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Electroless plating of silver on AZ31 magnesium alloy substrate
Surface & Coatings Technology 201 (2007) 4512 – 4517 www.elsevier.com/locate/surfcoat
Electroless plating of silver on AZ31 magnesium alloy substrate Hui Zhao, Jianzhong Cui ⁎ Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang, Liaoning 110004, PR China Received 21 July 2006; accepted in revised form 14 September 2006 Available online 13 November 2006
Abstract Electroless plating of silver on AZ31 magnesium alloys via electroless plating and organic coatings (organosilicon heat-resisting varnish), was studied. The organic coating was made by immersing the samples into organosilicon heat-resisting varnish. The applicability of this method was verified by a subsequent metallization process. In this method the organic coating acted as interlayer between the substrate and silver film. When the reaction started, silver would deposit virtually onto the interlayer surface. X-ray diffraction and SEM analysis were used to investigate the morphology of the interlayer and silver film. The silver film has a rather perfect crystal structure. And the result of the cross-cut test indicates the adhesion between the substrate and interlayer is good enough. The potentiodynamic polarization curves for corrosion studies of the coated magnesium alloys were performed in a corrosive environment of 3.5 wt.% NaCl at neutral pH (7). The result reveals: comparing with the substrate, the corrosion resistance of the coated AZ31 magnesium alloys increases distinctly. Moreover, the silver coating has perfectly antibacterial and decorative properties. The method proposed in this work is environmentally friendly: non-toxic chemicals are used. In addition, it provides a new concept for plating the metals, which are considered difficult to plate due to high reactivity. © 2006 Elsevier B.V. All rights reserved. Keywords: Organic coatings; Silver film; Electroless deposition; Corrosion
1. Introduction It is well known that magnesium is the lightest of all metals in the world. The density of magnesium is one quarter that of steel and only two-thirds that of aluminum. Consequently magnesium base alloys have obviously become the choice for weight reduction in portable microelectronics, telecommunications and aerospace application etc [1–3]. However, a relatively poor resistance to corrosion is a serious impediment against wider application of magnesium alloys. Therefore, some methods must be supplied to improve the corrosion resistance of magnesium and its alloys. One of the most significant ways to protect corrosion is to coat the base materials [4,5]. Among the various technologies available for coating, organic coatings are the most common, simple and cost effective mode of corrosion protection for metallic objects and structures [6]. However the suggestion of a suitable organic coating for a particular system is still a difficult task because the protection time and efficiency of organic coatings are quite limited and ⁎ Corresponding author. Tel.: +86 24 83681738; fax: +86 24 83681758. E-mail address: [email protected] (J. Cui). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.044
strongly related to the environment. In order to improve their protection efficiencies and prepare better quality coatings, their possible uses as primer or top coatings or blends have been studied by many researchers [7–11]. In recent years, electroless plating has attracted attention due to its virtues [8–10]. In the process of electroless deposition, a sharp edge receives the same thickness of deposit as does a blind hole. There are many reports about depositing silver on material surfaces [11–15]. Silver is recently promoted as a potential candidate due to its low room temperature bulk resistively (1.59 μΩ cm), relatively high melting point and expected higher electromigration resistance in comparison to the common Al and Cu metallization. In addition, silver has perfectly antibacterial and decorative properties. Nevertheless, the published literature about electroless deposited silver onto the surfaces of magnesium and its alloys is rather scanty. It is probably because electroless deposited directly silver on magnesium is difficult to carry out due to its high reactivity. In this paper, we produce a new method of coating silver film on the magnesium alloys via electroless deposition and organic protection coating. We present experimental data about coating process, microstructure of the interlayer and the silver film, and corrosion resistance of the coated samples.
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After plating, the samples were rinsed quickly with deionized water and dried in the oven with air blower at room temperature.
