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Microstructure and formation mechanism of cerium conversion coating on alumina borate whisker-reinforced AA6061 composite
Corrosion Science 50 (2008) 3185–3192
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Microstructure and formation mechanism of cerium conversion coating on alumina borate whisker-reinforced AA6061 composite Jin Hu *, Shawei Tang, Zaiyang Zhang School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 5 June 2008 Accepted 8 August 2008 Available online 22 August 2008 Keywords: A. Metal matrix composite B. SEM B. TEM C. Passive film
a b s t r a c t Cerium conversion coatings, formed on alumina borate whisker-reinforced aluminum composite with pre-treatment for different time, were characterized by SEM/EDX, TEM and electrochemical method. The structure and formation mechanism of the coatings were investigated in detail. It was found that the pre-treatment prior to coatings has strong influence on formation, growth and morphology as well as granular size of coatings. Two typical coating morphologies existed at the pre-treated substrate surfaces. Cracks only presented on thick coatings because of the higher deposition rate and the greater inner stress. Cracks preferably produced at the whisker/matrix interface and the grain boundary of matrix. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Rare earth metal conversion coating for the corrosion protection of aluminum has been widely researched in the recent years. It was generally recognized as an attractive alternative to chromate conversion coating for the protection of aluminum alloys because cerium compounds are non-toxic and are relatively cheap. To date, several methods have been used to form cerium-rich film on aluminum alloys [1–7]. It has been found that chemical passivation by immersion in rare earth metal chlorides can produce exceptional resistance to localized corrosion [6]. Corrosion inhibition by cerium salts is generally associated with the formation and precipitation of cerium oxides or hydroxides over cathodic sites on the metal surface [7]. Metal matrix composites (MMCs) have been considered for a wide range application [8–10]. One of the principal factors for reinforcing aluminum with ceramic whiskers is to provide excellent strength-to-weight ratio. Nevertheless, there are few studies on MMCs protection, probably due to a major interest on the improvement of its mechanical properties [11]. Greene and Mansfeld [12] studied the corrosion behavior of MMCs treated with cerium and found that the corrosion resistance can be increased, confirming the effectiveness of the cerium treatment [12,13]. Recently similar results were obtained by Hu et al. coating Al18B4O33w/Al composite with cerium [14,15]. Concretely, they found that the concentration of H2O2 in the cerium solution had a strong influence on the corrosion properties and the concen-
* Corresponding author. Tel.: +86 451 86415894; fax: +86 451 86413922. E-mail address: [email protected] (J. Hu). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.08.020
tration of CeCl3 in the test solution also had a significantly influence on the corrosion behavior and surface morphologies of the coated samples. In order to increase the available cerium ion concentration and to reduce the time required for successful surface modification, many other works have been published using different conditions to obtain Ce conversion layers on different MMCs, and investigating their corrosion behavior [13,16,17]. The pre-treatment of materials surfaces prior to coating process is recognized and accepted as one of the most critical steps in a surface operation [18–21]. The quality and performance of coated parts in service can be directly related and correlated to the initial pre-treatment of the surface to be coated. Some researches [18,22] studied the effect of surface pre-treatment on coating surface morphology and salt fog performance. They found that surface preparation (desmutting and degreasing) produces coatings with the best salt fog performance than coatings deposited on substrates prepared by others procedures due to formation of cracks in the coating or insufficient coating thickness. The cerium conversion coating on Al18B4O33w/Al composite with excellent corrosion resistance has been reported in our previous study [14]. Our previous studied also suggested that the pretreatment time prior to coating process also has a distinct influence on the corrosion properties of aluminum matrix composite. Relatively little is known about the microstructure evolution of coating on the pre-treated composite substrates. In the present work, the effect of HF pre-treatment of the composite surfaces prior to coating process on the morphologies of the coated Al18B4O33w/AA6061 composite was evaluated. Particular attention was paid to the interface between the substrate and the coating. More specifically, the various morphologies of
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coatings, the distribution and the characteristics on the substrate with different pre-treatment conditions prior to coating process were investigated, and a possible formation mechanism is proposed.
2. Experimental 2.1. Material and specimen preparation The alumina borate whisker (20% volume fraction) reinforced 6061 aluminum composite (Al18B4O33w/AA6061) as substrate was fabricated by squeeze casting. The whisker had a 0.5–1 lm in diameter and 10–30 lm in length (Shikoku Chemical Co. Ltd). The whisker preform and the mould were preheated at 480– 520 °C, and then 6061Al alloy at 700–750 °C was cast into the mould with a pressure of 80–100 MPa. Disc-samples with a thickness of 4 mm and a diameter of 15 mm were cut from the bulk composite, which were desmutted and ground using SiC paper, followed by polishing using a diamond paste, and ultrasonic cleaning in acetone. 2.2. Chemical pre-treatment Pre-treatment has previously been identified as an important step in the fabrication of cerium oxide conversion coatings. Pretreatment processes that included deoxidation or activation steps. To further define the relationship among pre-treatment, coating performance and coating surface morphology, the chemical pretreatment steps that were designed HF solution, and the concentration of HF is 2.5 mol/L. Before the composite was modified by cerium-based conversion coatings, a chemical pre-treatment of the composite was carried out. The samples were pre-treated by immersion in an aqueous solution of HF for 20 s (sample A) and 180 s (sample B), respectively, at room temperature, followed by rinsing in deionized water.
