Preparation of chitosan/phosphate composite coating on Mg alloy (AZ31B) via one-step chemical conversion method

•Chitosan can be co-deposited with phosphate to form a dense composite coating on magnesium alloys.•Chitosan forms a deposit with Zn2+ and co-deposits with phosphate to form a composite coating.•The phosphate/chitosan composite coating can improve the corrosion resistance of magnesium. The purpose of this study is to develop chitosan/phosphate composite films on magnesium alloys to improve their corrosion resistance and broaden their applications in aerospace. Phosphate/chitosan composite films were successfully prepared by adding ultra-high deacetylated chitosan in a phosphate bath. The chemical composition of the prepared composite film was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR), and the morphology and fracture of the composite film were characterized by scanning electron microscope (SEM). Potentiodynamic polarization curves and electrochemical impedance spectroscopy were used to study the corrosion behavior of the coated alloys. The results showed that when pH=2.5 and the chitosan concentration was 5.0 g/L, chitosan was deposited in the phosphate coating in the form of inclusions. In addition, the potentiodynamic polarization curves of the composite films show that the corrosion potential is positively shifted by 0.6 V compared to the monolayer phosphate coating, indicating improved corrosion resistance. This work shows that highly deacetylated chitosan can be co-deposited with phosphate to form a dense composite film on the surface of magnesium alloys in one step, thereby improving the corrosion resistance of the alloy. [Display omitted]