Study of corrosion protection of the composite films on A356 aluminum alloy

JOURNAL OF RARE EARTHS, Vol. 29, No. 10, Oct. 2011, P. 991

Study of corrosion protection of the composite films on A356 aluminum alloy SUN Huanhuan (ᄭ⛩⛩)1, WANG Hui (⥟䕝)2, MENG Fanling (ᄳ޵⦆)1 (1. School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, China; 2. Norsom Press Shop, Shanghai GM (Shenyang) Norsom Motors Company Limited, Shenyang 110044, China) Received 6 April 2011; revised 24 May 2011

Abstract: Composite films were fabricated on A356 aluminum alloy by combined anodizing and rare earth deposition. The corrosion protection effect and corrosion behavior of the composite films in 3.5% NaCl solution were studied by electrochemical impedance spectroscopy (EIS). SEM observation indicated that the rare earth Ce film completely sealed the porous structure of the anodic film, and the composite films composed of anodic film and Ce film were compact and integrated. According to the characteristics of EIS, the EIS plots of the composite films at different immersion times were simulated using the equivalent circuits of Rsol(QceRce)(QaRa), Rsol(QceRce)(QpRp)(QbRb) and Rsol(QpRp)(QbRb) models, respectively. The test results showed that the Ce film at the outer layer of the composite films had good protection effect at the initial stage of the immersion corrosion. It effectively helped the anodic film at the inner layer to prevent chloride irons from penetrating the aluminum alloy matrix. After 18 days, the Ce film lost its anticorrosive property, and the anodic film took the leading role of the corrosion protection. When the corrosion time was up to 42 days, the aluminum matrix was not corroded yet. Thus, the higher protection degree of the composite films for A356 aluminum alloy was attributed to the synergism effects of anodic film and rare earth Ce film. Keywords: aluminum alloy; anodizing; rare earth Ce film; EIS; corrosion resistance

Cast aluminum alloys are widely used to produce many complex parts, due to their good liquidity. Aluminum alloys exposed to the air are naturally protected by a layer of oxide film. However, this layer is heterogeneous and does not provide adequate corrosion protection in many environments[1]. The anodizing technology is an effective method to improve the corrosion resistance of the aluminum and aluminum alloys, and it has been universally applied. The anodic films are known to present a duplex structure[2]: a thick porous layer separated from the metal matrix by a thin non-porous layer called the barrier layer. Due to their porous structure, the anodic film is sensitive to aggressive environments. So it is necessary for the anodic film to proceed to seal post treatment. Many researches have testified that the chromate sealing is an excellent method of corrosion protection for anodized aluminum. Regretfully, enacted environmental laws have imposed severe restrictions on chromate use in many countries because of its high toxicity and consequent environmental hazards[3]. Therefore, it is essential to investigate other nontoxic sealing techniques with equal or even better corrosion protection for anodized aluminum. Rare earth (RE) salts are widely recognized as an attractive alternative to chromate for the protection of aluminum alloys because they are non-toxic and relatively cheap[4]. Also, in many researches rare earth salts have been shown to be promising cathodic inhibitors against uniform and localized corrosion of a large variety of aluminum alloys[3]. Consequently, depositing rare earth compounds on porous structure of anodized film is also attempted to improve the corro-

sion resistance of the anodized aluminum alloy. Some studies find out that the rare earth elements can not penetrate into the anodized film by adding rare earth salts into the solution during anodizing[5,6]. However, it can be achieved that rare earth compounds are deposited on the porous structure of anodized aluminum by chemical treatment and electrolytic deposition in rare earth solutions[7–9]. At present, the studies of the structures and properties of anodic film after rare earth depositing have been reported. But the studies on the corrosion resistance of the anodized aluminum after rare earth sealing are relatively limited. In the present work, composite films were fabricated on cast A356 aluminum alloy by combined anodizing and rare earth deposition. The corrosion protection effect and corrosion behavior of the composite films in NaCl solution were studied by electrochemical impedance spectroscopy (EIS). The corrosion process of the composite films was revealed, and their protection mechanism was analyzed.

