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Achieving non-adsorptive anodized film on Al-2024 alloy: Surface and electrochemical corrosion investigation
Surfaces and Interfaces 15 (2019) 78–88
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Achieving non-adsorptive anodized film on Al-2024 alloy: Surface and electrochemical corrosion investigation
T
M. Faizan Khana,c, , A. Madhan Kumarb, , Anwar Ul-Hamidc, Luai M. Al-Hemsc ⁎
⁎
a
Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia c Center for Engineering Research, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia b
ARTICLE INFO
ABSTRACT
Keywords: AFM Al2024 alloys Anodized film Corrosion EIS XPS
Naturally formed oxide film on the surface of aluminum reduces corrosion rate to significant extent. Anodizing of aluminum is done to enhance this feature by increasing film thickness deliberately to desired level. In the current work, anodizing of Al-2024 alloy was carried out in sulfuric-acetic acid anodizing bath of different concentrations along with gradually increased applied potential. The structure and morphology of the developed anodized film was investigated with and without the addition of acetic acid. Structural and surface analysis results revealed the difference in the anodized layer formation with the presence of acetic acid in sulfuric acid bath. Electrochemical investigation was carried out to evaluate the surface protective performance of the formed anodized film in 3.5% NaCl solution. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) revealed that lowest corrosion rate with highest polarization resistance was achieved due to the presence of CH3COOH in the anodizing bath. Non-adsorptive and less porous anodized film noticed to be formed with the presence of CH3COOH in bath solution.
1. Introduction Anodizing investigation of Al alloys has been remained under deep interest since from a long time and attracted significant attention of researcher from decades [1–7]. In electrochemical process, when potential is applied high enough on the anode part of the electrochemical cell then this allows oxygen to accumulate on the Al (anode) surface and results film formation, is called anodization. In fact, film is developed as a result of electrochemical reaction in between metal surface and electrolyte ions and an overall increase in volume occur on Al substrate as a result of anodizing. The formation of anodic oxide layer is the result of the movement of Al3+ and O2− ions towards each other across the barrier layer region [8]. The O2− movement becomes the primary cause of oxide layer formation on metal-film interface. Fieldassisted flow of alumina at a certain region does not allow to the formation of continuous oxide layer and, hence, a porous oxide layer is developed [9]. The detailed mechanism about porous film formation and growth can be understood somewhere else [10–14].
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Anodizing film can be thin, thick, dense or porous depending on the employed potential, bath composition and concentration. The surface morphology of the film is porous by nature and determines the adsorptive characteristics as well as abrasion resistance. In the latter case, coatings with a less number of smaller diameter pores will have higher resistance to abrasion than coatings with a large number of greater diameter pores. This makes sense as the density of the anodized coating is proportional to the amount of oxide formed and hence the pores size starts decreasing with continuous oxide formation thus improving the abrasion resistance of the coating [15]. Peter et al. [16] studied the anodization of Al alloys and found the optimum conditions by varying the electrolytic composition of sulfuric/tartaric acid, sulfuric/carbolic/ boric acid and sulfuric/oxalic/boric acids. Their investigation revealed that corrosion current density found to be decreased leading to reduce corrosion rate by increasing polarization resistance (RP). It was also observed in their study that anodized film developed in sulfuric/tartaric acid bath was better than the rest of the baths. Li et al. [17] studied anodizing on Al-Si substrate by three different techniques; (i) hard
Corresponding author. E-mail address: [email protected] (A.M. Kumar).
https://doi.org/10.1016/j.surfin.2019.02.005 Received 25 September 2018; Received in revised form 7 January 2019; Accepted 11 February 2019 Available online 12 February 2019 2468-0230/ © 2019 Published by Elsevier B.V.
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far literature survey reveals that all the developed anodized films possess porous adsorptive surface due to the presence of pores over its surface. This porous surface causes a big trouble by inviting the corrosive media to flow inside the film through these pores and deteriorate the film severely and, thus, reduces the overall integrity of the film. Externally applied sol-gels are being utilized to block these pores [27] but these gels add the additional cost on industrial level and their compatibility towards echo-friendly environment yet has not been proven. In the current work, the presence of acetic acid in varying concentration of sulfuric acid bath is investigated under different applied potentials to understand its impact on the anodized film thickness and non-adsorptive morphology of Al-2024 substrate. Evaluating surface morphology of the anodized film and the electrochemical performance in corrosive environment is the part of this research work.
Table 1 2024 aluminum alloy elemental compositions determined by XRF. Element
Al
Zn
Mg
Cu
Fe
Si
Mn
Ni
Ti
wt.%
93.97
0.085
1.29
3.80
0.16
0.063
0.58
0.004
0.03
Table 2 Chemical composition, temperature and holding time for surface treatment. Surface Treatment
Solution composition
Temp. (°C)
Time (min)
Chemical polishing
54 mL H3PO4 (85%) 2 mL HNO3 (66.4%) 15 mL CH3COOH (99%) 13 mL H2O 35 mL HNO3 (66.4%) 65 mL H2O
90
4
25
2
Deoxidation
2. Experimental work 2.1. Material 2024 Aluminum alloy sheet, having thickness of 3 mm, was cut for preparing samples to investigate anodization. The composition of the material was determined by x-ray fluorescence (XRF) technique and given in Table 1.
anodizing (HA), (ii) simple anodizing and (iii) modified anodizing (MA). They found different hardness against each film developed by different techniques and each film was sufficiently resistive to corrosion. However, they reported that modified anodizing technique was more beneficial due to its environmental friendly process. Wielage et al. [18] found the effect of aluminum substrate surface (roughness) on the anodized film thickness and morphology. Forn, et al. [19] studied the effect of additional particles in Al-substrate and noticed increased film thickness by the presence of discussed particles. The tribological properties of this thick oxide layer were reported tremendous in sense of wear resistance. Fratila-Apachitei et al. [20] reported the growth of anodized film by taking three different aluminum substrates and applied three different current densities in 2.25 M H2SO4 bath at 0 °C. Similarly, Morks et al. [21] did their experiment to grow the anodic oxide layer on aluminum alloy in 5-sulfosalicylic acid electrolyte and studied the corrosion resistance in different concentrations of electrolytes at different temperatures. Saeedikhani et al. [22] demonstrated the improvement in corrosion resistance of 2024-T3 aluminum alloy by anodization in mixed electrolyte containing 10% sulfuric acid, 5% boric acid and 2% phosphoric acid. Zhang et al. [23] investigated the addition of boric acid in the presence of sulfuric acid electrolyte to study the effect of current density-time response, growth rate and morphology of the porous anodic film. Shanmuga et al. [24] did their research on AC anodization technique by taking Al substrate in an electrolyte containing 10% sulfuric acid, 3% sodium sulfate, organic additives SLS and gelatin. They revealed that the presence of gelatin increases the compactness of the coating. Xiang Feng et.al revealed through their research that the addition of citric acid in sulfuric acid bath improved anticorrosion behavior of the anodized film [25]. Ying-dong et.al showed in their research that the addition of adipic acid in sulfuric acid bath enhanced dielectric properties and corrosion resistance [26]. So
2.2. Surface preparation For the pretreatment of aluminum alloy, samples were first of all degreased using acetone solvent followed by rinsing in distilled water. The samples were then grinded up to 800 grit size of SiC abrasive paper followed by alkaline cleaning in a solution of 12 g NaOH/100 ml of distilled water at 60 °C for 3 min. In the next step, chemical polishing of the samples were carried out in a mixture whose composition is provided in Table 2. Finally, the samples were treated in a solution (as per the technique given in Table 2) to deoxidize the surface. 2.3. Anodizing and sealing Aluminum strips were anodized under two applied potentials (15 V and 25 V) in electrolytic bath of sulfuric acid (H2SO4) whose composition and concentration was changed with and without the addition of acetic acid (CH3COOH). For each anodizing process, potential was provided by using D.C power supply whereas time and temperature was kept constant to 30 min and 25 °C, respectively. The scheme for each anodizing process has been provided in Table 3. After completing anodizing process, samples were cleaned rigorously using distilled water and then sealing was carried out by treatment in near-boiling distilled water to convert oxide lining of the pores from amorphous oxide to a non-hydrate. However, sealing mechanism of the developed anodized film not only increases the volume of the film but also blocks the pore openings by rendering the film morphology to get transformed from adsorptive to non-adsorptive [19].
Table 3 Electrolytic bath composition, concentration and applied potential used for anodizing of aluminum at room temperature for 30 min. Sample
Electrolyte composition
Voltage (V)
Sample
Electrolyte composition
Voltage (V)
AN-1 AN-3 AN-5
5% wt. H2SO4 10% wt. H2SO4 5% wt. H2SO4 +2% CH3COOH
15 15 15
AN-2 AN-4 AN-6
5% wt. H2SO4 10% wt. H2SO4 5% wt. H2SO4 +2% CH3COOH
25 25 25
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Fig. 1. Film thickness of different anodized Al samples achieved at (a) 5%H2SO4 @15 V, (b) 5%H2SO4 @25 V, (c) 10%H2SO4 @15 V, (d) 10%H2SO4 @25 V, (e) 5% H2SO4/2%AA @15 V, (f) 5%H2SO4/2%AA @25 V.
2.4. Microstructure analysis
AFM, USA) instrument. The acquired AFM images were detected with non-contact mode of Au coated silicon cantilevers with a resonance frequency of 26 kHz with a spring constant of 1.6 N/m under air environment. The chemical structure of anodized Al samples was characterized by an Attenuated Total Reflectance-Infrared spectrometer (Thermo scientific, with universal ATR attachment, range 500–4000 cm−1).
