Cyclic Voltammetry Studies of PEO Processes in Alkaline and Silicate/Phosphate Electrolytes and Resulting Coatings

Abstract

A cyclic voltammetry (CV) method was employed to clarify the electrochemical behaviour of Al in alkaline silicate/phosphate electrolytes over a voltage range of 0 to 550V and study the growth process of plasma electrolytic oxidation (PEO) coatings on rectangular shape Al foil samples. The non-linear behaviour of the current – voltage curve reflects a combination of three processes, including dissolution, passivation and formation of PEO coating under discharge conditions. Different thickness distributions of oxide layer and residual aluminium along the sample length were observed. Similarly, the total Al consumed for the coating process at the edge of sample was more than in the middle. Silicate addition obstruct anodic dissolution of Al, which enhanced the oxide growth. While phosphate, in addition of promoting alpha phase formation it is also enhancing strong metal passivation and allow the breakdown voltage to be easily obtained. Breakdown voltage depends on the concentration and electrolyte composition. Under the studied conditions, we can also conclude that the sparking voltage decreases when electrolyte resistivity decrease which is consistent with Ikonopisov equation. However, which’s more surprising and unexpected in K electrolyte, is the earlier emission starting prior to the current starts increasing. There are a number of considerations used to determine the breakdown voltage such as reaching of maximum voltage, rapid voltage fluctuation. However, the appearance of visible sparking is not a criteria for detecting the dielectric breakdown as concluded in our study although many literatures considered this as criteria.