1 Experimental The compositions of A356 aluminum alloy employed are Al-7.0%Si-0.35%Mg-0.01% others. The 30 mm×30 mm×6 mm specimens were cut from the alloy for the subsequent use. The specimens were polished successively with 180#, 600#, 1000# abrasive paper, dusted with acetone, surface activated with 1:1 HNO3 for 30 s and rinsed with distilled water. Then the specimens were anodized at a constant current density of 1.0 A/cm2 in 16.0 wt.% sulphuric acid and 1.0 wt.% organic

Corresponding author: SUN Huanhuan (E-mail: [email protected]; Tel.: +86-24-24680841) DOI: 10.1016/S1002-0721(10)60584-4

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acid mixed electrolytes at 10–12 ºC for 60 min. The Pt sheet served as the cathode. After anodizing, the specimen was rinsed thoroughly with distilled water. Subsequently, the anodized A356 alloy was immersed into the solution containing 6.0 g/L Ce(NO)3 and 3.0 g/L H2O2 for 150 min at room temperature and rinsed with distilled water and dried with air. After sealing, the films on aluminum alloy became yellow. The thickness of anodized film increased approximately 1–1.5 ȝm (examined by an MiniTest 600 thickness indicator). The Ce sealed anodized A356 alloy was obtained. The cross-section of the anodic film was examined by an Olympus LECO optical microscope, and the morphologies of the anodized A356 alloy with or without Ce sealing layer were observed by a SIRION-200 scanning electron microscope. The electrochemical impedance spectroscopy (EIS) measurement is carried out in 3.5% NaCl solution on a CHI 660C electrochemical workstation, and surface changes of the samples were also observed at different immersion times. The corrosion medium was prepared with analytical grade sodium chloride and distilled water. The working electrode was the tested sample. Pt electrode and saturated calomel electrode (SCE) were used as the counter and the reference electrode, respectively. Before the measurement, the sample was immersed into the NaCl solution for 60 min to reach a steady open circuit potential (OCP) at room temperature. The EIS was measured in the frequency range from 105–10–1 Hz and the imposition of a 5 mV amplitude sinewave was involved. The impedance data were analyzed using ZSimpWin 3.10 software.

2 Results and discussion 2.1 Microstructures of the composite films The surface and cross-section morphologies of anodized A356 alloy are shown in Figs. 1(a) and (b), respectively. Fig. 1(a) shows that the surface of anodic film is not uneven. There are some micro-pores and cavities at local sites of the film surface, which shall be generated because Si particles are plucked from the film surface during anodizing. The anodized A356 alloy presents porous structure (about 20 nm), which is a common characteristic of anodic film of aluminum and aluminum alloys[10]. The thickness of anodic film is uniform and is about 18–20 ȝm, as shown in Fig. 1(b). Moreover, it can be found that a few of white Si are occluded

in anodic film. During anodizing, these Si particles do not take part in the formation film but rather disturb the growth of anodic film[11]. The effect of these Si particles on the thickness of the anodized film is less, due to their small sizes. Considering the structure of the anodized A356 alloy, an effective post treatment is expected to seal the porous anodic film and further enhance the corrosion resistance of the A356 alloy. In this work, rare earth (Ce) depositing treatment was used to seal anodized A356 alloy. Sealing layer was fabricated on anodized A356 alloy by a spontaneous reaction in a water-based solution containing Ce(NO)3 and H2O2. After Ce sealing, a layer of yellow Ce film covers on the anodic film surface. Scanning electron microscope observation finds out that the Ce film is composed of many spherical particulates, as shown in Fig. 1(c). These spherical deposits compactly cover the whole surface and seal the porous structure of anodized aluminum alloy. The size of bigger particulates is approximately 100 nm. Simultaneously, it can be seen that some smaller particulates are reserved between the bigger ones. They are the initial stages of the lager particulates. It is obvious that the sizes of the spherical deposits are far bigger than these of the porous structures. However, the rare earth Ce compound particulates only deposit on the surface of the porous anodic film, but not enter into the inner of the anodized aluminum, which is testified by the author[12] and Yu et al.[7] These spherical particulates mainly consist of there-valence and four-valence cerium oxide and cerium hydroxide, which is shown elsewhere[12]. Based on the above analysis, it is concluded that the composite films composed of anodic film and rare earth Ce film can been successfully fabricated on A356 aluminum alloy by combined anodizing and rare earth deposition, and the schematic diagram is shown in Fig. 2. And the composite films are uniform, compact and integrated. 2.2 Corrosion protection effect and corrosion behavior of the composite film 2.2.1 Corrosion characteristics of the anodic film without Ce sealing In order to detailedly reveal the corrosion process and protection mechanism of the composite films, the EIS of the anodic film without rare earth Ce sealing of the A356 alloy at different immersion times is firstly studied in 3.5% NaCl solution, as shown in Fig. 3.

