Formation mechanism of a self-sealing pore micro-arc oxidation film on AM60 magnesium alloy

• A novel self-sealing pore MAO film is developed on AM60 Mg alloy.
• Fluorides are beneficial in improving the compactness of initial passive film.
• Different melting points of film constituents are key factor for pore self-sealing.
• High titanium oxide content greatly enhances the compactness of MAO film.

Ceramic films are formed on AM60 magnesium alloy by micro-arc oxidation (MAO) using a new fluorotitanate electrolyte system. Compared to the films obtained with traditional electrolytes, the film has the characteristics of self-sealing pores and different chemical compositions. To investigate the film's growth mechanism, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used to characterize the phase structure and chemical composition. Mott–Schottky (M–S) curve measurements were used to study the electrical properties of the passive layer and the influence of the fluorine during initial film deposition. The NaF content in the new electrolyte plays an important role in improving the compactness of the initial film. By increasing oxidation voltages in the MAO process, an increase of titanium oxides in the film is produced. Different melting points of film constituents and high titanium oxide content in the MAO film are key factors for forming self-sealing pores.