A new concept for electroless nickel plating: aluminium as reducing agent

Abstract

A facile electroless plating strategy to obtain nickel coatings on copper substrates was designed to simplify the plating baths and procedure. The plating baths contained only nickel sulfate and ammonia. The aluminium connected to the copper substrates served as the electron source for nickel deposition. The nickel coatings obtained via this approach were tested and proved to possess excellent anticorrosion behavior.

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1 Introduction

Copper is widely used in electricity systems, electronics, machinery and decoration fields for its promising electrical conductivity, thermal conductivity, mechanical workability and fine appearance.¹ However, copper is prone to oxidation and corrosion in humid environments,² which generally results in the premature failure of copper products.³ Various methods have been developed to inhibit the oxidation and corrosion of copper, and one of the effective methods is surface coating treatment.^4,5 As a well-known method for preparing coatings or films of metals and their alloys, electroless plating is a convenient and effective technique to improve the corrosion resistance of copper while eliminating the need for complicated equipment and dangerous power sources.⁶ The utilization of electroless nickel plating has grown steadily during the last decade because of the promising corrosion and wear resistance of nickel based coatings.⁷ However, electroless nickel plating has its problems, which emerge from the complexity of the plating bath. A traditional electroless nickel plating bath⁸ is complex and usually contains complexing agents, nickel salts, reducing agents, buffering agents, stabilizers, etc. Reducing agents, including NaH₂PO₂,⁹ KBH₄ ¹⁰ and N₂H₄·H₂O,¹¹ have been used to prepare nickel coatings, and these costly and hazardous reducing agents usually lead to decreased stability of the plating baths. In addition, nickel cannot be electrolessly deposited on copper automatically because copper is inert to the hypophosphite oxidation reaction;¹² therefore, complicated sensitization or activation steps¹³ are generally required.

Aluminium or iron wire can also be used to initiate electroless nickel plating on copper and its alloys by bringing them in contact with the substrates;^14–16 in addition, electroless Fe–P plating was first achieved by coupling copper substrates with aluminium foil.¹⁷ Using the same technique, by introducing other elements, multicomponent iron alloys were also prepared;^18,19 the plating rate of iron alloys without coupled aluminium is too slow to be observed.²⁰

As mentioned above, aluminium is used to initiate or promote the electroless plating process; the reason is that aluminium can serve as the electron source, but aluminium is rarely used as a reducing agent to prepare electroless nickel coatings directly. Therefore, inspired by the above phenomena, we designed a facile electroless plating strategy to prepare nickel coatings on copper substrates in this paper. Aluminium was used instead of traditional and costly reducing agents in order to simplify the electroless nickel plating bath and process. The used plating bath only contained nickel sulphate and ammonia, and the plating process was to immerse copper substrates connected with aluminium into the above plating bath directly at room temperature. The plating bath was stable and could be used repeatedly, as it contained no reducing agent. The kinetics of the electroless nickel plating process was studied by electrochemical methods. The obtained nickel coatings were tested and proved to have excellent corrosion resistance.

2 Experimental

The copper substrates (99.9% purity) were etched with 5 wt% HCl solution for 30 s; they were then connected with aluminium foils and immersed into the plating bath without any sensitization or activation steps. The plating bath contained NiSO₄·6H₂O (20 g L⁻¹) and ammonia (25 wt%, 500 mL L⁻¹). After plating for 30 min at 20 °C, the samples were rinsed with de-ionized water and ethanol and then dried in a nitrogen stream. The traditional electroless nickel coating was prepared from a typical bath containing NiSO₄·6H₂O (30 g L⁻¹), NaH₂PO₂·H₂O (10 g L⁻¹), C₆H₅O₇(NH₄)₃ (65 g L⁻¹), and NH₄Cl (50 g L⁻¹). Operating conditions were pH 9 (adjusted with 5 M NaOH solution), temperature 85 °C.⁷ The pure aluminium foil was connected with the copper substrates for several minutes to induce the electroless nickel plating; otherwise, the nickel coating could not be obtained.