Fig.1 SEM morphologies (a) Porous structure of the anodic film; (b) Cross-section of the anodic film; (c) Rare earth Ce film

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Fig. 2 Schematic diagram of the composite films

Fig. 3 EIS analysis of anodic film on A356 alloy (a) Nyquist plots (2–15 d); (b) Nyquist plots (18–42 d); (c) Bolt plots of frequency vs phase angle; (d) Bolt plots of frequency vs impedance

Fig. 3(a) is the Niquist plots of anodic film without Ce sealing in a short immersion time (within 15 d). These plots all exhibit two clear capacitance arcs. The anodic film of aluminum alloy is composed of a thick porous layer and a thin non-porous barrier layer. The high frequency range of EIS plots reflects the information of resistance Rp and capacitance Cp of the porous layer, while the low frequency range reflects the information of resistance Rb and capacitance Cb of the barrier layer[13]. However, the capacitance values of Cp and Cb have some differences from pure capacitance element C, because anodic film of A356 alloy is very uneven. So constant phase angle elements Qp and Qb are used to substitute for capacitance Cp and Cb, respectively[14]. At a short immersion time, the anodic film can effectively protect the matrix of the A356 alloy and prevent the penetrations of chloride irons. The equivalent circuit

Rsol(QpRp)(QbRb) shown in Fig. 4(a) is suitable to simulate the EIS plots, where Rs is the resistance of the solution. Additionally, it can be noticed that capacitance arc radium at high frequency will decrease with immersion time increasing within 15 d. And the peak value of the phase moves to the low frequency region, as shown in Fig. 3(c), the impedance value of the anodic film reduces, as shown in Fig. 3(d). It is indicated that the corrosion resistance of the anodic film gradually decreases with the increase of the corrosion time. The polarization resistance Ra of the whole anodic film (the combination of Rp and Rb) in 15 days’ immersion still exceeds 10 Kȍ·cm2. After 18 days’ immersion, a little white corrosion product is observed (can be seen with the naked eyes) on the local region of the surface of anodic film. Here, the high frequency range of Niquist plots presents a half circle of capacity arcs. And the low frequency range shows a

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line, which indicates that the EIS exhibits the characteristics of diffuse control (Fig. 3(b)). The corrosion medium has already penetrated through the localized regions of barrier layer of the anodic film and entered into the aluminum matrix, which leads to the corrosion reaction of A356 alloy. At the same time, the equivalent circuit is designed by the model Rsol(Qa(Ra(Qs(RsW)))) as shown in Fig. 4(b), where Ra is the resistance of the broken anodic film, Qa is the capacity of the broken anodic film, Rs is the electrochemical transaction resistance of the aluminum matrix, Qs is the phase element of the aluminum matrix and W is the transmission line parameter (Warberg impedance). Moreover, the radium of capacitance arcs is also increasing with the immersion time increasing. After 30 days’ immersion, the polarization resistance Ra reduces to about 4.4 Kȍ·cm2. However, the radium of capacitance arcs is increased after 42 days’ immersion. The reason shall be that the corrosion products at the interface of the anodic film and aluminum alloy block the corrosion progress of the aluminum matrix in corrosion medium

