Electrodeposition of aluminum on aluminum surface from molten salt

Transcription

1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.24 No.6 pp December 2011 Electrodeposition of aluminum on aluminum surface from molten salt Wenmao HUANG, Xiangyu XIA, Bin LIU, Yu LIU, Haowei WANG and Naiheng MA State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai , China Manuscript received 7 March 2011; in revised form 29 June 2011 The surface morphology, microstructure and composition of the aluminum coating of the electrodeposition plates in AlCl 3 -NaCl-KCl molten salt with a mass ratio of 8:1:1 were investigated by SEM and EDS. The binding force was measured by splat-cooling method and bending method. The results indicate that the coatings with average thicknesses of 12 and 9 µm for both plates treated by simple grinding and phosphating are compacted, continuous and well adhered respectively. Tetramethylammonium chloride (TMAC) can effectively prevent the growth of dendritic crystal, and the anode activation may improve the adhesion of the coating. Binding force analysis shows that both aluminum coatings are strongly adhered to the substrates. KEY WORDS Aluminum; Electrodeposition; Surface morphology; Molten salt 1 Introduction Aluminum matrix composites have attracted considerable attention in recent years because their high stiffness, high elastic modulus, good dimensional stability and low coefficient of thermal expansion, and these advantages make them be a ideal for applications in aerospace, military and automobile sectors [1 3]. However, aluminum matrix composites or aluminum alloys exhibit inferior corrosion resistance as compared to Al [4 7]. Some studies have been done to improve the corrosion resistance of the composite [8 14], such as anodic oxidation and chemical oxidation [8 12], but the reinforcement of the composite leads to the discontinuities in the oxide film [9,15 17]. To solve this problem, electrodeposition of aluminum coating on the aluminum matrix composites or aluminum alloys can be adopted before anodic oxidation. The electrodeposition has an ability to produce an ideal coating with higher adhesive strength, and Al is an excellent coating metal for its high corrosion resistance against chemical and atmospheric attack, low potential difference with aluminum matrix composites or aluminum alloys and good mechanical properties [18 20]. The electrodeposition of Al on aluminum matrix composites or aluminum alloys in molten salt, such as AlCl 3 -NaCl-KCl with the advantages of long service life, stability and low Corresponding author. Professor, PhD; Tel: ; Fax: address: nhma@sjtu.edu.cn (Naiheng MA)

2 444 price [21], is an adapted electrodeposition technology. The main component of the aluminum matrix composites or aluminum alloys is aluminum. So a research into the electrodeposition of Alon aluminum surface from AlCl 3 - NaCl-KCl molten salt was done in this paper, acts as the preliminary research for electrodeposition of Al on aluminum matrix composites or aluminum alloys. 2 Experimental The molten salt was prepared by the following compositions: anhydrous AlCl 3, NaCl and KCl, with the mass ratio AlCl 3 :NaCl:KCl of 8:1:1. NaCl and KCl measured were put into resistance furnace and dried at 400 C for 8 h. Then they were mixed with anhydrous AlCl 3 in a radius flask inside a glove box full of Ar gas. Tetramethylammonium chloride (TMAC) (2.5% in mass fraction) was mixed into the flask and finally the flask was heated to the electrodepositing temperature (140 C). Aluminum rods of 2 mm diameter were used as the anode. Al plates (20 mm 8 mm 1 mm) were used as the cathode. Prior to the experiment, Al rods and Al plates were cleaned ultrasonically in deionized water for 3 min, in anhydrous alcohol for 5 min and finally in acetone for 5 min. Then the Al plates were transferred to the glove box for pretreatment. One Al plate was treated by grinding with SiC sandpapers. The other Al plate was treated with orthophosphoric acid (50% mass fraction) for about 20 s. Anode activation was carried out directly in the molten salt by transforming the cathode into the anode, the Al plates after anode activation then acted as the cathode electrodes. The electrodepositing parameters were as follows: cathode current density is 3 A/dm 2 ; deposition time is 120 min and the experimental temperature is 140 C. The distance between the anode and the cathode is 15 mm. A sketch of the experimental setup used for electrodeposition is illustrated in Fig.1. The morphology, microstructure and compositions of the coating were analyzed by scanning electronic microscope (SEM) and energy disperse spectroscopy (EDS). The effects of TMAC and anode activation on electrodeposition were investigated. The binding force between the aluminum coating and substrate was measured by splat-cooling method and bending method. 3 Results and Discussion 3.1 Electrode reactions The anodic reaction is: Al 3e Al 3+ (1) The cathode reaction is: 4Al 2 Cl 7 + 3e Al + 7AlCl 4 (2) AlCl 4 + 3e Al + 4Cl (3) The aluminum anode dissolves into the molten salt, and Al 3+ is formed, as shown in Eq.(1). It is clear from comparison of Eqs.(2) and (3) that AlCl 4 influences the reaction process. If Eqs.(2) and (3) occur simultaneously, the electroplating process will remain

