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Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced AA6061 composite
Applied Surface Science 253 (2007) 8879–8884 www.elsevier.com/locate/apsusc
Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced AA6061 composite J. Hu a,*, X.H. Zhao a, S.W. Tang a, W.C. Ren b, Z.Y. Zhang a b
a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China College of Mechanical and Electrical Engineering, Harbin Engineering University, Harbin 150006, PR China
Received 22 April 2007; received in revised form 26 April 2007; accepted 26 April 2007 Available online 7 May 2007
Abstract Cerium-based conversion coatings on Al18B4O33w/6061Al composite surface were obtained by immersing the composite into a solution containing various concentrations of CeCl3. Results indicate that the susceptibility to pitting for the conversion-coated composites was much lower than that of the untreated composite, and the corrosion resistance of the coated composites was improved markedly; moreover, the concentration of CeCl3 in the cerium solution affects significantly the corrosion behaviors of the coated composites. The coating obtained from a solution containing 7.5 g CeCl3 into 1000 ml produced better corrosion resistance on the composite due to the surface being almost covered by conversion coating. EDX and XPS experimental results indicated that the coatings were made up of oxygen, cerium, and aluminum. # 2007 Elsevier B.V. All rights reserved. Keywords: Cerium; Conversion coating; Aluminum matrix composite; Corrosion
1. Introduction Aluminum and its alloys are widely used as a structural material due to its low cost, excellent strength-to-weight ratio. The demand for materials of superior mechanical, thermal and electrical properties has focused attention on aluminum metal matrix composites (MMCs). However, MMCs have a more inhomogeneous structure than alloys due to presence of reinforcement, which will change the corrosion behavior significantly [1,2]. Various surface modification procedures are being evaluated for Al alloys and their composites to improve the corrosion resistance. Chromate conversion coatings have been widely applied for corrosion protection of aluminum alloys due to their high electrical conductivity and high efficiency/cost ratio [3,4]. However, the recent recognition that chromates are both highly toxic and carcinogenic has led to extensive worldwide research to develop effective alternative methods. Recent patents cover a wide variety of a new coating formulation [5–8]. One of the most promising is based on rare earth elements. Some researches demonstrated that treatment with aqueous solutions
* Corresponding author. Tel.: +86 451 86415894; fax: +86 451 86413922. E-mail address: [email protected] (J. Hu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.085
of rare earth salts (cerium, lanthanum, etc) effectively inhibit the corrosion of aluminum alloys [9–11]. It has been found that chemical passivation by immersion in rare earth metal chlorides can produce exceptional resistance to localized corrosion [12]. 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 [13]. Relatively little is known about the effectiveness of corrosion protection of aluminum borate whisker reinforced aluminum composites (Al18B4O33w/Al), which has been considered for a wide range application because of their high specific strength, high modulus and low cost [14–16]. The purpose of this paper is to report on characterization results for cerium conversion coating formed by immersion process that resulted in improved salt corrosion performance. The effect of the concentrations of CeCl3 in the Ce-salt solution was investigated using electrochemical method and weight loss test. 2. Experimental 2.1. Material The Al18B4O33w/6061Al composite was fabricated by squeeze casting. The volume fraction of the whiskers in the
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composite was about 20%. The whisker had a 0.5–1 mm in diameter and 10–30 mm in length (Shikoku Chemical Co. Ltd). The strength of the whisker is 8 GPa and the module is 400 GPa. The composition of the 6061 alloy: 0.2–0.6 wt.% Cu, 0.5–0.9 wt.% Mg, 0.5–0.9 wt.% Si, 0.2 wt.% Mn and balance Al. The whisker preform and the mould were preheated at 480– 520 8C, and then 6061Al alloy at 780–800 8C was cast into the mould with a pressure of 80–100 MPa.
of XPS measurements was of 1.6 mm2. The sputter rate was 5 nm min1.
2.2. Ce-conversion coating process
3. Results and discussion
Before treatment, the sample surfaces were desmutted and polished 2000-grit silican carbide (SiC) abrasive papers. Subsequently, a chemical pretreatment of the composites was carried out as follows: ‘‘degreasing treatment’’ by immersion in an aqueous solution of NaOH at pH 12 for 1 min at room temperature. After the chemical pretreatment a cerium-based conversion coating was obtained on the pretreated samples by immersion in cerium solution for 10 min at 30 8C. 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 5, 7.5 and 10 g CeCl3 into 1000 ml of deionized water with 100 ml/L H2O2 concentrations, respectively.
