The influence of silicon on the formation of phosphate coatings for low-carbon IF steels

Abstract

It is essential to clarify the influence of silicon on the phosphating process for low-carbon IF steels. In the present work, the formation of phosphate coatings for the IF steels with 0.01 wt%, 0.1 wt% and 0.5 wt% silicon was experimental determined by the combination of electrochemical characterization with X-rays photoelectron spectroscopy (XPS), focused Ion beam and transmission electron microscope (FIB-TEM), X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. It was confirmed that the silicon layer formed on the IF steels surface was mainly composed by Si, SiO2, SiC, Si3+ and oxides of Fe. Thicker silicon layers were formed with the increase of silicon concentrations, which inhibited the electrochemical reaction on the steels surface during the phosphating process. The diffusion of the ions including Me2+, PO43− and H+ to the surface of the substrate was severely restricted by the thick silicon layer, leading to the deterioration of the phosphating performance. The influence mechanism of silicon was finally proposed for a deep understanding of the phosphating process for IF steels.