Achieving non-adsorptive anodized film on Al-2024 alloy: Surface and electrochemical corrosion investigation

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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
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  Contents lists available at ScienceDirect Surfaces and Interfaces  journal homepage: www.elsevier.com/locate/surfin Achieving non-adsorptive anodized film on Al-2024 alloy: Surface andelectrochemical corrosion investigation M. Faizan Khan a,c, ⁎ , A. Madhan Kumar b, ⁎ , Anwar Ul-Hamid c , Luai M. Al-Hems c a  Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia b 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 A R T I C L E I N F O  Keywords: AFMAl2024 alloysAnodized filmCorrosionEISXPS A B S T R A C T Naturally formed oxide film on the surface of aluminum reduces corrosion rate to significant extent. Anodizingof aluminum is done to enhance this feature by increasing film thickness deliberately to desired level. In thecurrent work, anodizing of Al-2024 alloy was carried out in sulfuric-acetic acid anodizing bath of differentconcentrations along with gradually increased applied potential. The structure and morphology of the developedanodized film was investigated with and without the addition of acetic acid. Structural and surface analysisresults revealed the difference in the anodized layer formation with the presence of acetic acid in sulfuric acidbath. Electrochemical investigation was carried out to evaluate the surface protective performance of the formedanodized film in 3.5% NaCl solution. Potentiodynamic polarization (PDP) and electrochemical impedancespectroscopy (EIS) revealed that lowest corrosion rate with highest polarization resistance was achieved due tothe presence of CH 3 COOH in the anodizing bath. Non-adsorptive and less porous anodized film noticed to beformed with the presence of CH 3 COOH in bath solution. 1. Introduction Anodizing investigation of Al alloys has been remained under deepinterest since from a long time and attracted significant attention of researcher from decades [1–7]. In electrochemical process, when po-tential is applied high enough on the anode part of the electrochemicalcell then this allows oxygen to accumulate on the Al (anode) surfaceand results film formation, is called anodization. In fact, film is devel-oped as a result of electrochemical reaction in between metal surfaceand electrolyte ions and an overall increase in volume occur on Alsubstrate as a result of anodizing. The formation of anodic oxide layer isthe result of the movement of Al 3+ and O 2− ions towards each otheracross the barrier layer region [8]. The O 2− movement becomes theprimary cause of oxide layer formation on metal-film interface. Field-assisted flow of alumina at a certain region does not allow to the for-mation of continuous oxide layer and, hence, a porous oxide layer isdeveloped [9]. The detailed mechanism about porous film formationand growth can be understood somewhere else [10–14].Anodizing film can be thin, thick, dense or porous depending on theemployed potential, bath composition and concentration. The surfacemorphology of the film is porous by nature and determines the ad-sorptive characteristics as well as abrasion resistance. In the latter case,coatings with a less number of smaller diameter pores will have higherresistance to abrasion than coatings with a large number of greaterdiameter pores. This makes sense as the density of the anodized coatingis proportional to the amount of oxide formed and hence the pores sizestarts decreasing with continuous oxide formation thus improving theabrasion resistance of the coating [15]. Peter et al. [16] studied the anodization of Al alloys and found the optimum conditions by varyingthe electrolytic composition of sulfuric/tartaric acid, sulfuric/carbolic/boric acid and sulfuric/oxalic/boric acids. Their investigation revealedthat corrosion current density found to be decreased leading to reducecorrosion rate by increasing polarization resistance (R P ). It was alsoobserved in their study that anodized film developed in sulfuric/tartaricacid bath was better than the rest of the baths. Li et al. [17] studiedanodizing on Al-Si substrate by three different techniques; (i) hard https://doi.org/10.1016/j.surfin.2019.02.005Received 25 September 2018; Received in revised form 7 January 2019; Accepted 11 February 2019 ⁎ Corresponding author.  