2.5. Electrochemical measurements A M273A model potentiostat with a saturated calomel reference electrode (SCE) and a graphite counter electrode were used. All potentials are given against the reference electrode. The testing sample was used as working electrode. The open circuit potential (OCP) as a function of time for the pre-treated samples was recorded in cerium bath at 30 °C. 3. Results 3.1. Surface characterization of pre-treated surfaces Typical surface morphologies of pre-treated surfaces by SEM observation are shown in Fig. 1. It can be seen that the whisker is uniformly distributed in the matrix of Al18B4O33w/AA6061 composite and the surface of the sample A is smooth (Fig. 1a). In addition, it can be observed that more whiskers and defects appeared at the surface of sample B and the surface of the sample B is very uneven, which suggested that more aluminum can be etched with the increasing of pre-treatment time and more whiskers and defects (crevices or pores) of matrix can appear at the surface of the composite (Fig. 1b). 3.2. Microstructure and composition of coatings Fig. 2 shows the evolution of the surface morphologies of the coatings obtained on the pre-treated samples by SEM observation. A uniform chemical conversion coating morphology presents in
2.3. Ce conversion coating process After the chemical pre-treatment a cerium-based conversion coating was obtained on the pre-treated samples by immersion in cerium solution for 10 min at 30 °C. The solution pH was adjusted to 2.85 with NaAc + HAc and was kept constant during the coating process and then rinsed with distilled water and dried with air. A typical coating solution was prepared by dissolving 7.5 g CeCl3 into 1000 ml of deionized water with 100 ml/L H2O2 concentrations. 2.4. Microstructure analysis Morphologies of the conversion coatings on the composite were observed using SEM/EDX analysis was carried out on a Hitachi S3000N scanning electron microscope operated at 20 keV. Cross-sectional and plan-view microstructures were carried out on a Philips CM12 TEM with an operating voltage of 120 kV. EDAX was employed during TEM observations to examine the phase composition in the composite. Cross-sectional specimens for the TEM observation were prepared by standard mechanical polishing, dimpling, and ion milling methods. Plan-view specimens were prepared by back thinning by mechanically polishing, dimpling, and ion milling to remove the substrate from the coating. Ion milling, which was performed by Ar+ bombardment at 5 keV using a precision polishing system. Initially, the angle of ion milling was set at 10°; at the final stage, an angle 5° was used and processing was performed at 3 keV.
Fig. 1. Typical surface morphologies of the pre-treated surfaces for (a) 20 s, (b) 180 s.
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Fig. 2. Morphologies and composition of coatings obtained on pre-treated samples (a) morphology of coatings on sample A, (b) EDX spectrum of coatings on sample A, (c) morphology of coatings on sample B and (d) EDX spectrum of coatings on sample B.
Fig. 2a, and the coatings have almost no cracking; the coatings were composed of fine and compact granular morphology. The coatings were even and thin, substrate topographies left by chemical pre-treatment, such as whisker and etch pits, were visible through the coatings. The chemical composition of the coating was analyzed by EDX as shown in Fig. 2b. The SEM investigation performed on coatings deposited on sample B shows a larger difference in the morphologies compared with coatings deposited on sample A. A cracked ‘‘dry mud-like” structure can be clearly seen in Fig. 2c, and the distribution of the cracks is regular. The coatings are uneven and thick, however, substrate topographies can also be clearly observed. The morphology of coatings on the surface consisted of nodules, which were identified by EDX to be cerium-rich phases. These phenomena suggested that increasing the pre-treatment time caused the coated surface morphology to change. The chemical composition of the coating was analyzed by EDX as shown in Fig. 2d. SEM observation indicates that under the two pre-treatment conditions the morphologies of the deposited coatings are remarkably different. The cracking of the coating on the substrate pretreated for 180 s was most likely caused by deeply etching prior to coating. Plan-view TEM micrographs and corresponding SAED patterns of the cerium conversion coatings obtained on both pre-treatment samples are shown in Figs. 3 and 4, respectively. The TEM images and SAED patterns clearly indicate that under the both pre-treatment conditions the deposited coatings were composed of nanocrystalline particles with a sphere-like morphology. However, the
particles of the coating deposited on sample A (Fig. 3a) were much finer (2–5 nm) than that of the coating deposited on sample B (7– 18 nm) (Fig. 4a), which suggested that the surface pre-treatment for a shorter time can inhibit the growth of coating particles. In addition, extensive film cracking were visible in the TEM for the deposited coating on sample B. The current study has shown that the size of the coating granular size was a strong function of the pre-treatment prior to coating. The longer the pre-treatment time, the larger the granular size of coatings. It is interested noticed that the deposited coatings were also composed of amorphous morphology under long pre-treatment condition (as shown in Fig. 4c), which suggested that the deposition process is complicated under the long time pre-treatment prior to coating results in both crystalline and amorphous states morphologies present on the pre-treated sample surface. 3.3. Interface between coatings and substrate Fig. 5 presents cross-sectional TEM characterization showing the evolution of the coatings obtained on sample A. As expected, a thin, continuous coating (70–80 nm) formed on sample A (Fig. 5a). This indicated that the cerium conversion coating can uniformly deposit on the substrate with short time pre-treatment. The coatings can deposit on either the side surface of whisker or the transverse section of whisker (Fig. 5b). There is neither defect at the interface between the coatings and the substrate nor cracks in the coatings, indicating the coatings had good adhesion to the substrate.
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tion at interface owing to the difference of stress states or the formation of crevices. Fig. 7 expresses the OCP as a function of time for the pre-treated substrates immersed in cerium bath at 30 °C. The OCP decreased rapidly to about 795 mV (SCE) at a very short time (about 90 s) for the coating deposited to sample A, and the OCP decreased gradually to 759 mV (SCE) at a longer time (about 815 s) for the coating deposited to sample B. Subsequently, the potentials rose as the coating developed over the sample surfaces, and finally approached to a constant value of about 751 mV for sample A and 716 mV for sample B, respectively. 4. Discussion 4.1. Electrochemical driving of coating formation
Fig. 3. Plan-view TEM micrographs and corresponding SAED pattern of the cerium conversion coatings obtained on sample A (a) morphology and (b) SAED pattern.