The anodized samples were analyzed for measuring film thickness and morphological study. Samples were cold mounted followed by grinding using up to 1200 emery paper. In the next step, polishing of the mounted samples was carried out by diamond paste and HF was used to etch the anodized aluminum samples. Scanning electron microscope (SEM) JEOL JSM-6480 LV was used for obtaining images as well as measuring anodized film thickness. X-Ray Photoelectron Spectroscopic (XPS) measurements have been done to evaluate the particular surface elemental compositions as result of the alterations, produced by the relevant anodization techniques. The surface topography of anodized samples was analyzed through AFM (Agilent 5500
2.5. Electrochemical corrosion testing The general corrosion resistance of the anodized samples was investigated by potentiodynamic polarization (PDP) technique under room conditions using Gamry 3000 potentiostat. The potentiodynamic
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0.2 mV/s. Electrochemical impedance spectroscopy (EIS) was measured by applying AC voltage having an amplitude of 10 mV and the frequency was set from 100 K to 10 mHz. 3. Results and discussion 3.1. Anodizing film thickness The film thicknesses obtained by variation of the applied voltage and electrolytic composition under fixed temperature (25 °C) and time (25 min) have been provided in Fig. 1. In addition to this, the film thicknesses of different anodized samples have also been depicted graphically in Fig. 2. It can be seen through the graphs (Fig. 2) that higher applied voltage of 25 V provided thicker anodized film (AN-2, AN-4 and AN-6) compared to lower applied voltage of 15 V (AN-1, AN-3 and AN-5). However, when the applied voltage was fixed to 15 V then the sample which was anodized in a mixer of 2%wt. acetic/5%wt. sulfuric acid (AN-5) provided thicker anodized film (9.5 µm) compared to the samples which were anodized just in the presence of sulfuric acid (5 wt.% = 7.8 µm, 10 wt.% = 8.5 µm). Interesting thing to note that the addition of only 2 wt.% acetic acid in 5 wt.% sulfuric acid (AN-5) provided thicker anodized film even than 10 wt.% sulfuric acid (AN-3). Nevertheless, 10 wt.% sulfuric (AN-3) provided slightly thicker anodized film compared to 5 wt.% sulfuric acid (AN-1). It is worth noting from the anodized thickness analysis that concentration of the bath solution has less effect for achieving thickened anodized film compared to the applied potential. The same trend of changing the film thickness with the composition of electrolyte was seen to repeat when the applied voltage was enhanced to 25 V. Again the addition of 2 wt.% acetic acid in 5 wt.% sulfuric acid (AN-6 = 12.4 µm) provided nearly equal film thickness to the anodized film developed in 10 wt.% sulfuric acid (AN-4 = 12.7 µm) bath. It can be stated that anodized film thickness for the case of aluminum increased with enhanced applied voltage and concentration of sulfuric acid in the electrolyte. But little addition of acetic acid (2 wt.%)
Fig. 2. The effect of applied voltage and electrolyte composition on anodizing film thickness at 25 °C for 30 min.
polarization (PDP) measurements were carried out by a conventional three-electrode cell using a saturated calomel electrode (SCE) as a reference electrode and a graphite rod as a counter electrode. When the electrochemical system was stable, the measurements were taken in a 3.5% NaCl solution (sea water) and the corrosion rates were determined for comparison. Before conducting electrochemical test, open circuit potential (OCP) of all the tested samples was measured until to get stabilized potential. For conducting PDP, the applied initial and final voltage was set to ± 250 mV whereas the scan rate was fixed to
Fig. 3. X-Ray diffraction patterns of anodized Al samples under different electrolytic compositions and applied potentials.
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Fig. 4. Scanning electron micrographs (SEM) of anodized surface (a) 5%H2SO4 @15 V, (b) 5%H2SO4 @25 V, (c) 10%H2SO4 @15 V, (d) 10%H2SO4 @25 V, (e) 5% H2SO4/2%Acetic acid @15 V, (f) 5%H2SO4/2%Acetic acid @25 V.
along with sulfuric acid cut off the concentration level of sulfuric acid in the electrolytic bath to achieve the required anodized film by fixing the applied voltage. In other words, in the presence of acetic acid, overall less concentration of sulfuric acid can provide the same thickness which should be achieved at higher concentration of sulfuric acid. To obtain required anodized film, decreasing the concentration of sulfuric acid and replacing it partially with acetic acid is a step towards ecofriendly anodizing.
only Al peaks appeared. However, once anodizing was done in different electrolytes then it can be seen that anodized film was formed and mainly composed of two phases i.e. γ-Al2O3, α-Al2O3 [25,28]. As the thickness and volume of the porous anodized film is very less compared to bare Al, therefore the x-rays are diffracted from base aluminum with greater peaks intensity which is in agreement with the study of Yerokhin [28]. From the XRD patterns of each anodized sample, it is cleared that voltage played an effective role along with electrolyte percentage of the bath solution for the formation of crystalline phases. For example, for AN-1, in 5% H2SO4 bath solution, it can be seen that 15 V assisted in the formation of just α-Al2O3 phase whereas AN-2, at the same electrolytic percentage, formed an additional γ-Al2O3 crystalline phases when voltage was enhanced to 25 V. Nevertheless,
3.2. Surface characterization X-ray diffraction pattern of different anodized Al-2024 samples have been provided in Fig. 3. It can be seen that, for the bare Al-2024 sample,
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Fig. 5. ATR-IR spectra of AN-4 and AN-6 anodized samples.
Fig. 6. (a) XPS survey spectra, (b) Al2p and (c) O1s deconvolution spectra of AN-4 and AN-6 samples.
increase of electrolytic percentage at both voltages (15 V and 25 V) assisted the formation of both discussed crystalline phases. The only difference in the XRD pattern from AN-2 to AN-6 samples was noted to be the variation of phase intensities of both γ-Al2O3, α-Al2O3.
Table 4 Surface roughness parameters of TNZ substrates. Substrates
Ra (µm)
Rp (µm)
Rq (µm)
Rz (µm)
Rv (µm)
AN-2 AN-4 AN-6
0.0784 0.132 0.0761
0.306 0.440 0.293
0.105 0.163 0.101
0.547 0.780 0.583
0.197 0.34 0.29
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partially blocked the pore openings of the developed anodized film. But still pores openings can be seen on top surface. Nevertheless, once the applied anodizing potential was increased to 25 V, then those agglomerated particles got bigger in size and took spherical shape and can be seen from Fig. 4f. These spherical shaped agglomerated particles blocked the pore openings of the anodized film. The relation in between increased potential and the morphology of particles could not be understood clearly, however, it seems that the applied increased potential set a gradient towards increased kinetic movement of the particle thus assisting them to get bigger in size. ATR-IR spectra of anodized Al alloy samples is presented in Fig. 5. IR spectra of acetic acid that we recorded is also presented for the reference. In the case of AN-4, the characteristic vibrations of AleO bonds in the region of 500–800 cm−1 which confirm that the anodized layer mainly composed of Al2O3 [29]. On the other hand, IR spectra of AN-6 samples, the peaks appeared in the region of 1000–2000 cm–1 are associated with the vibrational frequencies of bonds in individual structural units of acetic acid molecules and in adsorbed carbon-containing structures, contains absorption bands with different intensities. Attachment of carboxylate groups by metal ions causes a change in the IR spectrum of the acid in the region of the valence vibrations of CeO bonds in this group. The obtained results further corroborates that acetate ions exist in the anodic layer with coordination bonding to Al cations. The obtained data is well agreement with the previous reports [30,31]. The chemical composition and the state of surfaces of anodized Al alloy samples were analyzed using XPS measurements and the resultant survey spectra is displayed in Fig. 6a. From the observation of survey
Table 5 Potentiodynamic polarization parameters of Al-2024 tested in 3.5% NaCl solution at 25ᵒC. Sample no.
βa (V/decade) (x10−3)
βc (V/decade) (x10−3)
icorr (10−6 A/cm2)
Ecorr (mV)
Corrosion rate (mpy)
Bare-Al AN-1 AN-2 AN-3 AN-4 AN-5 AN-6
45 34 59 32 269 70 61
665 82 78 61 232 483 165
4.1 1 0.003 0.72 0.001 0.018 0.0004
−581.0 −576 −534 −635 −632 −604 −446
0.643 0.553 0.002 0.12 0.001 0.009 0.0003
Surface morphology of anodized Al-2024 films can be seen from the scanning electron microscopic (SEM) images provided in Fig. 4 (a–f). It can be seen from the images that those samples (AN-1, AN-2, AN-3, AN4) which were anodized in just sulfuric acid (H2SO4) bath provided the same surface morphology regardless of the concentration of (H2SO4) and applied potential. It can be seen from Fig. 4 (a–d) that all the samples possessed porous anodized film. However, the addition of 2 wt. % acetic acid in the electrolyte solution brought a tremendous change in the surface morphology of the anodized film and can be seen from the provided images in Fig. 4 (e and f). It can be seen from Fig. 4e that, at 15 V, in the presence of 2 wt.% acetic acid, the nucleation of particles happened followed by agglomeration. The agglomerated particles
Fig. 7. AFM surface topographic images of (a) AN-2 (b) AN-4 and (c) AN-6 samples.
