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Enhancing aluminum corrosion resistance by two-step anodizing process
SCT-18830; No of Pages 9 Surface & Coatings Technology xxx (2013) xxx–xxx
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Enhancing aluminum corrosion resistance by two-step anodizing process L. Bouchama a,⁎, N. Azzouz a, N. Boukmouche a, J.P. Chopart b, A.L. Daltin b, Y. Bouznit c a b c
Laboratory of the Interactions Materials–Environment (LIME), Jijel University, Jijel 18000, Algeria Laboratory of Materials Science and Engineering (LISM), University of Reims, France Laboratory of Materials: Elaborations–Properties–Applications, Jijel University, Jijel 18000, Algeria
a r t i c l e
i n f o
Article history: Received 22 March 2013 Accepted in revised form 25 August 2013 Available online xxxx Keywords: Aluminum Porous alumina Single-step anodizing Two-step anodizing EIS spectroscopy
a b s t r a c t Two-step anodizing process can be seen as an effective way to form an adhesive and continued layer on aluminum to enhance its corrosion resistance. Anodic alumina was produced by two types of anodization process, namely single-step and two-step anodizing. We have then evaluated their protection effectiveness against aluminum corrosion in aggressive medium (NaCl 3%). In both cases, X-ray diffraction reveals an amorphous nature of porous oxide films. The electrochemical impedance spectroscopy (EIS) results revealed a strong correlation between the corrosion performance and the morphology of the development anodic films. Also, these results indicated that the morphology and the corrosion resistance of anodic film formed by two-step anodizing process are very improved if compared to single one. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It is well known that aluminum and its alloys have a huge number of uses. These range from all sorts of packaging, through the aeroplanes, cars and train carriages. Aluminum is also vital in aircraft domains, building, construction industry and commonplace household objects. Although aluminum has a huge advantage when compared to other metals, it is not always completely impervious to corrosion; its protective oxide layer can become unstable when it is exposed to extreme pH levels. So, it is in high demand to develop highly effective and selective method to counter this phenomenon. Actually, many studies have been carried out and different protective methods have been reported. Corrosion is usually prevented by chemical passivation coating [1], polymer coating [2] and anodized coating [3]. Goeminne et al. [4] have used the conversion layers on aluminum alloys to improve the corrosion resistance and the adhesion of organic coatings. On the other hand, Bonnel et al. [5] have used a traditional organic solvent coating and a waterborne coating deposited on aluminum. Accordingly, some electrochemical processes can be used to obtain oxidized thick and rigid layers which are benefits to improve the physical and chemical properties of aluminum [6]. Some authors attempt to incorporate corrosion inhibitors into the anodic film during the anodizing process [7]. Anodization of aluminum is an electrochemical method that consists of converting aluminum to its oxide (Al2O3) by applying an external current in the presence of an electrolyte; it is a widely studied and accepted way in industry for protecting against corrosion and abrasion [8]. The most widely used electrolyte for anodizing is an acid, such as a sulfuric ⁎ Corresponding author. Tel.: +213 794317413; fax: +213 34502692. E-mail address: [email protected] (L. Bouchama).
acid solution [9,10]. Generally, the morphology of the porous films has been characterized as a closed-packed array of columnar hexagonal cells, each one contains central pore that passes normally to the substrate surface, with a compact barrier-type film at the base of individual pores, which contributes to the overall anticorrosion performance [11]. The porous structure of these films makes them highly absorbent and hence prone to fouling and corrosion in aggressive environments. Industrially, this has traditionally been accomplished by immersion in boiling deionized water, a procedure known as hydrothermal sealing (HTS) [12]. A two-step anodizing method was used by Masuda et al. [13] to obtain highly ordered and perfect hexagonal alumina nanostructure over a wide range of area under proper anodizing conditions in oxalic and sulfuric acids. The application of this process of anodization confers on alumina films' self-organized porous structures with hexagonal cells distributed in more homogeneous ways [14]. This technique becomes promising for the preparation of such ordered porous structure due to its low-cost, relatively easy and simple to use, and also efficient if the factors of corrosion resistance are increased compared to those obtained on conventionally elaborated alumina films. The procedure involves two separate anodization processes: the first anodization process consists of a short period of anodization by forming the disordered porous structure. After the removal of the oxide, an array of highly ordered dimples is formed. These dimples act as initiation sites for growing highly ordered porous structure [14]. In this paper, we aimed to study the corrosion performance of the anodic films formed in sulfuric acid by single and two-step anodizing processes. From appropriate electrical equivalent circuits, representative parameters of corrosion process can be extracted. Therefore, this research has correlated the morphology of these new anodic films to their corrosion behavior in NaCl solution determined by using EIS measurements.
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2. Experimental In this work, pure aluminum (Fig. 1) has been used as the substrate material with an exposed area of 4 cm2 and the rest of the surface was coated with resin. The preparation process principally involves two successive stages: surface pretreatment and anodization process. Firstly, all samples were dry-polished using abrasive paper (grid #120 to #1200) to obtain a surface roughness of about Ra: 1 μm. The aluminum (50 mm × 30 mm × 0.5 mm) was degreased in acetone and then annealed at 500 °C for 5 h to remove mechanical stresses and recrystallize the aluminum. The surface of aluminum samples was electropolished in a mixed solution of perchloric acid and ethanol (1:4 in volume) under a constant voltage of 23 V for 2 min below 5 °C to obtain smooth-surfaced samples for anodization. Finally, the samples were rinsed with ethanol and dried. Anodic oxidation of aluminum was performed in an electrochemical cell, where the aluminum sheets were used as anode and another one as a counter-electrode. For samples treated by single-step anodizing process, the anodic oxidation of aluminum was run for a given duration (in the range of 7 h) in potentiostatic mode (25 V) using a sulfuric acid solution (1 M) thermally regulated (3 °C b T b 7 °C, typically at 5 °C). In the case of two-step anodizing process, the procedure involves two separate successive anodization processes; the first anodization step consists of a short period of anodization in potentiostatic mode (25 V) using a sulfuric acid solution (1 M) thermally regulated (3 °C b T b 7 °C, typically at 5 °C) for 3.30 h forming a disordered porous structure. The aluminum oxide formed during the first anodization step was removed by chemical etching in the bath consisting of 0.6 M phosphoric acid at 25 °C for 3.30 h followed by rinsing in distilled water and drying (Fig. 2). The second anodizing step was performed immediately after the oxide removal under the same conditions for 7 h [15]. The different stages of two-step anodizing aluminum are illustrated in Fig. 3. X-ray diffractograms were recorded with an X'Pert Pro apparatus from PANalytical using Cu Kα radiation (λ = 1.5425 A°). The surface morphology and elemental composition of the samples were investigated using a Quanta 200 scanning electron microscope (SEM). FTIR spectroscopy was used to determine the nature of the chemical bonds present in the anodic films. Alumina films were scratched, crushed and mixed with KBr in the form of pellets. The spectra were obtained using a spectrometer model FTIR-8400S SHIMADZU in the transmission mode. Electrochemical behavior of anodic films was studied under the potentiostatic control of a Radiometer Analytical type VoltaLab 40 PGZ301 potentiostat/galvanostat controlled by a computer using a Voltamaster 4 software for treatment and data acquisition, using the
Al
20000
100
KCnt
15000
10000
Fig. 2. The top surface images of step oxide removal forming in the first anodization step.
