The effects of duty cycles on pulsed current electrodeposition of ZnNiAl₂O₃ composite on steel substrate: Microstructures, hardness and corrosion resistance

Highlights

• Embedded Al₂O₃ nano particles in ZnNi film help to increase a coating's hardness.

• Phase transformation of Ni₅Zn₂₁ and Ni₃Zn₂₂ film could be influenced by duty cycle.

• Ni₅Zn₂₁ (gamma phase) is more corrosion resistant than Ni₃Zn₂₂ (delta phase).

Abstract

In this study, we examine the effect of duty cycles (33%, 50% and 67%) under square-wave galvanostatic pulses on the electrodeposition of zinc-nickel-alumina (ZnNiAl₂O₃) composites from a sulfate bath. XRD results showed that the dominant phases of the ZnNiAl₂O₃ electrodeposits were mixtures of Zn₂₁Ni₅ and Zn₂₂Ni₃ phases together with as Al₂O₃. The Ni content measured in the electrodeposits using EDS varied from 9.73 to 13.47 wt%. SEM results showed that finer and smoother surface electrodeposits were obtained by pulsed current electrodeposition at a low (33%) duty cycle. In addition, the corrosion properties of the electrodeposits were characterized by Tafel plots and electrochemical impedance spectroscopy (EIS), while the microhardness of the electrodeposits was measured by a Vickers hardness tester. In summary, this study revealed that pulsed current electrodeposition at a 33% duty cycle led to a finer and smoother surface morphology, an enhanced strength, a greater corrosion resistance, and a higher Ni content in the ZnNiAl₂O₃ composite coatings compared to plating at higher duty cycles or plating through DC electrodeposition.

Introduction

Due to the energy shortage and environmental concerns, fuel cells are expected to be one of the most promising renewable source of energy [1], [2], [3]. Despite of its great advantages such as high efficiency, modular and environmental acceptability [4], corrosion still exists a problem especially the bipolar plates for proton exchange membrane fuel cells (PEMFC) and the interconnect in the a working solid oxide fuel cell (SOFC) stack [5].

Great efforts are being made to improve the corrosion resistance of zinc and zinc alloy coatings for use under harsh conditions. ZnNi alloys have attracted much attention due to their high corrosion resistance and mechanical erosion durability. Electrodeposition of zinc and nickel alloys offers an attractive and alternative method for producing coatings with a high Ni content in the form of a thin and uniform deposit on substrates. Coatings of Ni exhibit good mechanical properties as well as high corrosion resistance, thus increasing demand for industrial applications such as fuel cell, petroleum pipe line and high temperature electrochemical devices [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].

Metal matrix composites (MMCs) are known for their superior properties, e.g., they are hard, self-lubricating and resistant to oxidation, wear, high temperatures and corrosion. Plating, hot dipping or chemical/physical vapor deposition may be used for the production of MMCs. MMC production in electrolytes should be designed bearing in mind the ease of use, the production cost, and pollution control [19], [20], [21], [22], [23], [24], [25], [26].

ZnNi coated composites reported in the literature include ZnNiAl₂O₃, ZnNiTiO₂, and ZnNiSiC [19], [20], [21], [22], [23], [24], [25], [26]. Integration of micro or nano-ceramic particles in an electrodeposit suspension during electrodeposition allows the particles to be encapsulated into the coating matrix. This process, however, has been found to be problematic as a result of the particles' large surface area and high surface energy, which contribute to uneven nano-particle deposition with detrimental consequences [19], [20], [21], [22], [23].

Newer methods, such as pulsed electrodeposition, are designed to produce coatings better mechanical and anti-corrosion properties. It has been observed that pulse plating improves coating quality by reducing grain size and improving corrosion resistance compared to that of DC electro-plating. Moreover, pulse electro-deposition was reported to allow for a better control of deposit composition as it has been shown that by increasing Ni content, the coating's grain refinement can be accomplished down to the nano scale. Pulse electrodeposited ZnNi coatings had been found to have increased surface smoothness with reduced porosity and increased hardness as well as better ductility [24], [25], [26], [27], [28], [29], [30], [31], [32].

The process of pulsed electrodeposition involves the use of a pulsed current. The average current is calculated as:

$$I_m=I_c\left(T_{on}/T_{on}+T_{off}\right)$$

where I_m is the average current density, I_c is the cathodic current density, T_on is the time of the cathodic pulse (on-time) and T_off is the time between pulses (off-time). The duty cycle is the fraction of a period or cycle during which the current is applied, as shown in Fig. 1.

The pulse electrodeposition technique is capable of co-depositing oxide nano-particles, thereby reinforcing metal coatings. In the research of Ghaziof and co-workers [25], [26], [27], [28], the electrodeposition of MMCs by DC and pulsed plating methods was compared by varying the pulse frequency at a fixed duty cycle of 50%. They found that although pulse frequency did not significantly affect the chemical and mechanical properties of the deposited ZnNiAl₂O₃ composite coatings, it increased microhardness, reduced through-thickness microcracks and enhanced corrosion resistance relative to that of DC plated MMCs. However, to the best of our knowledge, the effects of duty cycle, changing the ratio of on and off time under the same or controlled frequency, on ZnNiAl₂O₃ composite electrodeposition have not yet been investigated. It is expected that the duty cycle will influence the deposit properties.

Therefore, in this paper we examine the effects of the duty cycle on the nano structures and properties of ZnNiAl₂O₃ composite coatings. Details such as the surface morphology, Ni content, phase structure, microhardness and corrosion resistance of ZnNiAl₂O₃ composite coatings from each duty cycle are discussed. In addition, the effect of the duty cycle on the phase transformation of ZnNi is reported.