Progress in Organic Coatings 56 (2006) 227–233

Study of corrosion resistance of colored anodized aluminum with Quinizarinusing cyclic voltammetry and impedance measurement methods

A. Ahmadi a,∗, P. Norouzi b, M.R. Ganjali b

a Department of Chemistry, Islamic Azad University, Karaj Branch, Karaj, Iranb Department of Chemistry, Tehran University, Tehran, Iran

Received 2 August 2005; received in revised form 23 May 2006; accepted 29 May 2006

Abstract

The effect of coloring condition of aluminum with Quinizarin (1,4-dihydroxy anthraquinone) on the corrosion of Al in sulfuric acid solutionwas studied using cyclic and AC voltammetry and impedance measurement methods. The changes in the corrosion resistance of colored aluminumplates were evaluated by measuring the magnitude of impedance and cyclic voltammetric responses of anodized and colored plates.

The current and threshold potential of corrosion responses strongly depend on the applied conditions during anodizing, coloring and sealingstages. In addition, significant changes in impedance at AC voltammogram were observed, when the plates were colored under various conditions.Icc©

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n this study, the surfaces of the aluminum plates were also studied by scanning electron microscopy (SEM). Comparison of SEM images of theolored and uncolored aluminum specimens showed that the uncolored surface contained a significant number of pits. The results indicated thatoloring aluminum with Quinizarin and sealing it with boiled water could reduce corrosion of this metal.

2006 Elsevier B.V. All rights reserved.

eywords: Quinizarin; Colored aluminum; Anodizing; Cyclic voltammetry; Impedance measurement

. Introduction

Anodizing aluminum in sulfuric acid develops a porous oxidelm [1]. The structure of the porous film has been described asclosed packed array of columnar hexagonal cells each consist-

ng of a central porous film perpendicular to the macroscopicluminum surface [2]. The pores are separated from the metaly a thin barrier-type film layer, which is impressed on theluminum metal surface as a close-packed array of hemispher-cal scallops [3,4]. The existence of porous layer makes thexide highly absorbent, and hence, suited to coloring. Amonghe existing organic and inorganic pigments, organic dyes haveeen attracted a great attention for coloring aluminum, becausef their color range low cost [5]. The characteristic of thexide layer is a complex superposition of various effects, whichre influenced by the processing variables. There are numer-us papers which show different strategies to create conversionayers which defined voids and specific properties, e.g. for mem-ranes colored surfaces, etc. [3–6]. The quality of a colored

aluminum is strongly affected by conditions imposed duringanodizing, coloring and sealing. On the other hand, from thepoint of view of corrosion resistance, influence of coloring on theelectrochemical properties of the oxide film can be very impor-tant [7]. In this regard, it is expected that absorption of organicdye on the oxide film can change the electrical characteristicsand morphology of the aluminum oxide. It has been shown thatthe presence of organic compounds, such as carboxylic acid,oxalic acid, citric acid and isopropyl alcohol [8] in the anodiz-ing or sealing bath, can have a strong effect on characteristics ofthe formed oxide film on the aluminum surface.

It is well known that the processes inside the barrier layerare electrochemical in nature, while the processes inside theporous layer are chemical and physical in nature [9]. However,because of the complexity of the electrochemistry processesinside porous oxide, a detailed mechanism of the influence ofdye on the oxide film is very difficult or even impossible toprovide. This is rather attributed to the difficulties in studyingelectrokinetic’s phenomena inside the barrier layer (where theoxide production and film growth exclusively take place) and indetermining accurately structural features of pores [10].

∗ Corresponding author. Tel.: +98 21 88008800; fax: +98 261 4418156.E-mail address: abbas ahmady 3957@yahoo.com (A. Ahmadi).

In recent years, various new electrochemical methods havebeen employed to study the growth of porous oxide films on

300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2006.05.007

228 A. Ahmadi et al. / Progress in Organic Coatings 56 (2006) 227–233

aluminum and the effect of organic compounds and dye onthe process [11–13]. In this work, by using cyclic voltammetryand impedance measurement in potential and frequency domain,changes in corrosion resistance of colored aluminum preparedunder various conditions were investigated. Here, the corro-sion resistance was evaluated based on the changes in thresholdpotential of corrosion and the rate of corrosion was appraisedbased on values of the peak current at the cyclic voltammogram,and value of the impedance at the AC voltammogram.

