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Materials Chemistry and Physics 76 (2002) 228–235 Study on the rare earth sealing procedure of the porous film of anodized Al6061/SiCp Xingwen Yu a,b...

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Materials Chemistry and Physics 76 (2002) 228–235

Study on the rare earth sealing procedure of the porous film of anodized Al6061/SiCp Xingwen Yu a,b,∗ , Chuanwei Yan b , Chunan Cao b b

a Laboratory for Solid State Ionics, Institute of Physics, Chinese Academy of Science, P.O. Box 603, Beijing 100080, PR China State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Science, Shenyang 110016, PR China

Received 13 June 2001; accepted 4 September 2001

Abstract The rare earth (RE) sealing procedure of the porous film of anodized aluminum based metal matrix composites (MMC) Al6061/SiCp was studied with the field emission scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS). The results show that RE solution gets into the pores of the anodized coating to form cerium oxide/hydroxides precipitation at the beginning of sealing. At the same time, the spherical deposits formed on the surface of the anodized coating create a barrier to the permeation of RE solution getting into the pores. When the pore-structured anodizing film is covered all with the spherical deposits. RE conversion coating will form on the surface of the anodized coating. The reaction of the coating formation was investigated by employing cyclic voltammetry. The results indicate that accelerator H2 O2 acts as the source of O2 by carrying chemical reaction in course of coating formation. In the mean time, it may carry electrochemical reaction to generate alkaline condition to accelerate the coating formation The porous structure of the film is beneficial to the precipitation of the cerium hydroxides film. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth sealing; Anodized A1606l/SiCp ; Scanning electron microscope; Cyclic voltammetry

1. Introduction Chromate sealing has been used as a standard method of corrosion protection for anodized aluminum for a long time. However, the recent recognition that chromates are both highly toxic and carcinogenic has led to extensive worldwide research to develop effective alternative methods. Rare earth (RE) metal conversion coating for the corrosion protection of aluminum alloys has been widely researched in the recent years. It was widely recognized as an attractive alternative to chromate conversion coatings 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. Typical processes included: (i) exposure to aerated aqueous solution of 1000 ppm CeCl3 and 100 mol m−3 NaCl for up to 1 week [1,2]; (ii) immersion of the aluminum alloys in a aerated solution containing cerium salts and film formation accelerant such as H2 O2 and KMnO4 for various times which are usually less than 30 min depending on different process [3–7]; (iii) Ce–Mo process described by Mansfeld and coworkers [8–13]. It ∗ Corresponding author. Tel.: +86-10-82649050; fax: +86-10-82649050. E-mail address: [email protected] (X. Yu).

should be noted, however, that among these processes almost all cerium conversion coatings were to be deposited directly on the matrix of aluminum alloys and little attention was paid to its application in anodizing of aluminum alloys. Because anodizing is one of the most widely used technique for corrosion protection and decoration of aluminum alloys, it is of interest to explore the possibility of forming cerium conversion coatings on porous film of anodized aluminum. In the aerospace industry, the demand for improvement of the mechanical performance and weight savings has focused attention to new materials, including aluminum based metal matrix composites (MMC). These materials are formed by the addition of a second phase, generally a ceramic material, to an aluminum matrix. They have the advantages of the improved mechanical properties and the decreased weight. In contrast to aluminum alloys, relatively little is known about the effectiveness of corrosion protection of aluminum MMC. Authors have studied the RE sealing process for the porous anodized Al6061/SiCp . The corrosion resistance of the RE sealed coating was examined to be comparable to that of chromate sealing anodic coatings, which is discussed elsewhere [14]. Here, the purpose of this paper is to study the RE sealing procedure and the reactions of coating formation with scanning electron microscope (SEM) and cyclic voltammerty.

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 5 3 6 - 3

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Table 1 Major alloying elements of 6061 alloy (wt.%) Alloy

Cu

Mg

Mn

Fe

Si

Zn

Ni

Ti

Others

Al

6061

0.15–0.4

0.8–1.2

0.15

0.7

0.4–0.8

0.25



0.15

0.15

Balance

Table 2 Contents and mechanical properties of Al606l/SiCp Matrix

Reinforcement

Bending strength (MPa)

Hardness (Hv)

6061

SiC particles (3.5 ␮m, 45%)

600

200

Fig. 1. Surface images of RE sealing anodized coatings at different times: (a) unsealed; (b) 2 min; (c) 5 min; (d) 10 min; (e) 20 min; (f) 30 min; (g) 40 min; (h) 60 min; (i) 90 min; (j) 120 min; (k) 180 min; (l) 240 min.

