Compositional and Structural Analysis of Iron Phosphate Conversion Coatings by Soft X-ray Emission Spectroscopy

Iron phosphate conversion coatings that precipitate on steel prevent corrosion and improve paint adhesion. Although this coating is composed of FePO4 and iron oxides, separating the chemical species of the constituent elements using conventional analytical methods is challenging; hence, the detailed structure and deposition mechanism of the coating are not understood. This study used X-ray absorption and emission spectroscopy to analyze the coatings. The results showed that the main component of the coatings was FePO4, mixed with FeO for short treatment times and γ-Fe2O3 for long treatment times. These results suggested a new reaction pathway between FeO and FePO4, generated by eluted Fe2+ ions within solutions.