Fig. 6 is a set of cross-sectional TEM characterization showing the evolution of the coatings obtained on the sample B. From Fig. 6, a correspondingly thick coating (100–180 nm) appeared on the substrate as shown in Fig. 6a, some cracks attain the substrate surface. Cracks can appear at the end of whisker (Fig. 6b) or the side surface of whisker (Fig. 6c) along with grain boundary of the matrix (Fig. 6d). These results show that the coatings split to be more easily at some defect place. In both Figs. 5a and 6a, the coatings seem to have a quite uniform thickness and to follow the irregularities of the substrates. Moreover, the thickness of the coatings obtained on the pre-treated substrates increases with the pre-treatment time of the substrates, but defects and cracks in the coatings also increase with the pre-treatment time of the substrates. Cracks preferred forma-
The investigation indicated that conversion coating formation mainly due to corrosion of anode, the transportation of species in liquid phase and metal/solution interface, and formation and growth of nucleus. The course started from corrosion of substrate, maintained the correlativity and went along simultaneously [23]. The interface in the composite will act as an active region [24]. In corrosion medium (such as HF solution), whisker severed as a cathode formed micro-cells with the aluminum matrix around them, aluminum matrix in relation to the adjacent Al18B4O33 whisker severed as an anodic were dissolved away, since the potential of the matrix in this region diverge from equilibrium potential [24]. The cathodic sites on the pre-treated surfaces will increase with the pre-treatment time. From Fig. 1b, it can be found that the long time pre-treatment in HF solution appears to etch the surface more aggressively. A lot of whisker appeared on the surface of the substrate, indicating the aluminum matrix in the vicinity interface was corroded away and whisker can be emerged from the substrate. The longer the pre-treatment time prior to the coating is, the more the number of the micro-cells in the composite becomes and the higher the activation degree of the surface achieves. The difference in the number of cathodic sites present on the substrate surface with different pre-treatment time influences the formation, growth and morphology along with granular size of the conversion coating on the composite surface. Cerium conversion coatings have been proposed to deposit as a result of a pH rise at the surface of aluminum matrix composite which is generated as part of the coating reactions [23]. As can be observed, the coatings were not only deposited over whisker but also deposited over aluminum matrix, since the deposition process was driven by the electrochemical potential differences between the aluminum matrix and whisker. Whisker served as a cathode, the reaction that the reduction of hydrogen ion to liberate hydrogen gas happened at whisker, which caused local pH rise in the vicinity of whisker and resulted in the cerium depositing on whisker preferential. Matrix Al served as an anode can be dissolved during the electrochemical process that leads to cerium conversion starts by etching the initially covering oxide layer to allow electronic access with the underlying metal. The deposition of Ce-coatings is possible because of the chemical heterogeneity of the aluminum alloy surface, which establishes the local electrochemical cells. 4.2. Characteristic of coating formation When sample A was immersed into cerium salt solution, Ce was uniformly precipitated on the substrate owing to the micro-cells is uniformly distributed on the pre-treated substrate surface because of the uniform distribution of whisker. A compact and uniform conversion coating morphology presents in Fig. 2a.
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Fig. 4. Plan-view TEM micrographs and corresponding SAED pattern of the cerium conversion coatings obtained on sample B (a) crystalline morphology, (b) SAED pattern and (c) amorphous morphology.
When sample B was immersed into cerium salt solution, more Ce was precipitated with regard to the conversion process, because whole material surface was cathodically activated. Since the presence of plentiful micro-cells on the pre-treated substrate surface and the number of cathodic sites were increased, the precipitation reaction occurred more rapidly, thus leading to a faster coating deposition rate during coating process, resulted in not only the thickness but also the unevenness of the coatings increase. The heterogenous nucleation and quick deposition of the coatings took place on sample B may be the primary reason to form amorphous structure and obtain crystalline coatings with large particle.
Indeed, EDX analyses of many sample surfaces have shown that the longer the pre-treatment time, the higher the Ce content in the surface layer. A substantial fraction of Al was detected during analysis because the size of the deposits was small compared to the interaction depth of the probe beam. The change of Ce content on the different coated surfaces indicated that the longer the pre-treatment time, the thicker the coatings under the same deposition conditions. Since the longer the pre-treatment time, the higher the activity of sample surface, and the quicker the nucleation rate of coatings on pre-treated surfaces. Therefore, the thickness of coatings increases with the chemical pre-treatment time prior to coatings.
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process of coatings, resulting in large numbers of cracks appear at the substrate surface of sample B. In addition, the cracks can also be caused by the heterogeneous nucleation and growth of coatings due to the more defects presented on the substrate surface for a long time pre-treatment. The faster deposition rate of the coatings caused large stress existed at the coatings, thereby more cracks presented at the coatings. 4.4. Electrochemical behavior of coating formation
Fig. 5. Cross-sectional TEM characterization of the coatings obtained on sample A (a) coating deposited on the substrate, (b) coating deposited on whisker.