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Fig. 1. (Continued ).

2. Experimental 2.1. Material The material studied is Al606l/SiCp . Major alloying elements of 6061 alloy are listed in Table 1. Compositions and mechanical properties of Al606l/SiCp are listed in Table 2. 2.2. Preparation of anodized film. Al606l/SiCp samples were first polished with 300#, 600# and 1000# abrasive paper, dusted by cotton saturated with acetone and degreased in organic solvent. Then, the samples were pretreated in boiling water for 10 min. Lastly, the specimens were anodized golvanostatically in stirred aqueous solution of 100 dm3 m−3 H2 SO4 at a current density of

l.5 A dm−2 and room temperature. The Pb sheets served as the cathode. After anodized, the specimens were rinsed fully in distilled water for subsequent use. 2.3. SEM and EDS analysis SEM was performed on an Amary 1910 field emission SEM at 20 kV and X-ray energy dispersive spectroscopy (EDS) spectra was collected and analyzed using Link ISIS system. 2.4. Cyclic voltammetry analysis Cyclic voltammetry analysis was conducted as follows: the samples were immersed in the solution containing

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Fig. 2. EDS analysis of anodized coatings at different sealing times: (a) unsealed; (b) 2 min; (c) 5 min; (d) 10 min; (e) 20 min; (f) 30 min; (g) 40 min; (h) 60 min; (i) 90 min; (j) 120 min; (k) 180 min; (l) 240 min.

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Fig. 2. (Continued ).

X. Yu et al. / Materials Chemistry and Physics 76 (2002) 228–235

Ce(NO3 )3 (open to air). The exposed area was 1 cm2 . Cyclic voltammetry measurements were carried out with an EG&G PARC M273 potentiostat. A saturated calomel electrode (SCE) was used as the reference electrode. A smooth Pt plate of 2.25 cm2 served as the counter electrode.

3. Results and discussion 3.1. SEM observation The anodized A16061/SiCp samples were immersed in the solution containing 5.0 g L−1 Ce(NO3 )3 and 0.5 g L−1 H2 O2 . SEM observation was conducted at different sealing times. The results are shown in Fig. 1.

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From Fig. 1, it can be observed that the porous structure of the anodized film was very clear for the unsealed sample (Fig. 1a). After immersion for 2 min, the pores on the sample began to be changed. The wall of the pores became thicker (Fig. 1b). Five minutes later, the spherical particles could be observed on the surface of the sample (Fig. 1c). After 10 min sealing, the pores became blurry (Fig. 1d). Then, the number and the size of the particles also increased with the increase in the sealing time. After 40 min sealing, the pores of the film were all covered with the particles (Fig. 1g). Then, with the increase in the immersion time, the particles went on to grow and integrate. At last, a layer of RE metal conversion coating was formed on the surface of the sample (Fig. 1i–l). Based on the analysis given in previous paragraphs, it can be seen that RE solution got into the pores of the anodized

Fig. 3. Cyclic voltammograms of anodized coatings in Ce(NO3 )3 solution containing different contents of H2 O2 : (a) 0 g L−1 ; (b) 0.125 g L−1 ; (c) 0.25 g L−1 ; (d) 0.5 g L−1 ; (e) 1.0 g L−1 ; (f) 2.0 g L−1 .

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coating to form cerium oxide/hydroxides precipitation at the beginning of sealing. At the same time, the spherical deposits formed on the surface of the anodized coating created a barrier to the permeation of RE solution from getting into the pores. When the pore-structured anodizing film was all covered with the spherical deposits, the RE conversion coating would form on the surface of the anodized coating. 3.2. The results of EDS EDS spectra of the anodized Al6061/SiCp immersion in the solution containing 5.0 g L−1 Ce(NO3 )3 and 0.5 g l−1 H2 O2 for different times are shown in Fig. 2. The data of the elements contents are also listed under each graph. Apparently, the Ce contents increased with the immersion time.

Also, it could be observed that the Ce content no longer increased after immersing the sample for 3 h. This indicated that the growth of the RE metal conversion coating stopped or became slower after 3 h immersion. 3.3. Cyclic voltammetry analysis Hinton and Arnott have researched the formation mechanism of the RE metal conversion coating in 1980s. They considered that the formation of the film centered on the precipitation of insoluble cerium oxides and hydroxides in alkaline conditions generated at the aluminum surface. These alkaline conditions occurred through electrochemical reduction reactions at cathodic sites on the surface. When the surface of an aluminum alloy was exposed into an aqueous environment containing CeCl3 , local electrochemical

Fig. 4. Cyclic voltammograms of the samples for anodizing different times: (a) 0 min; (b) 5 min; (c) 10 min; (d) 20 min; (e) 45 min; (f) 60 min.

