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Materials Letters 60 (20

Advanced nano-particles anti-corrosion ceria basedsol gel coatings for aluminum alloys

Abdel Salam Hamdy ⁎

Department of Surface Treatment and Corrosion Control, Central Metallurgical R & D Institute, CMRDI, P.O. Box 87, Helwan, Cairo, Egypt

Received 2 December 2005; accepted 18 January 2006Available online 10 February 2006

Abstract

Chromate conversion coatings can be successfully used for the corrosion protection of aluminum alloys. However, the environmental laws inmany countries have imposed severe restrictions on chromate use due to its high toxicity and consequent environmental hazards. Many attemptshave been made to find alternatives to chromating such as cerate. In this work, the effect of surface preparation prior to ceria treatment wasstudied. A series of specimens was prepared under the following conditions: (a) as polished, (b) directly treated, (c) etched, (d) oxide thickened,and (e) etching followed by oxide thickening. After surface preparation, the specimens dipped in ceria solutions prepared by sol–gel method.Electrochemical Impedance Spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance in 3.5% NaCl.The optimum conditions under which ceria treatments can provide good corrosion protection to the aluminum substrate were determined.Generally, ceria treatments improve the corrosion resistance due to the formation of protective oxide films which act as a barrier to oxygendiffusion to the metal surface. According to the EIS and polarization measurements, a combination between etching and oxide thickening has avital role on the corrosion protection performance of AA6061-T6 in NaCl.© 2006 Elsevier B.V. All rights reserved.

Keywords: Corrosion protection; Surface treatment; Aluminum alloys; Ceria; Sol–gel coatings

1. Introduction

The advent of chromate replacements for aluminum alloysbegan in the late 1970s. As regulations became increasinglystricter, commercial, academic and government facilities haverelied on a collaborative effort to come up with innovationsfor the corrosion protection of aluminum alloys [1]. Salts likecerate have been proposed as alternatives to chromateinhibitors, as have several other rare earth elements, becausethe rare earths behave as cathodic inhibitors in aluminum [2–14]. In previous work [2,8,9,12–14] we found that cerateconversion coatings prepared by conventional method im-prove the corrosion resistance of aluminum due to formationof protective oxides, which act as a barrier to oxygendiffusion to the metal surface. However, most of the proposedalternative coatings formulations have failed to produce thelevel of protection that chromates provide. Advances in the

⁎ Tel.: +20 25010640; fax: +20 25010639.E-mail address: ashamdy@cmrdi.sci.eg.

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.01.049

chemical tailorability of mixed alkoxide sol–gel coatings haveled to attempts to create a long-lived, environmentallycompliant, conversion coating for aluminum alloys [15]. Itwas shown that coating based on silica, ceria, vanadia andmolybdenum can be tailored to produce a functionallygradient coating that will provide covalent bonding for strongcoating adhesion and act as a barrier coating to limit thetransport of water to the surface of the alloy [16–20].

In this work, newly developed environmentally friendlysurface treatments based on ceria prepared by sol–gel methodwere proposed as alternatives to toxic chromate-basedsystems. The effect of ceria sol–gel coatings on the corrosionbehavior of Al6061 T6 in 3.5% NaCl solution will be studiedusing EIS and DC polarization techniques. A new surfacepreparation method consisted of pre-etching followed byoxide thickening before applying the sol–gel coating isproposed. Surface examination will be performed by scanningelectron microscopy (SEM), Energy dispersive X-ray (EDS),X-ray Photoelectron Spectroscopy (XPS) and Atomic ForceMicroscopy (AFM).

2634 A.S. Hamdy / Materials Letters 60 (2006) 2633–2637

2. Experimental

2.1. Materials

The specimens of AA6061 T6 alloy, in the form of 60×30mm taken from a sheet 3 mm thick, were abraded to 800finish with SiC grit papers, degreased in acetone, washed withdistilled water, and dried in dry air. The nominal compositionwas as follows (wt.%): 0.35 Cu; 0.95 Mg; 0.70 Fe; 0.50 Si;0.15 Mn; 0.15 Cr; 0.25 Zn; 0.15 Ti; remainder Al.

2.2. Sol synthesis and surface pre-treatment

Ceria sol was prepared by dissolving 0.465 g (0.005 M) ofCeCl3·7H2O and adding 0.48 g (0.01 M) citric acid and 250 mlC2H5OH. The solution was completed by distilled water to givea final concentration of 0.005 M cerium chloride.

