Morphology and corrosion resistance of composite coatings obtained by phosphating and molybdate post-sealing on galvanized steel

• Published in: Transactions of Nonferrous Metals Society of China Vol. 17; no. A01; pp. 198 - 203
• Main Author: 徐玉野 林碧兰 卢锦堂
• Format: Journal Article
• Language: English
• Published: 01.11.2007
• Subjects: 微观结构; 磷酸盐; 腐蚀测试; 钼酸盐; 镀锌钢材
• ISSN: 1003-6326

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 molyhdate 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.