2. Experimental 2.1. Specimen preparation In this study the substrates used for the silver films were AZ31 rolling sheets. They were supplied by Yingkou magnesium alloys Factory, China. The sheets were cut into strips of about 4 × 6 cm in size. The interlayer used was a kind of commercial product, 8604 organosilicon heat-resisting varnish (purchased from Changjiang Paint co, LTD, Jiangsu, China), which has favorable water proof, salts mist resistance, heat resistance. The process of electroless deposition was a five-stage procedure, including pre-cleaning, coating interlayer, surface roughing, surface sensitization and electroless deposition. Precleaning was carried out by immersion of the substrate in 30 ml l− 1 of HNO3 aqueous solution at room temperature for 1.5 min. After that, the samples were cleaned by water and dried completely in the drying oven. Coating interlayer was conducted via using the 8604 organosilicon heat-resisting varnish. In this process, firstly, submersed vertically the samples in the varnish; secondly, hung them in air for the sake of surface drying. The time was about 30 min; and then dried them at 180 °C for 1 h in the drying oven; finally, took out the samples and repeated above-mentioned coating steps once. Surface roughing solution was made up of 600 g l− 1 NaOH aqueous solution. The time of roughing was controlled to 30 min. Surface sensitization was conducted by immersion of the samples in an aqueous solution containing 20 ml/l HCl and 10 g/l SnCl2 at 30 °C for 8 min. After that, the specimens were rinsed in deionized water. Lastly, the specimens were immersed in the electroless plating bath which contained silver salts solution A; glucose-based reducing agent B; and deionized water; at room temperature for 30 min [16]. The two solutions A and B were mixed in a 1/1(v/v) ratio just before the bath was used. The reaction can be written as: nAgþ C6 H12 O6 þ 2=3nOH− →nAg↓ þ 1=2nRCOO− þ nH2O The specific process of the electroless plating is illustrated schematically in Fig. 1.
2.2. Specimen characterization Morphological analysis was conducted on the surfaces before and after etching the samples coated with interlayer, and electroless plating by a SSX-550 SEM with a BSE detector. The components in the deposited films were determined by an energy dispersed X-ray micro-analyzer (EDX) coupled to the SEM. Moreover, the morphological of the cross-section of the samples with interlayer was conducted. These specimens were fixed between two magnesium alloy sheets. Afterwards, the samples was sanded (with sandpaper of different grain size), followed by polishing with MgO2. Nonconducting specimens were coated with gold prior to analysis. The cross-cut test was employed to study the adhesion between the interlayer and the substrate. In the cross-cut test, two sets of 6 cuts with a spacing of 1 mm were made perpendicular to each other, thus making a lattice of 25 small blocks. Then a standardized tape was stuck on the lattice and pulled off with a constant force. Cross-cut adhesion was evaluated according to GB/T 9286-1998 [17] (eqv ISO 2409:1992). X-ray diffraction (XRD) analysis of before and after metallized samples was preformed with an X-ray diffractometer (X'Pert Pro MPD).The incident radiation was Cu kα. Data were collected form 2θ = 20 to 90°, in a θ/2θ geometry. For studying the corrosion resistance of samples coated with coatings, the potentiodynamic polarization curves were used. The samples were prepared by initially attaching an electrical connection wire to the surface with conducting glue. 1 cm × 1 cm surface of the sample was exposed and the rest was sealed with silica gel. The experiment was carried out in a corrosive environment of 3.5 wt.% NaCl aqueous solution (pH 6.9) at room temperature. The solution was prepared with AR grade NaCl. The potential of the sample was swept using the Autolab PGSTAT 302 electrochemical measurement system. The polarization started from about − 1750 to 250 mv at a scan rate of 1 mv/s to construct the Tafel plots (logarithmic variation
Fig. 1. Schematic diagram illustrating the process of the electroless plating.
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of current as a function of voltage) and derive the anodic and catholic Tafel constants. A standard calomel electrode (SCE) was used as a reference electrode and platinum mesh was used as a counter electrode. The samples were immersed in solution for 15 min prior to each polarization test, allowing the system to be stabilized. 3. Results and discussion A major source of the loss of corrosion protection in paint films is non-uniform film properties of defects in films [18]. Fig. 2 shows the cross-section SEM picture of the substrate after coating. The film is compact and symmetrical. The thickness of the film is about 45 μm. This figure depicts the interlayer homogeneously distributes onto the surface of the substrate. And the outcome of the cross-cut testing is 2. The value reveals that the adhesion between the interlayer and the substrate is good enough. Fig. 3 shows the micrographs of the interlayer surface before and after 30 min etching. The non-etched sample (Fig. 3a) represents a surface morphology preferentially amorphous. The surface is awfully hydrophobe and smooth. However, it is unfavorable to subsequent plating. The aim of the etching is to increase the hydrophilic character and roughness of the interlayer surface, allowing anchorage of the silver film during the following plating process. After 30 min etching, it is apparent that an increase in the micro-roughness occurs on the surface (Fig. 3b). The surface activity of the interlayer is generally low. Therefore, it must be sensitized. After etching, the samples were sensitized. In this way, a lot of stannous ions were adsorbed on the sample surface. These ions would ensure the electroless plating to conduct on the sample surface. After surface sensitization, the samples were completely immersed in the plating solution. It ensured the sample would be coated evenly. Fig. 4 shows the surface morphology of the silver film for different plating times. As described in Refs. [13,19], all phase transformations, including coating, involve the processes of nucleation and growth. During the earliest stage of film formation, a sufficient number of atoms or molecules condense and establish a permanent residence on the substrate. This is the so-called nucleation stage, followed by the growth stage. However, it is difficult to separate the two processes
Fig. 2. SEM photograph showing the cross sections of the interlayer on the substrate.