conventional three-electrode configuration, with a saturated calomel electrode (SCE) as reference and platinum plate as counter electrode. The exposed surface area of samples was 3 cm2. Electrochemical impedance measurement was carried out at room temperature by applying AC amplitude of 10 mV on open-circuit potential over the frequency range of 100 kHz to 1 mHz, with five points per decade. Electrochemical behavior of anodic films was studied by immerging of the anodized specimens in an unstirred and aerated 3% NaCl solution at different periods of immersion: 4, 6, 26, 28 and 30 h, respectively. Impedance diagrams were recorded immediately after anodization for different immersion times. The ZSimpWin software was used to adjust the experimental impedance data. 3. Results and discussion 3.1. Structural analysis The structural characteristics of anodic films, synthesized by singlestep and two-step anodizing processes in sulfuric acid are illustrated in Fig. 4. Amorphous alumina was obtained as it can be seen in Fig. 4. The XRD diffractograms exhibit intense peaks at 45° and 65° that can be assigned to face-centered cubic Al (JCPDS 04-0787). Similar results have been previously reported by Stojadinovic et al. [16]. They have found amorphous nature of anodic films formed by two-step anodizing process in sulfuric acid. Ghrib et al. [16] revealed an amorphization of the anodic films after annealing of the anodic films prepared by twostep anodizing process in sulfuric acid. Also, Vázquez et al. [17] have reported amorphous mesoporous alumina films produced by anodization in sulfuric acid after different pretreatment on aluminum substrate such as mechanical polishing and electropolishing. 3.2. Morphological analysis
5000
0
Al
0
5
10
15
Energy (KeV) Fig. 1. EDS analysis on surface of aluminum substrate.
20
Fig. 5 (a, b) shows the surface topography of anodic films prepared in sulfuric acid by single-step and two-step anodizing processes. We can see that the surface morphology of anodic films produced in two cases is completely different. The surface morphology of anodic films produced by single-step anodizing (Fig. 5a) contains heterogeneously distributed holes due to the field-assisted dissolution of the oxide [18]. It is noteworthy that the surface in this case presents many craters having
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Highly ordered dimples Barrier layer
Dissolution of porous layer
Second step of an odization
Al
First step of anodization
Porous layer Barrier layer
Fig. 3. Schematic illustration of the anodization process.
these cavities to the evolution of oxygen during anodic alumina growth. As indicated previously, some authors have demonstrated that oxygen gas can be generated within films. Crevecoeur et al. [22] checked that gaseous oxygen was present in anodic oxide layers in the anodization of pure aluminum. Thus, it is possible that oxygen gas can be produced in anodic processes under the conditions used in our work. Because of the presence of oxygen bubbles at some locations of the films, the anodic oxide will only grow around them. This leads to the production of cavities within the anodic films. The cavities shown in Fig. 5e are intermittent, which indicates that the evolution of oxygen gas is also discontinuous [23].
few microns in length. This phenomenon can be explained according to Patermarakis et al. [19] by local interconnection of the pore mouths leading to formation of many small craters (Fig. 5a). But for films produced by the two-step anodizing process, the surface is fully covered like what appears in Fig. 5b. This type of structure produced by the last one process was also observed in a previous study with anodization process in tertiary mixed acid without pre-immersing process [20]. Also, Dasquet et al. [21] have observed the same microstructure of anodic films obtained by anodization process on 1050 and 2024 T3 aluminum alloys in solutions of boric acid. From Fig. 5b, the cell structure on the surface of the anodic film is more uniform and closer than that of the single process. Fig. 5 (c, d) shows cross-section micrographs of alumina formed by single-step and two-step anodizing aluminum before the corrosion tests. Knowing that these films were produced with almost the same duration for the anodization process (7 h), the single-step anodized sample had the thickest (about 7 μm and 153 nm for porous and barrier layers, respectively) coating but was non-uniform and had a porous morphology as shown in Fig. 5c. In contrary, the two-step anodized sample had the thickest coating (about 6 μm and 615 nm for porous and barrier layers). Few cavities are apparent in the case of single-step anodized sample as shown in Fig. 5e. The cavities are shielded by the upper alumina films. Obviously, formation of the cavities cannot be explained by the field-assisted dissolution mechanism. We ascribe
8000
3.3. Elemental analysis In order to investigate the compositional analysis of the developed alumina films, representative EDS spectra are shown in Fig. 6. From Fig. 6a, the major elements composing the films are: Al, O, C and S. The existence of sulfur indicates the presence of sulfuric species in oxide films incorporated (diffusion) during the film growth [15,16]. In the present case, EDS analysis reveals a strong content of aluminum and low content of oxygen on the surface of anodic film. According to Wang et al. [24], aluminum and oxygen are found in the film with different distribution in the two layers; in the outer layer it is uniform while it changes in the inner oxide layer (Al increases and O decreases). From
a
Al
b
Al
3000
Counts
Counts
6000
4000
2000
1000
2000
Al
Al Al
0 10
20
30
40
2θ (ο)
50
60
70
0 10
20
30
40
50
60
70
2θ (ο)
Fig. 4. XRD pattern of anodic layer formed by: (a) single-step anodizing and (b) two-step anodizing.
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a
b
c Al
Barrier layer
Porous layer
Resin
d Al
Barrier layer
Porous layer
Al2O3
Resin
Al2O3
e
cavities
cavities and defects
Fig. 5. SEM micrographs of the anodic layer formed by: (a) single-step anodizing and (b) two-step anodizing, (c) cross-sections of porous alumina films formed by single-step anodizing and (d) cross-sections of porous alumina films formed by two-step anodizing and (e) cross-sections of porous alumina films formed by single-step anodizing (cavities and defects).
Fig. 6b, it can be found that the elements present are C, O, Al, S and P. The presence of S may originate from the solution (electrolyte), which embeds inside the anodic film. It is widely known that porous films formed
a
in sulfuric acid were contaminated by acid species (SO−2 4 ) [7]. The presence of a certain amount of P (Fig. 6b), presumably as PO−3 ions, indi4 cating a deep penetration of PO−3 anions into the oxide may originate 4
b 57,98 69,32 31,07 22,88 5,88
7,55
2,39 2,69
0,25 C
O
C Al
S
O
Al
P
S
Fig. 6. EDS spectrum of the anodic layer formed by: (a) single-step anodizing and (b) two-step anodizing.