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spectra, the presence of C, O and Al was identified without any contaminated elements which further confirms the formation of anodized films on Al samples. The photoelectron peak for Al2p Fig. 6b seems evidently at a binding energy of 75.05 eV, C1s at 281.25 and O1s at 531.50. The estimated binding energy from the monitored oxygen photoelectron lines positioned at 531.50 eV is the sign for the oxidation of Al in acid bath. The deconvoluted O1s spectra (Fig. 6c) are comprised of a chief peak with a binding energy of 530.13 eV and a minor peak with a binding energy of 529.24 eV. The peaks with 530.13 and 529.24 eV are ascribed to the hydrated alumina Al(OH)3, and oxyhydroxide AlOOH respectively [32]. From the acquired O1s spectra, it was indicated that the growth of uniform oxide layer is obtained on Al alloys by anodization process. Moreover, the calculated O:Al ratio for AN-4 and AN-6 samples is about 2.4 and 4.1 respectively, which further specifies that the higher O:Al ratio obtained in acetic acid bath due to the hydration of anodized layer. 3.3. Surface topographic analysis through AFM technique The surface topographic images of anodized surfaces were obtained using AFM techniques and the results are presented in Fig. 7 (a, b and c). The surface of AN-2 (Fig. 7a) exhibited non uniform surface with uneven pores, whereas AN-4 (Fig. 7b) was observed to be of typical porous topographic surface with a void micropores. On the other hand, AFM image of AN-6 (Fig. 7c) displayed uniform surface with a few micropores. In order to attain more vision on the surface topography and the porosity, line profile analysis was performed on the anodized surfaces and surface profile images are presented in Fig. 7. In addition, the change in the roughness of the PEO coatings was evaluated and the obtained parameters are summarized in Table 4. Generally, Rq (root mean square, RMS) and Ra (average surface roughness) are considered as significant parameters for investigating surface topographic features as its offering an indication about the frequency of deviances from a flat surface by inspecting a continuous surface profile [33]. It is clear that Rq comparatively reduces with acetic acid additions in the anodization bath, which showed that the presence of acetate ions on the anodized layer pointedly influenced the porosity and the surface roughness. The lower roughness values of AN-6 samples revealed smoother, more compact and comparatively less porous than other samples. The size and number of pores in AN-6 samples are reasonably low and small which indicated that the acetic acid in anodization bath pointedly impacts the surface topography of anodized Al alloy samples. 3.4. Electrochemical corrosion analysis Potentiodynamic polarization results of the anodized aluminum alloy samples tested in a 3.5% NaCl solution are given in Table 5. The PDP curves have been provided in Fig. 8 (a–c). It can be seen from the Fig. 8(a) and Table 5 that anodizing reduced the corrosion current density considerably compared to the bare Al-2024. Bare Al-2024 provided highest corrosion rate and corrosion current density. However, anodizing of Al in 5 wt.% H2SO4 bath solution at 15 V reduced both the current density and corrosion rate and provided 1 × 10−6A/ cm2 and 0.553 mpy, respectively. But anodizing done at 25 V (AN-2), in the same bath solution, reduced corrosion current density and corrosion rate dramatically and provided 3 × 10−9A/cm2 and 0.002 mpy, respectively. The current density and corrosion rate reduced 333 and 276 times, respectively, when the applied potential was increased from 15 V to 25 V. The dramatic reduction in corrosion rate and current density
Fig. 8. Potentiodynamic polarization (PDP) graphs of anodized Al-2024 samples tested in 3.5% NaCl solution.
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Table 6 Electrochemical impedance spectroscopy (EIS) simulations of Al-2024 anodized film in 3.5% NaCl. Sample
RSol (Ω.cm2)
R (Ω.cm2)
Rp (Ω.cm2)
Rb (Ω.cm2)
CPEPW S*sa
CPEp S*sa
CPEb S*sa
apw
ap
W S*s1/2
Bare-Al AN-1 AN-2 AN-3 AN-4 AN-5 AN-6
4.9 5.8 7.0 4.3 6.0 4.4 3.9
5.4 × 103 1.1 × 103 1.3 × 104 6.7 × 103 2.7 × 104 3.3 × 104 7.2 × 104
1.8 × 102 2.2 × 102 1.7 × 104 1.2 × 104 1.9 × 105 1.1 × 105 2.3 × 105
1.2 × 103 5.1 × 103 3.9 × 106 1.5 × 104 7.5 × 106 1.1 × 106 8.9 × 106
3.1 × 10−7 1.2 × 10−7 3.3 × 10−7 1.5 × 10−7 4.1 × 10−8 9.9 × 10−8 7.8 × 10−8
2 × 10−1 5.8 × 10−7 7.7 × 10−7 3.9 × 10−5 4.1 × 10−8 2.4 × 10−6 7.8 × 10−8
7.2 × 10−6 2.9 × 10−5 6.0 × 10−7 1.9 × 10−5 8.2 × 10−2 1.4 × 10−6 1.1 × 10−6
8.4 × 10−1 8.6 × 10−1 7.9 × 10−1 7.9 × 10−1 8.6 × 10−1 8.5 × 10−1 8.6 × 10−1
6.7 × 10−1 1 1 7.0 × 10−1 6.1 × 10−1 3.9 × 10−1 3.6 × 10−1
1.6 × 10−2 8.2 × 10−3 8.5 × 10−6 3.7 × 10−3 2.9 × 10−3 1.8 × 10−3 2.3 × 10−4
can be attributed due to the thicker anodized film formation at 25 V (10.5 µm) compared to film which developed at 15 V (7.8 µm). Anodizing of aluminum 2024 in 10 wt.% H2SO4 bath solution reduced both the current density and corrosion rate compared to 5 wt.% solution. Like in 5 wt.% solution, again anodizing potential of 25 V decreased current density and corrosion rate to significant extent compared to 15 V applied potential. It can be seen from Table 5 and Fig. 8(b) that, in 10 wt.% H2SO4 electrolyte solution, 25 V applied potential (AN-4) provided current density and corrosion rate 1 × 10−6 A/ cm2 and 0.001 mpy, respectively. This is 720 and 120 times less, respectively, compared to those samples which were anodized at 15 V. Reducing of current density and corrosion rate seems to be the function of thickness of anodized film. At 25 V, AN-2 developed film thickness of 10.5 µm and hence provided 3 times higher current density compared to AN-4 which developed 12.7 µm thicken film. Nevertheless, addition of just 2 wt.% acetic acid in 5 wt.%H2SO4 bath solution provided astonishing effect by reducing the current density and corrosion rate to many folds. It can be seen from Table 5 and Fig. 8(c) that, at 15 V, the addition of 2 wt.% acetic acid in 5 wt.% H2SO4 (AN-5) provided current density and corrosion rate of 1.8 × 10−8 A/cm2 and 0.009 mpy, respectively, which is 40 times less than its counterpart AN-3. Increased anodizing potential to 25 V (AN6), in the same bath composition, provided lowest current density and corrosion rate which was noted to be 4 × 10−10 A/cm2 and 0.0003 mpy. This current density and corrosion rate noted to be reduced 2.5 and 3.3 times, respectively, compared to the that sample (AN-4) which was anodized at the same potential (25 V) in 10 wt.% H2SO4 (without the presence of acetic acid). It is well cleared that the addition of acetic acid not only increased the thickness of the anodized film but also blocked the pore openings of the film which can be seen from Fig. 4(f). Electrochemical impedance spectroscopy (EIS) was done on anodized Al-2024 samples in 3.5% NaCl solution and the spectra obtained are provided in Fig. 9 (a–c). Fig. 10 provides the schematic illustration of porous anodized film by superimposing equivalent circuit on it. This equivalent circuit has already been reported somewhere else but without the Warburg element [27]. In this equivalent circuit, Rs represents solution resistance, R general resistance, Rp pore resistance and Rb barrier film resistance. Rb is considered actual corrosion resistance film [34]. Whereas, CPEp, CPEpw and CPEb represent constant phase elements at pore, pore walls and barrier film, respectively.