Four groups of specimens prepared under the followingconditions:

1. As-polished specimens (DT)2. Etching in 0.01 M KOH solution for 10 min (K)3. Oxide thickening in boiling distilled water for 1 h (B)4. Etching in 0.01 M KOH solution for 10 min followed by

oxide thickening in boiling water for 1 h (KB).

After the surface pretreatment, the specimens were dippedinto ceria sols for 10 min, dried at 110 °C for 30 min and heattreated at 500 °C for 30 min. A comparison will be madebetween the corrosion resistances obtained from the four groupsafter applying the ceria sol and as-polished samples without anysurface treatment (WT) after 30 days of immersion in 3.5%NaCl solution.

2.3. Methods

The corrosion behavior of the previous groups wasmonitored using electrochemical impedance spectroscopy(EIS) and DC polarization techniques during immersion in

Fig. 1. Nyquist plots after 30 days of immersion in NaCl. Blank WT (○), as-polishepre-treatments). Group 2 K (▴), etching followed by ceria treatment. Group 3 B (▪followed by boiling water for 1 h followed by ceria treatment.

3.5% NaCl solution open to air and at room temperature for upto 30 days.

A three-electrode set-up described elsewhere [12] was usedwith impedance spectra being recorded at the corrosionpotential Ecorr. A saturated calomel electrode (SCE) was usedas the reference electrode. It was coupled capacitively to a Ptwire to reduce the phase shift at high frequencies. EIS wasperformed between 0.01 Hz and 65 kHz frequency range usinga frequency response analyzer (Autolab PGSTAT 30, Eco-Cheime, The Netherlands). The amplitude of the sinusoidalvoltage signal was 10 mV.

DC polarization tests of specimens previously immersed for30 days in 3.5% NaCl solution were made at a scan rate of0.07 mV/s in the applied potential range from −0.15 to 0.7 VSCE

with respect to Ecorr using an Autolab PGSTAT 30 galvanostat/potentiostat, EcoCheime, The Netherlands. The exposed surfacearea was 2.54 cm2. All curves were normalized to 1 cm2.

SEM images were obtained using a digital scanning electronmicroscope Model JEOL JSM 5410, Oxford Instruments,Japan. Microprobe analysis was performed using energydispersive spectrometry, EDS, Model 6587, Pentafet Link,Oxford microanalysis group, UK. XRD analysis was performedusing Bruker axs, Model D8 ADVANCE, Germany.

AFM analysis was performed using Scanning ProbeMicroscope/Atomic Force Microscope SPM/AFM Dimension3100. XPS analysis was performed using XPS/ESCA Perkin-Elmer PHI 5100 ESCA System.

3. Results and discussion

A thick layer of Al-oxide was formed after immersion of the as-polished samples without any surface treatment (WT) after 30 daysof immersion in 3.5% NaCl solution. This layer increased theinsulating power of the passive film in addition to hindering the iondiffusion through the surface. This layer cannot protect the surfacecompletely and hence severe localized corrosion will take place inthe presence of Cl− ions.

According to EIS measurements (Fig. 1), cerium sol gel coatingsimprove the surface resistance of aluminum to localized corrosion. Thespecimens etched and oxide thickened prior to ceria treatment showed

d (without treatment). Group 1 DT (♦), directly treated in ceria (without surface), boiling water for 1 h followed by ceria treatment. Group 4 KB (◊) etching

Table 1Polarization data after 30 days in NaCl solution

Sample Ecorr

(mV)Passivecurrent (A)

Epit (mV) Perfect passivitydomain

As-polished −778 3×10−6 −740 Not presentCeria (KB) −738 0.5×10−6 −683 ≈8 mV

Fig. 3. SEM of the sample etched and oxide thickened prior to ceria treatmentafter 30 days in 3.5% NaCl (micro-cracking and agglomeration at a corrodedarea).

2635A.S. Hamdy / Materials Letters 60 (2006) 2633–2637

the best corrosion protection after 30 days of immersion in NaCl. Thesurface resistance of such specimens was about 7.4×104 Ω cm2

compared with 0.43×104 Ω cm2 for the as-polished specimens withoutceria treatment. Directly treated (DT) samples as well as the sampleswhich were etched (K) or oxide thickened (B) prior to ceria treatmentshowed surface resistance of 2.8×104, 5.0×104 and 3.5×104 Ω cm2,respectively.

