1. INTRODUCTION:

Aluminium and its alloys are widely used in aerospace and defence industries because of their relatively low densities and moderate strength. They are in their passive state in the pH range of 4-8.5 due to naturally formed barrier oxide film on their surface. This oxide film is 2-3 nm thick and is strongly bonded to the substrate. The oxide film can re-form immediately when damaged in most environments. However, when exposed to aggressive environments containing halides, aluminium and its alloys are susceptible to localized corrosion, such as pitting and crevice corrosion.

Various surface treatments, such as chemical conversion coatings, have been developed to protect Al alloys from corrosion. Among them, tin coatings for Al alloys are considered to be environmentally acceptable. There have been some studies on the formation and characterization of tin coatings deposited on Al and its alloys by the simple immersion method^1-3.

Immersion deposition is the simplest chemical process and a favorable process to control a very small amount of deposition. Immersion plating involves a cementation process where deposition takes place by charge transfer.

Although the plated deposits are mostly porous, the advantages of immersion plating are its simplicity and minor capital expense for equipment. Such plating is often used for aluminium substrates. Aluminium is known to cause problems in plating due to the presence of adherent oxide coating over its surface. To overcome this difficulty, immersion plating of Zn onto Al (zincating) is carried out prior to electroplating of any metal. However, Annamali^4,5avoided zincating and successfully deposited copper on to aluminium using Cl^- ion for destroying oxide film on Al surface.

The application of this immersion coating allows considerable improvement of the adhesion of the metal coating to the substrate, as well as in promoting the corrosion resistance of aluminium articles. The basis of this method is an exchange reaction leading to the dissolution of the electronegative metal phase (Al) and deposition of a more electropositive metal (e.g., Sn) during the so called ‘immersion (stannate) treatment’ of aluminium.

Tin coatings have been around for several years, both electrolytic and immersion. Both have suffered from several problems. These include whiskers and dendritic growth associated with high coating thickness, poor process control and high levels of copper/tin intermetallic formation. These impact the solderability performance of the coating. Hence the development of a novel immersion tin process to overcome these challenges and additionally meet the requirements of lead free soldering is need of the hour.

Number of organic and inorganic compounds was synthesized by using microwave heating^{6, 7}. It was reported by many researchers that the rate and efficiency of the reaction can be increased by using microwave heating^{8, 9}. Based on thorough literature survey, we came to know that there is no report on microwave synthesis of immersion deposition. This tempted us to use microwave for the immersion deposition of tin on aluminium.

In the present study, tin immersion coatings were applied to aluminium at various temperatures, time intervals from various bath compositions by using microwave heating. The surface properties, corrosion behaviour and characterization of the tin coated samples were studied. The influence of the process parameters and surface characteristics on the corrosion behaviour of tin coated samples was studied.

2. EXPERIMENTAL:

Rectangular samples of A1100 Al alloy of size 7 cm x 2 cm x 0.1 cm were used as substrate materials. All the chemicals used were of AR grade obtained from MERCH, India. Prior to the experiments, the polished front surfaces of panels were degreased with acetone. The natural oxide film adhered on aluminium sheet was dissolved in 5% sodium hydroxide for 5 min at 30^°C, and then immersed into 20% HNO₃ for 2 min, washed in running water followed by rinsing with distilled water and then dried and weighed (w₁).

Plating of tin was carried out by immersing the preweighed aluminium specimens (w₁) in bath containing (1–5 g/l) SnCl₂,30 ml/l HCl and 200 ml/l H₃PO₄and irradiating microwaves at 1/rpm at various bath temperatures and time durations. The bath solutions were prepared using triply distilled water. After the treatment, the specimens were washed with tap water, rinsed with deionized water, dried and weighed (w₂).

The details concerning the operation of the bath can be found in the paper of Huttunen-Saarivirta¹⁰. The bath contained hydrochloric acid for pH adjustment, phosphoric acid for bath stabilisation and stannous chloride for delivering Sn²⁺ions. Deposition times of 30, 60, 90, 120 and 150s were used at the bath temperature of 30, 40, and 50°C. The deposited aluminium specimens were then rinsed with deionised water dried overnight and then re-weighed (w₂). The weight difference of the plate before and after immersion deposition gives the weight of tin deposits.

