Enhanced uniformity of apatite coating on a PEO film formed on AZ31 Mg alloy by an alkali pretreatment

Abstract

Anodization by plasma electrolytic oxidation (PEO) and subsequent apatite coating were performed on a biodegradable AZ31 magnesium alloy to enhance its corrosion resistance and bioactivity in physiological solution. The PEO film itself (~48μm in thickness) exhibited low bioactivity, where only aggregated apatite particles were deposited locally on its surface as a result of the alternative immersion method (AIM) in Ca–phosphate solutions. The uniformity of apatite coating on the PEO film was markedly improved by pretreatment of the film in a dilute NaOH solution. The alkali treatment induced the formation of a nano-size platelet Mg(OH)2 layer on the film surfaces that drastically enlarged the effective surface area for the precipitation of apatite. A uniform apatite layer as thick as 1μm was successfully deposited on the hydroxide layer after AIM treatment. The enhanced uniformity of the apatite coating on an alkali- and AIM-treated surface significantly improved the corrosion resistance in both simulated body fluid (SBF) and NaCl solution, and the bioactivity in SBF.