Fig. 3. SEM micrographs of the interlayer surface before a), and after b) 30 min etching.
because of the unclear demarcation between the end of nucleation and the onset of nucleus growth. Film formation is classified into three basic categories: 1) island (or Volmer– Weber) mode, 2) layer (or Frank–van der Merwe) mode, 3) Straski–Krastanov mode (a combination of island and layer mode). Island growth occurs when the smallest stable clusters nucleate on the substrate and grow in three dimensions to form islands. This happens when atoms or molecules in the deposit are more strongly bound to each other than to the substrate. Many system of metals on insulators, alkali halide crystals, graphite, and mica substrates display this mode [19]. In this experiment, the interlayer is an insulator and the silver grains directly deposit on the interlayer surface. Therefore, silver deposition in this study falls in this category. When plated for 2 min, silver grains deposited on the sample surface (Fig. 4a and a′). The existence of silver was further confirmed by EDX element analysis (Fig. 4e). The EDX spectrum shows that the sample plated for 2 min scarcely contains carbon, oxygen, silicon and silver elements. It means that silver grains have deposited on the surface after plated for 2 min. Fig. 4a″ shows a back-scattered electron (BSE) image taken the same region as the SEM image (Fig. 4a′). In BSE images mode, image contrast is very sensitive to the chemical composition or the atomic number of the species present in the sample. Moreover, the result of EDX analysis (Fig. 4e) indicates that there are silver grains on the surface. Thus, as can be seen from Fig. 4a″, those white dots with similitude size are silver island. They are generally small and separated from each other, and randomly disperse on the sample surface. In the early stage, the island density is low, coming atoms would rather form new nucleus than join other islands, and hence the islands grow slowly in
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Fig. 4. SEM micrographs showing the growth of silver grains plating with time a) 2 min, b) 6 min, c) 12 min and d) 25 min. Note that a′), b′), c′) and d′) are the corresponding images with high magnifications; a″) BSE micrograph taken the same region as a′ on the sample plated for 2 min; and e) EDX spectrum of the sample plated for 2 min.
size but rapidly in number. Actually, at 2 min, without plating gold on the surface of the sample, the SEM images are misty. It is in agreement with the result observed in BSE. Soon, the small
island density saturates. In the following stage, most new deposited silver atoms can find a favorable island to join. The number of islands is almost fixed, and grows linearly with
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plating time (shown in Fig 4b and b′). Nevertheless, the surface plated for 6 min is still nonconducting (the SEM images are misty if the surface is not coated with gold). It indicates that the continuous film is not formed. Gradually, the growth of the islands decreases with size. When the islands are large to meet each other, they just overlap each other instead of merging. As can be seen in Fig.4c and c′, the continuous film is formed (the surface is conducting). After the first layer of sliver film is formed, silver grains deposit and grow to form many clusters on it (Fig 4d and d′). Finally, during the prolonged plating, silver ions in the solution become diluted and eventually reach a low level at which the whole reaction will be ceased. X-ray diffraction patterns are shown in Fig. 5. The substrates were plated in the same solution respectively for 2 min, 6 min, 15 min and 24 min. All the peaks other than those of silver are attributed to the interlayer and substrate (Fig. 5a). And with the increasing plating time, the intensities of these peaks are decreased, indicating the growth of silver film. As can be seen in Fig. 5, all types of silver deposit have a rather perfect crystal structure, especially in the direction Ag (111). The intensity of Ag (111) has obviously increased from 2 min to 24 min plating. At the beginning of the plating, there are only the peaks of Ag (111) (Fig. 5b). After plating for 6 min, there is a growing trend in the direction Ag (200) (Fig. 5c). And after plating for 15 min, an apparent Ag (200) peak appears (Fig. 5d). The intensity of the peak in the direction Ag (200) increases with plating time. There are no silver peaks of other direction until after plating for 15 min (Fig. 5d and e). Furthermore, comparing the curves in Fig. 5, there is no significant peak shift of silver, suggesting a well-crystallized lattice structure. Tafel polarization curves were employed to evaluate the effect of the coatings on the corrosion resistance characteristics of magnesium alloys. The standard electrode potential of pure magnesium was known to the lowest among engineering metallic materials, implying its poor corrosion resistance. Fig. 6 shows the polarization curves of the sample plated for 24 min and AZ31 sheet in a 3.5-wt.% NaCl solution. From these curves, it can be obviously seen, the corrosion potential (Ecorr) of the AZ31 sheet and silver coated sample are − 1.569 and − 1.259 v, respectively. There is about a 300-mv increase in the
Fig. 5. X-ray diffraction patterns of a) the uncoated sample, b)–e) silver films coated on the samples for different time.
