Production and Characterization of Zn-Ni Nano-Composite Coatings Using Sol-Enhanced Electrodeposition Method

Abstract

Zn-Ni coatings provide sacrificial corrosion protection for steel components and are widely used by automotive manufacturers and fasteners industries in particular. It has been reported that Zn-Ni coatings with single γ-Ni5Zn21 phase structure and Ni content in the range of 10-14 wt.% possess five times better corrosion resistance compared to the pure Zn coating. To have a commercially attractive Zn-Ni deposits, Ni content and phase structure are two important factors. However, it is difficult to obtain a single-phase coating as Zn-Ni coatings are mostly a mixture of intermetallic phases. Although Zn-Ni coatings are known to have much improved sacrificial protection, these alloys lose their protective abilities quickly when subjected to certain aggressive media, especially when combine with a high degree of wear. Composite coatings of single metal or alloy have been developed using electroplating to improve properties including corrosion and wear resistance, self-lubricating and high temperature stability. Typically, solid micro or nano sized powders are mixing into an electrolyte solution under agitation to produce nano/micro composite coatings using electrodeposition. By co-deposition of metal ions and micro/nano particles, metal matrix composite coatings are produced. Compared with the micro-composite coatings, nano-composite coatings attract more attention due to the significantly improved properties. However, it is always difficult to get composite coatings reinforced by highly dispersive nano-particles because nano-particles have very large surface areas. The high surface energy tends to cause agglomeration of the nano-particles during processing, and led to deterioration of the properties. The aim of this thesis research is to study and develop Zn-Ni based nano-composite coatings with improved corrosion and mechanical properties. This project is based on a sol-enhanced electroplating method, which is a novel technique recently developed in our group. In this method, sol solution containing nano-particles are added into the deposition bath instead of solid powders, leading to a truly dispersed nano particle distribution. The research was started with an investigation on production of Zn-Ni alloy coatings with optimum Ni content, single gamma phase microstructure and optimum corrosion and mechanical properties from a standard Zn-Ni bath. Then a systematic investigation was conducted to produce sol-enhanced Zn-Ni-Al2O3 nano-composite coatings for the first time. The effects of sol concentration on the microstructure and properties of the coatings were studied, and the underlying strengthening mechanism was investigated. Highly dispersed Al2O3 nano-particles were introduced into the Zn-Ni coating matrix, resulted in much improved corrosion and mechanical properties. Pulse electrodeposition was also employed to prepare sol-enhanced Zn-Ni-Al2O3 nano-composite coatings and the results were compared with DC and pulse deposited Zn-Ni alloy coatings. The effect of pulse electroplating on microstructure, corrosion resistance and incorporation of second phase into the coatings were investigated. The pulsed current plated coatings showed more uniform and compact coating layers with less through-thickness micro-cracks compared to DC ones. Pulse plating also provided coatings with higher microhardness and better corrosion resistance. However, considering sol- enhanced composite coatings, pulse plating may decrease the incorporation of alumina particles into the coating. The electro-crystallization process at the initial stage of the sol-enhanced composite plating was studied in details. The influence of alumina sol on the mechanism of Zn-Ni reduction and nucleation in a sulfate-based acidic bath was investigated. For this reason, cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were applied in conjunction with ESEM and XRD techniques. Based on the Scharifker-Hills model, the progressive nucleation was predominant for the Zn-Ni alloy deposit. However, for the sol-enhanced composite coating, the nucleation mode was closer to instantaneous nucleation. Palomar-Pardave model was used to calculate nucleation parameters describing the nucleation process such as number of active nucleation sites (N0) and nucleation rate (A). The results showed that incorporation of alumina nano-particles increased both N0 and A values. It was found that the alumina nano-particles provided additional nucleation active sites on cathode, changed the nucleation and growth mechanism and refined the crystal size of the deposits. The sol-enhanced composite plating technique is an easy operating and low cost technique which enables the production of Zn-Ni-based nano-composite coatings with superior mechanical and corrosion properties. It is believed that these Zn-Ni based composite coatings will have a wide range of applications in industries and could be considered as one of the Cd coating alternatives.