It must be mentioned that the main goal here was to show thatcoloring of aluminum could reduce corrosion resistance of thesurface. The details related to interpretation of the impedanceresponses will be discussed in later works. The effect of impor-tant parameters related to the coloring procedure (such asanodizing voltage and time, concentration of the acid anodiz-ing bath and coloring time) on the voltammetric and impedanceresponses of aluminum specimens were studied. Scanning elec-tron microscopy (SEM) was used for studying and comparingchanges in the physical appearance of specimen surfaces. Inorder to increase corrosion resistance and quality of coloredaluminum strips, it was tried to find an optimum condition forthe coloring procedure.

2. Experimental

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Fig. 1. The structure of Quinizarin.

3. Results

Fig. 2 shows the cyclic voltammograms for two anodized Alelectrodes, uncolored (curve a) and colored (curve b) in solutionof 0.1 M H2SO4. Anodizing of the plates were performed byapplying a direct current at potential 12 V (for 30 min) in 18%(w/v) H2SO4 solution and then for coloring, one of them wasplaced in a solution of 2 g/l of Quinizarin for 20 min at 50 ◦C.Sealing of the plate was performed in distilled water at 100 ◦C(for 30 min). It can be seen that for the uncolored plate, thecurrent increased (at the threshold potential), whereas for thecolored plate, there is no significant current change. The resultsof two experiments indicated that the observed shift in thresholdpotential (at uncolored Al) was related to oxidation of aluminumsurface.

Therefore, it can be suggested that the existing differencebetween the voltammograms a and b can be due to interactionof the dye with the oxide layer (see Section 4).

Fig. 3 shows the variation of AC impedance and admittancevoltammograms at frequency of 8800 mV/s for uncolored (a)and colored (b) plates.

In those voltammograms, the impedance and admittancemeasurements were performed with AC amplitude 20 mV,potential scan rate 50 mV/s. Also, the potential was set within arange where corrosion processes took place. The declining of theplate impedance in specific frequencies can result from a higherrscoh

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.1. General

All of the chemical compounds were purchased from Merckhemical Company (Darmstadt, Germany). The working elec-

rodes used in the experiments were plates (3 cm × 2 cm) of9.5% pure aluminum, which were sealed in polyethylene inrder to have a surface of 0.14 cm2.

.2. Methods

For cleaning the surface, the plates were etched in a 5% solu-ion of NaOH at room temperature and rinsed in a 50% (w/v)olution of HNO3, and then immersed in a brightening solutionH3PO4 85%, H2SO4 16% and HNO3 3%, w/v) and rinsed withistilled water. The prepared specimens, under found optimumonditions, were anodized with direct current (DC) at potentialsf 12 V (for 30 min) in 18% (w/v) H2SO4 solution (at 22 ◦C).fter this step, the plates were washed and placed in a solu-

ion of 2 g/l of Quinizarin (1,4-dihydroxy anthraquinone, dye)or 20 min at 50 ◦C. Sealing the electrode was performed in dis-illed water at 100 ◦C (for 30 min).

The structure of the dye is shown in Fig. 1. For studying theffect of the above conditions on the characteristics of the col-red specimens, they were varied as will be mentioned below.he cyclic and impedance voltammetric measurements wereerformed by using lab made Potentiostate [14,15]. The solutionor electrochemical measurements was 0.1 M H2SO4 solution.he auxiliary electrode was a Pt electrode (1 cm × 0.1 mm diam-ter) and the reference electrode was a graphite electrode.

ate of corrosion process. In addition, the existence of noise atome frequencies for the uncolored plate indicates progress oforrosion processes at the plate surface [16]. Whereas, in casef the colored plate, such noise does not exist, this can be due toigh corrosion resistance of the surface at that potential range.

Fig. 4 presents SEM micrographs of the uncolored (a) andolored (b) plates. As seen in Fig. 4(b), the sealing process of an

ig. 2. Cyclic voltammograms for two anodized Al plates: (a) uncolored andb) colored in solution of 0.1 M H2SO4, scan rate 50 mV/s.

A. Ahmadi et al. / Progress in Organic Coatings 56 (2006) 227–233 229

Fig. 3. AC admittance and impedance measurements of (a) uncolored and (b)colored plates, in solution of 0.1 M H2SO4, scan rate 50 mV/s, AC amplitude20 mV.

anodized electrode in presence of the dye produces a uniformsurface and apparently, the pits were sealed very well. Whereasin case of the uncolored plate, the surface is a little roughs andcontains some pits (Fig. 4a).