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from the electrolyte (chemical etching in phosphoric acid). Fig. 6b indicates that oxygen and aluminum concentrations are 57.98% and 31.07% (atomic %). The atomic ratio between oxygen and aluminum is nearly 1.86, which approaches to the theoretical value (1.5 for Al2O3). The explanation of the high C content is due to the use of graphite glue during analysis by SEM to ensure conductivity of specimens and their contacts with the specimen-holder. 3.4. Infrared spectroscopy The infrared spectra of the anodic films prepared by single-step anodizing (black line) and two-step anodizing (red line) processes are presented in Fig. 7. Both spectra have absorption bands at around 3640–3000 cm−1 and 1220–920 cm−1. Broad asymmetric absorption band between 3640 and 3000 cm−1 originates from H2O and/or OH group stretching vibrations are clearly visible, which may be due to the presence of physisorbed water. Also, the vibration of H2O in the range of 1640 to 1450 cm−1 is present. Typically band at 1650 cm−1 is the characteristic of the presence of water in the anodic films [25]. Relatively intense peaks appeared in the region of 3000–2800 cm−1 for the twostep anodized sample corresponding to the C\H stretching modes of aliphatic chains. In addition, from Fig. 7 a weak absorption band centered at 2339 cm−1 which reflects the carbon dioxide may be from the atmosphere and from impurities formed during the preparation of the pellets. Absorption bands at 1190 to 900 cm−1 are the results of S_O stretching vibrations [26]. Furthermore, we reveal a band corresponding to P\O bond, phosphorus is found therefore partially or completely in the form of phosphates [27]. This peak at 1240 cm−1 is observed only for twostep anodized sample which is assigned to the vibration arising from phosphate species incorporated into the film. Also, a weak band at 1730 cm−1 is observed for two-step anodized sample corresponding to carbonyl C_O normal stretching. For both samples, part of the spectrum from (420–757 cm−1) most bands was a characteristic of Al–O group of alumina. 3.5. Impedance spectra of anodic films Fig. 8 shows the evolution of the impedance diagrams versus immersion time in a 3% NaCl solution for the anodic films formed using both anodizing process in sulfuric acid. Nyquist plots are characterized by the presence of two capacitive loops. Several studies have already shown that the low frequency range corresponds to the barrier layer properties and the high and medium frequency ranges reflect the porous layer properties [28–32]. Particularly, the shapes of the single-
-1
8
4000
-1
707 cm
-1
1240 cm
-1
12
2913 cm -1 2863 cm
16
2339 cm
-1
1730 cm -1 1644 cm -1 1450 cm
-1
20
3420 cm
Transmittance ( %)
24
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1) Fig. 7. FTIR spectra of the anodic layer formed by: (black line) single-step anodizing and (red line) two-step anodizing. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5
step and two-step anodized specimens after 4 h of immersion in 3% NaCl solution are very similar. We observe that the diameter of the capacitive loop slightly increases with the increases of the immersion time for two-step anodized sample. As mentioned above, the anodic film on aluminum consists of a very thin compact layer and a thick porous layer, and the latter is composed of pores and walls of approximately hexagonal cells. In order to describe the electrochemical behavior of this system, the thin barrier layer and the porous layer are considered to be independent with each other. According to previous studies [30–33], the properties of each layer may be characterized by resistances and capacitances (in parallel and in series), describing their electronic and dielectric behaviors. Since the pioneering work of Hoar and Wood [28], various equivalent circuits have been proposed to model the response of porous anodic films. It is accepted that the porous anodic film may be modeled by the equivalent circuit proposed by Hitzig et al. [29], which is presented in Fig. 9. This model is successfully applied to explain the properties of the barrier and porous layers [34–40]. In this model, Rel is the resistance of the solution between the reference electrode and the anodic film. Rel is usually very small in NaCl solutions. One of the parallel branches in the circuit is formed by resistance Rw and the associate capacitance Cw; representing the walls of the cells. Rw and Cw are generally omitted since they are extremely high and extremely low, respectively. Indeed, the walls of hexagonal cells prevent the passage of current. The pore wall resistance and capacitance are neglected as it barely allows the conduction of ions through it [41]. In this way, equivalent circuit can be reduced as illustrated in Fig. 9b. The porous layer properties are characterized by capacitance Cp and resistance Rp. Barrier layer properties are described by capacitance Cb and resistance Rb, respectively. However, porous and barrier layers have heterogeneities that make their capacitive behavior better simulated by constant phase elements (CPEs) instead of simple capacitances (Cs). Hence, the EC can be simplified like what is presented in Fig. 9c, where CPEp represents the capacitance of the porous layer and CPEb represents the capacitance of the barrier layer. The constant-phase elements are used to account for the irregularities and variations of the properties of various layers [41]. According to Thompson et al. [34] CPE is defined as: α
CPE ¼ 1=ð2πJfCÞ
ð1Þ
αp and αb are affected by CPEp and CPEb, respectively, due to the nonideal capacitive behaviors [27]. Only when α = 1 can CPEp and CPEb be considered as real capacitances. For α = 0 the CPE is an ideal resistor. When α = 0.5 the CPE represents a Warburg impedance with diffusional character and for 0.5 b α b 1 the CPE describes a frequency dispersion of time constants due to local non-homogeneity in the dielectric material. In general, it is believed that the CPE is related to some type of heterogeneity of the electrode surface as well as to the fractal nature (roughness or porosity) of the surface [42]. Fitted experimental impedance diagrams are illustrated in Fig. 10. The parameters used to describe the electrochemical behavior of anodic films (porous and barrier layers) are presented in Table 1. Fig. 11 shows fit parameters for simulated spectrum to the resistance of the porous layer and barrier layer of anodic films for both cases in 3% NaCl solution. Rp values (Fig. 11a) reveal a different trend; a decrease of Rp for single-step anodizing process is observed with immersion times revealing a degradation of the porous layer which indicates that the porous layer is deteriorated wherein the anodic film presented more heterogeneity (more disorganized) and present many defects such as craters and cavities. It is well known that this structure favors the penetration of chloride ions in the oxide films, Cl− easily passed through the porous layer, causing its degradation, and therefore affecting its corrosion resistance. However, a gradual increase of Rp till the end of immersion time for two-step anodizing process is observed. Actually, Rp values of the anodic films are much higher in comparison with those formed by single one in NaCl medium. The Rp values are about (104 Ω cm2) at the beginning of immersion
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a
b
Fig. 8. Nyquist plots as a function of immersion time in 3% NaCl solution at 25 °C, for the anodic layer formed by: (a) single-step anodizing and (b) two-step anodizing.
time in the chloride solution then it increased during the whole immersion time. These values are relatively similar with those obtained by some authors [43,44]. This indicates that the porous layer in this case is not deteriorated due to the efficient protection of the film which prevents electrolyte penetration. The increase of Rp with immersion time in this case could be attributed to a self-sealing phenomenon [45,46]. As hydration and the aging process developed (as the exposure time increases), the anhydrous alumina reacts with absorbed water, which leads to voluminous hydrated alumina and resulting in a self-sealing effect. Thus, the resistance of the porous layer increases. The self-sealing mechanism involves degradation, gelling, agglomeration and precipitation process [31]. As there is a reduction of free water content inside the pores, it is in the formation of hydrated alumina, and the hydrated alumina permittivity is lower than the alumina and free water permittivity. The CPEp value obviously decreases. So, lower CPEp values indicate that there is less free water in the porous layer of anodic films and a better protectiveness of the porous layer from the corrosion solution. As the resistance of the porous layer of anodic films, higher Rp values show a more difficult penetration of the aggressive electrolyte into the anodic film on the aluminum. However, a remarkable increase in CPEp in the case of the twostep anodizing may be due to the contamination of anhydrous alumina
a
by sulfate and phosphate ions which can increase the anodic layer permittivity. At the same time, the anodic film may also be attacked by aggressive ions from the electrolyte. The conversion of alumina to hydrated alumina may be retarded because they have the aggressive ions in the electrolyte and also a hard penetration of electrolyte. For single-step anodizing sample, CPEp remains lower and constant during immersion time. CPEp depends principally on the oxide roughness not on the porous layer thickness [47]. The oxide roughness (porosity in surface) is more porous for the anodic film in this case, which is confirmed by micrograph SEM results (Fig. 5a). Therefore, the morphology of anodic film is an important factor which can explain these differences in behavior; the morphology of oxide films influenced both chloride adsorption and penetration. Indeed, the increase of porosity favors electrolyte penetration through the anodic film. So, single-step anodizing sample has the highest porosity and the quickest chloride penetration. Another parameter which can explain this fact is the compactness. Film formed by twostep anodizing process is more compact and denser; the electrolyte penetration through the passive film is hindered in this case. The parameter αp (Fig. 13a) takes into account the non-ideal capacitive behavior of porous layers. As the initial αp values are far from 1 for single-step anodizing process, the heterogeneity of porous layer seems
Rel
i)
Cw
Rel: electrolyte resistance Rw : hexagonal cells resistance Cw : hexagonal cells capacitance Rp: electrolyte resistance through pores Cp : electrolyte capacitance through pores Rb: barrier layer resistance Cb : barrier layer capacitance
ii)
Rw
Cp
Cb
Rp
Rb
i) wall of hexagonal cells ii) porous layer: pores iii) barrier layer
iii)
b
Cp
Cb
c
CPEp
CPEb
Rp
Rb
Rel
Rel
Rp
Rb
Fig. 9. Equivalent circuits (ECs) for modeling the behavior of anodic films. (a) General model, (b, c) simplified model.