4.3. Formation of cracked coating It is interesting to note that the morphologies of coatings by SEM and TEM observation indicated that the cracks only appear at the thick film, since they were only observed in sample submitted to the long pre-treatment period, the cracks did not occur at the thin film. It seems that the cracks are caused by stress relieve. Cracked ‘‘dry mud-like” structure of cerium conversion coatings was considered to be formed during drying step and to be caused by internal stress. Cracks preferred formed at interface between whisker and aluminum matrix owing to the difference of stress states or the formation of crevices. Subsequently, the cracks developed during growth
From Fig. 7, it can be found that the trend of both curves is almost same. During the immersion process of the two samples, the electrode potentials decrease at the beginning of immersion, subsequently increase, and finally are closed to constant values. This phenomenon is consistent with the study about chemical conversion coating on AZ91D magnesium alloy by Elesentriecy et al. [25,26]. In their paper, the OCP was shown as a function of time for AZ91D alloy immersed in a stannate bath, and they found the curve disclosed three stages during the coating process. They suggested that alloy dissolution was proceeding over the initial time of immersion and a decrease in OCP was showed at this period. Subsequently, the OCP showed an increase, indicating that initial substrate dissolution promoted the initiation of the coating process, and the substrate surface was almost fully covered by the coating film until the OCP remained almost constant. But our research found that the cerium conversion coating had already formed on the studied composite at the very beginning of immersion process. A direct proof is presented in Fig. 8. Coatings morphologies can be clearly seen in this figure, which strongly suggested that the conversion coating deposited on the pre-treated substrates accompanied by the decreasing of OCP. A thin coating with finer-grained structure had already been obtained on the surface of sample A by immersion in cerium solution for 0.5 min (as shown in Fig. 8a), and a thick coating with cracked structure had also been formed on the surface of sample B by immersion in cerium solution for 1.5 min (as shown in Fig. 8b), which gives a further proof that the deposition rate of the coatings on the surface of sample B is faster and the inner stress is greater during deposition process. The decrease of potential may be relative to the corrosion of aluminum matrix, local pH therewith rises in the vicinity of whisker and caused the conversion coating formation. The conversion coatings nucleate and grow continuously, until the whole surface was covered by formed coatings, accordingly resulted in the potential increase. The electrode potential maintained stable to a final value of about 751 mV for the coating deposited to sample A and 716 mV for the coating deposited to sample B, respectively. It is suggested that a dynamic balance was established between the coating growth rate and the coating dissolution rate. For the coatings deposited to sample A, a uniform nucleation took place on the substrate surface due to the micro-cells is uniformly distributed on the pre-treated substrate surface, the surface of sample A can be covered by the coatings in the initial stage of immersion (90 s), which blocked the transportation of species needed for the conversion coating formation resulted in the potential increase. In this case, the formation of coatings is difficult continuously and the growth rate of the coatings has also already decreased, whereas, the immersion time is persisted to 10 min but the coating is still thin and the grain is still finer (as shown in Fig. 2a). For the coatings deposited to sample B, the nucleation is very heterogeneous owing to the presence of plentiful micro-cells on the pre-treated substrate surface, a coating blocked the transportation of species can just be formed on the substrate surface after longer immersion time, the potential decreased with the immersion process until the immersion time exceeds 815 s. In this case, the formation and growth of coatings along with the corrosion of
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Fig. 6. Cross-sectional TEM characterization of the coatings obtained on sample B (a) coating deposited on the substrate, (b, c) cracks at the end of whisker, (d) cracks at grain boundary of the matrix.
aluminum matrix took place during the coating process, both crystalline and amorphous structure can present on the substrate surface simultaneously due to the complicated deposition process. Moreover, the continuous nucleation and growth of coatings on the surface by immersion in cerium solution for 10 min also caused a thick coating with cracked structure presented on the surface (as shown in Fig. 2c).
5. Conclusions (1) The longer the pre-treatment time, the more the whisker appear to be on the pre-treated substrate surface, the higher the activity of the pre-treated surface. The pre-treatment
prior to coating has strong influence on the formation and growth of coatings. (2) A compact, uniform and even coating can be formed on the substrate surface with pre-treatment for 20 s. The coating was crystalline and was about 70–80 nm in thickness. Both crystalline and amorphous states morphologies present on the substrate surface with pre-treatment for 180 s. A dry-mud cracked surface can be observed and the coating was about 100–180 nm in thickness. (3) Cracks only presented on the thick coating surface. The cracks mainly produced at the interface because of the higher deposition rate on whisker. The inner stress in dried coating resulted in the extensive cracks.
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References
Fig. 7. The relationship between OCP and time for coating formation process on the pre-treated composite surfaces with various time.
Fig. 8. Morphologies of coatings obtained on pre-treated samples (a) morphology of coatings on sample A by immersion in cerium solution for 0.5 min, (b) morphology of coatings on sample B by immersion in cerium solution for 1.5 min.
[1] X.W. Yu, C.W. Yan, C.N. Cao, Study on the rare earth sealing procedure of the porous film of anodized Al6061/SiCp, Mater. Chem. Phys. 76 (2002) 228–235. [2] B.Y. Johnson, J.E. Edington, M.J. O’Keefe, Effect of coating parameters on the microstructure of cerium oxide conversion coatings, Mater. Sci. Eng. A 361 (2003) 225–231. [3] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, The inhibition of aluminium alloy corrosion by cerous cations, Met. Forum 7 (1984) 211–217. [4] B.R.W. Hinton, Corrosion inhibition with rare earth metal salts, J. Alloy. Compd. 180 (1992) 15–25. [5] A.E. Hughes, R.J. Taylor, B.R.W. Hinton, L. Wilson, XPS and SEM characterization of hydrated cerium oxide conversion coatings, Surf. Interface Anal. 23 (1995) 540–550. [6] F. Mansfeld, F.J. Perez, Surface modification of Al/15%SiC metal matrix composite in molten salts containing CeCl3, Surf. Coat. Technol. 86–87 (1996) 449–453. [7] A.L. Rudd, C.B. Breslin, F. Mansfeld, The corrosion protection afforded by rare earth conversion coatings applied to magnesium, Corros. Sci. 42 (2000) 275– 288. [8] A.S. Hamdy, A.M. Beccaria, P. Traverso, Corrosion protection of aluminium metal-matrix composites by cerium conversion coatings, Surf. Interface Anal. 34 (2002) 171–175. [9] J. Hu, W.D. Fei, C. Li, Interfascial reactiion in alumina borate whisker-reinforced aluminum composite, J. Mater. Sci. Lett. 13 (1994) 1797–1799. [10] J. Hu, W.D. Fei, C.K. Yao, Aging behaviors of alumina borate whisker reinforced AC8A-Al composite, J. Mater. Sci. 36 (2001) 4817–4821. [11] A. Pardo, M.C. Merino, R. Arrabal, S. Merino, F. Viejo, A.E. Coy, Effect of La surface treatments on corrosion resistance of AA3xx.x/SiCp composites in salt fog, Appl. Surf. Sci. 252 (2006) 2794–2805. [12] H.J. Greene, F. Mansfeld, Corrosion protection of aluminum metal-matrix composites, Corrosion 53 (1997) 920–927. [13] C. Chen, F. Mansfeld, Corrosion protection of an Al6092/SiCp metal matrix composite, Corros. Sci. 39 (1997) 1075–1082. [14] J. Hu, X.H. Zhao, S.W. Tang, M.R. Sun, Corrosion protection of aluminum borate whisker reinforced AA6061 composite by cerium oxide-based conversion coating, Surf. Coat. Technol. 