3.1. Surface morphology
2.3. Electrochemical measurements Polarization curves were obtained using standard threeelectrode system in 3.5 wt.% NaCl solution (open to air). A M273A model potentiostat with a saturated calomel reference electrode (SCE) and a graphite counter electrode were used. The exposed area of the samples was about 1.5 cm2. The potential scan was started 10 min after the specimens were placed into the test solution when reached a steady open circuit potential. A scan rate of 0.5 mV/s was used. Tests were performed at room temperature in naturally aerated 3.5 wt.% NaCl solution. 2.4. Weight loss tests Weight loss tests were performed in specimens of noncerium and cerium conversion coated surfaces. The corrosion rate (Vcorr) was determined using the Eq. (1): V corr ¼
Dm St
(1)
where S is the exposed area (m2), t is the exposed time (h), and Dm is the average of the weight variation for each type of coated surfaces (mg). 2.5. X-ray photoelectron spectroscopy (XPS) The XPS measurements were made on a Physical Electronics model PHI5700 system with a standard AlKa Xray source (1486 eV) operated at 250 W. Detail scans were recorded for the Ce 3d, C1s and O1s regions. The analyzed area
2.6. Scanning electron microscopy (SEM) SEM and energy-dispersive X-ray (EDX) analysis were carried out on a Hitachi S-4700 scanning electron microscope operated at 20 keV.
Fig. 1 shows the surface morphologies for the cerium conversion coated samples. Some whiskers existed on the surface as shown in Fig. 1(a), which indicated that the coating did not cover the whole surface of the sample. As a matter of fact, the coating formed slowly as observed visible which led to a majority of the surface exposed. For 5 g/L CeCl3 concentration, the combination of Ce3+ and OH is limited on the surface owing to the presence of the lower Ce 3+ concentration in the coating solution. The surface appears covered by a protective layer with ‘‘drymud’’ morphology as shown in Fig. 1(b). A majority of the surface can be covered by the coating which was more uniform than coatings obtained by immersion in coating solutions with other Ce-salt concentrations. The EDX spectrum (as shown in Fig. 1(c)) indicated that the conversion coating was made up of oxygen, cerium and aluminum. Because of the aluminum is the main element of the substrate, therefore, the surface elements probably were mainly in the state of cerium oxide or cerium hydroxide. From Fig. 1(d), the mud morphology also can be seen on the surface. However, the coating did not cover the whole surface of the composite too. Bare patches in several areas can be found which indicated that the coating had a poor adhesion to the substrate. The concentration of the Ce-salt increased led to a part of Ce3+ and OH combine to form deposition in the solution. In this case, to keep the coating growing on the surface is difficult. 3.2. XPS analysis Fig. 2 shows the Ce 3d XPS spectrum of the conversion coatings. The characteristic peak for Ce4+ is at 918 eV, while Ce3+ has a characteristic peak near 905 eV [17]. Form Fig. 2, it can be found that the Ce 3d spectrum are different in these three cerium-based conversion coatings. For the coated surface was obtained by immersing the composite into 7.5 g/L CeCl3 solution, the position of satellite peak was at 918 eV, which contributed to Ce4+, Ce4+ is the predominant oxidation state for the sample. Characterization of coatings by XPS measurements has found that Ce was deposited as Ce4+ and Ce3+ on other coated surfaces. According to the depicted by Decroly and Petitjean [18], the formation of Ce(OH)4 (or CeO22H2O) could proceed in two main steps starting from the Ce(OH)3 gel and via the peroxide
J. Hu et al. / Applied Surface Science 253 (2007) 8879–8884
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Fig. 1. SEM images of the cerium conversion coatings obtained by immersion in test solutions contained various concentrations of CeCl3 (a) CeCl3 = 5 g/L; (b) CeCl3 = 7.5 g/L; (c) EDX analysis for the coated surface prepared from 7.5 g/L CeCl3 in test solution, and (d) CeCl3 = 10 g/L.
Ce(OH)3OOH which slowly oxidizes Ce(OH)3 into Ce(OH)4.
Fig. 2. Ce 3d XPS spectra for the conversion coatings obtained by immersion in various concentrations of CeCl3 solution.
2Ce3þ þ H2 O2 þ 2OH ! 2CeðOHÞ2þ 2
(2)
2CeðOHÞ2þ 2 þ 2OH ! CeO2 2H2 O
(3)
For the coated surface prepared from 5 g/L CeCl3 solution, the combination of Ce3+ and OH is limited on the surface owing to the lower CeCl3 concentration. In this case, the formation of coating on the surface is difficult and the formation of Ce4+ is also difficult, therefore, the coated surface is a mixture of Ce 4+ and Ce 3+. For the coated surface prepared from 10 g/L CeCl3 solution, the increasing of CeCl3 concentration results in the chance of the combining of Ce 3+ and OH increase (a great chance of the combining of Ce 3+ and OH), a part of Ce 3+ and OH combine to form deposition in the coating solution by the consumption of H2O2. In this case, the formation of Ce 4+ is also difficult due to the H2O2 concentration decreased, and therefore, the surface of coating is also a mixture of Ce 4+ and Ce 3+. For the coated surface prepared from 7.5 g/L CeCl3 solution, a conversion coating can be formed on the whole surface of the composite, limited deposition appeared in the coating solution, the formation of Ce4+ is easy on the surface relative to other coated surfaces. Characterization of coating by XPS found that the surface of the coating was made up of mainly Ce4+.