during the corrosion process, which results in the increase of the capacitance arc[15]. Here, the polarization resistance Ra was only about 5.7 Kȍ·cm2. 2.2.2 Corrosion characteristics of the composite film Based on the above analysis results of the corrosion process of anodic film without Ce sealing, the corrosion characteristics of the composite films obtained after Ce sealing treatment are further studied by electrochemical impedance spectroscopy (EIS) technology. Fig. 5 shows the Niquist plots and Bode plots of the EIS of the composite film samples in 3.5% NaCl. At the initial stage of the immersion corrosion (within 2 d), all the EIS plots present the larger capacitance arcs, as shown in Fig. 5(a), while the two obvious phase peaks appear on the Bolt plots as shown in Fig. 5(c). The equivalent circuit of the composite film is designed by the Rsol(QCeRCe)(QaRa) model as shown in Fig. 6(a), where RCe is the resistance of the rare earth Ce film, QCe is the capacitance of the Ce film, Ra is the resistance of the whole anodic film, Qa is the capacitance of

Fig. 4 Schematic diagrams of EIS equivalent circuit of anodic film in NaCl solution (a) Model Rsol(QpRp)(QbRb); (b) Model Rsol(Qa(Ra(Qs(RsW))))

Fig. 5 EIS analysis of the composite films (a) Nyquist plots (2–15 d); (b) Nyquist plots (18–42 d); (c) Bolt plots of frequency vs phase angle; (d) Bolt plots of frequency vs impedance

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Fig. 6 Schematic diagrams of EIS equivalent circuit of the composite film in NaCl solution (a) Model Rsol(QpRp)(QbRb); (b) Model Rsol(Qa(Ra(Qs(RsW))))

the whole anodic film. According to the above studies on the morphologies of the composite films, it is known that a layer of Ce compound film composed of three-valence and fourvalence cerium oxide and cerium hydroxide is formed on the anodized A356 alloy after Ce sealing treatment. The Ce film not only seals the porous structure, but also has excellent corrosion resistance[7,12]. When the time of immersion corrosion is short, only Ce film of the composite films is attacked by chloride irons. The anodic film does not contact with NaCl solution, and it, as a shield layer with good corrosion resistance, can isolate the corrosion medium and aluminum matrix and protect the aluminum alloy. The polarization resistance RCe in 2 days’ immersion exceeds 20 Kȍ·cm2. With the corrosion time increasing (5–15 d), the high frequency range of Bolt plots of EIS makes visible changes, which shows that a new corrosion reaction happens on the composite films. The anodic film is also corroded because the corrosion medium has penetrated into the porous structure of the anodic film through the rare earth Ce film. So the protection effect of the composite films has decreased. Here, the color of Ce film of the composite film has become light. The several black pits can be observed at the local region of the film only by naked eyes. But the Ce film covering on the anodic film still has protection effect for the anodic film. The EIS plots can be simulated by equivalent circuit Rsol(QCeRCe)(QpRp)(QbRb) as shown in Fig. 6(b). After 18 days’ immersion, the whole polarization resistance (the combination of RCe and Ra) is approximately 20 Kȍ·cm2. After 42 days, which is the longest corrosion time in this experimental, no white corrosion products are found on the surface of the composite film. However, the yellow color of the composite film has completely retired, which also indicated that the protection effect of Ce film has entirely vanished. Here, the shapes of EIS plots of the composite films are similar with these of the anodic film without Ce sealing. The EIS plots present the double capacitance arcs due to the collective protection of the porous layer and barrier layer of anodic film, as shown in Fig. 5(b). The corresponding equivalent circuit is designed by the Rsol(QpRp)(QbRb) model. The polarization resistance Ra of the whole anodic film is also about 10 Kȍ·cm2. Simultaneously, it is found that the impedance values of the composite films are obviously larger than the anodic films without sealing at the same immersion time by comparing Fig. 3(d) with Fig. 5(d). It is further testified that the

composite films have more excellent protection degree than the single anodic film. Moreover, the impedance value of the composite films after 42 days’ immersion is almost equal to that of the anodic film after 15 days’ immersion. After 42 days’ immersion, the composite still provides a better protection for the aluminum matrix, and the A356 alloy matrix is not corroded yet. Certainly, if the immersion time is increasing all along, the anodic film is also broken, and the protection effect of the composite film will finally lose.