Constant phase elements were used instead of capacitors because of the non-ideal nature of the developed film. The non-ideal behavior of the film reflects defects and surface roughness. Once the data obtained from electrochemical impedance spectroscopy was fitted on given equivalent circuit then the obtained values of circuit elements are provided in Table 6. The simulated model of porous anodized aluminum in Fig. 10 is in agreement with the referenced literature of sealed anodized film [35–38] and represents that how sealing mechanism took placed inside the pore openings. This reveals that when an unsealed porous anodized film was exposed to hot boiling water then these pores were filled initially by hot boiling. Afterwards, auto-sealing mechanism took placed progressively throughout the pore depth by dissolving anhydrous alumina from the pore walls. To next step, precipitation of these hydrated alumina particles took placed which resulted pore widening leading to progressive pore blocking [38,39]. From SEM images in Fig. 4 (a–d), it can be seen that partially sealing achieved of the pore openings of those samples which were anodized in just H2SO4 bath solution. However, the addition of 2 wt.% acetic acid in H2SO4 bath solution assisted to get fully sealed surface by choking the pores openings. But the true effect of acetic acid on the pore blocking could not be understood. However, it is assumed that the presence of acetic acid in the anodized bath solution introduced active polarized sites inside the pore openings. These active polarized sites are considered to be acetate ions (CH3COO−) which adsorbed water molecules and increased the tendency of dissolving anhydrous alumina from the pore walls leading towards auto-sealing mechanism by progressive precipitation to the depth of pores. The precipitation of these hydrated alumina particles took placed and choked the pore widening leading to progressive pore blocking (Figs. 4f and 10b). EIS parameters obtained after fitting equivalent circuit is in agreement with PDP results. It can be seen from Table 6 that, just like PDP, highest resistances achieved at 25 V regardless of the composition of bath solution. It can be seen that bare Al-2024 provided lowest pore and barrier resistance. However, the anodizing of aluminum in 5 wt.% solution at 15 V (AN-1) improved both of the discussed resistances compared to bare Al. The enhanced potential to 25 V (AN-2) for anodizing purpose increased the thickness of the film and, hence, the pore resistances (1.7 × 104 Ω cm2) and barrier film resistances (3.9 × 106 Ω cm2) as well. Barrier film resistance provided much higher resistance because of the defect free nature. Increasing the
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concentration of sulfuric acid in solution bath also increased the pore resistance and barrier film resistance. Like 5 wt.% H2SO4 solution, 10 wt.% H2SO4 bath solution at 25 V (AN-4) also provided tremendous pore and barrier film resistances compared to AN-3 and noted to be 1.9 × 105 Ω cm2 and 7.5 × 106 Ω cm2, respectively. The most significant improvement in the film resistance was noted when 2 wt.% acetic acid was introduced in the test bath solution for carrying out anodizing of Al. It can be seem from Table 6 that addition of acetic acid provided remarkable pore and barrier film resistances even at 15 V (AN5) and noted to be 1.1 × 105 Ω cm2 and 1.1 × 106 Ω cm2, respectively. Nevertheless, the enhanced anodizing potential to 25 V (AN-6) improved pore and barrier film resistances splendidly and noted to be highest compared to all other anodized samples. Pore resistance and barrier film resistance was noted to be 2.3 × 105 Ω cm2 and 8.9 × 106 Ω cm2, respectively. These improved resistances found to be 2 and 8 times better compared to its counterpart (AN-4). 4. Conclusions Al 2024 substrate was anodized with and without the presence of acetic acid in sulfuric acid bath to understand the impact of acetic acid on developed film morphology and thickness. The film was developed by applying DC voltage of different potential. The developed non adsorptive anodized film surface was characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and x-ray photoelectron spectroscopy (XPS). The electrochemical performance of the developed anodized film was evaluated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). From the achieved results, the following conclusion can be stated; 1. Anodized film thickness increased with the increased applied potential regardless of the bath composition. In addition to this, increased concentration of the sulfuric acid increased the film thickness. 2. Addition of just 2 wt.% of acetic acid in 5 wt.% sulfuric acid bath provided the film thickness nearly equal to film thickness achieved in 10 wt.% of sulfuric acid bath. It can be said that just little amount of acetic acid addition in sulfuric acid bath can cut the overall weight percentage of sulfuric acid to get the same thickness. 3. The XPS study assist the acquired results in electrochemical study and corroborated the assumptions for the alteration and enhancement of the corrosion protection performance and the surface morphological properties of anodized Al with the addition of acetic acid in sulfuric acid bath. 4. The presence of acetic acid in sulfuric acid bath provided non-adsorptive anodized film morphology which not has been reported yet before us. The reason of getting non-adsorptive film morphology with the presence of acetic acid can be attributed due to the presence of active polarized sites which are considered to be acetate ions (CH3COO–). These ions adsorbed water molecules and increased the tendency of dissolving anhydrous alumina from the pore walls leading towards auto-sealing mechanism by progressive precipitation to the depth of pores. The precipitation of these hydrated alumina particles took placed and choked the pore widening leading to progressive pore blocking.
Fig. 9. Electrochemical Impedance spectroscopy (EIS) spectra of anodized Al2024 in 3.5% Nacl Solution.
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Fig. 10. Schematic illustration of equivalent circuit superimposed on anodized Al-2024 pore opening and tested in 3.5% NaCl solution (a)-anodized in just H2SO4 (b)anodized in H2SO4/CH3COOH.
Acknowledgments
1994–2000. [18] B. Wielage, D. Nickel, G. Alisch, H. Podlesak, Th. Lampke, Surf. Coat. Technol. 202 (2007) 569–576. [19] A. Forn, J.A. Picas, M.T. Baile, E. Martin, V.G. García, Surf. Coat. Technol. 202 (2007) 1139–1143. [20] L.E. Fratila-Apachitei, J. Duszczyk, L. Katgerman, Surf. Coat. Technol. 157 (2002) 80–94. [21] M.F. Morks, A.S. Hamdy, N.F. Fahim, M.A. Shoeib, Surf. Coat. Technol. 200 (2006) 5071–5076. [22] M. Saeedikhani, M. Javidi, A. Yazdani, Trans. Nonferrous Met. Soc. China 23 (2013) 2551–2559. [23] L. Zhang, G.E. Thompson, M. Curioni, P. Skeldon, J. Electrochem. Soc. 160 (4) (2013) C179–C184. [24] S. Sigamani, P.R. Thangavelu, K.N. Srinivasan, M. Selvam, Int. J. Innov. Res. Sci. Eng. Technol. 3 (6) (2014). [25] M. Xiangfeng, W. Guoying, Ge. Hongliang, Yu. Yundan, C. Ying, H. Dettinger, Int. J. Electrochem. Sci. 8 (2013) 10660–10671. [26] LI. Ying-dong, Y. ZHANG, LI. Song-mei, ZHAO Pi-Zhi, Trans. Nonferrous Met. Soc. China 26 (2016) 492–500. [27] V.R. Capelossi, M. Poelman, I. Recloux, R.P.B. Hernandez, H.G. de Melo, M.G. Olivier, Electrochimica Acta 124 (2014) 69–79. [28] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, urf. Coat. Technol. 122 (1999) 73. [29] A.M. Kumar, R.S. Babu, S. Ramakrishna, Ana L.F. de Barros, Synth. Metal 234 (2017) 18–28. [30] K.V. Chernyakova, I.A. Vrublevsky, M.I. Ivanovskaya, D.A. Kotsikau, J. Appl. Spectros. 79 (2012) 76–82. [31] A.Q. Wang, T.D. Golden, Int. J. Electrochem. 482187 (2013) 1–10. [32] T.-S. Shih, Y.-W. Chiu, Appli. Surf. Sci. 351 (2015) 997–1003. [33] A.M. Kumar, M.A. Hussein, A.Y. Adesina, S. Ramakrishna, N. Al-Aqeeli, RSC Adv. 8 (2018) 19181–19195. [34] F. Mansfeld, M.W. Kendig, J. Electrochem. Soc. 135 (1988) 828–833. [35] J.A. González, V. López, A. Bautista, E. Otero, X.R. Nóvoa, J. Appl. Electrochem. 29 (1999) 229–238. [36] G. Boisier, A. Lamure, N. Pébère, N. Portail, M. Villatte, Surf. Coat. Technol. 203 (2009) 3420–3426. [37] Y. Huang, H. Shih, H. Huang, J. Daugherty, S. Wu, S. Ramanathan, C. Chang, F. Mansfeld, Corrosion Sci. 50 (2008) 3569–3575. [38] X.H. Zhao, Y. Zuo, J.M. Zhao, J.P. Xiong, Y.M. Tang, Surf. Coat. Technol. 200 (2006) 6846–6853. [39] V.L.J.A. Gonzalez, E. Otero, A. Bautista, J. Electrochem. Soc. 147 (2000) B75–B82.
We would like to thank to King Fahd University of Petroleum and Minerals, Dhahran, KSA for providing necessary support to conduct this research. References [1] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Trans. Inst. Metal Finish. 75 (1) (1997) 18. [2] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Corros. Sci. 39 (1997) 731. [3] X. Zhou, G.E. Thompson, H. Habazaki, K. Shimizu, P. Skeldon, G.C. Wood, Thin Solid Films 293 (1997) 327. [4] G.E. Thompson, H. Habazaki, K. Shimizu, M. Sakairi, P. Skeldon, X. Zhou, G.C. Wood, Aircraft Eng. Aerosp. Technol. 71 (3) (1999) 228. [5] P. Bailey, P. Skeldon, T.C.Q. Noakes, G.E. Thompson, H.H. M.Sakairi, K. Shimizu, Surf. Interf. Anal. 31 (2001) 480. [6] Y. Liu, F. Colin, P. Skeldon, G.E. Thompson, X. Zhou, H. Habazaki, K. Shimizu, Corros. Sci. 45 (2003) 1539. [7] S. Wernick, R. Pinner, The Surface Treatment and Finishing of Aluminum and its Alloys, (1959), p. 214. [8] J.A. Davies, B. Domeij, J.P.S. Pringle, F. Brown, J. Electrochem. Soc. 112 (1965) 675. [9] S.J. Garcia-Vergara, P. Skeldon, G.E. Thompson, H. Habazaki, Corros. Sci. 49 (2007) 3772. [10] G.E. Thompson, G.C. Wood, Corrosion: Aqueous Processes and Passive Films, (Treatise on Materials Science and Technology, vol. 23, in: J.C. Scully (Ed.), Academic Press Inc, London, 1983, p. 205. [11] K. Shimizu, K. Kobayashi, G.E. Thompson, G.C. Wood, Phil. Mag. A 66 (1992) 643. [12] J. Oh, C.V. Thompson, Electrochim. Acta 56 (2011) 4044. [13] D. Mercier, Q. Van Overmeere, R. Santoro, J. Proost, Electrochim. Acta 56 (2011) 1329. [14] K.R. Hebert, S.P. Albu, I. Paramasivam, P Schmuki, Nat. Mater. 11 (2012) 162. [15] V.F. Henley, The Anodic Oxidation of Aluminum and Its Alloys, second ed., (1982), p. 4. [16] P. John, PhD Thesis, Anodizing of Aluminum in Environmentally Acceptable Electrolytes, Department of Chemistry, Government College University Lahore, Pakistan, Session: 2004-2008. [17] X. Li, X. Nie, L. Wang, D.O. Northwood, Surf. Coat. Technol. 200 (2005)
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Achieving non-adsorptive anodized film on Al-2024 alloy: Surface and electrochemical corrosion investigation
T
M. Faizan Khana,c, , A. Madhan Kumarb, , Anwar Ul-Hamidc, Luai M. Al-Hemsc ⁎
⁎
a
Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia c Center for Engineering Research, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia b
ARTICLE INFO
ABSTRACT
Keywords: AFM Al2024 alloys Anodized film Corrosion EIS XPS
Naturally formed oxide film on the surface of aluminum reduces corrosion rate to significant extent. Anodizing of aluminum is done to enhance this feature by increasing film thickness deliberately to desired level. In the current work, anodizing of Al-2024 alloy was carried out in sulfuric-acetic acid anodizing bath of different concentrations along with gradually increased applied potential. The structure and morphology of the developed anodized film was investigated with and without the addition of acetic acid. Structural and surface analysis results revealed the difference in the anodized layer formation with the presence of acetic acid in sulfuric acid bath. Electrochemical investigation was carried out to evaluate the surface protective performance of the formed anodized film in 3.5% NaCl solution. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) revealed that lowest corrosion rate with highest polarization resistance was achieved due to the presence of CH3COOH in the anodizing bath. Non-adsorptive and less porous anodized film noticed to be formed with the presence of CH3COOH in bath solution.