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Fig. 3. XPS O1s spectra of the conversion coating prepared from 7.5 g/L CeCl3 in test solution: (a) the as-prepared surface of the conversion coating, and (b) surface of conversion coating after 10 min of sputtering.
Fig. 3 shows the XPS O1s regions for the coated composites. The O1s spectra from three conversion coatings are very similar to each other. The O1s spectrum for the cerium conversion coating was fitted with three components at 529.5, 531.8 and 533.4 eV for oxide anion, hydroxyl groups and adsorbed water [19]. After 10 min of sputtering, the adsorbed water on the surface of conversion coating disappeared. It suggested that the coexistence of different chemical environments attributed to cerium oxides (BE O1s = 529.5 eV) and cerium hydroxyl (BE O1s = 531.8 eV) [20]. A depth profile of the cerium coating (by immersion the sample into 7.5 g/L CeCl3 solution) is presented in Fig. 4. The depth profile can be divided into three regions: the surface (region 1), the bulk of the conversion coating (region 2) and the interface region with the composite (region 3). The composition changed sharply with sputtering time (and hence depth) in region 1. The O1s spectrum also changed significantly in region 1 (see Fig. 3(a) and (b)). For sputtering times longer than 5 min, i.e., through regions 2 and 3, only two components were required to fit the spectra. The lower one at 529.5 eV was attributed to oxygen anions, while the higher binding energy component 531.8 eV was attributed to hydroxyl groups in the cerium coating. There were only slight changes in the composition of the conversion coating throughout most of region 2. The percentage of oxygen anion remained constant for sputtering times up to 70 min. The percentage of cerium and aluminum remained constant for sputtering times up to 30 min, but thereafter, decreased and increased, respectively. It suggested that the substrate was emerged.
3.3. Polarization behavior Fig. 5 presents the polarization curves of the cerium conversion coated samples and untreated sample. It can be found that polarization behaviors were different for the coated composites by immersion them into the coating solution containing various concentration of CeCl3, but the corrosion resistance of the composites was improved by immersion in cerium salt solution. From Fig. 5, it can be found that the anodic current density increased rapidly with the potential. A lower pitting potential existed in the anodic polarization curve of the conversion coating prepared from the solution containing 5 g/L
Fig. 4. XPS depth profile through the conversion coating prepared from 7.5 g/L CeCl3 in test solution.
J. Hu et al. / Applied Surface Science 253 (2007) 8879–8884
Fig. 5. Polarization curves of untreated and coated samples for Al18B4O33w/ 6061Al composite.
CeCl3. The poor corrosion resistance was observed due to the presence of uncoated surface. However, the anodic current density increased slowly with the potential, which caused a large passive region in the anodic polarization curves of the conversion coatings prepared from the solutions containing 7.5 g/L and 10 g/L CeCl3. This suggested that the pitting potentials were increased and the coated composites were passivated in the passive regions. Such a situation is regarded as exhibiting resistance to localized corrosion because the composite is well protected by its passive film. Comparing the two of polarization curves, it can be found that the shapes of the curves are similar, and the pitting potentials are approximate, which indicated that the corrosion behaviors of two coated composites are similar. However, the current density of the former is higher than that of the latter, which indicated that the corrosion rate of the former is higher than that of the latter. 3.4. Weight loss tests Fig. 6 is the variation of weight as a function of immersion time in 3.5 wt.% NaCl for the coated samples and untreated sample. The results show the weight variation of the untreated sample increased markedly with the immersion time, which suggested that the corrosion rate of the noncerium conversion coated surface was bigger. From Fig. 6, it also can be found that the weight variation of the coated sample prepared from 5 g/L CeCl3 solution increased slowly with the immersion time, which clarified that the corrosion rate of the coated sample was decreased. However, the weight variation of the coated sample prepared from 10 g/L CeCl3 solution is limited with the increasing of immersion time which led to the corrosion rate is lower. It is worthy of note, the weight variation of the coated sample prepared from 7.5 g/L CeCl3 solution is almost same with the increasing of immersion time, which results in corrosion rate of the coated sample is the lowest. The results of the weight loss tests just confirm that the polarization studied in this work, it is efficient on protecting the composite against corrosion.
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Fig. 6. Variation of weight as a function of immersion time in 3.5 wt.% NaCl for the coated samples and untreated sample.