As will be shown later, the threshold potential of corrosionand current density of the anodic response, as well as, the qualityof colored surface, were strongly influenced by the applied con-ditions during coloring procedure. To show this, the importantparameters were changed individually in the next experiments.

3.1. Effect of anodizing conditions

Fig. 5 shows the dependence of the threshold potential of thecorrosion versus the anodizing time. It can be seen that, it reachesa minimum for 30 min of anodizing time. The correspondingmeasured impedance for those plates at that potential range, also,showed a minimum low noise level (at 30 min anodizing time).Moreover, visual inspection of the plates surface showed that thecolor of the specimens were more pronounced with the anodizingtime, which is an indication of penetration of high number of thedye molecules into the oxide film. In fact, it was noted that, whenperforming anodizing time later than 30 min, cause the coloredspecimens, in acidic solution in the cell, to have a faster decayof color. It was reported [17] that performing anodizing Al ina long time, produces a thick amorphous hydrated (gel) oxide,wsafi

as

the applied DC potential. The good examples for those are 10,12 and 14 V (Fig. 6). It was observed that for colored plates,which were anodized at potentials lower than 12 V (e.g. 10 V),the threshold potential of corrosion shifts to 500 mV and cor-rosion current density was considerable (ca. 2 × 10−5 A/cm2).Moreover, in those cases, the colors of the plates were very lightand the adsorbed dye molecules were released easily into theacidic solution. In fact, the colors of electrodes were varnishedafter 45 min.

The threshold potential of corrosion for the colored plates,which were anodized at potential higher than 12 V (e.g. 14 V),shift to positive potentials (>3000 mV) and no peak current wasobserved. Though, those plates had the darkest color, but theylost their color after 3 h in the electrochemical cell. Whereas,for the plates which were anodized at 12 V, we could see verygood stability of the colored electrode under similar conditions.In addition, only in this case, the recorded impedance responsecontained a very low noise level, which was an indication of agood corrosion resistance of the colored plates. Only good colorstability was observed for the plates which were anodized at12 V. However, the threshold potential of corrosion in these caseswas ca.1100 V with a peak current around 9.5 × 10−7 A/cm2.

3.2. Effect of acidic concentration

In the next experiment, the aluminum plates were anodized(Hf(ciicqt

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hich can dissolve in acidic solutions. Based on this, it can beuggested that in those cases, the dye molecules are absorbedt the amorphous hydrated oxide (the upper layer of the oxidelm). We saw a darker color on specimens.

In another experiment, the aluminum plates were anodizedt different DC potential to study the dependence of the corro-ion current density in the voltammogram of colored plates, on

at potential 12 V for 30 min), but in various concentration of2SO4 solution. Fig. 7 illustrates the recorded voltammograms

or colored plates which anodized in solution of 14% (a), 18%b) and 22% (c) (v/w) of H2SO4. As is seen, there are no signifi-ant changes in the shape of voltammograms, but slight changesn magnitude of the corrosion current, which may due to thencreased acidity of the solution inside pores [10]. In high con-entration of acid, the plates were destroyed. The best coloruality was found for the plates that were anodized at a concen-ration of 18% of sulfuric acid.

.3. Effect of coloring conditions

Fig. 8 shows the recorded cyclic voltammograms of thenodized Al plates (at 12 V), which were colored at 2 g/l of theye in 20, 30 and 40 min. It can be seen that 20 min (coloringime, curve a) in the current/threshold potential of corrosion ishe best time for coloring and more coloring time caused a largerorrosion current.

It appears that, this time might be needed for the dyeolecules to penetrate into the oxide layer.

. Discussion

The cyclic voltammogram and impedance measurements ofnodized and well-sealed aluminum (curve b in Figs. 2 and 3)how that the plate surface is passive at the potential range–2300 mV. In fact, due to formation of a thick oxide film,uch plate should be passive at potentials higher than this poten-ial [1]. However, when the plate is colored, under special bestonditions, the surface oxidation process of the plate takes

230 A. Ahmadi et al. / Progress in Organic Coatings 56 (2006) 227–233

Fig. 4. The SEM photos of (a) uncolored and (b) colored aluminum plates.

place at potentials higher than 2000 mV. This high corrosionresistance is because of adsorbed dye on the plat’s surface ofcorrosion processes at such high potential suggests the pos-sibility of higher corrosion resistance of the oxide film bythe adsorbed dye. Comparison of the SEM pictures of thesurfaces of the colored and sealed plates (Fig. 4b), confirmsthat Quinizarin molecules do not cause any pores, still remainopen after coloring and sealing processes. Unfortunately, lit-tle is known about the processes, which occur inside the poresduring the coloring procedure. Therefore, we can only makeassumptions based on the reported data and our experimentalresults.