1. Introduction Anodizing investigation of Al alloys has been remained under deep interest since from a long time and attracted significant attention of researcher from decades [1–7]. In electrochemical process, when potential is applied high enough on the anode part of the electrochemical cell then this allows oxygen to accumulate on the Al (anode) surface and results film formation, is called anodization. In fact, film is developed as a result of electrochemical reaction in between metal surface and electrolyte ions and an overall increase in volume occur on Al substrate as a result of anodizing. The formation of anodic oxide layer is the result of the movement of Al3+ and O2− ions towards each other across the barrier layer region [8]. The O2− movement becomes the primary cause of oxide layer formation on metal-film interface. Fieldassisted flow of alumina at a certain region does not allow to the formation of continuous oxide layer and, hence, a porous oxide layer is developed [9]. The detailed mechanism about porous film formation and growth can be understood somewhere else [10–14].
⁎
Anodizing film can be thin, thick, dense or porous depending on the employed potential, bath composition and concentration. The surface morphology of the film is porous by nature and determines the adsorptive characteristics as well as abrasion resistance. In the latter case, coatings with a less number of smaller diameter pores will have higher resistance to abrasion than coatings with a large number of greater diameter pores. This makes sense as the density of the anodized coating is proportional to the amount of oxide formed and hence the pores size starts decreasing with continuous oxide formation thus improving the abrasion resistance of the coating [15]. Peter et al. [16] studied the anodization of Al alloys and found the optimum conditions by varying the electrolytic composition of sulfuric/tartaric acid, sulfuric/carbolic/ boric acid and sulfuric/oxalic/boric acids. Their investigation revealed that corrosion current density found to be decreased leading to reduce corrosion rate by increasing polarization resistance (RP). It was also observed in their study that anodized film developed in sulfuric/tartaric acid bath was better than the rest of the baths. Li et al. [17] studied anodizing on Al-Si substrate by three different techniques; (i) hard
Corresponding author. E-mail address: [email protected] (A.M. Kumar).
https://doi.org/10.1016/j.surfin.2019.02.005 Received 25 September 2018; Received in revised form 7 January 2019; Accepted 11 February 2019 Available online 12 February 2019 2468-0230/ © 2019 Published by Elsevier B.V.
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far literature survey reveals that all the developed anodized films possess porous adsorptive surface due to the presence of pores over its surface. This porous surface causes a big trouble by inviting the corrosive media to flow inside the film through these pores and deteriorate the film severely and, thus, reduces the overall integrity of the film. Externally applied sol-gels are being utilized to block these pores [27] but these gels add the additional cost on industrial level and their compatibility towards echo-friendly environment yet has not been proven. In the current work, the presence of acetic acid in varying concentration of sulfuric acid bath is investigated under different applied potentials to understand its impact on the anodized film thickness and non-adsorptive morphology of Al-2024 substrate. Evaluating surface morphology of the anodized film and the electrochemical performance in corrosive environment is the part of this research work.
Table 1 2024 aluminum alloy elemental compositions determined by XRF. Element
Al
Zn
Mg
Cu
Fe
Si
Mn
Ni
Ti
wt.%
93.97
0.085
1.29
3.80
0.16
0.063
0.58
0.004
0.03
Table 2 Chemical composition, temperature and holding time for surface treatment. Surface Treatment
Solution composition
Temp. (°C)
Time (min)
Chemical polishing
54 mL H3PO4 (85%) 2 mL HNO3 (66.4%) 15 mL CH3COOH (99%) 13 mL H2O 35 mL HNO3 (66.4%) 65 mL H2O
90
4
25
2
Deoxidation
2. Experimental work 2.1. Material 2024 Aluminum alloy sheet, having thickness of 3 mm, was cut for preparing samples to investigate anodization. The composition of the material was determined by x-ray fluorescence (XRF) technique and given in Table 1.
anodizing (HA), (ii) simple anodizing and (iii) modified anodizing (MA). They found different hardness against each film developed by different techniques and each film was sufficiently resistive to corrosion. However, they reported that modified anodizing technique was more beneficial due to its environmental friendly process. Wielage et al. [18] found the effect of aluminum substrate surface (roughness) on the anodized film thickness and morphology. Forn, et al. [19] studied the effect of additional particles in Al-substrate and noticed increased film thickness by the presence of discussed particles. The tribological properties of this thick oxide layer were reported tremendous in sense of wear resistance. Fratila-Apachitei et al. [20] reported the growth of anodized film by taking three different aluminum substrates and applied three different current densities in 2.25 M H2SO4 bath at 0 °C. Similarly, Morks et al. [21] did their experiment to grow the anodic oxide layer on aluminum alloy in 5-sulfosalicylic acid electrolyte and studied the corrosion resistance in different concentrations of electrolytes at different temperatures. Saeedikhani et al. [22] demonstrated the improvement in corrosion resistance of 2024-T3 aluminum alloy by anodization in mixed electrolyte containing 10% sulfuric acid, 5% boric acid and 2% phosphoric acid. Zhang et al. [23] investigated the addition of boric acid in the presence of sulfuric acid electrolyte to study the effect of current density-time response, growth rate and morphology of the porous anodic film. Shanmuga et al. [24] did their research on AC anodization technique by taking Al substrate in an electrolyte containing 10% sulfuric acid, 3% sodium sulfate, organic additives SLS and gelatin. They revealed that the presence of gelatin increases the compactness of the coating. Xiang Feng et.al revealed through their research that the addition of citric acid in sulfuric acid bath improved anticorrosion behavior of the anodized film [25]. Ying-dong et.al showed in their research that the addition of adipic acid in sulfuric acid bath enhanced dielectric properties and corrosion resistance [26]. So
2.2. Surface preparation For the pretreatment of aluminum alloy, samples were first of all degreased using acetone solvent followed by rinsing in distilled water. The samples were then grinded up to 800 grit size of SiC abrasive paper followed by alkaline cleaning in a solution of 12 g NaOH/100 ml of distilled water at 60 °C for 3 min. In the next step, chemical polishing of the samples were carried out in a mixture whose composition is provided in Table 2. Finally, the samples were treated in a solution (as per the technique given in Table 2) to deoxidize the surface. 2.3. Anodizing and sealing Aluminum strips were anodized under two applied potentials (15 V and 25 V) in electrolytic bath of sulfuric acid (H2SO4) whose composition and concentration was changed with and without the addition of acetic acid (CH3COOH). For each anodizing process, potential was provided by using D.C power supply whereas time and temperature was kept constant to 30 min and 25 °C, respectively. The scheme for each anodizing process has been provided in Table 3. After completing anodizing process, samples were cleaned rigorously using distilled water and then sealing was carried out by treatment in near-boiling distilled water to convert oxide lining of the pores from amorphous oxide to a non-hydrate. However, sealing mechanism of the developed anodized film not only increases the volume of the film but also blocks the pore openings by rendering the film morphology to get transformed from adsorptive to non-adsorptive [19].
Table 3 Electrolytic bath composition, concentration and applied potential used for anodizing of aluminum at room temperature for 30 min. Sample
Electrolyte composition
Voltage (V)
Sample
Electrolyte composition
Voltage (V)
AN-1 AN-3 AN-5
5% wt. H2SO4 10% wt. H2SO4 5% wt. H2SO4 +2% CH3COOH
15 15 15
AN-2 AN-4 AN-6
5% wt. H2SO4 10% wt. H2SO4 5% wt. H2SO4 +2% CH3COOH
25 25 25
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Fig. 1. Film thickness of different anodized Al samples achieved at (a) 5%H2SO4 @15 V, (b) 5%H2SO4 @25 V, (c) 10%H2SO4 @15 V, (d) 10%H2SO4 @25 V, (e) 5% H2SO4/2%AA @15 V, (f) 5%H2SO4/2%AA @25 V.
2.4. Microstructure analysis
AFM, USA) instrument. The acquired AFM images were detected with non-contact mode of Au coated silicon cantilevers with a resonance frequency of 26 kHz with a spring constant of 1.6 N/m under air environment. The chemical structure of anodized Al samples was characterized by an Attenuated Total Reflectance-Infrared spectrometer (Thermo scientific, with universal ATR attachment, range 500–4000 cm−1).