2.1. Corrosion studies:

The corrosion resistance of the coating was studied by electrochemical impedance spectroscopy and Tafel polarization studies. All electrochemical measurements were performed using Electrochemical Workstation (Model No. CHI 760, CH instruments, USA) and all experiments were carried out at a constant temperature of 28±2°C. The cell consists of a three electrode system consists of a platinum electrode and a saturated calomel electrode (SCE) which were used as auxiliary and reference electrode. The working electrodes consists the tin coated and uncoated aluminium of 1cm²area. In order to minimize ohomic potential drop, the tip of the reference electrode was positioned very close to the surface of working electrodes by using a fine luggin capillary. The potentiodynamic polarization studies were carried out in 3.5% NaCl solution at a scan rate of 0.01 mV s^-1. In all cases, the OCP (Open circuit potential) was established first and then the polarization measurements were carried out. The polarization curves for aluminium in test solution with and without tin coating were recorded from -400 to -2000 mV. Electrochemical impedance studies were carried out with the same setup used for potentiodynamic polarization studies. The applied AC perturbation signal was about 10mV within the frequency range 100 kHz to 1 Hz. Based on the equivalent circuit, the R_p and capacitance were evaluated.

2.2. Surface examinations:

The surface morphology and topography of the tin immersion coatings were observed by scanning electron microscope (model: SEM-JSM-6390). All the samples tested with SEM were sputtered with thin gold film to prevent surface charging effects. The phase composition of the coatings was analyzed by X-Ray Diffraction.

3. Results and discussions:

3.1. Corrosion behaviour of the immersion coating:

The corrosion behaviour of the immersion coatings formed in various immersion conditions were evaluated through potentiodynamic polarization technique and electrochemical impedance spectroscopy. The polarization curve and niquist diagram are shown in figure 1(a), 2(a), 3(a) and 1(b), 2(b), 3(b). The corrosion potential (E_corr), corrosion current density (I_corr), rate of corrosion and polarization resistance (R_p) values were determined from Stern-Geary equation and the results are represented in table 1 (a-d).

R_p =

The shape of the Nyquist diagram is similar for all samples and the shape is like a semi circle. The impedance data are mainly capacitive. The Nyquist diagram for the coated sample has semi circle with a larger diameter and higher corrosion resistance compared to those of the uncoated sample.

As seen from the equivalent circuit, the impedance of the measured system between reference electrode (SCE) and working electrode (tin coated) consisted 3 parts; electrolyte, outer porous layer and inner compact barrier layer. The equivalent circuit consists of two R components in series with the solution resistance (R_s), and the outer layer resistance R_p parellel with capacitance C_dl. From the EIS curves, it can be seen that, the corrosion resistance of tin coated aluminium electrodes is four orders of magnitude than that of bare aluminium electrode.

Figure 1. Simplified equivalent circuit used for impedance data fitting of aluminium.

3.1.1. Effect of treatment time on the corrosion parameters

Figure 1(a) shows the Tafel polarization curves of the tin immersion coatings at various treatment times. The electrochemical behaviour of the coated and uncoated samples were studied by recording the anodic and cathodic potentiodynamic curves for various time intervals (30s, 60s, 90s, 120s and 150s) reported in figure 1(a). For bare aluminium the potential increases in the anodic region, which is characteristic of an active state and dissolution of the aluminium alloy. This correlates with observed fluctuations in the corrosion potential curve of the untreated sample. For the untreated sample, a passive plateau is observed at a potential of -1.1V.

Figure 1(a) Tafel polarization curve for tin immersion coatings obtained on aluminium under various treatment time (30 – 150 s).

The samples coated with tin exhibit a large passivation plateau compared to the sample without tin. So the tin coated samples are stable and protective in a domain of potentials between -0.5 to -0.75 V. The corrosion resistance of the samples with and without tin immersion coatings was evidently different. In contrast to the bare aluminium, the samples with tin immersion coatings all had the positive corrosion potential and lesser corrosion current density which is shown in Figure 1(a) and more polarization resistance.

The corrosion parameters of the immersion coating as a function of immersion time are presented in table 1(a). From the table, it can be observed that corrosion current density (I_corr) decreases on increasing the treatment time from 30 to 150s and corrosion potential (E_corr) and polarization resistance (R_p) increases as a function of immersion time. It has been shown that when the Al alloy is immersed in a coating solution, it dissolves and a corrosion film is formed at the initial stage of coating process^1-3. Subsequent metal dissolution then proceeds through this initially formed porous corrosion film. Thinning of the corrosion film also occurs simultaneously during metal dissolution². Continuous metal dissolution provides a sufficient concentration of Al³⁺ ions for the coating film deposition. Hence better corrosion resistance was obtained at 150s time of immersion.

Table 1(a) Effect of treatment time (30 – 150 s) on tafel parameters at 40°C.

