Growth behavior of lanthanum conversion coating on hot-dip galvanized steel

La conversion coatings in various growth stages were obtained by immersing hot-dip galvanized (HDG) sheets in the lanthanum nitrate solution for different times from 10 s to 240 min. A growth process model of the La conversion coating on hot-dip galvanized steel was suggested. It is shown that the conversion coating grows more quickly by the zinc grain boundaries, where the cracks occur first and develop gradually on the whole surface of the conversion coating. When the immersion time is increased (more than 60 min), some rod-like precipitations containing more Zn and less La are deposited on the conversion coating. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurement results reveal that, with increasing immersion time, the protective property of the La conversion coatings is gradually increased in the early growth stage and then decreased when the cracks develop.