201 (2006) 3814–3818. [15] J. Hu, X.H. Zhao, S.W. Tang, W.C. Ren, Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced AA6061 composite, Appl. Surf. Sci. 253 (2007) 8879–8884. [16] A. Pardo, M.C. Merino, R. Arrabal, F. Viejo, M. Carboneras, Influence of Ce surface treatments on corrosion behaviour of AA3xx.x/SiCp composites in 3.5 wt.% NaCl, Corros. Sci. 48 (2006) 3035–3048. [17] X.W. Yu, C.W. Yan, C.N. Cao, MMCs XPS analysis of the cerium conversion coating on the anodized Al6061/SiCp, J. Mater. Sci. Technol. 19 (2003) 157– 160. [18] W.G. Fahrenholtz, M.J. O’Keefe, H. Zhou, J.T. Grant, Characterization of ceriumbased conversion coatings for corrosion protection of aluminum alloys, Surf. Coat. Technol. 155 (2002) 208–213. [19] B.F. Rivera, B.Y. Johnson, M.J. O’Keefe, W.G. Fahrenholtz, Deposition and characterization of cerium oxide conversion coatings on aluminum alloy 7075–T6, Surf. Coat. Technol. 1 (2004) 349–356. [20] M. Bethencourt, F.J. Botana, M.J. Cano, M. Marcos, Advanced generation of green conversion coatings for aluminium alloys, Appl. Surf. Sci. 11 (2004) 278– 281. [21] A. Pardo, M.C. Merino, R. Arrabal, S. Merino, F. Viejo, M. Carboneras, Effect of Ce surface treatments on corrosion resistance of A3xx.x/SiCp composites in salt fog, Appl, Surf. Sci. 200 (2006) 2938. [22] K. Brunelli, M. Dabala, Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys, Corros. Sci. 47 (2005) 989–1000. [23] W.Q. Zhou, D.Y. Shan, E.H. Han, W. Ke, Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy, Corros. Sci. 50 (2008) 329–337. [24] J. Hu, W.Y. Chu, W.D. Fei, L.C. Zhao, Effect of interfacial reaction on corrosion behavior of alumina borate whisker reinforced 6061Al composite, Mater. Sci. Eng. A 374 (2004) 153–159. [25] H.H. Elsentriecy, K. Azumi, H. Konno, Improvement in stannate chemical conversion coatings on AZ91 D magnesium alloy using the potentiostatic technique, Electrochim. Acta 53 (2007) 1006–1012. [26] H.H. Elsentriecy, K. Azumi, H. Konno, Effect of surface pretreatment by acid pickling on the density of stannate conversion coatings formed on AZ91 D magnesium alloy, Surf. Coat. Technol. 202 (2007) 532–537.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Microstructure and formation mechanism of cerium conversion coating on alumina borate whisker-reinforced AA6061 composite Jin Hu *, Shawei Tang, Zaiyang Zhang School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 5 June 2008 Accepted 8 August 2008 Available online 22 August 2008 Keywords: A. Metal matrix composite B. SEM B. TEM C. Passive film
a b s t r a c t Cerium conversion coatings, formed on alumina borate whisker-reinforced aluminum composite with pre-treatment for different time, were characterized by SEM/EDX, TEM and electrochemical method. The structure and formation mechanism of the coatings were investigated in detail. It was found that the pre-treatment prior to coatings has strong influence on formation, growth and morphology as well as granular size of coatings. Two typical coating morphologies existed at the pre-treated substrate surfaces. Cracks only presented on thick coatings because of the higher deposition rate and the greater inner stress. Cracks preferably produced at the whisker/matrix interface and the grain boundary of matrix. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Rare earth metal conversion coating for the corrosion protection of aluminum has been widely researched in the recent years. It was generally recognized as an attractive alternative to chromate conversion coating for the protection of aluminum alloys because cerium compounds are non-toxic and are relatively cheap. To date, several methods have been used to form cerium-rich film on aluminum alloys [1–7]. It has been found that chemical passivation by immersion in rare earth metal chlorides can produce exceptional resistance to localized corrosion [6]. Corrosion inhibition by cerium salts is generally associated with the formation and precipitation of cerium oxides or hydroxides over cathodic sites on the metal surface [7]. Metal matrix composites (MMCs) have been considered for a wide range application [8–10]. One of the principal factors for reinforcing aluminum with ceramic whiskers is to provide excellent strength-to-weight ratio. Nevertheless, there are few studies on MMCs protection, probably due to a major interest on the improvement of its mechanical properties [11]. Greene and Mansfeld [12] studied the corrosion behavior of MMCs treated with cerium and found that the corrosion resistance can be increased, confirming the effectiveness of the cerium treatment [12,13]. Recently similar results were obtained by Hu et al. coating Al18B4O33w/Al composite with cerium [14,15]. Concretely, they found that the concentration of H2O2 in the cerium solution had a strong influence on the corrosion properties and the concen-
* Corresponding author. Tel.: +86 451 86415894; fax: +86 451 86413922. E-mail address: [email protected] (J. Hu). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.08.020
tration of CeCl3 in the test solution also had a significantly influence on the corrosion behavior and surface morphologies of the coated samples. In order to increase the available cerium ion concentration and to reduce the time required for successful surface modification, many other works have been published using different conditions to obtain Ce conversion layers on different MMCs, and investigating their corrosion behavior [13,16,17]. The pre-treatment of materials surfaces prior to coating process is recognized and accepted as one of the most critical steps in a surface operation [18–21]. The quality and performance of coated parts in service can be directly related and correlated to the initial pre-treatment of the surface to be coated. Some researches [18,22] studied the effect of surface pre-treatment on coating surface morphology and salt fog performance. They found that surface preparation (desmutting and degreasing) produces coatings with the best salt fog performance than coatings deposited on substrates prepared by others procedures due to formation of cracks in the coating or insufficient coating thickness. The cerium conversion coating on Al18B4O33w/Al composite with excellent corrosion resistance has been reported in our previous study [14]. Our previous studied also suggested that the pretreatment time prior to coating process also has a distinct influence on the corrosion properties of aluminum matrix composite. Relatively little is known about the microstructure evolution of coating on the pre-treated composite substrates. In the present work, the effect of HF pre-treatment of the composite surfaces prior to coating process on the morphologies of the coated Al18B4O33w/AA6061 composite was evaluated. Particular attention was paid to the interface between the substrate and the coating. More specifically, the various morphologies of
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coatings, the distribution and the characteristics on the substrate with different pre-treatment conditions prior to coating process were investigated, and a possible formation mechanism is proposed.
