Study on an elaborated method to improve corrosion resistance of zinc phosphate coating

Coating with dense and fine particles containing fewer cracks and lower porosity shows more improved protective properties due to limiting pathways between the environment and base metal. The main aim of present research is to introduce an innovative method that is called rephosphating to achieve this morphology. The outstanding point of the present investigation is to highlight the significant effect of surface pretreatment by secondary grinding of phosphated surface and then rephosphating of this surface to obtain a coating with appropriate properties. The SEM observations showed that this method has an obvious influence on the formation of a very uniform zinc phosphate coating on the plain carbon steel compared with the traditional method of phosphating. Furthermore, the protecting properties of phosphated and rephosphated samples were described and compared using the neutral salt spray and the electrochemical polarization tests. The results showed that rephosphating method had a beneficial effect on improving the corrosion resistance. As well, improved paint adhesion of rephosphated sample was observed compared with that of the phosphated sample. Finally, it was concluded that when rephosphating method can be used to repair damaged phosphated areas, the coating with more compact morphology and improved properties can be achieved.