Time (s)	E_corr(V)	I_corr (A/cm²)	R_corr (mpy)	R_p(W/cm²)
Bare	- 1.028	1.8509x10^-5	5.3676 x10^-9	11030
30	- 0.727	2.9950x10^-6	8.6855 x10^-10	111238
60	- 0.699	2.9826x10^-6	8.6495 x10^-10	132832
90	- 0.690	2.1468 x10^-6	6.225 x10^-10	287073
120	- 0.680	1.9861x10^-6	5.7596 x10^-10	320657
150	- 0.685	1.2390x10^-6	3.5931 x10^-10	526139

The corrosion resistance of the coated and uncoated samples determined from the magnitude of the impedance data at 40ºC is shown in figure 1(b) as a function of immersion time. The impedance parameters are given in the table 1(b). It can be seen that the resistance is very much higher for the coated samples than for the bare sample as expected, i.e., > 2000 W at short times (to more than 60s) to > 8000 W at very long times (to more than 120s). The corrosion resistance increases with increase of coating time from 30 s to 150s. It was suggested that once a layer of deposits was formed, deposition would thereafter proceed to the second layer over the first one resulting a strong coating over the surface. So the thickness increases with increasing time intervals.

Figure 1 (b) Niquist Impedance Spectra analysis for tin immersion coatings obtained on aluminium under various treatment treatment time (30 – 150 s).

Table 1 (b) Effect of treatment time (30 – 150 s) on impedance parameters at 40°C.

Treatment time (s)	R_s(W/cm²)	C_dl(F)	R_p (W/cm²)
Bare	1.92	21.82	185
30	5.269	11.52	611
60	5.759	9.312	2611
90	5.968	11.69	4615
120	6.979	10.12	5870
150	4.597	8.286	8677

3.1.2. Effect of treatment temperature on the corrosion parameters:

Figure 2(a) shows the Tafel polarization curves of the tin immersion coatings at various treatment temperatures. On increasing the temperature from 30 to 40°C, there is a rapid increase in potential from -0.7V to -0.55V. This indicates that on increasing the temperature from 30 to 40°C the coating formation is rapid and anodic corrosion occurs, so the potential increases. But, on further increasing the temperature from 40 to 50°C, there is sudden fall in potential to -0.65V. This indicates that at 50°C, the surface corrosion is initiated, so there is fall in plating rate as well as potential. Hence the optimum temperature for formation of more corrosion resistant immersion coating is 40°C.

Figure 2 (a) Tafel polarization curve for tin immersion coatings obtained on aluminium under various treatment temperatures (30-50ºC).

The corrosion parameters of the immersion coating as a function of bath temperatures are presented in table 2(a). From the table, it can be observed that the corrosion potential (E_corr) increases and corrosion current density (I_corr) decreases on increasing the temperature from 30 to 40°C and further increase of temperature to 50°C, the values are reversed. The polarization resistance (R_p) increases up to 40°C and then decreases. Surface corrosion is initiated at 45°C and is enhanced at higher temperature. Enhanced surface corrosion is also marked by sudden fall in the plating rate.

Table 2(a) Effect of temperature (30-50ºC) on tafel parameters for 150s.

Temp. (°C)	E_corr(V)	I_corr (A/cm²)	R_corr (mpy)	R_p(W/cm²)
Bare	- 1.028	1.8180x10^-5	5.3676x10^-9	11030
30	- 0.835	2.8708x10^-6	8.3253x10^-10	232750
40	- 0.685	1.2390x10^-6	3.5931 x10^-10	526139
50	- 0.749	2.0835x10^-6	6.0421x10^-10	343660

The corrosion resistance of the coated and uncoated samples determined from the magnitude of the impedance data at 150sis shown in figure 2(b) as a function of treatment temperatures. The impedance parameters are given in the table 2(b). On increasing the temperature from 30 to 40°C, the corrosion resistance was increases rapidly (> 8000W). But on further increasing the temperature to 50°C the corrosion resistance is decreased (> 5000W). This is due to the enhanced surface corrosion arises above 40°C. So there is fall in the plating rate and subsequently corrosion resistance was decreased.

Figure 2 (b) Niquist Impedance Spectra analysis for tin immersion coatings obtained on aluminium under various treatment temperatures (30-50ºC).

Table 2(b) Effect of treatment temperature (30-50ºC) on impedance for 150s.