3.5. Mechanism of improvement on corrosion resistance Aluminum is a very active metal. Exposed to a source of oxygen, it reacts to form a thin transparent oxide film over the whole of the exposed metal surface. This film controls the rate of corrosion and protects the substrate metal, if the film is damaged and cannot be repaired; corrosion of the substrate is very rapid. It is generally accepted that the reinforcement additions are detrimental to corrosion resistance. The presence of whisker led to the oxide film might be nonuniform. The location oxide film, which is weak, will be thinned by dissolution. In addition, the interface in the composite may be provided numerous microcrevice sites. The microcrevice may arise either as a result of poor bonding at the whisker-Al interface or from the presence of cracked whisker. These sites are potential locations for the development of location attacks. Pits preferred nucleation at interface owing to the difference of chemical potential or the formation of crevices. Al18B4O33w/6061Al composite contains more active regions and has a higher pitting corrosion tendency in NaCl solution than matrix alloy. The Cl can adsorb easily onto the surface of an untreated composite sample and severe pitting corrosion will occur. The composite coatings prepared from the coating solutions had higher corrosion resistance in 3.5 wt.% NaCl solution. The good corrosion resistance may be attributed to the coatings covering the composites, and supplying a gap that more active region existed at the surface of the composite. For the coated surface prepared from 5 g/L CeCl3 solution, the coating did not cover the whole surface of the sample due to the lower concentration of CeCl3 in the coating solution. The deposit was preferentially formed around the Al18B4O33 whiskers because the whisker will be cathodic with respect to Al matrix. Ce acts as cathodic inhibitor, reduces the cathodic current density and decreases the corrosion attack, and therefore, the presence of coating does not affect directly the anodic behavior.
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For the coated surfaces prepared from 7.5 and 10 g/L CeCl3 solutions, the surfaces were almost covered by conversion coatings. The existence of the conversion coating made the reduction of metal ion which enters into NaCl solution, while a passivation region existed at the anodic polarization curves (as shown in Fig. 5), and the anodic reactions were inhibited. In Fig. 5, it also can be found that the current densities of cathodic reaction shifted lower, and the cathodic reactions were also inhibited.
4. Conclusions The cerium-based chemical conversion coating is an effective method to improve the resistance to localized corrosion of the composite. The concentration of CeCl3 in the test solution had a significantly influence on the corrosion behavior and surface morphologies of the coated samples. The entire substrate surfaces were coated with Ce-coatings when the concentration of CeCl3 was 7.5 and 10 g/L in the conversion solution, which exhibited resistance to localized corrosion because the composite is well protected by its passive film. When the concentration of CeCl3 was 5 g/L in the conversion solution, the existence of the conversion coating inhibits the cathodic reaction of the composite owing to the deposit was preferentially formed on cathodic sites, which led to the cathodic current density reduce and the corrosion attack decrease.
Acknowledgement The authors would like to thank the support of the Postdoctoral Science-Research Development Foundation of Heilongjiang Province, No. LBH-Q05051 References [1] A.J. Trowsdale, B. Noble, S.J. Harras, G.E. Thompson, G.C. Wood, Corros. Sci. 38 (1996) 177. [2] J. Hu, K. Tian, W.Y. Chu, J. Mater. Sci. 40 (2005) 5147. [3] A.S. Hamdy, Surf. Coat. Technol. 200 (2006) 3786. [4] A.E. Hughes, R.J. Taylor, B.R.W. Hinton, L. Wilson, Surf. Interface Anal. 25 (1997) 223. [5] L.S. Sander, E.M. Musingo, W.J. Neill, US patent 4 921 552 (1990). [6] N. Das, US Patent 5 139 585 (1992). [7] C.E. Tomlinson, US Patent 5 380 374 (1995). [8] B.R.W. Hinton, J. Alloys Compd. 180 (1992) 15. [9] F. Mansfeld, S. Lin, S. Kim, H. Shih, Electrochim. Acta 34 (8) (1989) 1123. [10] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Mater. Forum 7 (4) (1984) 211. [11] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Mater. Forum 9 (3) (1986) 162. [12] F. Mansfeld, F.J. Perez, Surf. Coat. Technol. 86-87 (1996) 449. [13] A.L. Rudd, C.B. Breslin, F. Mansfeld, Corros. Sci. 42 (2000) 275. [14] J. Hu, W.D. Fei, C. Li, C.K. Yao, J. Mater. Sci. Lett. 13 (1994) 1797. [15] J. Hu, W.D. Fei, C.K. Yao, J. Mater. Sci. 36 (2001) 4817. [16] J.P. Tu, M. Matsumura, Scripta Mater. 40 (1999) 645. [17] L.S. Kasten, J.T. Grant, N. Grebasch, N. Voevodin, F.E. Arnold, M.S. Donley, Surf. Coat. Technol. 140 (2001) 11. [18] A. Decroly, J.-P. Petitjean, Surf. Coat. Technol. 194 (2005) 1. [19] A.E. Hughes, R.J. Taylor, B.R.W. Hinton, Surf. Interface Anal. 25 (1997) 223. [20] G. Praline, B.E. Koel, R.L. Hance, H.-I. Lee, J.M. White, J. Electron. Spectrosc. Relat. Phenom. 21 (1980) 17.
