barrier and porous layer (C and C respectively), whereas the nearly horib p zontal segment which joins them, at intermediate frequencies, must be controlled by the resistance of the porous layer (R ). The location of such branches allows p one to estimate the electrical properties of the barrier and porous layers at each degree of sealing, and hence to derive valuable information about the effect of any factor significantly influencing the quality of anodic films. The graphs in Fig. 2 were used to calculate the R , C and C values summarized in Table 1 for anodic p p b films obtained in oxalic and sulphuric acid baths. From
C and the expression b C=
ee S 0 , d
(1)
where e =8.85×10−14 F cm−1 is the dielectric constant 0 in vacuo, e=10 is the relative constant for alumina, S the electrode surface and d the dielectric thickness; the thickness of the barrier layer obtained in oxalic acid ˚ , which is approximately was calculated to be 420 A three times greater than that of the films formed in sulphuric acid. This is quite consistent with the expected thicknesses for the operating conditions used (15 V in sulphuric acid and 40 V in oxalic acid) as the barrier
V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
(a)
(b)
(c)
(d)
81
Fig. 9. Cross-sectional views of anodic films obtained in an oxalic acid bath at 40 V, followed by hydrothermal sealing for 60 min, as seen under TEM after (a) 0, (b) 2, (c) 6 and (d) 10 min of exposure to the electron beam.
Table 1 Electrochemical parameters (R , C and C ) derived from Fig. 2 for anodic films formed in oxalic and sulphuric acid baths; and structural features p p b inferred from Figs. 8(a) and 9(a) (pore diameter, w , cell diagonal, d , specific surface, S , and specific volume, V ) p c spec spec Film type
R (kV cm2) p
R (kV cm2) b
C (nF ) p
C (mF ) b
˚) w (A p
Unsealed sulphuric Sealed sulphuric Unsealed oxalic Sealed oxalic
– 550 – 583
– – – 1.6×105
– 6.2 – 2.5
0.54 0.50 0.21 0.25
200 600
˚ V−1 [2]. layer is estimated to thicken at a rate of 10 A In any case, these calculations were only approximate as no significant differences between the capacitances of the barrier layers, C , obtained in oxalic acid solutions b
˚) d (A c
S (m2/g) spec
V
520
13.2
0.066
1300
6.4
0.095
spec
(cm3/g)
were detected on changing the anodization voltage to 50 and 60 V — presumably because the effect of the increased thickness was offset by the increase in e resulting from the increased contamination of the alu-
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V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
mina with anions as the potential was raised, as previously revealed by the energy dispersive X-ray ( EDX ) technique [10]. It should be noted that EIS allows one to determine physico-chemical properties of the porous and barrier layers and provides information about the sealing and aging processes of a higher precision than obtained with any other technique ( Table 1). Thus, despite the high dispersion of data at low frequencies, the idealized semicircle of Fig. 7 provides an R value of b 1.6×108 V cm2 ( Table 1), which in turns yields a resistivity r =3.8×1013 V cm, acceptably consistent with b the r value assigned to alumina (≥1014 V cm) [19]. On the other hand, the fact that, at similar thicknesses of the porous layer, C is smaller for anodic films formed p in oxalic acid, suggests that such films also have a lower permittivity, e, than those formed in sulphuric acid. This is consistent with reported data in that contamination of alumina by anions in the anodizing bath increases in the following acid sequence [6 ]: chromic
Such a different interaction of water with the two types of capillary systems can be explained in thermodynamic terms using the Kelvin equation [20], ln
p p
0
=−
2Vl rRT
,
(2)
where p is the vapour pressure; p , V and l are the 0 saturation pressure, molar volume and surface tension of the fluid at temperature T; R is the ideal gas constant; and r is the pore radius. Water vapour will condensate within the pores as soon as it reaches pressure p in the Kelvin equation. At each p/p ratio (or at each RH ), a 0 critical pore size r will exist below which pores will c remain saturated with water and above which water will not be absorbed (or will be evaporated). Substitution of the water values at 20°C yields r values of 102, 210 c ˚ at the RH values 90%, and 1075 A 95% and 99% respectively. One can therefore expect the small, closed vessels containing water in their bottoms, which act as wet chambers at 95% RH, to cause the anodic films formed in sulphuric acid to autoseal; this, however, is not to be expected from anodic films obtained in oxalic acid. The micrograph sequences of Fig. 8(b)–(d) and Fig. 