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L. Bouchama et al. / Surface & Coatings Technology xxx (2013) xxx–xxx
a
7
b
Fig. 10. Fitting of the Nyquist plots immersed in 3% NaCl solution at 25 °C for 6 h: (a) single-step anodizing and (b) two-step anodizing.
important, it is strongly disorganized and present of many defects. Also, porous layer is modified in the composition and morphology because of their hydration. Indeed, it is contaminated by species from electrolyte such as sulfate ions (from sulfuric acid) [7]. Fig. 11b presents the variation of the fitted Rb values as a function of exposure time in the 3% NaCl solution for both anodized samples formed in sulfuric acid. For the anodic films formed by two-step anodizing process, the Rb values increased with immersion time and remain high, indicating that the barrier layer is unattacked. This is consistent with the fact that the porous layer is homogeneous and less porous (compactness of porous layer); this surface seems to be more effective against electrolyte penetration, retards further corrosion and prevents the degradation of the barrier layer. Thus, less porous surface improves corrosion performance by restriction of the entrance of chloride to the passive film [7]. In contrast, for single-step anodizing process, the Rb values decrease slowly during the immersion time, indicating that a significant degradation of the barrier layer due to the low protection afforded by the porous layer is due to the easy penetration of electrolyte through porous layers to attack barrier layer. The decrease of Rb during the immersion time is related to pitting and the increase of Rb at 26 h of immersion time can be explained by formation of corrosion products after pitting. It is well known that the capacitance Cb can be connected with the barrier layer thickness eb, by the relation: Cb ¼ ζ r ζ o S=eb
ð2Þ
where ζo = 8.85 10−14 F cm−1 is the dielectric constant in vacuum, ζr = 10 is the relative constant for alumina and S = 3 cm2 is the electrode surface [29]. Relation (2) is acceptable if αb is close to 1 [47], when the CPE is close to an ideal capacitance essentially at 6 h of immersion for two-step anodizing (see Table 1), this corresponds to a barrier layer that is relatively homogeneous, Cb values are connected to barrier layer thicknesses (eb); and eb is calculated to be approximately 617 nm. For anodic film formed by single-step anodizing process, the CPE is close to an ideal capacitance essentially at 26 h of immersion, eb is 160 nm. The eb values calculated with relation (2) are in good
agreement with those obtained by cross-section micrographs of the anodic films which are respectively 615 nm and 153 nm for two-step and single-step anodizing processes. The parameter αb (Fig. 13b) takes into account the non-ideal capacitive behavior of barrier layers [47]. Indeed, barrier layers can be modified in composition and in morphology [48]. This alters their homogeneity. The homogeneity of the barrier layer seems to decrease as the hydration process progresses, as the decrease of αb value indicates. After about 30 h, the αb value is close to 0.4, reflecting the increase of defects in the barrier layer for single-step anodized sample. Further investigations to determine the action of two-step anodizing process on the barrier layer would be necessary. The results show that the compactness and homogenization and absence of defects such as cavities in the anodic films may give different results, thus this novel process seems to increase barrier layer thickness. From Fig. 5 (c, d) cross-section micrographs of the anodic alumina films check that the barrier layer formed by this technique is thicker, more compact and homogeneous (see model proposed in Fig. 3) and this results in a development and growth of the barrier layer during the second anodization step, which explains why Rb value is the highest with this process, and therefore the barrier layer becomes more resistant against electrolyte penetration. The evolution of CPEb as a function of immersion time shows higher values of CPEb for the single-step anodizing attributed to a thinner barrier layer (Fig. 12). The better corrosion resistance of the two-step anodizing anodic film is not related to the greater thickness of its oxide layer but depends on the barrier layer thickness and the more organized structure; the presence of defects for the single-step anodizing anodic film is actually affected by its corrosion resistance.
4. Conclusion In this work, the corrosion resistance of anodic layers on aluminum produced by single-step and two-step anodizing processes was studied by EIS and correlated to the microstructure of the anodic films. A series
Table 1 Fitting results for the EIS data acquired for layers immersed in 3% NaCl solution for different time concerning both single-step and two-step anodizing processes. Elements circuit Single-step anodization
Two-step anodization
Immersion time (h)
Rel (Ω·cm2)
CPEp (Ω−1·cm−2·sα)
αp
CPEb (Ω1·cm−2·sα)
αb
Rel (Ω·cm2)
CPEp (Ω−1·cm−2·sα)
αp
CPEb (Ω−1·cm−2·sα)
αb
4 6 26 28 30
56.37 61.50 61.02 47.50 36.14
1.25 8.39 15.25 21.43 22.34
0.81 0.86 0.56 1.00 1.00
262800 0.68 0.11 30.49 39.80
0.47 0.88 0.98 0.46 0.43
60.72 57.11 51.11 50.93 46.40
0.35 8.47 1618.00 3816.00 3952.00
1.00 0.69 0.79 1.00 1.00
1.76 4.30 5.55 5.85 5.91
0.63 1.00 0.62 0.62 0.61
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of tests and characterization showed that the two-step anodizing process has significantly enhanced corrosion resistance due to formation of a continued adherent alumina layer on the metal surface. The XRD analysis indicated that porous oxide films formed by single-step and two-step anodizing processes are amorphous. The anodic film obtained by singlestep anodizing aluminum is strongly porous and disorganized, whereas the film obtained by two-step anodizing process is homogeneous, more compact and less porous. In contrast, the best corrosion resistance is observed for anodic films formed with two-step anodizing aluminum. References [1] [2] [3] [4]
A. Turnbull, Corros. Sci. 23 (1983) 833. S. Lin, H. Greene, Corrosion 48 (1992) 61. F. Mansfeld, S. Lin, S. Kim, H. Smh, Electrochim. Acta 34 (1989) 1123. G. Goeminne, H. Terryn, J. Vereecken, Electrochim. Acta 40 (1995) 479.