The anodized samples were analyzed for measuring film thickness and morphological study. Samples were cold mounted followed by grinding using up to 1200 emery paper. In the next step, polishing of the mounted samples was carried out by diamond paste and HF was used to etch the anodized aluminum samples. Scanning electron microscope (SEM) JEOL JSM-6480 LV was used for obtaining images as well as measuring anodized film thickness. X-Ray Photoelectron Spectroscopic (XPS) measurements have been done to evaluate the particular surface elemental compositions as result of the alterations, produced by the relevant anodization techniques. The surface topography of anodized samples was analyzed through AFM (Agilent 5500
2.5. Electrochemical corrosion testing The general corrosion resistance of the anodized samples was investigated by potentiodynamic polarization (PDP) technique under room conditions using Gamry 3000 potentiostat. The potentiodynamic
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0.2 mV/s. Electrochemical impedance spectroscopy (EIS) was measured by applying AC voltage having an amplitude of 10 mV and the frequency was set from 100 K to 10 mHz. 3. Results and discussion 3.1. Anodizing film thickness The film thicknesses obtained by variation of the applied voltage and electrolytic composition under fixed temperature (25 °C) and time (25 min) have been provided in Fig. 1. In addition to this, the film thicknesses of different anodized samples have also been depicted graphically in Fig. 2. It can be seen through the graphs (Fig. 2) that higher applied voltage of 25 V provided thicker anodized film (AN-2, AN-4 and AN-6) compared to lower applied voltage of 15 V (AN-1, AN-3 and AN-5). However, when the applied voltage was fixed to 15 V then the sample which was anodized in a mixer of 2%wt. acetic/5%wt. sulfuric acid (AN-5) provided thicker anodized film (9.5 µm) compared to the samples which were anodized just in the presence of sulfuric acid (5 wt.% = 7.8 µm, 10 wt.% = 8.5 µm). Interesting thing to note that the addition of only 2 wt.% acetic acid in 5 wt.% sulfuric acid (AN-5) provided thicker anodized film even than 10 wt.% sulfuric acid (AN-3). Nevertheless, 10 wt.% sulfuric (AN-3) provided slightly thicker anodized film compared to 5 wt.% sulfuric acid (AN-1). It is worth noting from the anodized thickness analysis that concentration of the bath solution has less effect for achieving thickened anodized film compared to the applied potential. The same trend of changing the film thickness with the composition of electrolyte was seen to repeat when the applied voltage was enhanced to 25 V. Again the addition of 2 wt.% acetic acid in 5 wt.% sulfuric acid (AN-6 = 12.4 µm) provided nearly equal film thickness to the anodized film developed in 10 wt.% sulfuric acid (AN-4 = 12.7 µm) bath. It can be stated that anodized film thickness for the case of aluminum increased with enhanced applied voltage and concentration of sulfuric acid in the electrolyte. But little addition of acetic acid (2 wt.%)
Fig. 2. The effect of applied voltage and electrolyte composition on anodizing film thickness at 25 °C for 30 min.
polarization (PDP) measurements were carried out by a conventional three-electrode cell using a saturated calomel electrode (SCE) as a reference electrode and a graphite rod as a counter electrode. When the electrochemical system was stable, the measurements were taken in a 3.5% NaCl solution (sea water) and the corrosion rates were determined for comparison. Before conducting electrochemical test, open circuit potential (OCP) of all the tested samples was measured until to get stabilized potential. For conducting PDP, the applied initial and final voltage was set to ± 250 mV whereas the scan rate was fixed to
Fig. 3. X-Ray diffraction patterns of anodized Al samples under different electrolytic compositions and applied potentials.
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Fig. 4. Scanning electron micrographs (SEM) of anodized surface (a) 5%H2SO4 @15 V, (b) 5%H2SO4 @25 V, (c) 10%H2SO4 @15 V, (d) 10%H2SO4 @25 V, (e) 5% H2SO4/2%Acetic acid @15 V, (f) 5%H2SO4/2%Acetic acid @25 V.
along with sulfuric acid cut off the concentration level of sulfuric acid in the electrolytic bath to achieve the required anodized film by fixing the applied voltage. In other words, in the presence of acetic acid, overall less concentration of sulfuric acid can provide the same thickness which should be achieved at higher concentration of sulfuric acid. To obtain required anodized film, decreasing the concentration of sulfuric acid and replacing it partially with acetic acid is a step towards ecofriendly anodizing.
only Al peaks appeared. However, once anodizing was done in different electrolytes then it can be seen that anodized film was formed and mainly composed of two phases i.e. γ-Al2O3, α-Al2O3 [25,28]. As the thickness and volume of the porous anodized film is very less compared to bare Al, therefore the x-rays are diffracted from base aluminum with greater peaks intensity which is in agreement with the study of Yerokhin [28]. From the XRD patterns of each anodized sample, it is cleared that voltage played an effective role along with electrolyte percentage of the bath solution for the formation of crystalline phases. For example, for AN-1, in 5% H2SO4 bath solution, it can be seen that 15 V assisted in the formation of just α-Al2O3 phase whereas AN-2, at the same electrolytic percentage, formed an additional γ-Al2O3 crystalline phases when voltage was enhanced to 25 V. Nevertheless,
3.2. Surface characterization X-ray diffraction pattern of different anodized Al-2024 samples have been provided in Fig. 3. It can be seen that, for the bare Al-2024 sample,
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Fig. 5. ATR-IR spectra of AN-4 and AN-6 anodized samples.
Fig. 6. (a) XPS survey spectra, (b) Al2p and (c) O1s deconvolution spectra of AN-4 and AN-6 samples.
increase of electrolytic percentage at both voltages (15 V and 25 V) assisted the formation of both discussed crystalline phases. The only difference in the XRD pattern from AN-2 to AN-6 samples was noted to be the variation of phase intensities of both γ-Al2O3, α-Al2O3.
Table 4 Surface roughness parameters of TNZ substrates. Substrates
Ra (µm)
Rp (µm)
Rq (µm)
Rz (µm)
Rv (µm)
AN-2 AN-4 AN-6
0.0784 0.132 0.0761
0.306 0.440 0.293
0.105 0.163 0.101
0.547 0.780 0.583
0.197 0.34 0.29
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partially blocked the pore openings of the developed anodized film. But still pores openings can be seen on top surface. Nevertheless, once the applied anodizing potential was increased to 25 V, then those agglomerated particles got bigger in size and took spherical shape and can be seen from Fig. 4f. These spherical shaped agglomerated particles blocked the pore openings of the anodized film. The relation in between increased potential and the morphology of particles could not be understood clearly, however, it seems that the applied increased potential set a gradient towards increased kinetic movement of the particle thus assisting them to get bigger in size. ATR-IR spectra of anodized Al alloy samples is presented in Fig. 5. IR spectra of acetic acid that we recorded is also presented for the reference. In the case of AN-4, the characteristic vibrations of AleO bonds in the region of 500–800 cm−1 which confirm that the anodized layer mainly composed of Al2O3 [29]. On the other hand, IR spectra of AN-6 samples, the peaks appeared in the region of 1000–2000 cm–1 are associated with the vibrational frequencies of bonds in individual structural units of acetic acid molecules and in adsorbed carbon-containing structures, contains absorption bands with different intensities. Attachment of carboxylate groups by metal ions causes a change in the IR spectrum of the acid in the region of the valence vibrations of CeO bonds in this group. The obtained results further corroborates that acetate ions exist in the anodic layer with coordination bonding to Al cations. The obtained data is well agreement with the previous reports [30,31]. The chemical composition and the state of surfaces of anodized Al alloy samples were analyzed using XPS measurements and the resultant survey spectra is displayed in Fig. 6a. From the observation of survey
Table 5 Potentiodynamic polarization parameters of Al-2024 tested in 3.5% NaCl solution at 25ᵒC. Sample no.
βa (V/decade) (x10−3)
βc (V/decade) (x10−3)
icorr (10−6 A/cm2)
Ecorr (mV)
Corrosion rate (mpy)
Bare-Al AN-1 AN-2 AN-3 AN-4 AN-5 AN-6
45 34 59 32 269 70 61
665 82 78 61 232 483 165
4.1 1 0.003 0.72 0.001 0.018 0.0004
−581.0 −576 −534 −635 −632 −604 −446
0.643 0.553 0.002 0.12 0.001 0.009 0.0003
Surface morphology of anodized Al-2024 films can be seen from the scanning electron microscopic (SEM) images provided in Fig. 4 (a–f). It can be seen from the images that those samples (AN-1, AN-2, AN-3, AN4) which were anodized in just sulfuric acid (H2SO4) bath provided the same surface morphology regardless of the concentration of (H2SO4) and applied potential. It can be seen from Fig. 4 (a–d) that all the samples possessed porous anodized film. However, the addition of 2 wt. % acetic acid in the electrolyte solution brought a tremendous change in the surface morphology of the anodized film and can be seen from the provided images in Fig. 4 (e and f). It can be seen from Fig. 4e that, at 15 V, in the presence of 2 wt.% acetic acid, the nucleation of particles happened followed by agglomeration. The agglomerated particles
Fig. 7. AFM surface topographic images of (a) AN-2 (b) AN-4 and (c) AN-6 samples.
