Effect of NH₄Cl on the microstructure, wettability and corrosion behavior of electrodeposited Ni–Zn coatings with hierarchical nano/microstructure

Highlights

• Transition from the Zn-rich to the Ni-rich Ni-Zn coatings by adding NH₄Cl

• Synergetic effect of NH₄Cl and H₃BO₃ on the formation of nano/microstructure

• Correlation between the PSD values of roughness and contact angle of the coatings

• Deposition of a corrosion resistant superhydrophobic Ni-rich Ni-Zn coating

Abstract

In this study, nickel‐zinc (Ni–Zn) superhydrophobic coatings with a hierarchical nano/microstructure were deposited through a two-step electrodeposition process on the copper substrate. The effect of adding NH₄Cl crystal modifier concentration on the composition, surface morphology, roughness and wettability of coatings were evaluated by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscopy (SEM), laser confocal scanning microscopy (LCSM) and contact angle measurements. Additionally, the corrosion behavior of selected coatings was investigated using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) techniques. The results showed that adding NH₄Cl could affect the composition and surface morphology of the Ni–Zn coatings. A transition from a Zn-rich Ni–Zn coatings with polyhedral shape to the Ni-rich Ni–Zn coatings with hierarchical nano/mico-conical structure was taken place through adding NH₄Cl as a crystal modifier. The Ni–Zn coating prepared with 400 g·L⁻¹ NH₄Cl showed superhydrophobic behavior with the highest water droplet contact angle of ~155°. The confocal microscopy results confirmed this coating has the highest roughness and power spectral density (PSD) values at entire spatial frequencies. In addition, the superhydrophobic coatings exhibited higher corrosion resistance compared to the other coatings.

Introduction

Superhydrophobic surfaces have recently received much attention due to their unique properties such as waterproof characteristics [1], anti-fouling properties [2], high corrosion resistance [3,4] and self-cleaning behavior [5]. A low surface energy with a specific micro/nano roughness pattern is required to achieve a superhydrophobic surface. Surface morphology plays a very important role in wettability of the superhydrophobic surfaces. Surface roughening improves hydrophobicity not only by increasing the solid-liquid interface (Wenzel state), but also by trapping the air between the rough surface and liquid droplets (Cassie state) [6]. The behavior of a water droplet on the rough hydrophobic surfaces can be described by different models. Accordingly, in the Wenzel's model the water drops penetrate into the surface roughness, while in the Cassie-Baxter's model the drop is suspended on the surface bumps. For hierarchical nano/microstructure surface, the wettability behavior is more complex. There are different wetting modes for the surface with hierarchical roughness depending on water penetrate either into the interspace of micro or nano features or into both [7].

Several methods are applied to create the hierarchical surface structure on metals. Among them, electrodeposition is a cost-effective method for producing superhydrophobic metallic coatings, which is capable to produce a wide range of surface structures with different properties [8,9]. As the metallic surfaces have high surface energy, water droplets spread out on the surfaces, indicating hydrophilic behavior and low water contact angle. Therefore, for obtaining a superhydrophobic surface (i.e. contact angle above 150°), it is necessary to deposit a low energy material on the roughened metallic surface. The surface energy reducing agent could be used to modify the chemistry of the rough surfaces and create a superhydrophobic surface with high corrosion resistance [10].

Ni–Zn alloy electrodeposited coatings are very popular in many fields including automotive, aerospace and electronics industries because of their high corrosion resistance [11,12]. However, Zn-rich Ni–Zn coatings are usually obtained through the electrodeposition process due to the anomalous co-deposition effect in Ni–Zn system [11,13]. This effect that corresponds to the preferential deposition of less noble metal, Zn, is attributed to the different mechanisms such as Zn hydroxide precipitation, underpotential deposition of zinc and slow kinetics of nickel deposition [14,15]. However, this is not always the case and the composition of electrodeposited Ni–Zn coatings can also be influenced by deposition conditions such as bath composition, current density, applied potential, deposition duration and temperature. For instance, it has been reported that employing a more negative deposition potential increases the nickel content in the Zn-rich Ni–Zn coatings [16]. Additionally, Roventi et al. [17] have reported that Ni-rich alloys could be obtained from a Watts type bath by controlling the deposition mass transfer of zinc through tuning the bath temperature, cathodic polarization and Zn ions concentration in the bath. Moreover, it has been reported that the use of additive materials like NH₄Cl into deposition bath, affects the composition of the coatings and facilitates the formation of Ni-rich coatings [18]. In addition, the presence of such additives is essential to obtain a coating with a rough morphology. Hashemzadeh et al. [19] reported that the presence of NH₄Cl in the nickel electrodeposition bath creates a suitable hierarchical micro/nano-conical structure that is responsible for inducing superhydrophobic behavior.

In this study, the effect of NH₄Cl concentration on the composition and surface morphology of the deposited coatings has been investigated to achieve optimal conditions for deposition Ni–Zn superhydrophobic coating. Considering widespread industrial applications of Ni–Zn coatings with a significant corrosion resistance, the development of a superhydrophobic behavior on these coatings can considerably improve their performance.