Morphology and corrosion resistance of composite coatings obtained by phosphating and molybdate post-sealing on galvanized steel
(1. College of Civil Engineering, Huaqiao University, Quanzhou 362021, China;
2. College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China)
2. College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China)
Abstract: The phosphated hot-dip galvanized (HDG) sheets were post-sealed with sodium molybdate solution to improve the corrosion resistance of phosphate coatings. The morphology, chemical composition and corrosion resistance of the coatings were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Tafel polarization measurements and neutral salt spray (NSS) tests, and were compared with those of the single coatings. The results show that post-sealing the phosphated HDG steel with molybdate solution, the pores among the zinc phosphate crystals are sealed with molybdate films containing Zn, P, O and Mo, and the continuous composite coatings are formed. The suppression of both the anodic and the cathodic processes of zinc corrosion on the samples are enhanced significantly. The synergistic corrosion protection effect of the single phosphate coatings and molybdate films for zinc is evident. The corrosion resistance of the composite coatings increases with phosphating time up to 300 s.
Key words: galvanized steel; phosphate coatings; molybdate; corrosion resistance; polarization curve