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spectra, the presence of C, O and Al was identified without any contaminated elements which further confirms the formation of anodized films on Al samples. The photoelectron peak for Al2p Fig. 6b seems evidently at a binding energy of 75.05 eV, C1s at 281.25 and O1s at 531.50. The estimated binding energy from the monitored oxygen photoelectron lines positioned at 531.50 eV is the sign for the oxidation of Al in acid bath. The deconvoluted O1s spectra (Fig. 6c) are comprised of a chief peak with a binding energy of 530.13 eV and a minor peak with a binding energy of 529.24 eV. The peaks with 530.13 and 529.24 eV are ascribed to the hydrated alumina Al(OH)3, and oxyhydroxide AlOOH respectively [32]. From the acquired O1s spectra, it was indicated that the growth of uniform oxide layer is obtained on Al alloys by anodization process. Moreover, the calculated O:Al ratio for AN-4 and AN-6 samples is about 2.4 and 4.1 respectively, which further specifies that the higher O:Al ratio obtained in acetic acid bath due to the hydration of anodized layer. 3.3. Surface topographic analysis through AFM technique The surface topographic images of anodized surfaces were obtained using AFM techniques and the results are presented in Fig. 7 (a, b and c). The surface of AN-2 (Fig. 7a) exhibited non uniform surface with uneven pores, whereas AN-4 (Fig. 7b) was observed to be of typical porous topographic surface with a void micropores. On the other hand, AFM image of AN-6 (Fig. 7c) displayed uniform surface with a few micropores. In order to attain more vision on the surface topography and the porosity, line profile analysis was performed on the anodized surfaces and surface profile images are presented in Fig. 7. In addition, the change in the roughness of the PEO coatings was evaluated and the obtained parameters are summarized in Table 4. Generally, Rq (root mean square, RMS) and Ra (average surface roughness) are considered as significant parameters for investigating surface topographic features as its offering an indication about the frequency of deviances from a flat surface by inspecting a continuous surface profile [33]. It is clear that Rq comparatively reduces with acetic acid additions in the anodization bath, which showed that the presence of acetate ions on the anodized layer pointedly influenced the porosity and the surface roughness. The lower roughness values of AN-6 samples revealed smoother, more compact and comparatively less porous than other samples. The size and number of pores in AN-6 samples are reasonably low and small which indicated that the acetic acid in anodization bath pointedly impacts the surface topography of anodized Al alloy samples. 3.4. Electrochemical corrosion analysis Potentiodynamic polarization results of the anodized aluminum alloy samples tested in a 3.5% NaCl solution are given in Table 5. The PDP curves have been provided in Fig. 8 (a–c). It can be seen from the Fig. 8(a) and Table 5 that anodizing reduced the corrosion current density considerably compared to the bare Al-2024. Bare Al-2024 provided highest corrosion rate and corrosion current density. However, anodizing of Al in 5 wt.% H2SO4 bath solution at 15 V reduced both the current density and corrosion rate and provided 1 × 10−6A/ cm2 and 0.553 mpy, respectively. But anodizing done at 25 V (AN-2), in the same bath solution, reduced corrosion current density and corrosion rate dramatically and provided 3 × 10−9A/cm2 and 0.002 mpy, respectively. The current density and corrosion rate reduced 333 and 276 times, respectively, when the applied potential was increased from 15 V to 25 V. The dramatic reduction in corrosion rate and current density
Fig. 8. Potentiodynamic polarization (PDP) graphs of anodized Al-2024 samples tested in 3.5% NaCl solution.
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Table 6 Electrochemical impedance spectroscopy (EIS) simulations of Al-2024 anodized film in 3.5% NaCl. Sample
RSol (Ω.cm2)
R (Ω.cm2)
Rp (Ω.cm2)
Rb (Ω.cm2)
CPEPW S*sa
CPEp S*sa
CPEb S*sa
apw
ap
W S*s1/2
Bare-Al AN-1 AN-2 AN-3 AN-4 AN-5 AN-6
4.9 5.8 7.0 4.3 6.0 4.4 3.9
5.4 × 103 1.1 × 103 1.3 × 104 6.7 × 103 2.7 × 104 3.3 × 104 7.2 × 104
1.8 × 102 2.2 × 102 1.7 × 104 1.2 × 104 1.9 × 105 1.1 × 105 2.3 × 105
1.2 × 103 5.1 × 103 3.9 × 106 1.5 × 104 7.5 × 106 1.1 × 106 8.9 × 106
3.1 × 10−7 1.2 × 10−7 3.3 × 10−7 1.5 × 10−7 4.1 × 10−8 9.9 × 10−8 7.8 × 10−8
2 × 10−1 5.8 × 10−7 7.7 × 10−7 3.9 × 10−5 4.1 × 10−8 2.4 × 10−6 7.8 × 10−8
7.2 × 10−6 2.9 × 10−5 6.0 × 10−7 1.9 × 10−5 8.2 × 10−2 1.4 × 10−6 1.1 × 10−6
8.4 × 10−1 8.6 × 10−1 7.9 × 10−1 7.9 × 10−1 8.6 × 10−1 8.5 × 10−1 8.6 × 10−1
6.7 × 10−1 1 1 7.0 × 10−1 6.1 × 10−1 3.9 × 10−1 3.6 × 10−1
1.6 × 10−2 8.2 × 10−3 8.5 × 10−6 3.7 × 10−3 2.9 × 10−3 1.8 × 10−3 2.3 × 10−4
can be attributed due to the thicker anodized film formation at 25 V (10.5 µm) compared to film which developed at 15 V (7.8 µm). Anodizing of aluminum 2024 in 10 wt.% H2SO4 bath solution reduced both the current density and corrosion rate compared to 5 wt.% solution. Like in 5 wt.% solution, again anodizing potential of 25 V decreased current density and corrosion rate to significant extent compared to 15 V applied potential. It can be seen from Table 5 and Fig. 8(b) that, in 10 wt.% H2SO4 electrolyte solution, 25 V applied potential (AN-4) provided current density and corrosion rate 1 × 10−6 A/ cm2 and 0.001 mpy, respectively. This is 720 and 120 times less, respectively, compared to those samples which were anodized at 15 V. Reducing of current density and corrosion rate seems to be the function of thickness of anodized film. At 25 V, AN-2 developed film thickness of 10.5 µm and hence provided 3 times higher current density compared to AN-4 which developed 12.7 µm thicken film. Nevertheless, addition of just 2 wt.% acetic acid in 5 wt.%H2SO4 bath solution provided astonishing effect by reducing the current density and corrosion rate to many folds. It can be seen from Table 5 and Fig. 8(c) that, at 15 V, the addition of 2 wt.% acetic acid in 5 wt.% H2SO4 (AN-5) provided current density and corrosion rate of 1.8 × 10−8 A/cm2 and 0.009 mpy, respectively, which is 40 times less than its counterpart AN-3. Increased anodizing potential to 25 V (AN6), in the same bath composition, provided lowest current density and corrosion rate which was noted to be 4 × 10−10 A/cm2 and 0.0003 mpy. This current density and corrosion rate noted to be reduced 2.5 and 3.3 times, respectively, compared to the that sample (AN-4) which was anodized at the same potential (25 V) in 10 wt.% H2SO4 (without the presence of acetic acid). It is well cleared that the addition of acetic acid not only increased the thickness of the anodized film but also blocked the pore openings of the film which can be seen from Fig. 4(f). Electrochemical impedance spectroscopy (EIS) was done on anodized Al-2024 samples in 3.5% NaCl solution and the spectra obtained are provided in Fig. 9 (a–c). Fig. 10 provides the schematic illustration of porous anodized film by superimposing equivalent circuit on it. This equivalent circuit has already been reported somewhere else but without the Warburg element [27]. In this equivalent circuit, Rs represents solution resistance, R general resistance, Rp pore resistance and Rb barrier film resistance. Rb is considered actual corrosion resistance film [34]. Whereas, CPEp, CPEpw and CPEb represent constant phase elements at pore, pore walls and barrier film, respectively.
