Effect of NH4Cl on the microstructure, wettability and corrosion behavior of electrodeposited Ni–Zn coatings with hierarchical nano/microstructure
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.
Section snippets
Materials and methods
Commercial disk-shaped pure copper plates with a surface area of 2 cm2 were used as the substrates. Copper substrates were mechanically grinded using SiC emery paper up to grade #3000 and then polished by alumina powder (average particle size of 0.05 μm) on a mat. After this step, the specimens were cleaned in ultrasonic bath with ethanol for 10 min, rinsed with distilled water and immediately dried. The specimens were then electrically polished for 1 min in a bath containing sodium carbonate
Chemical composition of the coatings
The chemical composition of the coatings deposited without and with adding NH4Cl crystal modifier were analyzed by EDS as shown in Fig. 1. The results showed that although the atomic ratio of Zn/Ni ion in the bath was ~0.15, the Zn/Ni atomic ratio in the coating deposited without crystal modifier was 3.54, indicating the deposition of a Zn-rich coating. This result confirms the anomalous co-deposition of the Ni–Zn alloy, which could be attributed to the either slow kinetics of Ni deposition
Conclusion
Ni-rich Ni–Zn superhydrophobic coatings with a hierarchical nano/microstructure were deposited through a two-step electrodeposition. The effect of NH4Cl crystal modifier concentration on the morphology, topography, wettability and corrosion behavior of Ni–Zn electrodeposited coatings was investigated. Results showed that a transition from anomalous co-deposition of Zn-rich coatings to normal co-deposition of Ni-rich coatings was happened by adding NH4Cl to the deposition bath. In addition,
CRediT authorship contribution statement
Fatemeh Soleimangoli:Investigation, Formal analysis, Writing - original draft.S. Alireza Hosseini:Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Project administration.Ali Davoodi:Supervision, Conceptualization.Ali Mokhtari:Resources.Mostafa Alishahi:Methodology, Writing - review & editing, Supervision.
Declaration of competing interest
There are no conflicts of interest to declare.
Acknowledgments
Hakim Sabzevari University and Golestan University are appreciated for providing characterization facilities. Additionally, the support from CEPLANT (Masaryk University, Brno, Czech Republic) is gratefully acknowledged.
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