Optimization of defect-free protective layer considering the geometrical linearity of condensed phosphates

The present study looked into the formation of a defect-free protective layer by considering the geometrical linearity of condensed phosphates. For this purpose, Al alloy sample treated by plasma electrolytic oxidation (PEO) in the aluminate-based electrolyte was immersed subsequently in the solutions with condensed phosphates, such as linear sodium tripolyphosphate (LP) and cyclic sodium hexamethaphosphate (CP). Microstructural observations revealed that the geometrical linearity of condensed phosphate would significantly modify the morphology of the oxide layer due to the incorporations of hydration and/or P-containing chemical compounds onto the oxide layer. Per the electrochemical assessment, it was found that the treatment by LP solution would provide superior corrosion resistance to the treatment by CP. This improvement was underlying by the formation of significant amounts of AlPO4 to fill the structural defects through partial hydrolysis of LP whose phenomenon was triggered by hydration of γ-Al2O3 oxide layer during sealing treatment.