Formation And Corrosion Resistance Performance Of Strontium Phosphate Conversion Coating On AZ31 Magnesium Alloy

As obviously confirmed in many literatures, magnesium and magnesium alloys, with superior physical and mechanical properties, are expected to be an excellent industrial material for various applications. However, the main problem that restricts larger scale use is their extremely poor corrosion resistance, especially in the corrosive medium with high chloride concentration. So seeking new methods to effectively improve the corrosion resistance of magnesium alloys is particularly important.In this article, a novel anticorrosive strontium phosphate conversion film was obtained successfully on AZ31 magnesium alloy. Strontium nitrate and ammonium dihydrogen phosphate were used as the precursor reagents with the concentration of 11 g/L and 10.5 g/L,respectively. The crystalline structure, micro morphology and chemical composition of the coating were characterized with X-ray diffraction (XRD), scanning electron microscopy(SEM) and energy dispersive spectroscopy (EDS). The results revealed that the coating was composed of Sr, P and O and it consists of primary hydroxyl strontium phosphate and less strontium hydrogen phosphate. The SEM images observed under the 1000 times magnification showed that the coating is mainly constituted by cuboid crystals mixed with a small amount of irregular shape crystals. The coating had film thickness of approximately 26.6?m. The corrosion resistance of strontium phosphate conversion coating on the AZ31 magnesium alloy was investigated by electrochemical measurement. The potentiodynamic polarization curves showed that the corrosion current density of the conversion coating decreased by more than three orders of magnitude as compared to that of the uncoated AZ31 magnesium alloy. The effects of temperature and deposition time on composition, morphology,thickness and corrosion resistance of the coatings were investigated. The experimental results revealed that the optimum temperature was 65? and the optimum deposition time was 50 min.In order to reduce the micro-cracks and pits in the coating, the specimen coated with strontium phosphate conversion film was sealed with silicate. The compactness of the coating has been greatly improved after sealing reaction. In addition to the strontium, phosphorus and oxygen, silicon was also detected due to the EDS results. The film thickness increased after sealing. The electrochemical performance of the specimens before and after sealing was evaluated and the results showed that the corrosion resistance was kind of improved after sealing treatment.A hydrophobic surface was fabricated on the strontium phosphate conversion film which is coating on the AZ31 magnesium alloy by fluoroalkylsilane modification. The static water contact angles on the hydrophobic surfaces increased with the extension of immersion time.EDS results elucidated that the content of strontium and phosphorus reduced while carbon increased, and silicon were detected simultaneously. As the immersion time went on, the corrosion resistance of hydrophobic surface increased significantly, but the changes of corrosion rate gradually become smaller.