Dual incorporation of SiO2 and ZrO2 nanoparticles into the oxide layer on 6061 Al alloy via plasma electrolytic oxidation: Coating structure and corrosion properties

The present work investigated the influence of SiO2 and ZrO2 incorporated nanoparticles on the coating structure and the corrosion behavior of 6061 Al alloy coated by plasma electrolytic oxidation (PEO). To achieve this purpose, a set of PEO treatments were conducted in alternating current condition at a constant current density of 130 mAcm(-2) in phosphate electrolytes containing each and both nanoparticles, respectively. Microstructure observations revealed that when SiO2 or/and ZrO2 nanoparticles were added to the electrolyte, both size and fraction of micropores tended to be decreased, which would be attributed to the incorporation of nanoparticles. SiO2 nanoparticles were preferentially embedded in the vicinity of micropores, meanwhile ZrO2 nanoparticles preferentially filled the cracks. m-ZrO2 was mostly converted to o-ZrO2 while SiO2 were partly converted to o-SiO2 and another part to mullite. Based on potentiodynamic polarization tests in 3.5 wt% NaCl solution, the coatings containing both SiO2 and ZrO2 nanoparticles exhibited excellent corrosion protection properties due to the combination of their roles as the micropores blocker and cracks filler so that the microstructural defects were minimized. (C) 2016 Elsevier B.V. All rights reserved.