NANOSTRUCTURAL INVESTIGATION AND ENGINEERING OF ELECTRODEPOSITED ZINC COATINGS AND THEIR CHROMIUM AND GRAPHENE-BASED PASSIVATION LAYERS

Abstract

Electrodeposited zinc is an important type of coating used widely for corrosion protection in various applications. Nevertheless, the understanding on the relationships between coating fabrication process, micro/nanostructure, and properties of the material is rather limited. This thesis aims to develop structure-corrosion property relationship frameworks for the electrodeposited zinc coating and its chromium conversion layer, and also explore the novel process, particularly top-coating of graphene, upon which the corrosion resistance of the zinc coating could be enhanced. The research works performed in this thesis are divided into 3 parts: (i) the processing-structure-property relationships in electrodeposited zinc, (ii) the effects of the EZ’s structure on the formation of Cr(III) passivation, and (iii) the feasibility of graphene-based coatings as a potential for protecting corrosion. A set of environmentally friendly alkaline non-cyanide zinc coatings with and without trivalent chromium passivation is prepared by the electrodeposition process for examination of their micro/nanostructure and corrosion properties. Three groups of plating additives are employed to modulate the structure of the galvanized coating. Using a suite of modern characterization techniques, including field emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD), focused ion beam (FIB), and transmission electron microscopy (TEM), it is determined that texture and interfacial defects which are controlled by additive incorporation play critical role in controlling the corrosion behavior of the coatings. Furthermore, polyquaternary amine salt is identified as an effective additive for zinc plating as it helps promotes a formation of a relatively thick amorphous-oxide phase in the chromate film resulting in high resistance to corrosion. Finally, the feasibility study of graphene-based top coatings for corrosion shows that graphene can be coated onto a substrate using the electroplating setup. The coating of graphene is found to be controlled by additives, applied current density, and deposition time. The resulting graphene coating shows appreciable corrosion resistance results, and thus it is promising to be used for top-coating of electrogalvanized zinc.