The effects of duty cycles on pulsed current electrodeposition of ZnNiAl2O3 composite on steel substrate: Microstructures, hardness and corrosion resistance
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:where Im is the average current density, Ic is the cathodic current density, Ton is the time of the cathodic pulse (on-time) and Toff 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.
Section snippets
Materials and electrodeposition electrolyte
Mild steel with a thickness of 1 mm was cut into 1 cm × 1 cm samples and a nickel plate with a thickness of 2 mm was cut into 5 cm × 2 cm samples. The chemical compositions of the respective plates using a spectrometer (Thermo Scientific, ARL3460) are as shown in Table 1. Mild steel was used as a substrate while the nickel plate samples served as anodes on both sides of the electrolysis cell. Both the mild steel and nickel plates were prepared by polishing with 120, 240, 400, 600, 1200 emery
Results and discussion
The ZnNiAl2O3 composite was electrodeposited using pulsed or DC current on mild steel substrates. Pulsed current electrodeposition was performed using 33%, 50% and 67% duty cycles. Transparent alumina nanoparticles incorporated in the coating was made possible by adding alumina sol-gel electrodeposit, which were immediately covered by the metal surface during plating.
Fig. 3a–c shows the XRD patterns of ZnNiAl2O3 composite coatings deposited at 33%, 50% and 67% duty cycles, respectively, whereas
Conclusion
This study examined and compared the effects of DC and pulsed current electrodeposition at different duty cycles (33%, 50% and 67%) of a zinc-nickel-alumina (ZnNiAl2O3) composite that was electroplated on a steel surface in a sulfate bath. The effects on structure showed that all coatings, regardless of the methods and conditions used in electrodeposition, produced both Zn21Ni5 and Zn22Ni3 phase structures, although at different content ratios. The surface morphology of ZnNiAl2O3 composite
Acknowledgements
The authors would like to acknowledge the financial support from Kasetsart University Research and Development Institute (KURDI). In addition, the authors would like to thank the support from the Department of Materials Engineering, and the Center of Advanced Studies in Industrial Technology, Faculty of Engineering, Kasetsart University.
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2022, Surface and Coatings TechnologyCitation Excerpt :In addition, PDC in the electrodeposition process is often influenced by pulse frequency and pulse duty-cycles. For a given pulse frequency, pulse duty-cycles (PC) depend upon the duration of applying (ton) and stopping (toff) electricity to the electrochemical systems until the completion of the electrodeposition process [14]. Furthermore, PC electrodeposition has several advantages over DC electrodeposition in terms of preventing whisker formation, improving adhesion strength between the coated layers and substrate, enhancing good surface finish, reducing porosity etc. [4,15–17].