2. Experimental 2.1. Material and specimen preparation The alumina borate whisker (20% volume fraction) reinforced 6061 aluminum composite (Al18B4O33w/AA6061) as substrate was fabricated by squeeze casting. The whisker had a 0.5–1 lm in diameter and 10–30 lm in length (Shikoku Chemical Co. Ltd). The whisker preform and the mould were preheated at 480– 520 °C, and then 6061Al alloy at 700–750 °C was cast into the mould with a pressure of 80–100 MPa. Disc-samples with a thickness of 4 mm and a diameter of 15 mm were cut from the bulk composite, which were desmutted and ground using SiC paper, followed by polishing using a diamond paste, and ultrasonic cleaning in acetone. 2.2. Chemical pre-treatment Pre-treatment has previously been identified as an important step in the fabrication of cerium oxide conversion coatings. Pretreatment processes that included deoxidation or activation steps. To further define the relationship among pre-treatment, coating performance and coating surface morphology, the chemical pretreatment steps that were designed HF solution, and the concentration of HF is 2.5 mol/L. Before the composite was modified by cerium-based conversion coatings, a chemical pre-treatment of the composite was carried out. The samples were pre-treated by immersion in an aqueous solution of HF for 20 s (sample A) and 180 s (sample B), respectively, at room temperature, followed by rinsing in deionized water.
2.5. Electrochemical measurements A M273A model potentiostat with a saturated calomel reference electrode (SCE) and a graphite counter electrode were used. All potentials are given against the reference electrode. The testing sample was used as working electrode. The open circuit potential (OCP) as a function of time for the pre-treated samples was recorded in cerium bath at 30 °C. 3. Results 3.1. Surface characterization of pre-treated surfaces Typical surface morphologies of pre-treated surfaces by SEM observation are shown in Fig. 1. It can be seen that the whisker is uniformly distributed in the matrix of Al18B4O33w/AA6061 composite and the surface of the sample A is smooth (Fig. 1a). In addition, it can be observed that more whiskers and defects appeared at the surface of sample B and the surface of the sample B is very uneven, which suggested that more aluminum can be etched with the increasing of pre-treatment time and more whiskers and defects (crevices or pores) of matrix can appear at the surface of the composite (Fig. 1b). 3.2. Microstructure and composition of coatings Fig. 2 shows the evolution of the surface morphologies of the coatings obtained on the pre-treated samples by SEM observation. A uniform chemical conversion coating morphology presents in
2.3. Ce conversion coating process After the chemical pre-treatment a cerium-based conversion coating was obtained on the pre-treated samples by immersion in cerium solution for 10 min at 30 °C. The solution pH was adjusted to 2.85 with NaAc + HAc and was kept constant during the coating process and then rinsed with distilled water and dried with air. A typical coating solution was prepared by dissolving 7.5 g CeCl3 into 1000 ml of deionized water with 100 ml/L H2O2 concentrations. 2.4. Microstructure analysis Morphologies of the conversion coatings on the composite were observed using SEM/EDX analysis was carried out on a Hitachi S3000N scanning electron microscope operated at 20 keV. Cross-sectional and plan-view microstructures were carried out on a Philips CM12 TEM with an operating voltage of 120 kV. EDAX was employed during TEM observations to examine the phase composition in the composite. Cross-sectional specimens for the TEM observation were prepared by standard mechanical polishing, dimpling, and ion milling methods. Plan-view specimens were prepared by back thinning by mechanically polishing, dimpling, and ion milling to remove the substrate from the coating. Ion milling, which was performed by Ar+ bombardment at 5 keV using a precision polishing system. Initially, the angle of ion milling was set at 10°; at the final stage, an angle 5° was used and processing was performed at 3 keV.
Fig. 1. Typical surface morphologies of the pre-treated surfaces for (a) 20 s, (b) 180 s.
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Fig. 2. Morphologies and composition of coatings obtained on pre-treated samples (a) morphology of coatings on sample A, (b) EDX spectrum of coatings on sample A, (c) morphology of coatings on sample B and (d) EDX spectrum of coatings on sample B.