It has been reported that chemically adsorbed organic com-pound can influence the processes inside pores [18]. Moreover,adsorbed species on oxide film can influence the corrosion resis-tance of aluminum and the structure of the oxide film [19].Likely, here, there is a possibility that some of dye moleculesbound to the porous wall and change the structural characteristicof the pores oxide film during the coloring or sealing procedure.It is believed that the adsorbed Quinizarin inside the pores canbe in two forms: physically bound (or free form) and chemicallybound to the porous wall. Based on this, it can be speculated thatthe chemically bonded dyes influence the structural characteris-tics of the oxide during the coloring and sealing process.

A. Ahmadi et al. / Progress in Organic Coatings 56 (2006) 227–233 231

Fig. 5. Effect of anodizing time on coating of aluminum plates with Quinizarin.

Because, in case, there is physical penetration the dyemolecules, separated from the surface, have a very effect onthe corrosion resistance of the plate. Therefore, the observedphenomena can be interpreted in terms of adsorption of the dyemolecules and interference with the closure of the pores duringsealing. Supporting evidences for the chemical bond dye comefrom the results of studying the anodizing and coloring condi-tions.

Moreover, it is reported that the barrier layer may becomepermeable during oxide growth [20], resulting in penetrationof species in the electrolyte. According to the mechanisms forfield-assisted dissolution of oxide film [21] in acidic solutions,

Fig. 6. Effect of anodizing voltage on coating of aluminum plates withQuinizarin.

during applying positive potentials, the process:

Al → Al3+ + 3e−

takes place at the Al–Al2O3 interface. This produces an equiva-lent amount of aluminum ions, which enter the oxide bulk. Whilethe migration of Al3+ species from the oxide surface towards theAl2O3 interface takes place, the Al3+ ions become free from thesurrounding anions in the oxide lattice and solvated by watermolecules and move toward the film surface driven by the elec-trical field. Based on this mechanism, for unsealed oxide film,the anodic process at the plate surface during scanning of thepotential is similar to the anodizing process, and accordingly,

232 A. Ahmadi et al. / Progress in Organic Coatings 56 (2006) 227–233

Fig. 7. Effect of acid concentration of anodizing bath on coating of aluminumplates with Quinizarin.

the existing oxidation response at voltammograms is the resultof formation of Al3+ [22].

As mentioned above, when a thick barrier layer is developed(e.g. by applying voltage higher than 12 V during anodizing)the corrosion response at the voltammograms is weakened oreven disappeared. Considering that the impedance processes infilm formation take place within a thin barrier layer of non-porous oxide, a thicker barrier layer resulting from anodizingat high voltage (or current density) provides greater corrosionresistance. This may because of slower migration of Al3+ andO2− species [23].

Based on the data that there is an inverse relationship betweenthe surface area (or volume) of the pores and the sealing time[24,25], therefore, for a very long sealing time, the volume ofpores decreases and the adsorbed dye molecules move toward

Fig. 8. Effect of coloring time on coating of aluminum plates with Quinizarin.

the upper layer of the oxide film. Moreover, due to this fact thatsealing can cause notable changes in the surface development ofporous aluminum oxides, unplugged pores facilitate the ejectionof Al3+ ion into the solution [26].

5. Conclusion

The voltammetric study showed that coloring aluminum isaccompanied with extensive changes in the electrochemicalproperties of the anodized surface of it. The coloring of alu-minum with Quinizarin can change the threshold potential ofcorrosion and may increase the breakdown potential of the bar-rier layer. There is a possibility of chemisorptions of the dye to

A. Ahmadi et al. / Progress in Organic Coatings 56 (2006) 227–233 233

the oxide layer. This could be via O− in OH− and C O of thedye molecules, which penetrate in the presence of vacancies inthe oxide. This speculation comes from the recorded voltammo-grams at various conditions of anodizing and coloring. Also, theresults showed that the best conditions for anodizing and col-oring with Quinizarin are: anodizing voltage, 12 V; anodizingtime, 20 min; acid concentrate of anodizing bath, 18%; coloringtime, 20 min.

However, still more work is needed to understand the mech-anism of the observation.

Acknowledgement

Research council of university of Tehran supported the work.

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