3 445 Fig.1 Sketch of the experimental setup used for electrodeposition. stable. In acidic electrolytes, the reduction of Al 2 Cl 7 on the cathode is the main reaction (Eq.(2)), while in alkalescent one, the main reaction is the reduction of AlCl 4 on the cathode (Eq.(3)) [22]. Previous studies have found that if Eq.(2) occurs first, the deposited coating will be brighter [23]. The molten salt prepared in this paper is an acidic one (AlCl 3 >50 mol pct), which tends to get a brighter deposited coating. 3.2 Electrodeposition SEM micrographs of the Al coatings are shown in Fig.2. As can be seen from the figure that both the Al coatings with and without phosphating are lamellar, and the surfaces are continuous, well adhered and compacted. EDS results of the aluminum coatings are shown in Fig.3. It is clearly seen from Fig.3a that aluminum and oxygen are the major composition on the coating layer. Fig.3b shows that, in addition to the major component of aluminum and oxygen, there are also small amounts of phosphorus on the coating layer. For the Al plate, the oxide film is removed by simple grinding, the surface was sufficiently clean to deposit a compacted and well adhered Al layer. For the Al plate pretreated by phosphating, a thin phosphate film is formed on the surface of the Al plate [24]. The electric field imposed on the phosphate film/molten salt interface and substrate was high enough to force the electrons to travel through the phos- Fig.2 SEM micrographs of the aluminum coatings: on simple grinding Al plate (a) and on phosphating treated Al plate (b).

4 446 phate film and were captured by Al 2 Cl 7, promoting the nucleation and growth of the Al on phosphate film and increasing the brightness of the coating according to Eq.(2). This has been discussed in the section 3.1. On the other hand, anode activation consumes some of the phosphate film, and increases the number of transmission electrons, thus contributing to the nucleation and growth of Al. Furthermore, the dielectric coefficient of the phosphate film is small [25], and its formation may lead to the increase in the cathode current density. The surface morphology of the Al coating is related to the current density, and it is found that a better plating effect can be obtained based on the selected current density [26 28]. Preliminary research has shown that, the surface morphology of the aluminum coating has a sponge-like appearance under low current density (lower than 1 A/dm 2 ), and under middle current density (2 6 A/dm 2 ), the surface morphology of the Al coating is bright, continuous and well adhered. However, when the current density is higher than 7 A/dm 2, the growth of dendritic crystal occurs. Furthermore, phosphate film is a good antioxidant [24], which can prevent the occurrence of oxidation behavior during the experiment. The morphologies of the cross-sections are illustrated in Fig.4. It is apparent that both of the coating/substrate interfaces are well bonded. Their thickness is evaluated by crosssections examination. For the electrodeposition of Al on Al plate pretreated by simple grinding (Fig.4a), the thickness of the coating is approximately 12 µm, while for Al plate pretreated by phosphating (Fig.4b), the thickness of the coating varies from 6 to 12 µm. By comparing Fig.2 with Fig.4, it is observed that there are some cracks on the surface of the coatings due to the effect of the residual stress and the existence of inclusions on the surface of Al, as well as the formation of the gas bubbles. As the growths of the Al coatings are lamellar, so the cracks on the former layer are mostly covered by the following layers, therefore both of the Al coatings are continuous and mostly complete. 3.3 Effect of TMAC on the surface morphology of the coating Preliminary researches have showed that under the same electrodepositing parameters and operational conditions, addition of TMAC and the amount of the addition till 2.5% (mass fraction), the dendrite has been occurred on the surface coating. When the amount of the addition is larger than 2.5% (mass fraction), the dendrite disappears and the coating was smoother as shown in Fig.2. It is sure that the addition of TMAC can prevent the growth of the dendritic crystal effectively. Fig.3 Chemical compositions of the Al coatings measured by EDS: on simple grinding aluminum plate (a) and on phosphating treated Al plate (b).