Fig. 2a, and the coatings have almost no cracking; the coatings were composed of fine and compact granular morphology. The coatings were even and thin, substrate topographies left by chemical pre-treatment, such as whisker and etch pits, were visible through the coatings. The chemical composition of the coating was analyzed by EDX as shown in Fig. 2b. The SEM investigation performed on coatings deposited on sample B shows a larger difference in the morphologies compared with coatings deposited on sample A. A cracked ‘‘dry mud-like” structure can be clearly seen in Fig. 2c, and the distribution of the cracks is regular. The coatings are uneven and thick, however, substrate topographies can also be clearly observed. The morphology of coatings on the surface consisted of nodules, which were identified by EDX to be cerium-rich phases. These phenomena suggested that increasing the pre-treatment time caused the coated surface morphology to change. The chemical composition of the coating was analyzed by EDX as shown in Fig. 2d. SEM observation indicates that under the two pre-treatment conditions the morphologies of the deposited coatings are remarkably different. The cracking of the coating on the substrate pretreated for 180 s was most likely caused by deeply etching prior to coating. Plan-view TEM micrographs and corresponding SAED patterns of the cerium conversion coatings obtained on both pre-treatment samples are shown in Figs. 3 and 4, respectively. The TEM images and SAED patterns clearly indicate that under the both pre-treatment conditions the deposited coatings were composed of nanocrystalline particles with a sphere-like morphology. However, the
particles of the coating deposited on sample A (Fig. 3a) were much finer (2–5 nm) than that of the coating deposited on sample B (7– 18 nm) (Fig. 4a), which suggested that the surface pre-treatment for a shorter time can inhibit the growth of coating particles. In addition, extensive film cracking were visible in the TEM for the deposited coating on sample B. The current study has shown that the size of the coating granular size was a strong function of the pre-treatment prior to coating. The longer the pre-treatment time, the larger the granular size of coatings. It is interested noticed that the deposited coatings were also composed of amorphous morphology under long pre-treatment condition (as shown in Fig. 4c), which suggested that the deposition process is complicated under the long time pre-treatment prior to coating results in both crystalline and amorphous states morphologies present on the pre-treated sample surface. 3.3. Interface between coatings and substrate Fig. 5 presents cross-sectional TEM characterization showing the evolution of the coatings obtained on sample A. As expected, a thin, continuous coating (70–80 nm) formed on sample A (Fig. 5a). This indicated that the cerium conversion coating can uniformly deposit on the substrate with short time pre-treatment. The coatings can deposit on either the side surface of whisker or the transverse section of whisker (Fig. 5b). There is neither defect at the interface between the coatings and the substrate nor cracks in the coatings, indicating the coatings had good adhesion to the substrate.
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tion at interface owing to the difference of stress states or the formation of crevices. Fig. 7 expresses the OCP as a function of time for the pre-treated substrates immersed in cerium bath at 30 °C. The OCP decreased rapidly to about 795 mV (SCE) at a very short time (about 90 s) for the coating deposited to sample A, and the OCP decreased gradually to 759 mV (SCE) at a longer time (about 815 s) for the coating deposited to sample B. Subsequently, the potentials rose as the coating developed over the sample surfaces, and finally approached to a constant value of about 751 mV for sample A and 716 mV for sample B, respectively. 4. Discussion 4.1. Electrochemical driving of coating formation
Fig. 3. Plan-view TEM micrographs and corresponding SAED pattern of the cerium conversion coatings obtained on sample A (a) morphology and (b) SAED pattern.
Fig. 6 is a set of cross-sectional TEM characterization showing the evolution of the coatings obtained on the sample B. From Fig. 6, a correspondingly thick coating (100–180 nm) appeared on the substrate as shown in Fig. 6a, some cracks attain the substrate surface. Cracks can appear at the end of whisker (Fig. 6b) or the side surface of whisker (Fig. 6c) along with grain boundary of the matrix (Fig. 6d). These results show that the coatings split to be more easily at some defect place. In both Figs. 5a and 6a, the coatings seem to have a quite uniform thickness and to follow the irregularities of the substrates. Moreover, the thickness of the coatings obtained on the pre-treated substrates increases with the pre-treatment time of the substrates, but defects and cracks in the coatings also increase with the pre-treatment time of the substrates. Cracks preferred forma-
The investigation indicated that conversion coating formation mainly due to corrosion of anode, the transportation of species in liquid phase and metal/solution interface, and formation and growth of nucleus. The course started from corrosion of substrate, maintained the correlativity and went along simultaneously [23]. The interface in the composite will act as an active region [24]. In corrosion medium (such as HF solution), whisker severed as a cathode formed micro-cells with the aluminum matrix around them, aluminum matrix in relation to the adjacent Al18B4O33 whisker severed as an anodic were dissolved away, since the potential of the matrix in this region diverge from equilibrium potential [24]. The cathodic sites on the pre-treated surfaces will increase with the pre-treatment time. From Fig. 1b, it can be found that the long time pre-treatment in HF solution appears to etch the surface more aggressively. A lot of whisker appeared on the surface of the substrate, indicating the aluminum matrix in the vicinity interface was corroded away and whisker can be emerged from the substrate. The longer the pre-treatment time prior to the coating is, the more the number of the micro-cells in the composite becomes and the higher the activation degree of the surface achieves. The difference in the number of cathodic sites present on the substrate surface with different pre-treatment time influences the formation, growth and morphology along with granular size of the conversion coating on the composite surface. Cerium conversion coatings have been proposed to deposit as a result of a pH rise at the surface of aluminum matrix composite which is generated as part of the coating reactions [23]. As can be observed, the coatings were not only deposited over whisker but also deposited over aluminum matrix, since the deposition process was driven by the electrochemical potential differences between the aluminum matrix and whisker. Whisker served as a cathode, the reaction that the reduction of hydrogen ion to liberate hydrogen gas happened at whisker, which caused local pH rise in the vicinity of whisker and resulted in the cerium depositing on whisker preferential. Matrix Al served as an anode can be dissolved during the electrochemical process that leads to cerium conversion starts by etching the initially covering oxide layer to allow electronic access with the underlying metal. The deposition of Ce-coatings is possible because of the chemical heterogeneity of the aluminum alloy surface, which establishes the local electrochemical cells. 4.2. Characteristic of coating formation When sample A was immersed into cerium salt solution, Ce was uniformly precipitated on the substrate owing to the micro-cells is uniformly distributed on the pre-treated substrate surface because of the uniform distribution of whisker. A compact and uniform conversion coating morphology presents in Fig. 2a.
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Fig. 4. Plan-view TEM micrographs and corresponding SAED pattern of the cerium conversion coatings obtained on sample B (a) crystalline morphology, (b) SAED pattern and (c) amorphous morphology.
When sample B was immersed into cerium salt solution, more Ce was precipitated with regard to the conversion process, because whole material surface was cathodically activated. Since the presence of plentiful micro-cells on the pre-treated substrate surface and the number of cathodic sites were increased, the precipitation reaction occurred more rapidly, thus leading to a faster coating deposition rate during coating process, resulted in not only the thickness but also the unevenness of the coatings increase. The heterogenous nucleation and quick deposition of the coatings took place on sample B may be the primary reason to form amorphous structure and obtain crystalline coatings with large particle.