Fig. 6. The potentiodynamic polarization curves of a) AZ31 sheets and b) the sample plated for 24 min in 3.5% NaCl at 30 °C.
matter of the Ecorr. The effect of the coatings on the polarization of the AZ31 sheet is clear in moving the Ecorr to the noble direction. It is also noted that once the scanned potential exceeded Ecorr, corrosion current density of the samples continually increased. However, the increase of the corrosion current density of the coated sheet is largely lower than that of the AZ31 alloy sheet. Furthermore, the corrosion current density of the coated sheet greatly stabilized when the scanned potential exceeded about − 0.6 v. All these explained that the coated samples had better corrosion resistance and protection characterization than the substrate. Compared with the as received samples, the corrosion sensibility of the coated sample in NaCl solution effectively decreased. 4. Conclusions We have developed a novel approach to coat magnesium alloys. The optimum silver film on the surface of magnesium alloys has been achieved by using electroless deposition and organic coatings. The compact interlayer homogeneously distributes onto the surface of the substrate. And the value of cross-cut test indicates that the adhesion between the substrate and interlayer is good enough. To improve the combination between the silver film and the interlayer, etching has been employed to increase surface roughness of the interlayer. The silver film growth process has been studied by SEM and X-ray diffraction investigations. The results of the corrosion test indicate that the corrosion of the coated samples is enhanced. Organic coating acts as the interlayer in this method and silver particles will deposit virtually on the surface of interlayer. The interlayer insulates the substrate and film. In this way, once the silver film broken, the interlayer can restrain the occurrence of double action between the substrate and the film. The method proposed in the paper can realize to directly plate other materials on the magnesium alloys, such as Cu and Ni or its alloys. In addition, it also can be used for plating the metals with high reactivity. The detailed research on this aspect is still under study.
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Acknowledgment The authors thank Yingkou Magnesium Alloy Factory for donating AZ31 magnesium alloy sheets. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. 302 (2001) 37. [2] G.S. Cole, A.M. Sherman, Mater. Charact. 35 (1995) 3. [3] A. Yfantis, I. Paloumpa, D. Schmeiber, D. Yfantis, Surf. Coat. Technol. 151–152 (2002) 400. [4] J. Jutta Majumdar, R. Galun, B.L. Mordike, I. Manna, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 361 (2003) 119. [5] Rajan Ambat, W. Zhou, Surf. Coat. Technol. 179 (2004) 124. [6] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [7] Tunç Tüken, Prog. Org. Coat. 55 (2006) 60. [8] Yi Li, Qinghua Lu, Xuefeng Qian, Zikang Zhu, Jie Yin, Appl. Surf. Sci. 233 (2004) 299. [9] W.C. Wang, R. Hvora, E.T. Kang, K.G. Neoh, Polym. Eng. Sci. 44 (2) (February 2004) 362.