E-mail address:  madhankumar@kfupm.edu.sa (A.M. Kumar). Surfaces and Interfaces 15 (2019) 78–88Available online 12 February 20192468-0230/ © 2019 Published by Elsevier B.V.    anodizing (HA), (ii) simple anodizing and (iii) modified anodizing(MA). They found different hardness against each film developed bydifferent techniques and each film was sufficiently resistive to corro-sion. However, they reported that modified anodizing technique wasmore beneficial due to its environmental friendly process. Wielage et al.[18] found the effect of aluminum substrate surface (roughness) on theanodized film thickness and morphology. Forn, et al. [19] studied theeffect of additional particles in Al-substrate and noticed increased filmthickness by the presence of discussed particles. The tribologicalproperties of this thick oxide layer were reported tremendous in senseof wear resistance. Fratila-Apachitei et al. [20] reported the growth of anodized film by taking three different aluminum substrates and ap-plied three different current densities in 2.25M H 2 SO 4  bath at 0 °C.Similarly, Morks et al. [21] did their experiment to grow the anodicoxide layer on aluminum alloy in 5-sulfosalicylic acid electrolyte andstudied the corrosion resistance in different concentrations of electro-lytes at different temperatures. Saeedikhani et al. [22] demonstratedthe improvement in corrosion resistance of 2024-T3 aluminum alloy byanodization in mixed electrolyte containing 10% sulfuric acid, 5% boricacid and 2% phosphoric acid. Zhang et al. [23] investigated the addi-tion of boric acid in the presence of sulfuric acid electrolyte to study theeffect of current density-time response, growth rate and morphology of the porous anodic film. Shanmuga et al. [24] did their research on ACanodization technique by taking Al substrate in an electrolyte con-taining 10% sulfuric acid, 3% sodium sulfate, organic additives SLS andgelatin. They revealed that the presence of gelatin increases the com-pactness of the coating. Xiang Feng et.al revealed through their re-search that the addition of citric acid in sulfuric acid bath improvedanticorrosion behavior of the anodized film [25]. Ying-dong et.alshowed in their research that the addition of adipic acid in sulfuric acidbath enhanced dielectric properties and corrosion resistance [26]. Sofar literature survey reveals that all the developed anodized films pos-sess porous adsorptive surface due to the presence of pores over itssurface. This porous surface causes a big trouble by inviting the cor-rosive media to flow inside the film through these pores and deterioratethe 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 theircompatibility towards echo-friendly environment yet has not beenproven. In the current work, the presence of acetic acid in varyingconcentration of sulfuric acid bath is investigated under different ap-plied potentials to understand its impact on the anodized film thicknessand non-adsorptive morphology of Al-2024 substrate. Evaluating sur-face morphology of the anodized film and the electrochemical perfor-mance in corrosive environment is the part of this research work. 2. Experimental work  2.1. Material 2024 Aluminum alloy sheet, having thickness of 3mm, was cut forpreparing samples to investigate anodization. The composition of thematerial was determined by x-ray fluorescence (XRF) technique andgiven in Table 1.  2.2. Surface preparation For the pretreatment of aluminum alloy, samples were first of alldegreased using acetone solvent followed by rinsing in distilled water.The samples were then grinded up to 800 grit size of SiC abrasive paperfollowed by alkaline cleaning in a solution of 12g NaOH/100ml 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 pro-vided in Table 2. Finally, the samples were treated in a solution (as perthe technique given in Table 2) to deoxidize the surface.  