Control of surface plasma discharge considering the crystalline size of Al substrate

摘要

This study investigated how to control surface plasma discharge during plasma electrolytic oxidation (PEO) in an alkaline-silicate electrolyte by taking the crystalline size of the metal substrate into account. For this purpose, PEO treatments under an alternating current condition at a current density of 100 mA/cm(2) were performed on Al samples with two different crystalline sizes of similar to 1 (fine-grain, FG) and similar to 50 mu m (coarse-grain, CG), respectively. It was found that plasma discharges appeared earlier at lower voltage in the case of CG sample than that of FG. Microstructural observations exhibited that plasma discharges formed on the surface of FG sample was likely to be more homogenous and finer than those on CG sample due to the distinctive occurrences of the initial passive films providing numerous sites where plasma discharge initiated. Accordingly, the oxide layer in FG sample would tend to be less porous and thicker than that in CG counterpart, which was confirmed qualitatively by potentiodynamic polarization tests in 3.5 wt.% NaCl solution. The nucleation and growth mechanisms of the oxide layer were elucidated in relation to characteristics of initial passive film and transport behavior of electrons across the crystalline boundaries in the metal substrate during PEO. (C) 2018 Elsevier B.V. All rights reserved.