Phosphate coating on stainless steel prepared in electrolyte with silica sol: Zinc segregation,post-treatment and corrosion resistance

Phosphate film is generally not compact, left with microscopic cracks or pores during the phosphating process. Silica sol is incorporated into the phosphating solution to fabricate phosphate films on stainless steel 316L by electrochemical method. It is found that at appropriate amount of silica sol (8 vol.-%), the cracks on the hopeite film are effectively filled, while they appear again when much more silica sol is added as observed by SEM images. Moreover, zinc is found simultaneously formed as detected by the energy dispersive spectroscopy and X-ray diffraction measurements. In order to improve the performances of the hopeite film, we use titania sol by dip coating to cover the hopeite film. The titania sol can improve the anticorrosion ability of the films with limited extent since its crack formation during curing. By adding polyvinyl butyral (2 wt-%) in this sol, the composite film exhibits great improvement in corrosion resistance resulting from the cracks remedy and synergy effect of zinc with titania gel.