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L. Bouchama et al. / Surface & Coatings Technology xxx (2013) xxx–xxx [18] J. Oh, C.V. Thompson, Electrochim. Acta 56 (2011) 4044. [19] G. Patermarakis, K. Moussoutzanis, J. Electroanal. Chem. 659 (2011) 176. [20] C.W. Tang, The study of anodic treatment of aluminum in tertiary mixed acid after high temperature pre-immersing, Tatung University, Taiwan, 2005. 73. [21] J.P. Dasquet, U.D. Caillard, E. Conforto, J.P. Bonino, R. Bes, Thin Solid Films 371 (2000) 183. [22] C. Crevecoeur, H.J. de Wit, J. Electrochem. Soc. 134 (1987) 808. [23] X. Yang, X. Zhu, H. Jia, T. Han, Monatsh. Chem. 140 (2009) 595. [24] H. Wang, H. Yi, H. Wang, Appl. Surf. Sci. 252 (2005) 1662. [25] W.M. de Azevedo, G.B. de Oliveira, E.F. da Silva Jr., H.J. Khoury, E.F. Oliveira de Jesus, Radiat. Prot. Dosim. 119 (2006) 201. [26] D. Philip, A. Eapen, G. Aruldhas, J. Solid State Chem. 116 (1995) 217. [27] F. Le Coz, Elaboration et caractérisations de films anodiques hautement ordonnés, obtenus à partir de substrats d'aluminium, Université Toulouse III — Paul Sabatier U.F.R. P.C.A, 2007. 106. [28] T.P. Hoar, G.C. Wood, Electrochim. Acta 7 (1962) 333. [29] J. Hitzig, K. Junttner, W.J. Lorentz, W. Paatsch, Corros. Sci. 24 (1984) 945. [30] J. Hitzig, K. Junttner, W.J. Lorentz, J. Electrochem. Soc. 133 (1986) 887. [31] J.J. Suay, E. Gimenez, T. Rodriguez, K. Habbib, J.J. Saura, Corros. Sci. 45 (2003) 611.
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Please cite this article as: L. Bouchama, et al., Surf. Coat. Technol. (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.08.046
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Enhancing aluminum corrosion resistance by two-step anodizing process L. Bouchama a,⁎, N. Azzouz a, N. Boukmouche a, J.P. Chopart b, A.L. Daltin b, Y. Bouznit c a b c
Laboratory of the Interactions Materials–Environment (LIME), Jijel University, Jijel 18000, Algeria Laboratory of Materials Science and Engineering (LISM), University of Reims, France Laboratory of Materials: Elaborations–Properties–Applications, Jijel University, Jijel 18000, Algeria
a r t i c l e
i n f o
Article history: Received 22 March 2013 Accepted in revised form 25 August 2013 Available online xxxx Keywords: Aluminum Porous alumina Single-step anodizing Two-step anodizing EIS spectroscopy
a b s t r a c t Two-step anodizing process can be seen as an effective way to form an adhesive and continued layer on aluminum to enhance its corrosion resistance. Anodic alumina was produced by two types of anodization process, namely single-step and two-step anodizing. We have then evaluated their protection effectiveness against aluminum corrosion in aggressive medium (NaCl 3%). In both cases, X-ray diffraction reveals an amorphous nature of porous oxide films. The electrochemical impedance spectroscopy (EIS) results revealed a strong correlation between the corrosion performance and the morphology of the development anodic films. Also, these results indicated that the morphology and the corrosion resistance of anodic film formed by two-step anodizing process are very improved if compared to single one. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It is well known that aluminum and its alloys have a huge number of uses. These range from all sorts of packaging, through the aeroplanes, cars and train carriages. Aluminum is also vital in aircraft domains, building, construction industry and commonplace household objects. Although aluminum has a huge advantage when compared to other metals, it is not always completely impervious to corrosion; its protective oxide layer can become unstable when it is exposed to extreme pH levels. So, it is in high demand to develop highly effective and selective method to counter this phenomenon. Actually, many studies have been carried out and different protective methods have been reported. Corrosion is usually prevented by chemical passivation coating [1], polymer coating [2] and anodized coating [3]. Goeminne et al. [4] have used the conversion layers on aluminum alloys to improve the corrosion resistance and the adhesion of organic coatings. On the other hand, Bonnel et al. [5] have used a traditional organic solvent coating and a waterborne coating deposited on aluminum. Accordingly, some electrochemical processes can be used to obtain oxidized thick and rigid layers which are benefits to improve the physical and chemical properties of aluminum [6]. Some authors attempt to incorporate corrosion inhibitors into the anodic film during the anodizing process [7]. Anodization of aluminum is an electrochemical method that consists of converting aluminum to its oxide (Al2O3) by applying an external current in the presence of an electrolyte; it is a widely studied and accepted way in industry for protecting against corrosion and abrasion [8]. The most widely used electrolyte for anodizing is an acid, such as a sulfuric ⁎ Corresponding author. Tel.: +213 794317413; fax: +213 34502692. E-mail address: [email protected] (L. Bouchama).
acid solution [9,10]. Generally, the morphology of the porous films has been characterized as a closed-packed array of columnar hexagonal cells, each one contains central pore that passes normally to the substrate surface, with a compact barrier-type film at the base of individual pores, which contributes to the overall anticorrosion performance [11]. The porous structure of these films makes them highly absorbent and hence prone to fouling and corrosion in aggressive environments. Industrially, this has traditionally been accomplished by immersion in boiling deionized water, a procedure known as hydrothermal sealing (HTS) [12]. A two-step anodizing method was used by Masuda et al. [13] to obtain highly ordered and perfect hexagonal alumina nanostructure over a wide range of area under proper anodizing conditions in oxalic and sulfuric acids. The application of this process of anodization confers on alumina films' self-organized porous structures with hexagonal cells distributed in more homogeneous ways [14]. This technique becomes promising for the preparation of such ordered porous structure due to its low-cost, relatively easy and simple to use, and also efficient if the factors of corrosion resistance are increased compared to those obtained on conventionally elaborated alumina films. The procedure involves two separate anodization processes: the first anodization process consists of a short period of anodization by forming the disordered porous structure. After the removal of the oxide, an array of highly ordered dimples is formed. These dimples act as initiation sites for growing highly ordered porous structure [14]. In this paper, we aimed to study the corrosion performance of the anodic films formed in sulfuric acid by single and two-step anodizing processes. From appropriate electrical equivalent circuits, representative parameters of corrosion process can be extracted. Therefore, this research has correlated the morphology of these new anodic films to their corrosion behavior in NaCl solution determined by using EIS measurements.
