Abstract:

The hot-dipped galvanized (HDG) steel sheet was phosphated and then post-sealed using molybdate solution. Also , the corrosion performances and electrochemical behaviors of the phosphated/molybdate post-sealed HDG steel sheet in 5 % N aCl solution were investigated , using Tafel polarization and electrochemical impedance spectroscopy (EIS) measurements. The results were finally compared with those of the steel sheet only with phosphating process. It is found that , after the post-sealing of the phosphated HDG samples with molybdate solution , the anodic and cathodic processes of the zinc corrosion are conspicuously suppressed , the corrosion protection efficiency markedly increases , and the electrochemical impedance increases by more than one order of magnitude. All this reveals a synergistic effect of the corrosion resistances of the phosphate coatings and the molybdate films. It is alsQ found that the low-frequency diffusion impedance increases with the immersion time at the initial stage of immersion in 5% NaCl solution , which demonstrates clearly the self-repairing capability of the composite coatings.

Key words: galvanized steel, phosphated coating, molybdate, corrosion, electrochemical behavior