The surface and cross-sectional morphology and structure of the coating were characterized by scanning electron microscopy (JSM 6701F and JSM 5600LV, JEOL) with energy dispersive X-ray spectroscopy and a glancing angle X-ray diffractometer (D/MAX2500, Rigaku). Electrochemical measurements were performed using an electrochemical analyser (μ Autolab III, Metrohm) with platinum as the counter electrode and Ag/AgCl electrode (saturated KCl) as the reference electrode. The potentials of copper, aluminium and copper connected with aluminium foil were measured in the plating bath and NiSO₄·6H₂O solution (20 g L⁻¹). To simulate partial cathodic or anodic reactions, linear sweep voltammetry experiments of copper and aluminium in NiSO₄ solution or plating bath were carried out. To investigate the corrosion behaviour of the nickel coatings, potentiodynamic polarization tests were performed in 3.5 wt% NaCl solution. The open circuit potential measurements were maintained up to 60 min. The potentiodynamic polarization tests were carried out at a scan rate of 2 mV s⁻¹.

3 Results and discussion

After the coating process, the copper substrates were covered by uniform coatings with a silver-white appearance. The coating consisted of granules with irregular shapes, ranging in size under 100 nm, as shown in Fig. 1a and b. The granules were tightly packed to ensure a compacted structure. The EDX analysis result confirmed that the coating was composed of pure nickel (insert image in Fig. 1a). The XRD result indicated the coating showed typical crystallized peaks of fcc Ni at 2θ = 44.6°, 52.1° and 76.2° (inset image in Fig. 1b), which corroborated the result of the EDX analysis. Although the cross-sectional morphology of the sample is given in Fig. 1c, it was difficult to distinguish the coating and substrate under the electron microscope, as nickel and copper are adjacent elements in the periodic table. Therefore, EDX 2-D elemental mappings of copper and nickel were performed across cross-sections of the nickel coated samples (Fig. 1d). It can be clearly observed that a thin layer of nickel, which was compact, continuous and uniform, exists at the surface of the copper substrate. The average thickness of the nickel coating was about 1.0 μm.

Fig. 1 (a) and (b): Surface morphology of the nickel coating obtained from the reducing agent-free bath; cross-sectional morphology (c) and corresponding elemental mapping (d) of the sample. The inset images in (a) and (b) are the EDX spectrum and XRD pattern, respectively.

Fig. 2a and b show the surface morphology of the coatings deposited from the traditional plating bath. The coating also showed a compact structure; however, the size of the nodules was estimated to be about 100 to 200 nm. Compared with the nickel coatings prepared from the reducing agent-free bath, the traditional nickel coatings consisted of nodules with larger sizes and showed clearer nodule boundaries. Because sodium hypophosphite was used as the reducing agent, the obtained nickel coatings contained 14.2 wt% phosphorus, as verified by the inset image in Fig. 2a. The cross-sectional morphology and corresponding elemental distribution mapping of the traditional nickel coating were given in Fig. 2c and d, from which a uniform and compact layer formed on the copper substrate can be observed. Elemental nickel and phosphorus were uniformly distributed throughout the coating and the coating thickness increased to about 2.1 μm as a result of using a reducing agent.

Fig. 2 (a) and (b): Surface morphology of the traditional nickel coating; cross-sectional morphology (c) and corresponding elemental mapping (d) of the sample. The inset image in (a) is the EDX spectrum.

Nickel coatings were successfully prepared on copper substrates from a simple plating bath. To explore the possible mechanism of the nickel plating process, a contrast experiment was performed. No coatings were obtained on the copper surface when copper substrates connected with aluminium were immersed into NiSO₄·6H₂O solution (20 g L⁻¹) for 30 min. The OCP-t technique was used to understand the reason. As shown in Fig. 3a, the potential of the copper substrate in the plating bath was −0.554 V, and it shifted negatively to about −1.152 V as the copper substrate was connected with aluminium foil. From Fig. 3b, the above phenomenon was more obvious. It was observed that the potential of the copper substrate decreased from −0.57 V to −0.71 V after it was connected with aluminium for 3 minutes. This indicated that nickel had already been deposited on the copper, owing to the inducing effect of coupled aluminium foil by shifting the potential of copper negatively. Meanwhile, as shown in Fig. 3c, the potential of the copper substrate in NiSO₄ solution was −0.04 V; it shifted negatively to about −0.25 V after the copper was connected with aluminium foil. Furthermore, from Fig. 3d, it can be seen that the potential of the copper substrate in NiSO₄ solution hardly changed after it was connected with aluminium for 3 minutes. This shows that although the copper substrate was connected with aluminium, its potential in NiSO₄ solution was not negative enough to induce nickel deposition.