0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.08.046
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2. Experimental In this work, pure aluminum (Fig. 1) has been used as the substrate material with an exposed area of 4 cm2 and the rest of the surface was coated with resin. The preparation process principally involves two successive stages: surface pretreatment and anodization process. Firstly, all samples were dry-polished using abrasive paper (grid #120 to #1200) to obtain a surface roughness of about Ra: 1 μm. The aluminum (50 mm × 30 mm × 0.5 mm) was degreased in acetone and then annealed at 500 °C for 5 h to remove mechanical stresses and recrystallize the aluminum. The surface of aluminum samples was electropolished in a mixed solution of perchloric acid and ethanol (1:4 in volume) under a constant voltage of 23 V for 2 min below 5 °C to obtain smooth-surfaced samples for anodization. Finally, the samples were rinsed with ethanol and dried. Anodic oxidation of aluminum was performed in an electrochemical cell, where the aluminum sheets were used as anode and another one as a counter-electrode. For samples treated by single-step anodizing process, the anodic oxidation of aluminum was run for a given duration (in the range of 7 h) in potentiostatic mode (25 V) using a sulfuric acid solution (1 M) thermally regulated (3 °C b T b 7 °C, typically at 5 °C). In the case of two-step anodizing process, the procedure involves two separate successive anodization processes; the first anodization step consists of a short period of anodization in potentiostatic mode (25 V) using a sulfuric acid solution (1 M) thermally regulated (3 °C b T b 7 °C, typically at 5 °C) for 3.30 h forming a disordered porous structure. The aluminum oxide formed during the first anodization step was removed by chemical etching in the bath consisting of 0.6 M phosphoric acid at 25 °C for 3.30 h followed by rinsing in distilled water and drying (Fig. 2). The second anodizing step was performed immediately after the oxide removal under the same conditions for 7 h [15]. The different stages of two-step anodizing aluminum are illustrated in Fig. 3. X-ray diffractograms were recorded with an X'Pert Pro apparatus from PANalytical using Cu Kα radiation (λ = 1.5425 A°). The surface morphology and elemental composition of the samples were investigated using a Quanta 200 scanning electron microscope (SEM). FTIR spectroscopy was used to determine the nature of the chemical bonds present in the anodic films. Alumina films were scratched, crushed and mixed with KBr in the form of pellets. The spectra were obtained using a spectrometer model FTIR-8400S SHIMADZU in the transmission mode. Electrochemical behavior of anodic films was studied under the potentiostatic control of a Radiometer Analytical type VoltaLab 40 PGZ301 potentiostat/galvanostat controlled by a computer using a Voltamaster 4 software for treatment and data acquisition, using the
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conventional three-electrode configuration, with a saturated calomel electrode (SCE) as reference and platinum plate as counter electrode. The exposed surface area of samples was 3 cm2. Electrochemical impedance measurement was carried out at room temperature by applying AC amplitude of 10 mV on open-circuit potential over the frequency range of 100 kHz to 1 mHz, with five points per decade. Electrochemical behavior of anodic films was studied by immerging of the anodized specimens in an unstirred and aerated 3% NaCl solution at different periods of immersion: 4, 6, 26, 28 and 30 h, respectively. Impedance diagrams were recorded immediately after anodization for different immersion times. The ZSimpWin software was used to adjust the experimental impedance data. 3. Results and discussion 3.1. Structural analysis The structural characteristics of anodic films, synthesized by singlestep and two-step anodizing processes in sulfuric acid are illustrated in Fig. 4. Amorphous alumina was obtained as it can be seen in Fig. 4. The XRD diffractograms exhibit intense peaks at 45° and 65° that can be assigned to face-centered cubic Al (JCPDS 04-0787). Similar results have been previously reported by Stojadinovic et al. [16]. They have found amorphous nature of anodic films formed by two-step anodizing process in sulfuric acid. Ghrib et al. [16] revealed an amorphization of the anodic films after annealing of the anodic films prepared by twostep anodizing process in sulfuric acid. Also, Vázquez et al. [17] have reported amorphous mesoporous alumina films produced by anodization in sulfuric acid after different pretreatment on aluminum substrate such as mechanical polishing and electropolishing. 3.2. Morphological analysis
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Fig. 5 (a, b) shows the surface topography of anodic films prepared in sulfuric acid by single-step and two-step anodizing processes. We can see that the surface morphology of anodic films produced in two cases is completely different. The surface morphology of anodic films produced by single-step anodizing (Fig. 5a) contains heterogeneously distributed holes due to the field-assisted dissolution of the oxide [18]. It is noteworthy that the surface in this case presents many craters having
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Highly ordered dimples Barrier layer
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Fig. 3. Schematic illustration of the anodization process.
these cavities to the evolution of oxygen during anodic alumina growth. As indicated previously, some authors have demonstrated that oxygen gas can be generated within films. Crevecoeur et al. [22] checked that gaseous oxygen was present in anodic oxide layers in the anodization of pure aluminum. Thus, it is possible that oxygen gas can be produced in anodic processes under the conditions used in our work. Because of the presence of oxygen bubbles at some locations of the films, the anodic oxide will only grow around them. This leads to the production of cavities within the anodic films. The cavities shown in Fig. 5e are intermittent, which indicates that the evolution of oxygen gas is also discontinuous [23].
few microns in length. This phenomenon can be explained according to Patermarakis et al. [19] by local interconnection of the pore mouths leading to formation of many small craters (Fig. 5a). But for films produced by the two-step anodizing process, the surface is fully covered like what appears in Fig. 5b. This type of structure produced by the last one process was also observed in a previous study with anodization process in tertiary mixed acid without pre-immersing process [20]. Also, Dasquet et al. [21] have observed the same microstructure of anodic films obtained by anodization process on 1050 and 2024 T3 aluminum alloys in solutions of boric acid. From Fig. 5b, the cell structure on the surface of the anodic film is more uniform and closer than that of the single process. Fig. 5 (c, d) shows cross-section micrographs of alumina formed by single-step and two-step anodizing aluminum before the corrosion tests. Knowing that these films were produced with almost the same duration for the anodization process (7 h), the single-step anodized sample had the thickest (about 7 μm and 153 nm for porous and barrier layers, respectively) coating but was non-uniform and had a porous morphology as shown in Fig. 5c. In contrary, the two-step anodized sample had the thickest coating (about 6 μm and 615 nm for porous and barrier layers). Few cavities are apparent in the case of single-step anodized sample as shown in Fig. 5e. The cavities are shielded by the upper alumina films. Obviously, formation of the cavities cannot be explained by the field-assisted dissolution mechanism. We ascribe
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3.3. Elemental analysis In order to investigate the compositional analysis of the developed alumina films, representative EDS spectra are shown in Fig. 6. From Fig. 6a, the major elements composing the films are: Al, O, C and S. The existence of sulfur indicates the presence of sulfuric species in oxide films incorporated (diffusion) during the film growth [15,16]. In the present case, EDS analysis reveals a strong content of aluminum and low content of oxygen on the surface of anodic film. According to Wang et al. [24], aluminum and oxygen are found in the film with different distribution in the two layers; in the outer layer it is uniform while it changes in the inner oxide layer (Al increases and O decreases). From
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Fig. 5. SEM micrographs of the anodic layer formed by: (a) single-step anodizing and (b) two-step anodizing, (c) cross-sections of porous alumina films formed by single-step anodizing and (d) cross-sections of porous alumina films formed by two-step anodizing and (e) cross-sections of porous alumina films formed by single-step anodizing (cavities and defects).