Temp. (°C)	R_s(W/cm²)	C_dl(F)	R_p (W/cm²)
Bare	1.92	21.82	185.3
30°C	6.059	14.06	4293
40°C	4.597	8.286	8677
50°C	2.857	10.39	5317

3.1.3. Effect of tin concentration on the corrosion parameters:

Figure 3(a) shows the Tafel polarization curves of the tin immersion coatings at various tin concentrations. The potential moves towards anodic in the case of higher tin concentration, but towards cathodic direction at lowest tin concentration. The Sn containing layer is very stable and protective over a long range of anodic potential. On increasing the concentration from 1 to 3 g/l, the potential increases from -0.7 to -0.65V. This indicates that on increasing the concentration of tin, more number of Sn²⁺ ions are formed. This enhances the plating rate and so the potential increases. On further increasing the tin concentration to 5 g/l the potential increases and reaching to -0.55V.

Figure 3 (a) Tafel polarization curve for tin immersion coatings obtained on aluminium under various Sn²⁺concentrations (1-5 g/l).

Table 3 (a) Effect of concentration of Sn²⁺ion (1-5 g/l) on tafel parameters for 150s at 40⁰C

Conc. (g/lit)	E_corr(V)	I_corr(A/cm²)	R_corr (mpy)	R_p(W/cm²)
Bare	 1.028	1.8509x10^-5	5.3676x10^-9	11030
1	 0.710	2.343x10^-6	6.7957x10^-10	238711
3	 0.695	2.1486 x10^-6	6.225 x10^-10	287073
5	 0.685	1.239x10^-6	3.5931 x10^-10	526139

The corrosion parameters of the immersion coating as a function of various tin concentrations are presented in table 3(a). The corrosion current density (I_corr) decreases on increasing the concentration and corrosion potential (E_corr) and polarization resistance (R_p) increase as a function of concentration. This indicates that on increasing the concentration, the corrosion resistance increases.

The corrosion resistance of the coated and uncoated samples determined from the magnitude of the impedance data at 40ºC for 150sare shown in figure 3(b) as a function of various tin concentration. The impedance parameters are given in the table 3(b). At a lower concentration (1 g/l), the corrosion resistance is less (> 900W), but on raising the concentration to 3 g/l, there is rapid increase of corrosion resistance (> 4000W). Further increase of concentration to 5 g/l, the corrosion resistance becomes doubled (> 8000W) and yields a thick and strong deposit which has more corrosion resistance. Since the concentration of tin increases there is more number of Sn²⁺ ions are produced near the surface, so the thickness and growth rate increases.

Figure 3 (b) Niquist Impedance Spectra analysis for tin immersion coatings obtained on aluminium under various Sn²⁺concentration (1-5 g/l).

Table 3(b) Effect of concentration of Sn²⁺ion (1-5 g/l) on impedance parameters of tin coated aluminium for 150s at 40ºC.

Conc. (g/l)	R_s(W/cm²)	C_dl(F)	R_p (W/cm²)
Bare	1.92	21.82	185.3
1	1.115	78.89	935
3	6.059	14.06	4293
5	4.597	8.286	8677

3.2. Surface examination:

3.2.1. Surface Morphological studies: Scanning electron microscopy

Figure 4 shows the scanning electron microscope image of tin immersion coating obtained from 5g/l SnCl₂, 30 ml/l HCland 200 ml/l H₃PO₄ at 40ºC. The deposit is thick and compact and the crystallites are in flake shape and distributed uniformly over the aluminium surface.

Figure 4 Scanning electron microscopy image of tin coated aluminium.

3.3. Phase compositional analysis: X-Ray diffraction method

Figure 5 shows the XRD pattern of tin coatings prepared from 5g/l SnCl₂, 30 ml/l HCland 200 ml/l H₃PO₄ at 40ºC for 150s. The XRD pattern indicates that these immersion coatings are mainly composed of tin only. The average size of the crystallites in the coating ranges from 110-460 nm. The crystallographic data discussed below are in perfect agreement with what reported by JCPDS International Centre for Diffraction Data (2003). The tin phase is appeared at 45.045° (d spacing=2.0109), 65.80° (d spacing=1.4181),79.47° (d spacing=1.205), 99.577° (d =1.0086) and 112.09° (d spacing=0.9286) with preferred orientations of (2 1 1), (2 1 1), (3 1 2), (4 2 2) and (5 1 2) respectively [JCPDS card= 86-2265, tetragonal/Body-centered, JCPDS card= 65-0297, tetragonal/Body-centered, JCPDS card= 04-0673, tetragonal/Body-centered JCPDS card= 65-0296, tetragonal/Body-centered and JCPDS card= 65-7657, tetragonal/Body-centered]. As shown in figure 5, no separate peak of aluminium is observed and only sharp peaks of tin are observed indicating that the layer was covered completely by tin during deposition.