Indeed, EDX analyses of many sample surfaces have shown that the longer the pre-treatment time, the higher the Ce content in the surface layer. A substantial fraction of Al was detected during analysis because the size of the deposits was small compared to the interaction depth of the probe beam. The change of Ce content on the different coated surfaces indicated that the longer the pre-treatment time, the thicker the coatings under the same deposition conditions. Since the longer the pre-treatment time, the higher the activity of sample surface, and the quicker the nucleation rate of coatings on pre-treated surfaces. Therefore, the thickness of coatings increases with the chemical pre-treatment time prior to coatings.
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process of coatings, resulting in large numbers of cracks appear at the substrate surface of sample B. In addition, the cracks can also be caused by the heterogeneous nucleation and growth of coatings due to the more defects presented on the substrate surface for a long time pre-treatment. The faster deposition rate of the coatings caused large stress existed at the coatings, thereby more cracks presented at the coatings. 4.4. Electrochemical behavior of coating formation
Fig. 5. Cross-sectional TEM characterization of the coatings obtained on sample A (a) coating deposited on the substrate, (b) coating deposited on whisker.
4.3. Formation of cracked coating It is interesting to note that the morphologies of coatings by SEM and TEM observation indicated that the cracks only appear at the thick film, since they were only observed in sample submitted to the long pre-treatment period, the cracks did not occur at the thin film. It seems that the cracks are caused by stress relieve. Cracked ‘‘dry mud-like” structure of cerium conversion coatings was considered to be formed during drying step and to be caused by internal stress. Cracks preferred formed at interface between whisker and aluminum matrix owing to the difference of stress states or the formation of crevices. Subsequently, the cracks developed during growth
From Fig. 7, it can be found that the trend of both curves is almost same. During the immersion process of the two samples, the electrode potentials decrease at the beginning of immersion, subsequently increase, and finally are closed to constant values. This phenomenon is consistent with the study about chemical conversion coating on AZ91D magnesium alloy by Elesentriecy et al. [25,26]. In their paper, the OCP was shown as a function of time for AZ91D alloy immersed in a stannate bath, and they found the curve disclosed three stages during the coating process. They suggested that alloy dissolution was proceeding over the initial time of immersion and a decrease in OCP was showed at this period. Subsequently, the OCP showed an increase, indicating that initial substrate dissolution promoted the initiation of the coating process, and the substrate surface was almost fully covered by the coating film until the OCP remained almost constant. But our research found that the cerium conversion coating had already formed on the studied composite at the very beginning of immersion process. A direct proof is presented in Fig. 8. Coatings morphologies can be clearly seen in this figure, which strongly suggested that the conversion coating deposited on the pre-treated substrates accompanied by the decreasing of OCP. A thin coating with finer-grained structure had already been obtained on the surface of sample A by immersion in cerium solution for 0.5 min (as shown in Fig. 8a), and a thick coating with cracked structure had also been formed on the surface of sample B by immersion in cerium solution for 1.5 min (as shown in Fig. 8b), which gives a further proof that the deposition rate of the coatings on the surface of sample B is faster and the inner stress is greater during deposition process. The decrease of potential may be relative to the corrosion of aluminum matrix, local pH therewith rises in the vicinity of whisker and caused the conversion coating formation. The conversion coatings nucleate and grow continuously, until the whole surface was covered by formed coatings, accordingly resulted in the potential increase. The electrode potential maintained stable to a final value of about 751 mV for the coating deposited to sample A and 716 mV for the coating deposited to sample B, respectively. It is suggested that a dynamic balance was established between the coating growth rate and the coating dissolution rate. For the coatings deposited to sample A, a uniform nucleation took place on the substrate surface due to the micro-cells is uniformly distributed on the pre-treated substrate surface, the surface of sample A can be covered by the coatings in the initial stage of immersion (90 s), which blocked the transportation of species needed for the conversion coating formation resulted in the potential increase. In this case, the formation of coatings is difficult continuously and the growth rate of the coatings has also already decreased, whereas, the immersion time is persisted to 10 min but the coating is still thin and the grain is still finer (as shown in Fig. 2a). For the coatings deposited to sample B, the nucleation is very heterogeneous owing to the presence of plentiful micro-cells on the pre-treated substrate surface, a coating blocked the transportation of species can just be formed on the substrate surface after longer immersion time, the potential decreased with the immersion process until the immersion time exceeds 815 s. In this case, the formation and growth of coatings along with the corrosion of
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Fig. 6. Cross-sectional TEM characterization of the coatings obtained on sample B (a) coating deposited on the substrate, (b, c) cracks at the end of whisker, (d) cracks at grain boundary of the matrix.
aluminum matrix took place during the coating process, both crystalline and amorphous structure can present on the substrate surface simultaneously due to the complicated deposition process. Moreover, the continuous nucleation and growth of coatings on the surface by immersion in cerium solution for 10 min also caused a thick coating with cracked structure presented on the surface (as shown in Fig. 2c).
5. Conclusions (1) The longer the pre-treatment time, the more the whisker appear to be on the pre-treated substrate surface, the higher the activity of the pre-treated surface. The pre-treatment
prior to coating has strong influence on the formation and growth of coatings. (2) A compact, uniform and even coating can be formed on the substrate surface with pre-treatment for 20 s. The coating was crystalline and was about 70–80 nm in thickness. Both crystalline and amorphous states morphologies present on the substrate surface with pre-treatment for 180 s. A dry-mud cracked surface can be observed and the coating was about 100–180 nm in thickness. (3) Cracks only presented on the thick coating surface. The cracks mainly produced at the interface because of the higher deposition rate on whisker. The inner stress in dried coating resulted in the extensive cracks.
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References
Fig. 7. The relationship between OCP and time for coating formation process on the pre-treated composite surfaces with various time.
Fig. 8. Morphologies of coatings obtained on pre-treated samples (a) morphology of coatings on sample A by immersion in cerium solution for 0.5 min, (b) morphology of coatings on sample B by immersion in cerium solution for 1.5 min.
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