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[10] S.C. Domenech, E. Lima Jr., V. Drago, J.C. De Lima, Appl. Surf. Sci. 220 (2003) 238. [11] Antonio Pérez Padilla, Jorge A. Rodríguez, Hugo A. Saitǘa, Desalination 114 (1997) 203. [12] J.E. Gray, P.R.K. Griffiths, Thin Solid Films 484 (2005) 196. [13] Fang Mei, Donglu Shi, Tsinghua Sci. Technol. 10 (6) (December 2005) 680. [14] Thin-Chia Huang, Ming-Chi Wei, Huey-Ing Chen, Sep. Purif. Technol. 32 (2003) 239. [15] V. Bogush, A. Inberg, N. Croitoru, V. Dubin, Y. Shachanm-Diamand, Microelectron. Eng. 70 (2003) 489. [16] Ning Li, Electroless Plating Utility Technology, 2004. [17] GB/T 9286-1998 eqv ISO 2409: Paints and varnishes-cross cut test for films, GB/T, China, 1999. [18] Gordon P. Bierwagen, Reflections on corrosion control by organic coatings, Prog. Org. Coat. 28 (1996) 43. [19] M. Ohring, The Materials Science of Thin Films, Academic Press, London, 1993.
Electroless plating of silver on AZ31 magnesium alloy substrate Hui Zhao, Jianzhong Cui ⁎ Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang, Liaoning 110004, PR China Received 21 July 2006; accepted in revised form 14 September 2006 Available online 13 November 2006
Abstract Electroless plating of silver on AZ31 magnesium alloys via electroless plating and organic coatings (organosilicon heat-resisting varnish), was studied. The organic coating was made by immersing the samples into organosilicon heat-resisting varnish. The applicability of this method was verified by a subsequent metallization process. In this method the organic coating acted as interlayer between the substrate and silver film. When the reaction started, silver would deposit virtually onto the interlayer surface. X-ray diffraction and SEM analysis were used to investigate the morphology of the interlayer and silver film. The silver film has a rather perfect crystal structure. And the result of the cross-cut test indicates the adhesion between the substrate and interlayer is good enough. The potentiodynamic polarization curves for corrosion studies of the coated magnesium alloys were performed in a corrosive environment of 3.5 wt.% NaCl at neutral pH (7). The result reveals: comparing with the substrate, the corrosion resistance of the coated AZ31 magnesium alloys increases distinctly. Moreover, the silver coating has perfectly antibacterial and decorative properties. The method proposed in this work is environmentally friendly: non-toxic chemicals are used. In addition, it provides a new concept for plating the metals, which are considered difficult to plate due to high reactivity. © 2006 Elsevier B.V. All rights reserved. Keywords: Organic coatings; Silver film; Electroless deposition; Corrosion
1. Introduction It is well known that magnesium is the lightest of all metals in the world. The density of magnesium is one quarter that of steel and only two-thirds that of aluminum. Consequently magnesium base alloys have obviously become the choice for weight reduction in portable microelectronics, telecommunications and aerospace application etc [1–3]. However, a relatively poor resistance to corrosion is a serious impediment against wider application of magnesium alloys. Therefore, some methods must be supplied to improve the corrosion resistance of magnesium and its alloys. One of the most significant ways to protect corrosion is to coat the base materials [4,5]. Among the various technologies available for coating, organic coatings are the most common, simple and cost effective mode of corrosion protection for metallic objects and structures [6]. However the suggestion of a suitable organic coating for a particular system is still a difficult task because the protection time and efficiency of organic coatings are quite limited and ⁎ Corresponding author. Tel.: +86 24 83681738; fax: +86 24 83681758. E-mail address: [email protected] (J. Cui). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.044
strongly related to the environment. In order to improve their protection efficiencies and prepare better quality coatings, their possible uses as primer or top coatings or blends have been studied by many researchers [7–11]. In recent years, electroless plating has attracted attention due to its virtues [8–10]. In the process of electroless deposition, a sharp edge receives the same thickness of deposit as does a blind hole. There are many reports about depositing silver on material surfaces [11–15]. Silver is recently promoted as a potential candidate due to its low room temperature bulk resistively (1.59 μΩ cm), relatively high melting point and expected higher electromigration resistance in comparison to the common Al and Cu metallization. In addition, silver has perfectly antibacterial and decorative properties. Nevertheless, the published literature about electroless deposited silver onto the surfaces of magnesium and its alloys is rather scanty. It is probably because electroless deposited directly silver on magnesium is difficult to carry out due to its high reactivity. In this paper, we produce a new method of coating silver film on the magnesium alloys via electroless deposition and organic protection coating. We present experimental data about coating process, microstructure of the interlayer and the silver film, and corrosion resistance of the coated samples.
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After plating, the samples were rinsed quickly with deionized water and dried in the oven with air blower at room temperature.
