IMPROVMENT OF CORROSION RESISTANCE OF PHOSPHATE COATINGS ON HOT-DIP GALVANIZED STEELS BY MOLYBDATE POST-TREATMENT

To improve the corrosion resistance of phosphate coatings further,the phosphated hot-dip galvanized (HDG) steels were post-treated using molybdate solutions.The effect of molybdate post-treatment on the compositions and corrosion resistance of phosphate coatings was investigated by SEM,EDS,electrochemical polarization measurement and salt spray test.When the HDG steels were post-treated with molybdate,the pores among zinc phosphate crystals on phosphate coatings were filled up by molybdate films,and continuous and compact composite coatings composed of phosphate coatings and molybdate films were formed on zinc layers.Polarization resistance Rp of the composite coatings increased greatly,corrosion current density decreased strongly,and the corrosion resistance was improved markedly.When phosphating time was 300 s and post-treating time was 50 s,the composite coatings with optimal corrosion resistance were obtained,and Rp increased for more than ten times than that of the single phosphate coatings.