Constant phase elements were used instead of capacitors because of the non-ideal nature of the developed film. The non-ideal behavior of the film reflects defects and surface roughness. Once the data obtained from electrochemical impedance spectroscopy was fitted on given equivalent circuit then the obtained values of circuit elements are provided in Table 6. The simulated model of porous anodized aluminum in Fig. 10 is in agreement with the referenced literature of sealed anodized film [35–38] and represents that how sealing mechanism took placed inside the pore openings. This reveals that when an unsealed porous anodized film was exposed to hot boiling water then these pores were filled initially by hot boiling. Afterwards, auto-sealing mechanism took placed progressively throughout the pore depth by dissolving anhydrous alumina from the pore walls. To next step, precipitation of these hydrated alumina particles took placed which resulted pore widening leading to progressive pore blocking [38,39]. From SEM images in Fig. 4 (a–d), it can be seen that partially sealing achieved of the pore openings of those samples which were anodized in just H2SO4 bath solution. However, the addition of 2 wt.% acetic acid in H2SO4 bath solution assisted to get fully sealed surface by choking the pores openings. But the true effect of acetic acid on the pore blocking could not be understood. However, it is assumed that the presence of acetic acid in the anodized bath solution introduced active polarized sites inside the pore openings. These active polarized sites are considered to be acetate ions (CH3COO−) which adsorbed water molecules and increased the tendency of dissolving anhydrous alumina from the pore walls leading towards auto-sealing mechanism by progressive precipitation to the depth of pores. The precipitation of these hydrated alumina particles took placed and choked the pore widening leading to progressive pore blocking (Figs. 4f and 10b). EIS parameters obtained after fitting equivalent circuit is in agreement with PDP results. It can be seen from Table 6 that, just like PDP, highest resistances achieved at 25 V regardless of the composition of bath solution. It can be seen that bare Al-2024 provided lowest pore and barrier resistance. However, the anodizing of aluminum in 5 wt.% solution at 15 V (AN-1) improved both of the discussed resistances compared to bare Al. The enhanced potential to 25 V (AN-2) for anodizing purpose increased the thickness of the film and, hence, the pore resistances (1.7 × 104 Ω cm2) and barrier film resistances (3.9 × 106 Ω cm2) as well. Barrier film resistance provided much higher resistance because of the defect free nature. Increasing the
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concentration of sulfuric acid in solution bath also increased the pore resistance and barrier film resistance. Like 5 wt.% H2SO4 solution, 10 wt.% H2SO4 bath solution at 25 V (AN-4) also provided tremendous pore and barrier film resistances compared to AN-3 and noted to be 1.9 × 105 Ω cm2 and 7.5 × 106 Ω cm2, respectively. The most significant improvement in the film resistance was noted when 2 wt.% acetic acid was introduced in the test bath solution for carrying out anodizing of Al. It can be seem from Table 6 that addition of acetic acid provided remarkable pore and barrier film resistances even at 15 V (AN5) and noted to be 1.1 × 105 Ω cm2 and 1.1 × 106 Ω cm2, respectively. Nevertheless, the enhanced anodizing potential to 25 V (AN-6) improved pore and barrier film resistances splendidly and noted to be highest compared to all other anodized samples. Pore resistance and barrier film resistance was noted to be 2.3 × 105 Ω cm2 and 8.9 × 106 Ω cm2, respectively. These improved resistances found to be 2 and 8 times better compared to its counterpart (AN-4). 4. Conclusions Al 2024 substrate was anodized with and without the presence of acetic acid in sulfuric acid bath to understand the impact of acetic acid on developed film morphology and thickness. The film was developed by applying DC voltage of different potential. The developed non adsorptive anodized film surface was characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and x-ray photoelectron spectroscopy (XPS). The electrochemical performance of the developed anodized film was evaluated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). From the achieved results, the following conclusion can be stated; 1. Anodized film thickness increased with the increased applied potential regardless of the bath composition. In addition to this, increased concentration of the sulfuric acid increased the film thickness. 2. Addition of just 2 wt.% of acetic acid in 5 wt.% sulfuric acid bath provided the film thickness nearly equal to film thickness achieved in 10 wt.% of sulfuric acid bath. It can be said that just little amount of acetic acid addition in sulfuric acid bath can cut the overall weight percentage of sulfuric acid to get the same thickness. 3. The XPS study assist the acquired results in electrochemical study and corroborated the assumptions for the alteration and enhancement of the corrosion protection performance and the surface morphological properties of anodized Al with the addition of acetic acid in sulfuric acid bath. 4. The presence of acetic acid in sulfuric acid bath provided non-adsorptive anodized film morphology which not has been reported yet before us. The reason of getting non-adsorptive film morphology with the presence of acetic acid can be attributed due to the presence of active polarized sites which are considered to be acetate ions (CH3COO–). These ions adsorbed water molecules and increased the tendency of dissolving anhydrous alumina from the pore walls leading towards auto-sealing mechanism by progressive precipitation to the depth of pores. The precipitation of these hydrated alumina particles took placed and choked the pore widening leading to progressive pore blocking.
Fig. 9. Electrochemical Impedance spectroscopy (EIS) spectra of anodized Al2024 in 3.5% Nacl Solution.
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Fig. 10. Schematic illustration of equivalent circuit superimposed on anodized Al-2024 pore opening and tested in 3.5% NaCl solution (a)-anodized in just H2SO4 (b)anodized in H2SO4/CH3COOH.
Acknowledgments
1994–2000. [18] B. Wielage, D. Nickel, G. Alisch, H. Podlesak, Th. Lampke, Surf. Coat. Technol. 202 (2007) 569–576. [19] A. Forn, J.A. Picas, M.T. Baile, E. Martin, V.G. García, Surf. Coat. Technol. 202 (2007) 1139–1143. [20] L.E. Fratila-Apachitei, J. Duszczyk, L. Katgerman, Surf. Coat. Technol. 157 (2002) 80–94. [21] M.F. Morks, A.S. Hamdy, N.F. Fahim, M.A. Shoeib, Surf. Coat. Technol. 200 (2006) 5071–5076. [22] M. Saeedikhani, M. Javidi, A. Yazdani, Trans. Nonferrous Met. Soc. China 23 (2013) 2551–2559. [23] L. Zhang, G.E. Thompson, M. Curioni, P. Skeldon, J. Electrochem. Soc. 160 (4) (2013) C179–C184. [24] S. Sigamani, P.R. Thangavelu, K.N. Srinivasan, M. Selvam, Int. J. Innov. Res. Sci. Eng. Technol. 3 (6) (2014). [25] M. Xiangfeng, W. Guoying, Ge. Hongliang, Yu. Yundan, C. Ying, H. Dettinger, Int. J. Electrochem. Sci. 8 (2013) 10660–10671. [26] LI. Ying-dong, Y. ZHANG, LI. Song-mei, ZHAO Pi-Zhi, Trans. Nonferrous Met. Soc. China 26 (2016) 492–500. [27] V.R. Capelossi, M. Poelman, I. Recloux, R.P.B. Hernandez, H.G. de Melo, M.G. Olivier, Electrochimica Acta 124 (2014) 69–79. [28] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, urf. Coat. Technol. 122 (1999) 73. [29] A.M. Kumar, R.S. Babu, S. Ramakrishna, Ana L.F. de Barros, Synth. Metal 234 (2017) 18–28. [30] K.V. Chernyakova, I.A. Vrublevsky, M.I. Ivanovskaya, D.A. Kotsikau, J. Appl. Spectros. 79 (2012) 76–82. [31] A.Q. Wang, T.D. Golden, Int. J. Electrochem. 482187 (2013) 1–10. [32] T.-S. Shih, Y.-W. Chiu, Appli. Surf. Sci. 351 (2015) 997–1003. [33] A.M. Kumar, M.A. Hussein, A.Y. Adesina, S. Ramakrishna, N. Al-Aqeeli, RSC Adv. 8 (2018) 19181–19195. [34] F. Mansfeld, M.W. Kendig, J. Electrochem. Soc. 135 (1988) 828–833. [35] J.A. González, V. López, A. Bautista, E. Otero, X.R. Nóvoa, J. Appl. Electrochem. 29 (1999) 229–238. [36] G. Boisier, A. Lamure, N. Pébère, N. Portail, M. Villatte, Surf. Coat. Technol. 203 (2009) 3420–3426. [37] Y. Huang, H. Shih, H. Huang, J. Daugherty, S. Wu, S. Ramanathan, C. Chang, F. Mansfeld, Corrosion Sci. 50 (2008) 3569–3575. [38] X.H. Zhao, Y. Zuo, J.M. Zhao, J.P. Xiong, Y.M. Tang, Surf. Coat. Technol. 200 (2006) 6846–6853. [39] V.L.J.A. Gonzalez, E. Otero, A. Bautista, J. Electrochem. Soc. 147 (2000) B75–B82.
We would like to thank to King Fahd University of Petroleum and Minerals, Dhahran, KSA for providing necessary support to conduct this research. References [1] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Trans. Inst. Metal Finish. 75 (1) (1997) 18. [2] H. Habazaki, K. Shimizu, P. Skeldon, G.E. Thompson, G.C. Wood, X. Zhou, Corros. Sci. 39 (1997) 731. [3] X. Zhou, G.E. Thompson, H. Habazaki, K. Shimizu, P. Skeldon, G.C. Wood, Thin Solid Films 293 (1997) 327. [4] G.E. Thompson, H. Habazaki, K. Shimizu, M. Sakairi, P. Skeldon, X. Zhou, G.C. Wood, Aircraft Eng. Aerosp. Technol. 71 (3) (1999) 228. [5] P. Bailey, P. Skeldon, T.C.Q. Noakes, G.E. Thompson, H.H. M.Sakairi, K. Shimizu, Surf. Interf. Anal. 31 (2001) 480. [6] Y. Liu, F. Colin, P. Skeldon, G.E. Thompson, X. Zhou, H. Habazaki, K. Shimizu, Corros. Sci. 45 (2003) 1539. [7] S. Wernick, R. Pinner, The Surface Treatment and Finishing of Aluminum and its Alloys, (1959), p. 214. [8] J.A. Davies, B. Domeij, J.P.S. Pringle, F. Brown, J. Electrochem. Soc. 112 (1965) 675. [9] S.J. Garcia-Vergara, P. Skeldon, G.E. Thompson, H. Habazaki, Corros. Sci. 49 (2007) 3772. [10] G.E. Thompson, G.C. Wood, Corrosion: Aqueous Processes and Passive Films, (Treatise on Materials Science and Technology, vol. 23, in: J.C. Scully (Ed.), Academic Press Inc, London, 1983, p. 205. [11] K. Shimizu, K. Kobayashi, G.E. Thompson, G.C. Wood, Phil. Mag. A 66 (1992) 643. [12] J. Oh, C.V. Thompson, Electrochim. Acta 56 (2011) 4044. [13] D. Mercier, Q. Van Overmeere, R. Santoro, J. Proost, Electrochim. Acta 56 (2011) 1329. [14] K.R. Hebert, S.P. Albu, I. Paramasivam, P Schmuki, Nat. Mater. 11 (2012) 162. [15] V.F. Henley, The Anodic Oxidation of Aluminum and Its Alloys, second ed., (1982), p. 4. [16] P. John, PhD Thesis, Anodizing of Aluminum in Environmentally Acceptable Electrolytes, Department of Chemistry, Government College University Lahore, Pakistan, Session: 2004-2008. [17] X. Li, X. Nie, L. Wang, D.O. Northwood, Surf. Coat. Technol. 200 (2005)
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