2. Experimental 2.1. Specimen preparation In this study the substrates used for the silver films were AZ31 rolling sheets. They were supplied by Yingkou magnesium alloys Factory, China. The sheets were cut into strips of about 4 × 6 cm in size. The interlayer used was a kind of commercial product, 8604 organosilicon heat-resisting varnish (purchased from Changjiang Paint co, LTD, Jiangsu, China), which has favorable water proof, salts mist resistance, heat resistance. The process of electroless deposition was a five-stage procedure, including pre-cleaning, coating interlayer, surface roughing, surface sensitization and electroless deposition. Precleaning was carried out by immersion of the substrate in 30 ml l− 1 of HNO3 aqueous solution at room temperature for 1.5 min. After that, the samples were cleaned by water and dried completely in the drying oven. Coating interlayer was conducted via using the 8604 organosilicon heat-resisting varnish. In this process, firstly, submersed vertically the samples in the varnish; secondly, hung them in air for the sake of surface drying. The time was about 30 min; and then dried them at 180 °C for 1 h in the drying oven; finally, took out the samples and repeated above-mentioned coating steps once. Surface roughing solution was made up of 600 g l− 1 NaOH aqueous solution. The time of roughing was controlled to 30 min. Surface sensitization was conducted by immersion of the samples in an aqueous solution containing 20 ml/l HCl and 10 g/l SnCl2 at 30 °C for 8 min. After that, the specimens were rinsed in deionized water. Lastly, the specimens were immersed in the electroless plating bath which contained silver salts solution A; glucose-based reducing agent B; and deionized water; at room temperature for 30 min [16]. The two solutions A and B were mixed in a 1/1(v/v) ratio just before the bath was used. The reaction can be written as: nAgþ C6 H12 O6 þ 2=3nOH− →nAg↓ þ 1=2nRCOO− þ nH2O The specific process of the electroless plating is illustrated schematically in Fig. 1.
2.2. Specimen characterization Morphological analysis was conducted on the surfaces before and after etching the samples coated with interlayer, and electroless plating by a SSX-550 SEM with a BSE detector. The components in the deposited films were determined by an energy dispersed X-ray micro-analyzer (EDX) coupled to the SEM. Moreover, the morphological of the cross-section of the samples with interlayer was conducted. These specimens were fixed between two magnesium alloy sheets. Afterwards, the samples was sanded (with sandpaper of different grain size), followed by polishing with MgO2. Nonconducting specimens were coated with gold prior to analysis. The cross-cut test was employed to study the adhesion between the interlayer and the substrate. In the cross-cut test, two sets of 6 cuts with a spacing of 1 mm were made perpendicular to each other, thus making a lattice of 25 small blocks. Then a standardized tape was stuck on the lattice and pulled off with a constant force. Cross-cut adhesion was evaluated according to GB/T 9286-1998 [17] (eqv ISO 2409:1992). X-ray diffraction (XRD) analysis of before and after metallized samples was preformed with an X-ray diffractometer (X'Pert Pro MPD).The incident radiation was Cu kα. Data were collected form 2θ = 20 to 90°, in a θ/2θ geometry. For studying the corrosion resistance of samples coated with coatings, the potentiodynamic polarization curves were used. The samples were prepared by initially attaching an electrical connection wire to the surface with conducting glue. 1 cm × 1 cm surface of the sample was exposed and the rest was sealed with silica gel. The experiment was carried out in a corrosive environment of 3.5 wt.% NaCl aqueous solution (pH 6.9) at room temperature. The solution was prepared with AR grade NaCl. The potential of the sample was swept using the Autolab PGSTAT 302 electrochemical measurement system. The polarization started from about − 1750 to 250 mv at a scan rate of 1 mv/s to construct the Tafel plots (logarithmic variation
Fig. 1. Schematic diagram illustrating the process of the electroless plating.