5 447 Fig.4 Morphologies of the cross-sections of the coating/matrix interface: (a) aluminum electrodeposited on simple grinding aluminum plate; (b) aluminum electrodeposited on phosphating treated aluminum plate. 3.4 Effect of anode activation on electrodeposition After anode activation, the surface layer of the Al exhibited many holes of the nanometer size in diameter. As mentioned in the literature [28], the electrodeposition was controlled by nucleation and growth processes of the coating, which then affect the shape of the coating. Increasing the time of anode activation resulted in the increase of the holes size and the surface layer becomes rough, which is favorable to the nucleation and growth of the coating, and consequently improved the adhesion of the achieved coating. 3.5 Binding force Splat-cooling and bending methods were used to measure the binding force between the coating and the substrate. After electrodeposition, the aluminum cathode was immediately placed into a mixture of ice and water, the interface between the coating and substrate observed shows that the coating combined well with substrate, without any cracks and spallings. The bending test was performed by bending one side of aluminum cathode to 180 degree, and the coating surface do not exhibit any fracture and the Al cathode remain smooth. 4 Conclusion The aluminum coatings were obtained successfully on simple grinding and phosphating treated Al plates in AlCl 3 -NaCl-KCl molten salt. The Al coatings are compacted and continuous, the average thicknesses are 12 and 9 µm for both aluminum plates pretreated by simple grinding and phosphating, respectively. The addition of TMAC may effectively prevent the growth of dendritic crystal, and the anode activation improves the adhesion of the coating. Binding force analysis shows that both of the aluminum coatings are combined well with the substrates. REFERENCES [1] C.K. Fang, C.C. Huang and T.H. Chuang, J Mater Sci 30 (1999) 643. [2] Z.Y. Tian, Met Mater Metall Eng 36 (2008) 3. [3] D. Zhang, G.D. Zhang and Z.Q. Li, Mater China 29 (2010) 1. [4] J. Hu, Y.B. Li and H.L. Wang, Mater Lett 62 (2008) 1715.

6 448 [5] C.Y. Wang, G.H. Wu and Q. Zhang, J Chin Soc Corros Prot 28 (2008) 59. [6] H.J. Greene and F. Mansfeid, Corrosion 53 (1997) 920. [7] B. Torres, M. Campo and A. Ureña, Surf Coat Technol 201 (2007) [8] C.L. He, D.Y. Lou, J.M. Wang and Q.K. Cai, Thin Solid Film 519 (2011) [9] H.J. Greene and F. Mansfeld, Corrosion 53 (1997) 920. [10] Q.Z. Li, Y.M. Tang and Y. Zuo, Mater Chem Phys 120 (2010) 676. [11] A. Pardo, M.C. Merino, R. Arrabal, F. Viejo, M. Carboneras and J.A. Muñoz, Corros Sci 48 (2006) [12] S.W. Tang, J. Hu and X.H. Zhao, Corros Sci 53 (2011) [13] S. Candan, Corros Sci 51 (2009) [14] A.J. López, A. Ureña, M.D. López and J. Rams, Surf Coat Technol 202 (2008) 755. [15] M. Shahid, J Mater Sci Lett 32 (1997) [16] A.J. Trowsdale, B. Noble, S.J. Harris, I.S.R. Gibbins, G.E. Thompson and G.C. Wood, Corros Sci 38 (1996) 177. [17] J. Zhu and L.H. Hihara, Corros Sci 52 (2010) 406. [18] L.P. Zhang, X.J. Yu and Y.H. Dong, Trans Nonferrous Met Soc China 20 (2010) s245. [19] L. Barchi, U. Bardi and S. Caporali, Prog Org Coat 67 (2010) 146. [20] S. Caporali, A. Fossati and A. Lavacchi, Corros Sci 50 (2008) 534. [21] S.M. Zhang and Y.Q. Zhou, Corros Prot 2 (2000) 57. [22] Q.F. Li and Z.X. Qiu, Rare Met Mater Eng 24 (1995) 59. [23] P. Rolland and G. Mamantov, J Electrochem Soc 123 (1976) [24] W.D. Yang, J.F. Huanga and L.Y. Cao, Inorg Chem Ind 41 (2009) 1. [25] X.P. Wang and S. Tian, Rare Met Mater Eng 34 Suppl (2005) 716. [26] J.S. Li, Z.H. Yang and X.H. Wang, J Central South Univ (Sci Technol) 39 (2008) 672. [27] T. Tetsuya, N. Toshiyuki and I. Yasuhiko, Electrochim Acta 47 (2002) [28] Q.Y. Feng, Z.M. Ding and L.S. Jia, Mater Prot 37 (2004) 1.