Corrosion resistance of cerium-based conversion coatings on alumina borate whisker reinforced AA6061 composite J. Hu a,*, X.H. Zhao a, S.W. Tang a, W.C. Ren b, Z.Y. Zhang a b
a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China College of Mechanical and Electrical Engineering, Harbin Engineering University, Harbin 150006, PR China
Received 22 April 2007; received in revised form 26 April 2007; accepted 26 April 2007 Available online 7 May 2007
Abstract Cerium-based conversion coatings on Al18B4O33w/6061Al composite surface were obtained by immersing the composite into a solution containing various concentrations of CeCl3. Results indicate that the susceptibility to pitting for the conversion-coated composites was much lower than that of the untreated composite, and the corrosion resistance of the coated composites was improved markedly; moreover, the concentration of CeCl3 in the cerium solution affects significantly the corrosion behaviors of the coated composites. The coating obtained from a solution containing 7.5 g CeCl3 into 1000 ml produced better corrosion resistance on the composite due to the surface being almost covered by conversion coating. EDX and XPS experimental results indicated that the coatings were made up of oxygen, cerium, and aluminum. # 2007 Elsevier B.V. All rights reserved. Keywords: Cerium; Conversion coating; Aluminum matrix composite; Corrosion
1. Introduction Aluminum and its alloys are widely used as a structural material due to its low cost, excellent strength-to-weight ratio. The demand for materials of superior mechanical, thermal and electrical properties has focused attention on aluminum metal matrix composites (MMCs). However, MMCs have a more inhomogeneous structure than alloys due to presence of reinforcement, which will change the corrosion behavior significantly [1,2]. Various surface modification procedures are being evaluated for Al alloys and their composites to improve the corrosion resistance. Chromate conversion coatings have been widely applied for corrosion protection of aluminum alloys due to their high electrical conductivity and high efficiency/cost ratio [3,4]. However, the recent recognition that chromates are both highly toxic and carcinogenic has led to extensive worldwide research to develop effective alternative methods. Recent patents cover a wide variety of a new coating formulation [5–8]. One of the most promising is based on rare earth elements. Some researches demonstrated that treatment with aqueous solutions
* Corresponding author. Tel.: +86 451 86415894; fax: +86 451 86413922. E-mail address: [email protected] (J. Hu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.085
of rare earth salts (cerium, lanthanum, etc) effectively inhibit the corrosion of aluminum alloys [9–11]. It has been found that chemical passivation by immersion in rare earth metal chlorides can produce exceptional resistance to localized corrosion [12]. 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 [13]. Relatively little is known about the effectiveness of corrosion protection of aluminum borate whisker reinforced aluminum composites (Al18B4O33w/Al), which has been considered for a wide range application because of their high specific strength, high modulus and low cost [14–16]. The purpose of this paper is to report on characterization results for cerium conversion coating formed by immersion process that resulted in improved salt corrosion performance. The effect of the concentrations of CeCl3 in the Ce-salt solution was investigated using electrochemical method and weight loss test. 2. Experimental 2.1. Material The Al18B4O33w/6061Al composite was fabricated by squeeze casting. The volume fraction of the whiskers in the
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composite was about 20%. The whisker had a 0.5–1 mm in diameter and 10–30 mm in length (Shikoku Chemical Co. Ltd). The strength of the whisker is 8 GPa and the module is 400 GPa. The composition of the 6061 alloy: 0.2–0.6 wt.% Cu, 0.5–0.9 wt.% Mg, 0.5–0.9 wt.% Si, 0.2 wt.% Mn and balance Al. The whisker preform and the mould were preheated at 480– 520 8C, and then 6061Al alloy at 780–800 8C was cast into the mould with a pressure of 80–100 MPa.
of XPS measurements was of 1.6 mm2. The sputter rate was 5 nm min1.
2.2. Ce-conversion coating process
3. Results and discussion
Before treatment, the sample surfaces were desmutted and polished 2000-grit silican carbide (SiC) abrasive papers. Subsequently, a chemical pretreatment of the composites was carried out as follows: ‘‘degreasing treatment’’ by immersion in an aqueous solution of NaOH at pH 12 for 1 min at room temperature. After the chemical pretreatment a cerium-based conversion coating was obtained on the pretreated samples by immersion in cerium solution for 10 min at 30 8C. 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 5, 7.5 and 10 g CeCl3 into 1000 ml of deionized water with 100 ml/L H2O2 concentrations, respectively.
