Supercritical carbon dioxide treatment of hot dip galvanized steel as a surface treatment before coating

Supercritical carbon dioxide (scCO2) treatment was employed for rapid formation of a zinc patina layer on hot dip galvanized (HDG) steel. In the presence of H2O and a Cu precursor, an artificial patina consisting of two distinctive phases was formed: a dense ~ 1 μm layer of anhydrous ZnCO3 adjacent to native zinc coating, and a needle-like porous structure showing resemblance to hydrozincite (Zn5(CO3)2(OH)6). The artificial patina layer significantly decreased the surface free energy of HDG, which was evidenced also by good wettability by a polyester melamine coating. Furthermore, the needle-like patina surface structure stayed intact through the coating process, indicating improved coating adhesion. ScCO2 treatment facilitates rapid and impurity-free surface treatment of hot dip galvanized steel, and could be used to tailor novel adhesion and corrosion promoting surface morphologies.