The initiation and formation of a double-layer phosphate conversion coating on steel

Abstract

Although phosphating technology is developing all the time, the formation of phosphate chemical conversion (PCC) coatings still requires further study, particularly in terms of the subcrystalline and/or amorphous base layer at the bottom of the coating. The present study provides some additional insights into the mechanism of formation of a PCC coating on alloy steel using field emission SEM (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). The results indicate that the PCC coating has a double-layer structure, namely an amorphous base layer in contact with the steel substrate, and an outer crystalline phosphate layer. The formation of the PCC coating can be divided into four stages: electrochemical dissolution of the substrate, deposition of an amorphous phase, phosphate crystallization and growth, and the dynamic balance between coating dissolution and formation.