3.1. Surface morphology
2.3. Electrochemical measurements Polarization curves were obtained using standard threeelectrode system in 3.5 wt.% NaCl solution (open to air). A M273A model potentiostat with a saturated calomel reference electrode (SCE) and a graphite counter electrode were used. The exposed area of the samples was about 1.5 cm2. The potential scan was started 10 min after the specimens were placed into the test solution when reached a steady open circuit potential. A scan rate of 0.5 mV/s was used. Tests were performed at room temperature in naturally aerated 3.5 wt.% NaCl solution. 2.4. Weight loss tests Weight loss tests were performed in specimens of noncerium and cerium conversion coated surfaces. The corrosion rate (Vcorr) was determined using the Eq. (1): V corr ¼
Dm St
(1)
where S is the exposed area (m2), t is the exposed time (h), and Dm is the average of the weight variation for each type of coated surfaces (mg). 2.5. X-ray photoelectron spectroscopy (XPS) The XPS measurements were made on a Physical Electronics model PHI5700 system with a standard AlKa Xray source (1486 eV) operated at 250 W. Detail scans were recorded for the Ce 3d, C1s and O1s regions. The analyzed area
2.6. Scanning electron microscopy (SEM) SEM and energy-dispersive X-ray (EDX) analysis were carried out on a Hitachi S-4700 scanning electron microscope operated at 20 keV.
Fig. 1 shows the surface morphologies for the cerium conversion coated samples. Some whiskers existed on the surface as shown in Fig. 1(a), which indicated that the coating did not cover the whole surface of the sample. As a matter of fact, the coating formed slowly as observed visible which led to a majority of the surface exposed. For 5 g/L CeCl3 concentration, the combination of Ce3+ and OH is limited on the surface owing to the presence of the lower Ce 3+ concentration in the coating solution. The surface appears covered by a protective layer with ‘‘drymud’’ morphology as shown in Fig. 1(b). A majority of the surface can be covered by the coating which was more uniform than coatings obtained by immersion in coating solutions with other Ce-salt concentrations. The EDX spectrum (as shown in Fig. 1(c)) indicated that the conversion coating was made up of oxygen, cerium and aluminum. Because of the aluminum is the main element of the substrate, therefore, the surface elements probably were mainly in the state of cerium oxide or cerium hydroxide. From Fig. 1(d), the mud morphology also can be seen on the surface. However, the coating did not cover the whole surface of the composite too. Bare patches in several areas can be found which indicated that the coating had a poor adhesion to the substrate. The concentration of the Ce-salt increased led to a part of Ce3+ and OH combine to form deposition in the solution. In this case, to keep the coating growing on the surface is difficult. 3.2. XPS analysis Fig. 2 shows the Ce 3d XPS spectrum of the conversion coatings. The characteristic peak for Ce4+ is at 918 eV, while Ce3+ has a characteristic peak near 905 eV [17]. Form Fig. 2, it can be found that the Ce 3d spectrum are different in these three cerium-based conversion coatings. For the coated surface was obtained by immersing the composite into 7.5 g/L CeCl3 solution, the position of satellite peak was at 918 eV, which contributed to Ce4+, Ce4+ is the predominant oxidation state for the sample. Characterization of coatings by XPS measurements has found that Ce was deposited as Ce4+ and Ce3+ on other coated surfaces. According to the depicted by Decroly and Petitjean [18], the formation of Ce(OH)4 (or CeO22H2O) could proceed in two main steps starting from the Ce(OH)3 gel and via the peroxide
J. Hu et al. / Applied Surface Science 253 (2007) 8879–8884
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Fig. 1. SEM images of the cerium conversion coatings obtained by immersion in test solutions contained various concentrations of CeCl3 (a) CeCl3 = 5 g/L; (b) CeCl3 = 7.5 g/L; (c) EDX analysis for the coated surface prepared from 7.5 g/L CeCl3 in test solution, and (d) CeCl3 = 10 g/L.
Ce(OH)3OOH which slowly oxidizes Ce(OH)3 into Ce(OH)4.
Fig. 2. Ce 3d XPS spectra for the conversion coatings obtained by immersion in various concentrations of CeCl3 solution.
2Ce3þ þ H2 O2 þ 2OH ! 2CeðOHÞ2þ 2
(2)
2CeðOHÞ2þ 2 þ 2OH ! CeO2 2H2 O
(3)
For the coated surface prepared from 5 g/L CeCl3 solution, the combination of Ce3+ and OH is limited on the surface owing to the lower CeCl3 concentration. In this case, the formation of coating on the surface is difficult and the formation of Ce4+ is also difficult, therefore, the coated surface is a mixture of Ce 4+ and Ce 3+. For the coated surface prepared from 10 g/L CeCl3 solution, the increasing of CeCl3 concentration results in the chance of the combining of Ce 3+ and OH increase (a great chance of the combining of Ce 3+ and OH), a part of Ce 3+ and OH combine to form deposition in the coating solution by the consumption of H2O2. In this case, the formation of Ce 4+ is also difficult due to the H2O2 concentration decreased, and therefore, the surface of coating is also a mixture of Ce 4+ and Ce 3+. For the coated surface prepared from 7.5 g/L CeCl3 solution, a conversion coating can be formed on the whole surface of the composite, limited deposition appeared in the coating solution, the formation of Ce4+ is easy on the surface relative to other coated surfaces. Characterization of coating by XPS found that the surface of the coating was made up of mainly Ce4+.