Cyclic potentiodynamic tests (Table 1) confirmed these resultswhere the pitting potential, Epit, of ceria treated specimens is 57 mVmore nobel than the as-polished specimens. The corrosion potential,Ecorr, of ceria treated specimens is also 40 mV more nobel than theas polished specimens. These results confirm the previous observa-tions that cerium behave as cathodic inhibitors in aluminum [2–7,12–14]. The pitting area under the loop of ceria is too small to thatobtained from as-polished specimens. An onset perfect passivitydomain of about 8 mV was detected in the ceria treated specimens.Conversely, as-polished specimens did not show any perfectpassivity domain.

SEM (Fig. 2) showed that the specimens which were etched andoxide thickened prior to ceria treatment had a more smooth appearancethan the directly treated (DT) specimens. The film formed in the firstcase is more compact and distributed uniformly over the surface. Thisis may be due to formation of a compact Si-rich oxide film distributeduniformly over the surface of aluminum substrate. Conversely, thesamples which were directly treated with ceria without any surfacemodification showed non-uniform surface distribution which suggeststhe distribution depends on the substrate microstructure [7] resulting inpitting and crevice corrosion as well as micro-cracking.

SEM image at corroded area of the specimens which were etchedand oxide thickened prior to ceria treatment (Fig. 3) revealed presenceof Al2O3 agglomerates and micro-cracked film. EDS and XRD

Fig. 2. SEM of the samples etched and oxide thickened followed by ceria treatment ancorroded area).

analyses of the above did not detect the presence of cerium indicatingthat cerium may be present as a very thin film [2].

This result was evidenced by AFM surface topography (Fig. 4)which revealed compact surface film of smooth appearance for theetched specimens while the non-etched ones showed agglomeratedlayer. Consequently; etched specimens are more effective in reducingthe number of pits. This observation could be answered why the pittingarea under the loop of the etched samples is too small to that obtainedfrom as polished specimens (Table 1). These results confirm theimportance of the etching process to improve the localized corrosionresistance by formation of a protective Al-oxide film incorporated withcerium which acts as a barrier to oxygen diffusion to the metal surfacein addition to the cathodic inhibitive effect.

XPS analysis (Fig. 5) revealed presence of relatively higher amountof Al 2s and Al 2p for the samples etched followed by oxide thickeningcompared with the non-etched samples. The amount of cerium detectedin all cases was too small to be accounted. Based on the results of SEM,AFM andXPS, we suggest that the corrosion protection is mainly due tothe surface distribution of the Al-oxide film. If this film will distributeuniformly over the surface, the resistance to localized corrosion will

d directly treated samples after 30 days in 3.5% NaCl (surface distribution at non

Fig. 4. AFM surface topography image of etched and non-etched oxide thickened samples before corrosion.

2636 A.S. Hamdy / Materials Letters 60 (2006) 2633–2637

increase and vice versa. Since the etching process was the only factorchanged between two treatments, we can conclude that the etchingprocess prior to oxide thickening has a dual effect. The first is to enhancethe distribution of Al-oxide uniformly. The second is to preventagglomeration of the surface Al-oxide film and consequently, enhancesformation of uniformly distributed compact oxide film of smoothappearance. Therefore, the pitting corrosion resistance was improved.

4. Conclusions

1. Direct treatment with ceria sol gel coatings (without surfacemodification) cannot offer good corrosion protection inNaCl.

2. Cerium treatments improve the localized corrosion resis-tance of aluminum in NaCl and behave as cathodic inhibitorsin aluminum.

3. A combination between etching and oxide thickeningprocesses prior to ceria coatings plays a vital role in themechanism of corrosion protection of Al-alloys. The bestcorrosion resistance was observed for the specimens thatpassed with etching together with oxide thickening processesprior to ceria sol gel coatings.

4. XPS analysis revealed presence of higher amount of Al-oxide for the samples etched followed by oxidethickening compared with the non-etched samples. AFMshowed that film formed in the samples etched followedby oxide thickening is more compact and has smootherappearance than the oxide thickening samples withoutetching.

5. Etching process prior to oxide thickening plays animportant role to enhance the distribution of Al-oxideuniformly and to prevent agglomeration of the surface Al-

Fig. 5. Al 2s and Al 2p ratios of corroded KB and B.

2637A.S. Hamdy / Materials Letters 60 (2006) 2633–2637

oxide film and consequently the pitting corrosion resis-tance was improved.