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of current as a function of voltage) and derive the anodic and catholic Tafel constants. A standard calomel electrode (SCE) was used as a reference electrode and platinum mesh was used as a counter electrode. The samples were immersed in solution for 15 min prior to each polarization test, allowing the system to be stabilized. 3. Results and discussion A major source of the loss of corrosion protection in paint films is non-uniform film properties of defects in films [18]. Fig. 2 shows the cross-section SEM picture of the substrate after coating. The film is compact and symmetrical. The thickness of the film is about 45 μm. This figure depicts the interlayer homogeneously distributes onto the surface of the substrate. And the outcome of the cross-cut testing is 2. The value reveals that the adhesion between the interlayer and the substrate is good enough. Fig. 3 shows the micrographs of the interlayer surface before and after 30 min etching. The non-etched sample (Fig. 3a) represents a surface morphology preferentially amorphous. The surface is awfully hydrophobe and smooth. However, it is unfavorable to subsequent plating. The aim of the etching is to increase the hydrophilic character and roughness of the interlayer surface, allowing anchorage of the silver film during the following plating process. After 30 min etching, it is apparent that an increase in the micro-roughness occurs on the surface (Fig. 3b). The surface activity of the interlayer is generally low. Therefore, it must be sensitized. After etching, the samples were sensitized. In this way, a lot of stannous ions were adsorbed on the sample surface. These ions would ensure the electroless plating to conduct on the sample surface. After surface sensitization, the samples were completely immersed in the plating solution. It ensured the sample would be coated evenly. Fig. 4 shows the surface morphology of the silver film for different plating times. As described in Refs. [13,19], all phase transformations, including coating, involve the processes of nucleation and growth. During the earliest stage of film formation, a sufficient number of atoms or molecules condense and establish a permanent residence on the substrate. This is the so-called nucleation stage, followed by the growth stage. However, it is difficult to separate the two processes
Fig. 2. SEM photograph showing the cross sections of the interlayer on the substrate.
Fig. 3. SEM micrographs of the interlayer surface before a), and after b) 30 min etching.
because of the unclear demarcation between the end of nucleation and the onset of nucleus growth. Film formation is classified into three basic categories: 1) island (or Volmer– Weber) mode, 2) layer (or Frank–van der Merwe) mode, 3) Straski–Krastanov mode (a combination of island and layer mode). Island growth occurs when the smallest stable clusters nucleate on the substrate and grow in three dimensions to form islands. This happens when atoms or molecules in the deposit are more strongly bound to each other than to the substrate. Many system of metals on insulators, alkali halide crystals, graphite, and mica substrates display this mode [19]. In this experiment, the interlayer is an insulator and the silver grains directly deposit on the interlayer surface. Therefore, silver deposition in this study falls in this category. When plated for 2 min, silver grains deposited on the sample surface (Fig. 4a and a′). The existence of silver was further confirmed by EDX element analysis (Fig. 4e). The EDX spectrum shows that the sample plated for 2 min scarcely contains carbon, oxygen, silicon and silver elements. It means that silver grains have deposited on the surface after plated for 2 min. Fig. 4a″ shows a back-scattered electron (BSE) image taken the same region as the SEM image (Fig. 4a′). In BSE images mode, image contrast is very sensitive to the chemical composition or the atomic number of the species present in the sample. Moreover, the result of EDX analysis (Fig. 4e) indicates that there are silver grains on the surface. Thus, as can be seen from Fig. 4a″, those white dots with similitude size are silver island. They are generally small and separated from each other, and randomly disperse on the sample surface. In the early stage, the island density is low, coming atoms would rather form new nucleus than join other islands, and hence the islands grow slowly in
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Fig. 4. SEM micrographs showing the growth of silver grains plating with time a) 2 min, b) 6 min, c) 12 min and d) 25 min. Note that a′), b′), c′) and d′) are the corresponding images with high magnifications; a″) BSE micrograph taken the same region as a′ on the sample plated for 2 min; and e) EDX spectrum of the sample plated for 2 min.
