Microstructural Analysis of Iron Phosphate Conversion Coatings by FE-SEM and XPS

The microstructure and chemical composition of iron phosphate conversion coatings were investigated by surface/cross-sectional observation using FE-SEM/STEM and chemical state analysis using XPS. As a result, it was suggested that the conversion reaction progressed while the surface oxide on the base material remained, and the fine particles of coatings components were deposited and agglomerated to cover the material surface. The upper layer of coatings was mainly composed of iron phosphate, while the interface between the coatings and base material was deposited iron oxide. The proportion of iron phosphate and iron oxide was almost equal at the initial stage, and the amount of iron phosphate increased as the coating thickness.