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Fig. 3. XPS O1s spectra of the conversion coating prepared from 7.5 g/L CeCl3 in test solution: (a) the as-prepared surface of the conversion coating, and (b) surface of conversion coating after 10 min of sputtering.
Fig. 3 shows the XPS O1s regions for the coated composites. The O1s spectra from three conversion coatings are very similar to each other. The O1s spectrum for the cerium conversion coating was fitted with three components at 529.5, 531.8 and 533.4 eV for oxide anion, hydroxyl groups and adsorbed water [19]. After 10 min of sputtering, the adsorbed water on the surface of conversion coating disappeared. It suggested that the coexistence of different chemical environments attributed to cerium oxides (BE O1s = 529.5 eV) and cerium hydroxyl (BE O1s = 531.8 eV) [20]. A depth profile of the cerium coating (by immersion the sample into 7.5 g/L CeCl3 solution) is presented in Fig. 4. The depth profile can be divided into three regions: the surface (region 1), the bulk of the conversion coating (region 2) and the interface region with the composite (region 3). The composition changed sharply with sputtering time (and hence depth) in region 1. The O1s spectrum also changed significantly in region 1 (see Fig. 3(a) and (b)). For sputtering times longer than 5 min, i.e., through regions 2 and 3, only two components were required to fit the spectra. The lower one at 529.5 eV was attributed to oxygen anions, while the higher binding energy component 531.8 eV was attributed to hydroxyl groups in the cerium coating. There were only slight changes in the composition of the conversion coating throughout most of region 2. The percentage of oxygen anion remained constant for sputtering times up to 70 min. The percentage of cerium and aluminum remained constant for sputtering times up to 30 min, but thereafter, decreased and increased, respectively. It suggested that the substrate was emerged.
3.3. Polarization behavior Fig. 5 presents the polarization curves of the cerium conversion coated samples and untreated sample. It can be found that polarization behaviors were different for the coated composites by immersion them into the coating solution containing various concentration of CeCl3, but the corrosion resistance of the composites was improved by immersion in cerium salt solution. From Fig. 5, it can be found that the anodic current density increased rapidly with the potential. A lower pitting potential existed in the anodic polarization curve of the conversion coating prepared from the solution containing 5 g/L
Fig. 4. XPS depth profile through the conversion coating prepared from 7.5 g/L CeCl3 in test solution.
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Fig. 5. Polarization curves of untreated and coated samples for Al18B4O33w/ 6061Al composite.
CeCl3. The poor corrosion resistance was observed due to the presence of uncoated surface. However, the anodic current density increased slowly with the potential, which caused a large passive region in the anodic polarization curves of the conversion coatings prepared from the solutions containing 7.5 g/L and 10 g/L CeCl3. This suggested that the pitting potentials were increased and the coated composites were passivated in the passive regions. Such a situation is regarded as exhibiting resistance to localized corrosion because the composite is well protected by its passive film. Comparing the two of polarization curves, it can be found that the shapes of the curves are similar, and the pitting potentials are approximate, which indicated that the corrosion behaviors of two coated composites are similar. However, the current density of the former is higher than that of the latter, which indicated that the corrosion rate of the former is higher than that of the latter. 3.4. Weight loss tests Fig. 6 is the variation of weight as a function of immersion time in 3.5 wt.% NaCl for the coated samples and untreated sample. The results show the weight variation of the untreated sample increased markedly with the immersion time, which suggested that the corrosion rate of the noncerium conversion coated surface was bigger. From Fig. 6, it also can be found that the weight variation of the coated sample prepared from 5 g/L CeCl3 solution increased slowly with the immersion time, which clarified that the corrosion rate of the coated sample was decreased. However, the weight variation of the coated sample prepared from 10 g/L CeCl3 solution is limited with the increasing of immersion time which led to the corrosion rate is lower. It is worthy of note, the weight variation of the coated sample prepared from 7.5 g/L CeCl3 solution is almost same with the increasing of immersion time, which results in corrosion rate of the coated sample is the lowest. The results of the weight loss tests just confirm that the polarization studied in this work, it is efficient on protecting the composite against corrosion.
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Fig. 6. Variation of weight as a function of immersion time in 3.5 wt.% NaCl for the coated samples and untreated sample.