size but rapidly in number. Actually, at 2 min, without plating gold on the surface of the sample, the SEM images are misty. It is in agreement with the result observed in BSE. Soon, the small
island density saturates. In the following stage, most new deposited silver atoms can find a favorable island to join. The number of islands is almost fixed, and grows linearly with
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plating time (shown in Fig 4b and b′). Nevertheless, the surface plated for 6 min is still nonconducting (the SEM images are misty if the surface is not coated with gold). It indicates that the continuous film is not formed. Gradually, the growth of the islands decreases with size. When the islands are large to meet each other, they just overlap each other instead of merging. As can be seen in Fig.4c and c′, the continuous film is formed (the surface is conducting). After the first layer of sliver film is formed, silver grains deposit and grow to form many clusters on it (Fig 4d and d′). Finally, during the prolonged plating, silver ions in the solution become diluted and eventually reach a low level at which the whole reaction will be ceased. X-ray diffraction patterns are shown in Fig. 5. The substrates were plated in the same solution respectively for 2 min, 6 min, 15 min and 24 min. All the peaks other than those of silver are attributed to the interlayer and substrate (Fig. 5a). And with the increasing plating time, the intensities of these peaks are decreased, indicating the growth of silver film. As can be seen in Fig. 5, all types of silver deposit have a rather perfect crystal structure, especially in the direction Ag (111). The intensity of Ag (111) has obviously increased from 2 min to 24 min plating. At the beginning of the plating, there are only the peaks of Ag (111) (Fig. 5b). After plating for 6 min, there is a growing trend in the direction Ag (200) (Fig. 5c). And after plating for 15 min, an apparent Ag (200) peak appears (Fig. 5d). The intensity of the peak in the direction Ag (200) increases with plating time. There are no silver peaks of other direction until after plating for 15 min (Fig. 5d and e). Furthermore, comparing the curves in Fig. 5, there is no significant peak shift of silver, suggesting a well-crystallized lattice structure. Tafel polarization curves were employed to evaluate the effect of the coatings on the corrosion resistance characteristics of magnesium alloys. The standard electrode potential of pure magnesium was known to the lowest among engineering metallic materials, implying its poor corrosion resistance. Fig. 6 shows the polarization curves of the sample plated for 24 min and AZ31 sheet in a 3.5-wt.% NaCl solution. From these curves, it can be obviously seen, the corrosion potential (Ecorr) of the AZ31 sheet and silver coated sample are − 1.569 and − 1.259 v, respectively. There is about a 300-mv increase in the
Fig. 5. X-ray diffraction patterns of a) the uncoated sample, b)–e) silver films coated on the samples for different time.
Fig. 6. The potentiodynamic polarization curves of a) AZ31 sheets and b) the sample plated for 24 min in 3.5% NaCl at 30 °C.
matter of the Ecorr. The effect of the coatings on the polarization of the AZ31 sheet is clear in moving the Ecorr to the noble direction. It is also noted that once the scanned potential exceeded Ecorr, corrosion current density of the samples continually increased. However, the increase of the corrosion current density of the coated sheet is largely lower than that of the AZ31 alloy sheet. Furthermore, the corrosion current density of the coated sheet greatly stabilized when the scanned potential exceeded about − 0.6 v. All these explained that the coated samples had better corrosion resistance and protection characterization than the substrate. Compared with the as received samples, the corrosion sensibility of the coated sample in NaCl solution effectively decreased. 4. Conclusions We have developed a novel approach to coat magnesium alloys. The optimum silver film on the surface of magnesium alloys has been achieved by using electroless deposition and organic coatings. The compact interlayer homogeneously distributes onto the surface of the substrate. And the value of cross-cut test indicates that the adhesion between the substrate and interlayer is good enough. To improve the combination between the silver film and the interlayer, etching has been employed to increase surface roughness of the interlayer. The silver film growth process has been studied by SEM and X-ray diffraction investigations. The results of the corrosion test indicate that the corrosion of the coated samples is enhanced. Organic coating acts as the interlayer in this method and silver particles will deposit virtually on the surface of interlayer. The interlayer insulates the substrate and film. In this way, once the silver film broken, the interlayer can restrain the occurrence of double action between the substrate and the film. The method proposed in the paper can realize to directly plate other materials on the magnesium alloys, such as Cu and Ni or its alloys. In addition, it also can be used for plating the metals with high reactivity. The detailed research on this aspect is still under study.
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Acknowledgment The authors thank Yingkou Magnesium Alloy Factory for donating AZ31 magnesium alloy sheets. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. 302 (2001) 37. [2] G.S. Cole, A.M. Sherman, Mater. Charact. 35 (1995) 3. [3] A. Yfantis, I. Paloumpa, D. Schmeiber, D. Yfantis, Surf. Coat. Technol. 151–152 (2002) 400. [4] J. Jutta Majumdar, R. Galun, B.L. Mordike, I. Manna, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 361 (2003) 119. [5] Rajan Ambat, W. Zhou, Surf. Coat. Technol. 179 (2004) 124. [6] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [7] Tunç Tüken, Prog. Org. Coat. 55 (2006) 60. [8] Yi Li, Qinghua Lu, Xuefeng Qian, Zikang Zhu, Jie Yin, Appl. Surf. Sci. 233 (2004) 299. [9] W.C. Wang, R. Hvora, E.T. Kang, K.G. Neoh, Polym. Eng. Sci. 44 (2) (February 2004) 362.
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