9(a)–(d) reveal that exposure to the electron beam causes a series of microstructural and nanostructural changes in the anodic films which are much more rapid and marked in those obtained in sulphuric acid than in those formed in oxalic acid; this is probably a result of the widely acknowledged stronger contamination of the films with anions from the anodizing bath in the former case. Such changes have been ascribed to drying, sintering, agglomeration and crystallization by Thompson et al. [5]. Presumably, on a very small time scale they reproduce the changes caused by hydration reactions in pore walls and pore fillings during aging, effects which last over very long periods as shown by EIS studies [14–16 ]. A gel consisting of pseudo-boehmite and aluminium hydroxide might be formed within the pores which would precipitate as nanoparticles, alternating with empty spaces of similar size [Fig. 9(a)]. According to Wefers [4], such a gel can retain excess water at a rate of up to 40 g per 100 g of Al O . In any case, pore 2 3 fillings will be thermodynamically unstable and the water they contain, in conjunction with the acid anions retained on cell walls, will promote a series of transformations leading to gradual widening of the pores (dissolution step); boundaries with cell walls gradually becoming more diffuse [Fig. 9(b)–(d)]. A gradual homogenization takes place which appears to have little or no effect on the boundary between adjacent cells, which consists of virtually pure Al O [6 ]. 2 3 As can be inferred by comparing Figs. 9(a) and 10,
V. Lo´pez et al. / Surface and Coatings Technology 124 (2000) 76–84
83
encompasses the initial pores and the cell walls in sealed films [Fig. 11(b)]. In natural environments, the initial nanoprecipitates undergo an aging process which decreases the free energy through recrystallization and agglomeration [4], both of which can occur over long periods [11]. The changes in the resistance (Fig. 3) and capacitance of the porous layer measured by EIS after months, years or even decades of aging reflect the underlying aging process [14–16 ].
5. Conclusions
Fig. 10. Cross-sectional views of an anodic film obtained in an oxalic acid bath at 40 V, followed by hydrothermal sealing for 20 min, as seen under TEM.
obtained after two different sealing times, pore size appears to increase with an increase in this variable. However, the dissolution of pore walls and the increase in pore diameter also take place during the subsequent aging, at a rapid pace under the action of the electron beam [Fig. 9(a)–(d)] and much more slowly under natural exposure conditions. The micrographs of Fig. 9(a)–(d) illustrate the extent to which the scheme of Fig. 11(a), proposed by Thompson et al. for anodic films, conforms to the facts, even as regards the suggested size for the nanocrystals (≤2.5 nm) [7]. With time, the nanocrystalline structure
1. Anodic films obtained in oxalic acid baths reach the quality thresholds imposed by industrial sealing control tests within a shorter time than do those formed in sulphuric acid baths. 2. Once sealed, both types of anodic film behave qualitatively identically in relation to aging. 3. Anodic films obtained in sulphuric acid undergo rapid absorption of water and gradual autosealing in highly moist atmospheres; by contrast, films obtained in oxalic acid remain virtually unaltered under such conditions. 4. Differential contamination by anions from the anodizing bath results in a structure consisting of three distinct zones: pores filled with nanocrystals of colloid precipitates alternating with empty spaces of similar sizes, cell walls and their boundaries, which are much darker and consist of pure or scarcely contaminated alumina. Acknowledgement This research was funded by the Comisio´n Interministerial de Ciencia y Tecnologı´a (CICyT ) of the Spanish Ministry of Education and Culture within the framework of Project MAT98-0797-C02-01.
References
Fig. 11. (a) Schematic depiction of a unsealed anodic film according to Thompson et al. [7]. The black boundary around each cell represents the region of relatively pure alumina. The crystallites are probably uneven and of size ≤2.5 nm. (b) Proposed scheme for a sealed film.
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[15] J.A. Gonza´lez, V. Lo´pez, E. Otero, A. Bautista, R. Lizarbe, C. Barba, J.L. Baldonedo, Corros. Sci. 39 (1997) 1109. [16 ] R. Lizarbe, V. Lo´pez, J.A. Gonza´lez, Pinturas y Acabados Industriales 35 (207) (1993) 70. [17] R. Lizarbe, Rev. Metal. (Madrid) 22 (1986) 235. [18] R. Lizarbe, Rev. Metal. (Madrid) 24 (1988) 402. [19] Goodfellow Catalogue (1996/97) 409. [20] S.J. Gregg, K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, London, 1967, p. 162.