A chromium-free chemical conversion coating for Mg-Li alloy by phytic acid solution

An environment acceptable chromium-free process for Mg-Li alloy by phytic acid solution was developed,aiming at increasing the corrosion resistance of the alloy.The surface morphology,chemical structure of the conversion coatings were determined by scanning electron microscope(SEM) and X-ray diffraction(XRD).The results showed that there were some micro cracks in the homogeneous phytic acid conversion coating,on which white flower-shaped substance was formed.In this paper,the formation mechanism of the Mg-Li alloy in phytic acid solution has been discussed.And the formation process of the conversion coating analyzed by the evolution of the open circuit potential.The characters afforded by phytic acid and chromium acid conversion coatings on Mg-Li alloy had been studied using polarization curves,hydrogen evolution detection and drib experimentation.The results show that compared with the coatings formed in chromate solution,the corrosion potential of Mg-Li alloy coated by phytic acid solution increased from-1.65——1.40V.In addition,the hydrogen evolution rate of the conversion alloy decreased obviously.It means phytic acid conversion can improve the corrosion resistant of Mg-Li alloy more effectively.