Fig. 6b, it can be found that the elements present are C, O, Al, S and P. The presence of S may originate from the solution (electrolyte), which embeds inside the anodic film. It is widely known that porous films formed
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from the electrolyte (chemical etching in phosphoric acid). Fig. 6b indicates that oxygen and aluminum concentrations are 57.98% and 31.07% (atomic %). The atomic ratio between oxygen and aluminum is nearly 1.86, which approaches to the theoretical value (1.5 for Al2O3). The explanation of the high C content is due to the use of graphite glue during analysis by SEM to ensure conductivity of specimens and their contacts with the specimen-holder. 3.4. Infrared spectroscopy The infrared spectra of the anodic films prepared by single-step anodizing (black line) and two-step anodizing (red line) processes are presented in Fig. 7. Both spectra have absorption bands at around 3640–3000 cm−1 and 1220–920 cm−1. Broad asymmetric absorption band between 3640 and 3000 cm−1 originates from H2O and/or OH group stretching vibrations are clearly visible, which may be due to the presence of physisorbed water. Also, the vibration of H2O in the range of 1640 to 1450 cm−1 is present. Typically band at 1650 cm−1 is the characteristic of the presence of water in the anodic films [25]. Relatively intense peaks appeared in the region of 3000–2800 cm−1 for the twostep anodized sample corresponding to the C\H stretching modes of aliphatic chains. In addition, from Fig. 7 a weak absorption band centered at 2339 cm−1 which reflects the carbon dioxide may be from the atmosphere and from impurities formed during the preparation of the pellets. Absorption bands at 1190 to 900 cm−1 are the results of S_O stretching vibrations [26]. Furthermore, we reveal a band corresponding to P\O bond, phosphorus is found therefore partially or completely in the form of phosphates [27]. This peak at 1240 cm−1 is observed only for twostep anodized sample which is assigned to the vibration arising from phosphate species incorporated into the film. Also, a weak band at 1730 cm−1 is observed for two-step anodized sample corresponding to carbonyl C_O normal stretching. For both samples, part of the spectrum from (420–757 cm−1) most bands was a characteristic of Al–O group of alumina. 3.5. Impedance spectra of anodic films Fig. 8 shows the evolution of the impedance diagrams versus immersion time in a 3% NaCl solution for the anodic films formed using both anodizing process in sulfuric acid. Nyquist plots are characterized by the presence of two capacitive loops. Several studies have already shown that the low frequency range corresponds to the barrier layer properties and the high and medium frequency ranges reflect the porous layer properties [28–32]. Particularly, the shapes of the single-
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5
step and two-step anodized specimens after 4 h of immersion in 3% NaCl solution are very similar. We observe that the diameter of the capacitive loop slightly increases with the increases of the immersion time for two-step anodized sample. As mentioned above, the anodic film on aluminum consists of a very thin compact layer and a thick porous layer, and the latter is composed of pores and walls of approximately hexagonal cells. In order to describe the electrochemical behavior of this system, the thin barrier layer and the porous layer are considered to be independent with each other. According to previous studies [30–33], the properties of each layer may be characterized by resistances and capacitances (in parallel and in series), describing their electronic and dielectric behaviors. Since the pioneering work of Hoar and Wood [28], various equivalent circuits have been proposed to model the response of porous anodic films. It is accepted that the porous anodic film may be modeled by the equivalent circuit proposed by Hitzig et al. [29], which is presented in Fig. 9. This model is successfully applied to explain the properties of the barrier and porous layers [34–40]. In this model, Rel is the resistance of the solution between the reference electrode and the anodic film. Rel is usually very small in NaCl solutions. One of the parallel branches in the circuit is formed by resistance Rw and the associate capacitance Cw; representing the walls of the cells. Rw and Cw are generally omitted since they are extremely high and extremely low, respectively. Indeed, the walls of hexagonal cells prevent the passage of current. The pore wall resistance and capacitance are neglected as it barely allows the conduction of ions through it [41]. In this way, equivalent circuit can be reduced as illustrated in Fig. 9b. The porous layer properties are characterized by capacitance Cp and resistance Rp. Barrier layer properties are described by capacitance Cb and resistance Rb, respectively. However, porous and barrier layers have heterogeneities that make their capacitive behavior better simulated by constant phase elements (CPEs) instead of simple capacitances (Cs). Hence, the EC can be simplified like what is presented in Fig. 9c, where CPEp represents the capacitance of the porous layer and CPEb represents the capacitance of the barrier layer. The constant-phase elements are used to account for the irregularities and variations of the properties of various layers [41]. According to Thompson et al. [34] CPE is defined as: α
CPE ¼ 1=ð2πJfCÞ
ð1Þ
αp and αb are affected by CPEp and CPEb, respectively, due to the nonideal capacitive behaviors [27]. Only when α = 1 can CPEp and CPEb be considered as real capacitances. For α = 0 the CPE is an ideal resistor. When α = 0.5 the CPE represents a Warburg impedance with diffusional character and for 0.5 b α b 1 the CPE describes a frequency dispersion of time constants due to local non-homogeneity in the dielectric material. In general, it is believed that the CPE is related to some type of heterogeneity of the electrode surface as well as to the fractal nature (roughness or porosity) of the surface [42]. Fitted experimental impedance diagrams are illustrated in Fig. 10. The parameters used to describe the electrochemical behavior of anodic films (porous and barrier layers) are presented in Table 1. Fig. 11 shows fit parameters for simulated spectrum to the resistance of the porous layer and barrier layer of anodic films for both cases in 3% NaCl solution. Rp values (Fig. 11a) reveal a different trend; a decrease of Rp for single-step anodizing process is observed with immersion times revealing a degradation of the porous layer which indicates that the porous layer is deteriorated wherein the anodic film presented more heterogeneity (more disorganized) and present many defects such as craters and cavities. It is well known that this structure favors the penetration of chloride ions in the oxide films, Cl− easily passed through the porous layer, causing its degradation, and therefore affecting its corrosion resistance. However, a gradual increase of Rp till the end of immersion time for two-step anodizing process is observed. Actually, Rp values of the anodic films are much higher in comparison with those formed by single one in NaCl medium. The Rp values are about (104 Ω cm2) at the beginning of immersion
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L. Bouchama et al. / Surface & Coatings Technology xxx (2013) xxx–xxx
a
b
Fig. 8. Nyquist plots as a function of immersion time in 3% NaCl solution at 25 °C, for the anodic layer formed by: (a) single-step anodizing and (b) two-step anodizing.
time in the chloride solution then it increased during the whole immersion time. These values are relatively similar with those obtained by some authors [43,44]. This indicates that the porous layer in this case is not deteriorated due to the efficient protection of the film which prevents electrolyte penetration. The increase of Rp with immersion time in this case could be attributed to a self-sealing phenomenon [45,46]. As hydration and the aging process developed (as the exposure time increases), the anhydrous alumina reacts with absorbed water, which leads to voluminous hydrated alumina and resulting in a self-sealing effect. Thus, the resistance of the porous layer increases. The self-sealing mechanism involves degradation, gelling, agglomeration and precipitation process [31]. As there is a reduction of free water content inside the pores, it is in the formation of hydrated alumina, and the hydrated alumina permittivity is lower than the alumina and free water permittivity. The CPEp value obviously decreases. So, lower CPEp values indicate that there is less free water in the porous layer of anodic films and a better protectiveness of the porous layer from the corrosion solution. As the resistance of the porous layer of anodic films, higher Rp values show a more difficult penetration of the aggressive electrolyte into the anodic film on the aluminum. However, a remarkable increase in CPEp in the case of the twostep anodizing may be due to the contamination of anhydrous alumina
a
by sulfate and phosphate ions which can increase the anodic layer permittivity. At the same time, the anodic film may also be attacked by aggressive ions from the electrolyte. The conversion of alumina to hydrated alumina may be retarded because they have the aggressive ions in the electrolyte and also a hard penetration of electrolyte. For single-step anodizing sample, CPEp remains lower and constant during immersion time. CPEp depends principally on the oxide roughness not on the porous layer thickness [47]. The oxide roughness (porosity in surface) is more porous for the anodic film in this case, which is confirmed by micrograph SEM results (Fig. 5a). Therefore, the morphology of anodic film is an important factor which can explain these differences in behavior; the morphology of oxide films influenced both chloride adsorption and penetration. Indeed, the increase of porosity favors electrolyte penetration through the anodic film. So, single-step anodizing sample has the highest porosity and the quickest chloride penetration. Another parameter which can explain this fact is the compactness. Film formed by twostep anodizing process is more compact and denser; the electrolyte penetration through the passive film is hindered in this case. The parameter αp (Fig. 13a) takes into account the non-ideal capacitive behavior of porous layers. As the initial αp values are far from 1 for single-step anodizing process, the heterogeneity of porous layer seems
Rel
i)
Cw
Rel: electrolyte resistance Rw : hexagonal cells resistance Cw : hexagonal cells capacitance Rp: electrolyte resistance through pores Cp : electrolyte capacitance through pores Rb: barrier layer resistance Cb : barrier layer capacitance
ii)
Rw
Cp
Cb
Rp
Rb
i) wall of hexagonal cells ii) porous layer: pores iii) barrier layer
iii)
b
Cp
Cb
c
CPEp
CPEb
Rp
Rb
Rel
Rel
Rp
Rb
Fig. 9. Equivalent circuits (ECs) for modeling the behavior of anodic films. (a) General model, (b, c) simplified model.