Fig. 3 OCP of copper, aluminium and copper connected with aluminium in the plating bath (a) and NiSO₄ solution (c); OCP of copper in the plating bath (b) and NiSO₄ solution (d) dependence on time with or without connected aluminium.

To ulteriorly reveal the nickel plating mechanism, the partial anodic and cathodic polarization curves were measured. As shown in Fig. 4a, the cathodic nickel reducing and anodic aluminium oxidation curves in NiSO₄ solution intersected at one point where the current density was 1.32 μA cm⁻². According to the mixed potential theory,²¹ the deposition reaction was carried out with a current density of 1.32 μA cm⁻². Meanwhile, the cathodic and anodic curves in the plating bath intersected at one point where the current density was 353 μA cm⁻² (Fig. 4b). This means the nickel deposition reaction in the plating bath was carried out with a current density of 353 μA cm⁻², which was about 260 times greater than that in NiSO₄ solution.

Fig. 4 I–E curves for reduction of Ni²⁺ ions and oxidation of aluminium: (a) in NiSO₄ solution, (b) in plating bath.

The nickel plating process can be expressed by two half-cell reactions:

Anodic reaction:²²

Al → Al³⁺ + 3e⁻ (1)

Al + 4OH⁻ → [Al(OH)₄]⁻ + 3e⁻ (2)

Cathodic reaction:

Ni²⁺ + 2e⁻ → Ni (3)

According to reactions (1) and (3), it was supposed that nickel ions should be reduced to nickel atoms and plated on copper while aluminium was oxidized simultaneously. However, as the thin layer of aluminium oxide and aluminium hydroxide were formed, they would become barriers preventing the oxidizing reaction of aluminium in NiSO₄ solution. Therefore, nickel coatings could not be obtained on copper substrates from NiSO₄ solution. However, adding ammonia to the NiSO₄ solution could promote the oxidizing reaction of aluminium (reaction (2)). The generated electrons then transferred to the copper owing to the galvanic cell formed between copper and aluminium, and the nickel ions in the plating bath gained electrons and were reduced to nickel atoms. The aluminium served as the electron source for nickel deposition, and thus led to the continuous process of nickel plating. The ammonia also acted as a complexing agent to prevent precipitation.

To compare the corrosion resistance properties of the nickel coatings prepared by the current method and the traditional method, potentiodynamic polarization tests were carried out in 3.5 wt% NaCl solution. As described in Fig. 5, compared with bare copper (E_corr = −324.45 mV), the corrosion potential (E_corr) of the current nickel coating shifted in the positive direction to −294.83 mV, indicating the protective nature of the current nickel coating. Meanwhile, the corresponding corrosion current densities (i_corr) were 3.221 μA cm⁻² and 230.0 μA cm⁻² for the current nickel coated copper and bare copper, respectively. This indicated that the corrosion of copper could be strongly restrained by the protective capability of the nickel coating prepared by the current facile approach. The traditional nickel coating possessed a slightly higher E_corr value (E_corr = −252.72 mV) than the current nickel coating; however, it had a relatively higher i_corr value (i_corr = 11.28 μA cm⁻²). The corresponding corrosion inhibition efficiencies (η_p) of the current and traditional nickel coatings reached 98.6% and 95.1% respectively, which could be calculated using the following equation:²³

where i⁰_corr and i_corr are the corrosion current density values of bare copper and nickel coated copper, respectively. Although the nickel coating prepared by the current facile method had a thinner coating, it exhibited similar or even slightly better anticorrosion properties than the traditional electroless nickel coating. It was speculated that this was because the current nickel coating had a smaller granule size and a denser structure.

Fig. 5 Potentiodynamic polarization curves of bare copper and the copper substrates coated by the current and traditional nickel coatings in 3.5 wt% NaCl solution.

4 Conclusions

In summary, a facile and direct electroless plating strategy was designed to prepare nickel coatings on copper substrates. Compared with traditional electroless nickel plating, the current plating bath containing nickel sulphate and ammonia was simple and the plating process carried out at room temperature was free from sensitization or activation steps. The aluminium foil that connected with the copper substrate served as the electron source for nickel deposition instead of traditional reducing agents. The obtained nickel coatings exhibited excellent corrosion resistance, which was similar or even slightly better than that of a traditional electroless nickel coating.

Acknowledgements

Authors acknowledge research funding from the National Natural Science Foundation of China (Grant NO. 51305434).

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