2.3. Anodizing and sealing  Aluminum strips were anodized under two applied potentials (15Vand 25V) in electrolytic bath of sulfuric acid (H 2 SO 4 ) whose compo-sition and concentration was changed with and without the addition of acetic acid (CH 3 COOH). For each anodizing process, potential wasprovided by using D.C power supply whereas time and temperature waskept constant to 30 min and 25 °C, respectively. The scheme for eachanodizing process has been provided in Table 3. After completing an-odizing process, samples were cleaned rigorously using distilled waterand then sealing was carried out by treatment in near-boiling distilledwater to convert oxide lining of the pores from amorphous oxide to anon-hydrate. However, sealing mechanism of the developed anodizedfilm not only increases the volume of the film but also blocks the poreopenings by rendering the film morphology to get transformed fromadsorptive to non-adsorptive [19]. Table 1 2024 aluminum alloy elemental compositions determined by XRF. Element Al Zn Mg Cu Fe Si Mn Ni Tiwt.% 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 54mL H3PO4 (85%) 90 42mL HNO3 (66.4%)15mL CH3COOH (99%)13mL H2ODeoxidation 35mL HNO3 (66.4%) 25 265mL H2O Table 3 Electrolytic bath composition, concentration and applied potential used for anodizing of aluminum at room temperature for 30 min. Sample ElectrolytecompositionVoltage (V) Sample ElectrolytecompositionVoltage (V)AN-1 5% wt. H 2 SO 4  15 AN-2 5% wt. H 2 SO 4  25AN-3 10% wt. H 2 SO 4  15 AN-4 10% wt. H 2 SO 4  25AN-5 5% wt. H 2 SO 4  +2%CH 3 COOH15 AN-6 5% wt. H 2 SO 4  +2%CH 3 COOH25  M.F. Khan, et al.  Surfaces and Interfaces 15 (2019) 78–88 79   2.4. Microstructure analysis The anodized samples were analyzed for measuring film thicknessand morphological study. Samples were cold mounted followed bygrinding using up to 1200 emery paper. In the next step, polishing of the mounted samples was carried out by diamond paste and HF wasused to etch the anodized aluminum samples. Scanning electron mi-croscope (SEM) JEOL JSM-6480 LV was used for obtaining images aswell as measuring anodized film thickness. X-Ray PhotoelectronSpectroscopic (XPS) measurements have been done to evaluate theparticular surface elemental compositions as result of the alterations,produced by the relevant anodization techniques. The surface topo-graphy of anodized samples was analyzed through AFM (Agilent 5500AFM, USA) instrument. The acquired AFM images were detected withnon-contact mode of Au coated silicon cantilevers with a resonancefrequency of 26kHz with a spring constant of 1.6N/m under air en-vironment. The chemical structure of anodized Al samples was char-acterized by an Attenuated Total Reflectance-Infrared spectrometer(Thermo scientific, with universal ATR attachment, range500–4000 cm −1 ).  2.5. Electrochemical corrosion testing  The general corrosion resistance of the anodized samples was in-vestigated by potentiodynamic polarization (PDP) technique underroom conditions using Gamry 3000 potentiostat. The potentiodynamic Fig. 1.  Film thickness of different anodized Al samples achieved at (a) 5%H 2 SO 4  @15V, (b) 5%H 2 SO 4  @25V, (c) 10%H 2 SO 4  @15V, (d) 10%H 2 SO 4  @25V, (e) 5%H 2 SO 4 /2%AA @15V, (f) 5%H 2 SO 4 /2%AA @25V.  M.F. Khan, et al.  Surfaces and Interfaces 15 (2019) 78–88 80  polarization (PDP) measurements were carried out by a conventionalthree-electrode cell using a saturated calomel electrode (SCE) as a re-ference electrode and a graphite rod as a counter electrode. When theelectrochemical system was stable, the measurements were taken in a3.5% NaCl solution (sea water) and the corrosion rates were de-termined for comparison. Before conducting electrochemical test, opencircuit potential (OCP) of all the tested samples was measured until toget stabilized potential. For conducting PDP, the applied initial andfinal voltage was set to±250mV whereas the scan rate was fixed to0.2mV/s. Electrochemical impedance spectroscopy (EIS) was measuredby applying AC voltage having an amplitude