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cells were established. At anodic sites, metal dissolution would occur. At cathodic sites, cathodic reactions such as the reduction of oxygen (O2 + 2H2 O + 4e → 4OH− ) occurred. The reaction led to an excess of OH− , with consequent higher alkalinity [2]. In our work, the accelerator H2 O2 was added to the solution. So the formation mechanism of the Ce conversion coating would be not the same as that proposed by Hinton. Cyclic voltammograms of the anodized Al606l/SiCp in Ce(NO3 )3 solution containing different contents of H2 O2 are shown in Fig. 3. It could be observed that there was no peek for the solution contained no H2 O2 (Fig. 3a). When adding H2 O2 to the solution, the peeks with the potential range of −0.6 to −0.8 V appeared in the cyclic voltammetry curves and the current density of the peeks became higher with the increasing content H2 O2 . This indicated that H2 O2 carried the reduction reaction on the surface of the anodized sample: H2 O2 +2e → 2OH− . However, during the coat formation, the samples were immersed in the solution containing Ce(NO3 )3 and H2 O2 . No additional current was added to the samples. It was known that H2 O2 tended to decompose to O2 in the solution without additional current. The O2 would carry the cathodic reactions at the cathodic sites of the anodized Al6061/SiCp . Thus, higher alkalinity was obtained which could accelerate the precipitation of cerium hydroxides. On the other hand, H2 O2 might carry the reduction reaction directly on the surface of the sample to generate the OH− . Cyclic voltammograms in Ce(NO3 )3 + H2 O2 of the A1606l/SiCp anodized for different times are shown in Fig. 4. It could be observed that there was no peek in the curves of the sample without anodizing. The peeks with the potential range of −0.6 to −0.8 V appeared in the cyclic voltammetry curves for the samples anodized for different times. The current density of the peeks became higher with the increasing of the anodized time. This indicated that the anodizing time affected the precipitation of cerium hydroxides. The anodizing time directly affected the porous structure of the anodized film. Thus, the precipitation of cerium hydroxides film was directly related to the porous structure of the film. The current density of the peeks in the cyclic voltammetry curves increased with the increasing of

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the anodized time. This indicated that the porous structure of the film was beneficial to the precipitation of the film. 4. Conclusions At the beginning of sealing, RE solution gets into the pores of the anodized coating to form cerium oxide/hydroxides precipitation. At the same time, the spherical deposits formed on the surface of the anodized coating create a barrier to the permeation of RE solution from getting into the pores. When the pore-structured anodizing film is all covered with the spherical deposits, RE conversion coating will form on the surface of the anodized coating. During the coating formation, the accelerator H2 O2 acts as the source of O2 by carrying the chemical reaction. In the mean time, it may be carries the electrochemical reaction to generate alkaline condition to accelerate the coating formation. The porous structure of the film was beneficial to the precipitation of the cerium hydroxides film. Acknowledgements This work has been carried out with the support of The Chinese Postdoctoral Science Fund and The Special Funds for the Major State Basic Research Projects G 19990650. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

B.R.W. Hinton, et al., Met. Forum 7 (4) (1984) 211–217. B.R.W. Hinton, Mater. Forum 9 (3) (1986) 162–165. L. Wilson, US Patent 88,066397 (1988). A. Kindler, US Patent 5192374 (9 March 1993). R.N. Miller, US Patent 5221371 (22 June 1993). R.N. Miller, et al., US Patent 5419790 (30 May 1995). R.N. Miller, US Patent 5356492 (18 Oct 1994). F. Mansfeld, et al., J. Electrochem. Soc. 138 (12) (1991) L74–L75. F. Mansfeld, et al., J. Electrochem. Acta 37 (12) (1992) 2277–2282. F. Mansfeld, US Patent 5194138 (16 March 1993). F. Mansfeld, Br. Corros. J. 29 (3) (1994) 192–200. F. Mansfeld, et al., Mater. Sci. Eng. A198 (1995) 51–61. C. Chen, F. Mansfeld, Corros. Sci. 39 (6) (1997) 1075–1082. X. Yu, C. Cao, Z. Yao, Mater. Sci. Lett. 19 (2000) 1907.