References

[1] R.L. Twite, G.P. Bierwagen, Progress in Organic Coatings 33 (1998) 91.[2] A.S. Hamdy, A.M. Beccaria, P. Traverso, Journal of Applied Electro-

chemistry 35 (2005) 473.[3] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Materials Performance 8 (1987)

42.[4] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Materials Forum 9 (1986) 162.[5] A.E. Hughes, B.R.W. Hinton, R. Taylor, M. Henderson, K. Nelson, L.

Wilson, S.A. Nugent, International Patent application No. PCT/AU94/00539, International Patent No.WO95/08008.

[6] S. Lin, H. Shih, F. Mansfeld, Corrosion Science 33 (1992) 1331.[7] M. Dabalà, L. Armelao, A. Buchberger, L. Calliari, 2nd International

Symposium on Aluminum Surface Science and Technology, 21–25 May,UMIST, Manchester, England, 2000, Paper No. 97.

[8] A.S. Hamdy, Corrosion Protection of Aluminum Composites by Silicate/Cerate Conversion Coatings, Surface Coatings and Technology, in press(Available online).

[9] A.S. Hamdy, Ph.D. dissertation, Faculty of Science, University of Cairo,Feb. 2003.

[10] K. Aramaki, Corrosion Science 43 (2001) 591.[11] A.R. Trueman, A.E. Hughes, G. McAdam, B.R.W. Hinton, 2nd

International Symposium on Aluminum Surface Science and Technology,21–25 May, UMIST, Manchester, England, 2000, p. 270.

[12] A.S. Hamdy, A.M. Beccaria, R. Spiniello, Corrosion Prevention andControl 48 (2001) 101.

[13] A.S. Hamdy, A.M. Beccaria, Corrosion protection of aluminum metal–matrix composites by cerium conversion coatings, Surface and InterfaceAnalysis 34 (2002) 171–175.

[14] A.S. Hamdy, A.M. Beccaria, Cerium conversion coatings: a new era ofchrome-free pretreatments, 21st Annual Conference Corrosion Problems

in Industry, Dec. 17–20, Egyptian Corrosion Society, Sharm El-Sheikh,Red Sea City, Egypt, 2002.

[15] L.S. Kasten, J.T. Grant, N. Grebasch, N. Voevodin, F.E. Arnold, M.S.Donley, Surface and Coatings Technology 140 (2001) 11.

[16] A.S. Hamdy, A.A. Ismail, A.K. Ismail, D.P. Butt, Novel anti-corrosionnano-sized vanadia-based thin films prepared by sol–gel method foraluminum alloys, 4th Japanese-Mediterranean Workshop on AppliedElectromagnetic Engineering for Magnetic, Superconducting and NanoMaterials, (JAPMED'4), Sheraton Cairo Hotel, Egypt, September 17–20,2005.

[17] A.S. Hamdy, A.K. Ismail, D.P. Butt, A.A. Ismail, Comparative Studies onAnticorrosion Nano-Particle Silica-, Ceria-and Molybdate Based ThinFilms Adsorbed on Aluminum Alloys, Presented at the InternationalSymposium on the Role of Adsorbed Films and Particulate Systems inNano and Biotechnologies, August 24-26, 2005, Gainesville, Florida. Co-organized by the Particle Engineering Research Center (PERC) and theCenter for Surface Science and Engineering, University of Florida, USA.

[18] A.S. Hamdy, Review: corrosion protection via sol–gel derived nano-sizedthin films, 22nd Annual Conference Corrosion Problems In Industry,Dec. 8–11, Egyptian Corrosion Society, Ain El-Sukhna beach, Red SeaResort, Egypt, 2003.

[19] A. S. Hamdy, A.K. Ismail, D.P. Butt, A.A. Ismail, Sol-gel Prepared Anti-Corrosion Vanadia-Based Ceramic Coatings for 6061-T6 AluminumAlloy, Presented at the International Symposium on the Role of AdsorbedFilms and Particulate Systems in Nano and Biotechnologies, August 24-26, 2005, Gainesville, Florida. Co-organized by the Particle EngineeringResearch Center (PERC) and the Center for Surface Science andEngineering, University of Florida, USA.

[20] A.S. Hamdy, A.A. Ismail, A.K. Ismail, Newly developed silica-basedceramic coatings prepared by sol–gel technology for aluminum alloys,Presented at the 23rd Annual Conference Corrosion Problems in Industry,Dec. 6–9, Egyptian Corrosion Society, Nozha Beach Resort Red Sea,Egypt, 2004.