of 10mV and the fre-quency was set from 100K to 10 mHz. 3. Results and discussion 3.1. Anodizing film thickness The film thicknesses obtained by variation of the applied voltageand electrolytic composition under fixed temperature (25°C) and time(25 min) have been provided in Fig. 1. In addition to this, the filmthicknesses of different anodized samples have also been depictedgraphically in Fig. 2. It can be seen through the graphs (Fig. 2) that higher applied voltage of 25V provided thicker anodized film (AN-2,AN-4 and AN-6) compared to lower applied voltage of 15V (AN-1, AN-3and AN-5). However, when the applied voltage was fixed to 15V thenthe sample which was anodized in a mixer of 2%wt. acetic/5%wt.sulfuric acid (AN-5) provided thicker anodized film (9.5µm) comparedto the samples which were anodized just in the presence of sulfuric acid(5wt.% = 7.8µm, 10wt.% = 8.5µm). Interesting thing to note thatthe addition of only 2wt.% acetic acid in 5wt.% sulfuric acid (AN-5)provided thicker anodized film even than 10wt.% sulfuric acid (AN-3).Nevertheless, 10wt.% sulfuric (AN-3) provided slightly thicker ano-dized film compared to 5wt.% sulfuric acid (AN-1). It is worth notingfrom the anodized thickness analysis that concentration of the bathsolution has less effect for achieving thickened anodized film comparedto the applied potential.The same trend of changing the film thickness with the compositionof electrolyte was seen to repeat when the applied voltage was en-hanced to 25V. Again the addition of 2wt.% acetic acid in 5wt.%sulfuric acid (AN-6=12.4µm) provided nearly equal film thickness tothe anodized film developed in 10wt.% sulfuric acid (AN-4=12.7µm)bath. It can be stated that anodized film thickness for the case of alu-minum increased with enhanced applied voltage and concentration of sulfuric acid in the electrolyte. But little addition of acetic acid (2wt.%) Fig. 2.  The effect of applied voltage and electrolyte composition on anodizingfilm thickness at 25°C for 30min. Fig. 3.  X-Ray diffraction patterns of anodized Al samples under different electrolytic compositions and applied potentials.  M.F. Khan, et al.  Surfaces and Interfaces 15 (2019) 78–88 81  along with sulfuric acid cut off the concentration level of sulfuric acid inthe electrolytic bath to achieve the required anodized film by fixing theapplied voltage. In other words, in the presence of acetic acid, overallless concentration of sulfuric acid can provide the same thickness whichshould be achieved at higher concentration of sulfuric acid. To obtainrequired anodized film, decreasing the concentration of sulfuric acidand replacing it partially with acetic acid is a step towards ecofriendlyanodizing. 3.2. Surface characterization X-ray diffraction pattern of different anodized Al-2024 samples havebeen provided in Fig. 3. It can be seen that, for the bare Al-2024 sample,only Al peaks appeared. However, once anodizing was done in differentelectrolytes then it can be seen that anodized film was formed andmainly composed of two phases i.e. γ-Al 2 O 3 , α-Al 2 O 3  [25,28]. As thethickness and volume of the porous anodized film is very less comparedto bare Al, therefore the x-rays are diffracted from base aluminum withgreater peaks intensity which is in agreement with the study of Yer-okhin [28]. From the XRD patterns of each anodized sample, it iscleared that voltage played an effective role along with electrolytepercentage of the bath solution for the formation of crystalline phases.For example, for AN-1, in 5% H 2 SO 4  bath solution, it can be seen that15V assisted in the formation of just α-Al 2 O 3  phase whereas AN-2, atthe same electrolytic percentage, formed an additional γ-Al 2 O 3  crys-talline phases when voltage was enhanced to 25V. Nevertheless, Fig. 4.  Scanning electron micrographs (SEM) of anodized surface (a) 5%H 2 SO 4  @15V, (b) 5%H 2 SO 4  @25V, (c) 10%H 2 SO 4  @15V, (d) 10%H 2 SO 4  @25V, (e) 5%H 2 SO 4 /2%Acetic acid @15V, (f) 5%H 2 SO 4 /2%Acetic acid @25V.  M.F. Khan, et al.  Surfaces and Interfaces 15 (2019) 78–88 82
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