Pre-treatment of Zn surfaces for droplet corrosion studies

In this work, the significance of pre-treatment techniques to prepare stable and reproducible surfaces for corrosion and coatings studies on zinc-based materials is presented. The stability and consistency of Zn surface films were investigated using chemical and physical pre-treatment methods, whereas the effectiveness of pre-treatments was assessed by water contact angle measurements using droplet profiles. Morphology and composition of pre-treated Zn surfaces were investigated using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Chemical etchants, such as HNO3 (acidic) and NaOH (alkaline), were found to be aggressive towards the Zn metal surface and responsible for the formation of various oxides of inhomogeneous thickness. Air plasma pre-treatment proved to be effective in removal of hydrocarbon contaminants, leaving the surface highly hydrophilic irrespective of the fact that the thickness and roughness of native surface oxides remained unchanged. Zn surfaces pre-treated with organic solvents were found to be hydrophobic in nature in contrast to Zn surfaces pre-treated with abrasives like SiC paper, which were found to be relatively hydrophobic. Further, SEM-EDS analysis showed contamination of these surfaces with SiC. As expected the last surface pre-treatment step exerts a large influence over the final characteristics of the surface film when a number of pre-treatments are applied sequentially to the same surface. This work demonstrates that Zn surfaces pre-treated with diamond particles of sizes 9, 6, 3 and 1 μm show a moderate hydrophobic character. SEM-EDS analysis showed that surface films prepared via diamond polishing are uncontaminated. Additionally, surface film thickness was estimated using Auger signals produced during X-ray Photoelectron Spectroscopy (XPS). Surface depth profiling through depth sputtering revealed that the surface film is composed of ZnO and had a thickness of 1.98 nm. Specifically, the objective of this study is to lay the groundwork for future research on Zn corrosion in which consistent and reproducible surfaces are paramount to produce meaningful results. However, this work may also find more general utility in showing how different surface pre-treatments affect surface reactivity and in turn could impact adhesion to Zn or other metal surfaces.