Effects of Na3PO4 on plasma electrolytic oxidation and subsequent-sealing of Al alloy

초록

Aluminum and its alloys have been widely used due to the unique physical properties such as high strength per weigh, and thermal/electrical conductivity. Generally, thin oxide layer (1-5 nm thick) is naturally formed on the surface of aluminum and alloys. However, since the natural oxide layer is ineffective to resist the corrosion in harsh environments, dense coating usually is applied to effectively improve the corrosion resistance of metal. Plasma electrolytic oxidation (PEO) is a one of powerful method to prepare stable oxide layers on the metals such as Al, Mg and Ti. However, the oxide layer prepared by PEO process has inherent defects including micropores and cracks on the surface, causing that corrosion resistance does not highly improved than we expected. Herein, the influence of Na3PO4 was investigated on the porosity of surface after thick-oxide coated Al by PEO. After optimizing the condition of PEO process, we employed a sealing process in which NaAlO2 was coated on the microporous Al by chemical and thermal treatment to additionally reduce the porosity. The surface characterization was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, in order to measure the corrosion resistance, linear sweep voltammetry (LSV) was carried out in 3.5 wt% NaCl from -2.0 V to 0 V at a scan rate of 1 mV s-1. All the results showed the concentration of Na3PO4 affected the porosity of surface, and subsequent-sealing effectively-further reduce the porosity.