3 Conclusions (1) A layer of uniform, compact and integrated composite film could be fabricated in A356 alloy by the combined anodizing and rare earth deposition. (2) The single anodic film could protect A356 aluminum matrix to some extent in chloride iron solution. However, if the immersion time was more than 18 d, the protect effect would lose and the aluminum alloy was corroded. (3) The composite films composed of the anodic film and Ce film had excellent protection effect for A356 alloy. The protection degree of the composite films was superior to the single anodic film. At the initial stages of immersion corrosion, the protection effect of Ce film was obvious. With the immersion time increasing, its protection effect gradually decreased and lost finally. After 42 days’ immersion, the A356 alloy under the composite films is not corroded yet.

References: [1] Fanny Snogan, Christine Blanc, Georges Mankowski, Nadine Pébère. Characterisation of sealed anodic film on 7050 T74 and 2214 T6 aluminum alloys. Surf. Coat. & Tech., 2002, 154: 94. [2] Konieczny J, DobrzaĔski L A, Labisz K, Duszczyk J. The influence of cast method and anodizing parameters on structure and layer thickness of aluminum alloys. J. Mater. Process. Technol., 2004, 157-158: 718. [3] Bethencourt M, Botana F J, Calvino J J, Marcos M, Rodríguezchacón M A. Lanthanide compounds as environmentallyfriendly corrosion inhibitors of aluminum alloys: a review. Corros. Sci., 1998, 40: 1803. [4] Shao M H, Huang R S, Fu Y, Hu R G, Lin C J. Investigation on the formation mechanism of Ce conversion films on 2024 aluminum alloy. Acta Phys. Chim. Sin. (in Chin.), 2002, 18(9): 791. [5] Li L J, Li D, Peng M X, Zhang S T, Lei J L. Study on the depo-

996 sition of cerium on oxide coatings of aluminum alloy by ellipsometry. Materials Protection (in Chin.), 2001, 34: 18. [6] Peng M X, Li D, Li G Q, Guo B L. Study of Ce in anodized aluminum by electrochemical deposition technology. Corrosion Science and New Advances for Protection Technology. Beijing: Chemical Industry Press (in Chin.), 1999. 98. [7] Yu Xingwen, Cao Chunan. Electrochemical study of the corrosion behavior of Ce sealing of anodized 2024 aluminum alloy. The Solid Films, 2003, 423: 252. [8] Li G Q, Li D, Li J Q, Guo B L, Peng M X. Cerium conversion coatings formed on anodized aluminum by cathodic electrolysis. Journal of Chinese Society for Corrosion and Protection (in Chin.), 2001, 21(3): 150. [9] Liang K, Liang C H, Huang N B. Forming cerium composite coatings on aluminum films by AC electrolytic deposition. Journal of the Chinese Rare Earth Society (in Chin.), 2009, 27(1): 110. [10] Keller F, Hunter M S, Robinson D L. Structural features of oxide coatings on aluminum. J. Electrochemical Society, 1953,

JOURNAL OF RARE EARTHS, Vol. 29, No. 10, Oct. 2011 100(9): 411. [11] Fratila-Apachitei L E, Tichelaar F D, Thompson G E, Terryn H, Skeldon P, Duszczyk J, Katgerman L. A transmission electron microscopy study of hard anodic oxide layers on AlSi(Cu) alloys. Electrochimica Acta, 2004, 49: 3169. [12] Sun H H, Ma N H, Chen D, Li X F, Wang H W. Fabrication and analysis of anti-corrosion coatings on in-situ TiB2p reinforced aluminum matrix composite. Surf. & Coat. Tech., 2008, 203(3-4): 329. [13] Chung S C, Cheng J R, Chiou S D. EIS behavior of anodized zinc in chloride environments. Corros. Sci., 2000, 42: 1249. [14] Zhao X, Zuo Y, Zhao J M. Electrochemical properties of anodized aluminum films in sodium chloride solution. The Chinese Journal of Nonferrous Metals (in Chin.), 2004, 14(4): 562. [15] Wang J, Cao C N, Lin H C. Features of AC impedance of pitting corroded electrodes during pits propagation. Journal of Chinese Society of Corrosion and Protection (in Chin.), 1989, 9(4): 271.