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L. Bouchama et al. / Surface & Coatings Technology xxx (2013) xxx–xxx
a
7
b
Fig. 10. Fitting of the Nyquist plots immersed in 3% NaCl solution at 25 °C for 6 h: (a) single-step anodizing and (b) two-step anodizing.
important, it is strongly disorganized and present of many defects. Also, porous layer is modified in the composition and morphology because of their hydration. Indeed, it is contaminated by species from electrolyte such as sulfate ions (from sulfuric acid) [7]. Fig. 11b presents the variation of the fitted Rb values as a function of exposure time in the 3% NaCl solution for both anodized samples formed in sulfuric acid. For the anodic films formed by two-step anodizing process, the Rb values increased with immersion time and remain high, indicating that the barrier layer is unattacked. This is consistent with the fact that the porous layer is homogeneous and less porous (compactness of porous layer); this surface seems to be more effective against electrolyte penetration, retards further corrosion and prevents the degradation of the barrier layer. Thus, less porous surface improves corrosion performance by restriction of the entrance of chloride to the passive film [7]. In contrast, for single-step anodizing process, the Rb values decrease slowly during the immersion time, indicating that a significant degradation of the barrier layer due to the low protection afforded by the porous layer is due to the easy penetration of electrolyte through porous layers to attack barrier layer. The decrease of Rb during the immersion time is related to pitting and the increase of Rb at 26 h of immersion time can be explained by formation of corrosion products after pitting. It is well known that the capacitance Cb can be connected with the barrier layer thickness eb, by the relation: Cb ¼ ζ r ζ o S=eb
ð2Þ
where ζo = 8.85 10−14 F cm−1 is the dielectric constant in vacuum, ζr = 10 is the relative constant for alumina and S = 3 cm2 is the electrode surface [29]. Relation (2) is acceptable if αb is close to 1 [47], when the CPE is close to an ideal capacitance essentially at 6 h of immersion for two-step anodizing (see Table 1), this corresponds to a barrier layer that is relatively homogeneous, Cb values are connected to barrier layer thicknesses (eb); and eb is calculated to be approximately 617 nm. For anodic film formed by single-step anodizing process, the CPE is close to an ideal capacitance essentially at 26 h of immersion, eb is 160 nm. The eb values calculated with relation (2) are in good
agreement with those obtained by cross-section micrographs of the anodic films which are respectively 615 nm and 153 nm for two-step and single-step anodizing processes. The parameter αb (Fig. 13b) takes into account the non-ideal capacitive behavior of barrier layers [47]. Indeed, barrier layers can be modified in composition and in morphology [48]. This alters their homogeneity. The homogeneity of the barrier layer seems to decrease as the hydration process progresses, as the decrease of αb value indicates. After about 30 h, the αb value is close to 0.4, reflecting the increase of defects in the barrier layer for single-step anodized sample. Further investigations to determine the action of two-step anodizing process on the barrier layer would be necessary. The results show that the compactness and homogenization and absence of defects such as cavities in the anodic films may give different results, thus this novel process seems to increase barrier layer thickness. From Fig. 5 (c, d) cross-section micrographs of the anodic alumina films check that the barrier layer formed by this technique is thicker, more compact and homogeneous (see model proposed in Fig. 3) and this results in a development and growth of the barrier layer during the second anodization step, which explains why Rb value is the highest with this process, and therefore the barrier layer becomes more resistant against electrolyte penetration. The evolution of CPEb as a function of immersion time shows higher values of CPEb for the single-step anodizing attributed to a thinner barrier layer (Fig. 12). The better corrosion resistance of the two-step anodizing anodic film is not related to the greater thickness of its oxide layer but depends on the barrier layer thickness and the more organized structure; the presence of defects for the single-step anodizing anodic film is actually affected by its corrosion resistance.
4. Conclusion In this work, the corrosion resistance of anodic layers on aluminum produced by single-step and two-step anodizing processes was studied by EIS and correlated to the microstructure of the anodic films. A series
Table 1 Fitting results for the EIS data acquired for layers immersed in 3% NaCl solution for different time concerning both single-step and two-step anodizing processes. Elements circuit Single-step anodization
Two-step anodization
Immersion time (h)
Rel (Ω·cm2)
CPEp (Ω−1·cm−2·sα)
αp
CPEb (Ω1·cm−2·sα)
αb
Rel (Ω·cm2)
CPEp (Ω−1·cm−2·sα)
αp
CPEb (Ω−1·cm−2·sα)
αb
4 6 26 28 30
56.37 61.50 61.02 47.50 36.14
1.25 8.39 15.25 21.43 22.34
0.81 0.86 0.56 1.00 1.00
262800 0.68 0.11 30.49 39.80
0.47 0.88 0.98 0.46 0.43
60.72 57.11 51.11 50.93 46.40
0.35 8.47 1618.00 3816.00 3952.00
1.00 0.69 0.79 1.00 1.00
1.76 4.30 5.55 5.85 5.91
0.63 1.00 0.62 0.62 0.61
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L. Bouchama et al. / Surface & Coatings Technology xxx (2013) xxx–xxx
b
a 60
40
Rb (KΩ.cm2)
Rp (KΩ.cm2)
60
Two-step anodization
Single-step anodization
40 Two-step anodization Single-step anodization
20
20 0 0
5
10
15
20
25
0
30
5
10
Immersion time (h)
15
20
25
30
Immersion time (h)
Fig. 11. Variation of Rp and Rb as a function of immersion time for anodic layer formed by: (a) single-step anodizing and (b) two-step anodizing.
b
250000
3000
CPEb (Ω-1.cm-2.sα)
CPEp (Ω-1.cm-2.sα)
300000
a
4000
200000 Single-step anodization
150000
2000
Two-step anodization
100000
1000
Single-step anodization
Two-step anodization
50000
0
0 0
5
10
15
20
25
0
30
5
10
15
20
25
30
Immersion time (h)
Immersion time (h)
Fig. 12. The changes in the CPEp and CPEb parameters of the anodic films in 3% NaCl solution with immersion time.
a
b 1,0
1,0
0,9 0,9
Two-step anodization
Single-step anodization
0,8
αp
αb
0,8 0,7
0,7 0,6 Two-step anodization
0,5
0,6 Single-step anodization
0,5
5
10
15
20
25
0,4 30
5
10
15
20
25
30
Immersion time (h)
Immersion time (h)
Fig. 13. The changes in the αP and αb parameters of the anodic films in 3% NaCl solution with immersion time.
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