Anodising and Plasma Electrolytic Oxidation for the Surface Modification of Aluminium Alloys: Review

Aluminium (Al) and its alloys are attractive for a variety of applications due to its advantages like light weight, ease of processing and high thermal/electrical conductivities. However, it suffers from shortcomings in terms of strength, wear resistance and corrosion resistance. Anodising is commonly used to improve the surface modification of Al alloys. This paper presents a review of anodising and related processes and updates the current status in this area. The chemical structure and the size of the oxide film are influenced by factors like temperature, type of electrolyte and current density/ applied voltage. Depending on the process parameters, the oxide film formed in this process can be thin, non-porous, thick, and porous. The hardness of the coating in anodizing is influenced by the parameters like coating thickness, voltage, current density, and temperature. Further, it is required to regulate the metal/oxide or film/solution interface at which the barrier film grows. In Plasma Electrolytic Oxidation (PEO), the modified version of anodising, and the selection of operating conditions influence coating morphology, structure and composition which in turn affect the wear resistance and corrosion properties. The properties of the coating in case of PEO process are influenced by the nature/structure of the base material, type/composition of electrolyte, temperature of the electrolyte during the process, oxidation time, and electrical parameter (current density and voltage). Understanding of the effect of these parameters on coating properties opens new vista for better application prospects.