3.5. Mechanism of improvement on corrosion resistance Aluminum is a very active metal. Exposed to a source of oxygen, it reacts to form a thin transparent oxide film over the whole of the exposed metal surface. This film controls the rate of corrosion and protects the substrate metal, if the film is damaged and cannot be repaired; corrosion of the substrate is very rapid. It is generally accepted that the reinforcement additions are detrimental to corrosion resistance. The presence of whisker led to the oxide film might be nonuniform. The location oxide film, which is weak, will be thinned by dissolution. In addition, the interface in the composite may be provided numerous microcrevice sites. The microcrevice may arise either as a result of poor bonding at the whisker-Al interface or from the presence of cracked whisker. These sites are potential locations for the development of location attacks. Pits preferred nucleation at interface owing to the difference of chemical potential or the formation of crevices. Al18B4O33w/6061Al composite contains more active regions and has a higher pitting corrosion tendency in NaCl solution than matrix alloy. The Cl can adsorb easily onto the surface of an untreated composite sample and severe pitting corrosion will occur. The composite coatings prepared from the coating solutions had higher corrosion resistance in 3.5 wt.% NaCl solution. The good corrosion resistance may be attributed to the coatings covering the composites, and supplying a gap that more active region existed at the surface of the composite. For the coated surface prepared from 5 g/L CeCl3 solution, the coating did not cover the whole surface of the sample due to the lower concentration of CeCl3 in the coating solution. The deposit was preferentially formed around the Al18B4O33 whiskers because the whisker will be cathodic with respect to Al matrix. Ce acts as cathodic inhibitor, reduces the cathodic current density and decreases the corrosion attack, and therefore, the presence of coating does not affect directly the anodic behavior.
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For the coated surfaces prepared from 7.5 and 10 g/L CeCl3 solutions, the surfaces were almost covered by conversion coatings. The existence of the conversion coating made the reduction of metal ion which enters into NaCl solution, while a passivation region existed at the anodic polarization curves (as shown in Fig. 5), and the anodic reactions were inhibited. In Fig. 5, it also can be found that the current densities of cathodic reaction shifted lower, and the cathodic reactions were also inhibited.
4. Conclusions The cerium-based chemical conversion coating is an effective method to improve the resistance to localized corrosion of the composite. The concentration of CeCl3 in the test solution had a significantly influence on the corrosion behavior and surface morphologies of the coated samples. The entire substrate surfaces were coated with Ce-coatings when the concentration of CeCl3 was 7.5 and 10 g/L in the conversion solution, which exhibited resistance to localized corrosion because the composite is well protected by its passive film. When the concentration of CeCl3 was 5 g/L in the conversion solution, the existence of the conversion coating inhibits the cathodic reaction of the composite owing to the deposit was preferentially formed on cathodic sites, which led to the cathodic current density reduce and the corrosion attack decrease.
Acknowledgement The authors would like to thank the support of the Postdoctoral Science-Research Development Foundation of Heilongjiang Province, No. LBH-Q05051 References [1] A.J. Trowsdale, B. Noble, S.J. Harras, G.E. Thompson, G.C. Wood, Corros. Sci. 38 (1996) 177. [2] J. Hu, K. Tian, W.Y. Chu, J. Mater. Sci. 40 (2005) 5147. [3] A.S. Hamdy, Surf. Coat. Technol. 200 (2006) 3786. [4] A.E. Hughes, R.J. Taylor, B.R.W. Hinton, L. Wilson, Surf. Interface Anal. 25 (1997) 223. [5] L.S. Sander, E.M. Musingo, W.J. Neill, US patent 4 921 552 (1990). [6] N. Das, US Patent 5 139 585 (1992). [7] C.E. Tomlinson, US Patent 5 380 374 (1995). [8] B.R.W. Hinton, J. Alloys Compd. 180 (1992) 15. [9] F. Mansfeld, S. Lin, S. Kim, H. Shih, Electrochim. Acta 34 (8) (1989) 1123. [10] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Mater. Forum 7 (4) (1984) 211. [11] B.R.W. Hinton, D.R. Arnott, N.E. Ryan, Mater. Forum 9 (3) (1986) 162. [12] F. Mansfeld, F.J. Perez, Surf. Coat. Technol. 86-87 (1996) 449. [13] A.L. Rudd, C.B. Breslin, F. Mansfeld, Corros. Sci. 42 (2000) 275. [14] J. Hu, W.D. Fei, C. Li, C.K. Yao, J. Mater. Sci. Lett. 13 (1994) 1797. [15] J. Hu, W.D. Fei, C.K. Yao, J. Mater. Sci. 36 (2001) 4817. [16] J.P. Tu, M. Matsumura, Scripta Mater. 40 (1999) 645. [17] L.S. Kasten, J.T. Grant, N. Grebasch, N. Voevodin, F.E. Arnold, M.S. Donley, Surf. Coat. Technol. 140 (2001) 11. [18] A. Decroly, J.-P. Petitjean, Surf. Coat. Technol. 194 (2005) 1. [19] A.E. Hughes, R.J. Taylor, B.R.W. Hinton, Surf. Interface Anal. 25 (1997) 223. [20] G. Praline, B.E. Koel, R.L. Hance, H.-I. Lee, J.M. White, J. Electron. Spectrosc. Relat. Phenom. 21 (1980) 17.