Figure 5 XRD pattern of tin coated aluminium.

3.4. Mechanism:

The following reactions are assumed to takes place during immersion plating^{11, 12}. When no dissolved oxygen is present, immersion plating proceeds as coupled reactions of Al oxidation and Sn deposition. These reactions are described as follows.

2Al + 3HCl + H₃PO₄® Al (PO₄) + AlCl₃ +6H⁺ + 6e^- (Reaction. 1)

E⁰= -1.70 V vs. SHE

Sn²⁺ + 2e^- ® Sn (Reaction. 2)

E⁰= -0.14 V vs. SHE

After the entire Al surface is oxidized or covered with Sn, reaction (1) does not proceed, leading to no change in the amount of Sn.

In the presence of dissolved oxygen, the reduction of oxygen is possible in addition to reactions (1) and (2).

O₂ + 4H⁺ + 4e^- ® 2H₂O (Reaction. 3)

E⁰= 1.23 V vs. SHE

The amount of the deposit increases as long as reaction (1) is available, but it starts to decrease due to the lack of available sites for reaction (1). The surface is covered with Sn at this stage. Since the standard reduction potential of reaction (3) is much higher than that of reaction (2), reaction (2) proceeds toward the reverse direction i.e, Sn dissolution. The re-dissolution rate of Sn depends only on the activity of dissolved oxygen. These processes cause the deposition of Sn and its re-dissolution on the Al surface.

4. CONCLUSIONS:

1. Temperature plays a significant role in deciding the surface morphology of plated tin. A temperature between 30 to 40°C produces strong and smooth deposit. At 50°C the deposit deteriorates and a porous structure was obtained.

2. The rate of tin deposition increases linearly as the initial tin concentration in the bath increases. A thin coating was obtained at higher tin concentration (5 g/l).

4. XRD confirms that sharp peaks of tin are observed indicating that the layer was covered completely by tin during deposition.

5. REFERENCES:

1. Gonzalez-Nunez MA, Nunez-Lopez CA, Skeldon P, Thompson GE, Karimzadeh H, Lyon P and Wilks TE, A non-chromate conversion coating for magnesium alloys and magnesium –based metal matrix composites, Corrosion Science, 37; 1995; 1763-1772.

2. Gonzalez-Nunez MA, Skeldon P, Thompson GE and Karimzadeh H, Kinetics of the development of a non-chromate conversion coating for magnesium alloys and magnesium –based metal matrix composites, Corrosion, 55; 1999; 1136-1143.

3. Lin CS, Lin HC, Lin KM and Lai WC, Formation and properties of stannate conversion coatings on magnesium alloys, Corrosion Science, 48; 2006; 93-109.

4. Annamalai V and Murr LE, Effects of the source of chloride ion and surface corrosion patterns on the kinetics of the Copper-Aluminium cementation system, Hydrometallurgy, 3; 1978; 249-263.

5. Annamalai V and Hiskey JB, A kinetic study of copper cementation on pure aluminium, Society of Mining Engineers, 1978; 650-659.

6. David R Baghurst, Michael D and Mingos P, Application of microwave heating techniques for the synthesis of solid state inorganic compounds, Journal of Chemical Society, Chemical Communications, 12; 1988; 829-830.

7. Srivastava YK, Eco friendly microwave assisted synthesis of some chalcones, Rasayan Journal of Chemistry, 1; 2008; 884-886.

8. Phani AR and Santucci S, Evaluation of structural and mechanical properties of aluminum oxide thin films deposited by a sol-gel process: Comparison of microwave to conventional anneal, Journal of Non-Crystalline Solids, 352; 2006; 4093-4100.

9. Sanjaya B and Shivashankar SA, Microwave irradiation-assisted method for the deposition of adherent oxide films on semiconducting and dielectric substrates, Thin Solid Films, 518; 2010; 5905-5911.

10. Saarivirta EH, Observations on the uniformity of immersion tin coatings on copper, Surface Coating Technology, 160; 2002; 288-294.

11. Sham TK, Naftel SJ and Coulthard I, M3,2‐edge x‐ray absorption near‐edge structure spectroscopy: An alternative probe to the L3,2‐edge near‐edge structure for the unoccupied densities of d states of 5d metals, Journal of Applied Physics, 79; 1996; 7134-7138.

12. Coulthard I and Sham TK, Novel Preparation of Noble Metal Nanostructures Utilizing Porous Silicon, Solid State Communications, 105; 1998; 751-754.

Received on 11.09.2011 Modified on 02.10.2011

Asian J. Research Chem. 4(11): Nov., 2011; Page 1726-1731