Effect of Pre-Treatment of AZ91 Mg Alloy on PEO Film Formation

Abstract

This study investigated electrochemical behavior and morphological changes of AZ91 Mg alloy during pre-treatment in hydrofluoric acid solution and its effect on the PEO film formation behavior. The pre-treatment was conducted up to 20 min in 1 M HF solution at 25 ± 1 ℃ and the pre-treated AZ91 Mg alloy was anodically oxidized for the formation of PEO films at 100 mA/cm2 of 300 Hz AC in 0.1 M NaOH + 0.4 M Na2SiO3 solution. The electrochemical behavior of AZ91 Mg alloy during the pre-treatment was studied with immersion time using open-circuit potential (OCP) measurement and in-situ observation of gas bubble formation on the specimen. Surface morphologies of AZ91 Mg alloy after pre-treatment for various durations were examined by Scanning electron microscopy (SEM) and compositional analyses were performed using Energy dispersive spectrometer (EDS). PEO film morphologies and properties were analyzed by SEM, Electrochemical impedance spectroscopy (EIS) and film thickness measurement. Vigorous generation of hydrogen bubbles were observed upon immersion and its generation rate decreased with immersion time, suggesting the formation of a protective thin film of MgF2 on AZ91 Mg alloy surface. It was also found that β-phase in AZ91 Mg alloy was dissolved during pre-treatment in 1 M HF solution, its dissolution rate significantly decreased after 3 min of immersion time. Growth rate of PEO films on AZ91 Mg alloy largely increased with increasing pre-treatment time and the number and size of defects in the PEO films were reduced by the pre-treatment, representing that more uniform PEO films can be formed on AZ91 Mg alloy by employing the HF pre-treatment process.