Steam initiated hydrotalcite conversion coatings: Application to environmental Al alloy surface treatment

A facile process of exploiting high-temperature steam to deposit environmentally friendly hydrotalcite (HT) coatings on Al alloy 6060 was developed in a spray system. Scanning electron microscopy showed the formation of a continuous and conformal coating comprised of a compact mass of crystallites. A range of coating processes based on the formation of HT surface layers has been developed to examine its effect on the coating's thickness and corrosion resistance properties. These varieties include pre-coating cleaning (grid blasting vs. chemical etching), metal species in HT compounds (Al-Zn HT coating vs. Al-Li HT coating), oxidizer additives (K2S2O8, Na2SO4, NH4NO3, KNO3), and post-coating treatment (Mg(CH3COO)2, Mg(CH3COO)2 + Ce(NO3)3 + H2O2, Mg(CH3COO)2 + La(NO3)3). Results showed that grid blasting can increase the coating surface area, while chemical etching improves the chemical bonding connection between the coating layer and substrate, and therefore leads to a higher corrosion resistance in the filiform corrosion test. Al-Li HT coatings on the Al alloy are much thicker than the Al-Zn HT coatings that formed from the same procedures. This variation is likely caused by the pH condition of coating solution, which is essential to the co-precipitation of two or more cations in HT structure. The formation of Al-Zn HT coating was enhanced by oxidizer and ammonium salt additions. The thickness of Al-Zn HT coating decreases patterning after the oxidizing and complexing capability of additives in the following order: K2S2O8 > Na2SO4 > NH4NO3 > KNO3. Post-coating rinsing by Mg(CH3COO)2, in addition to Ce- and La- based salts, is capable of sealing the porous Al-Li HT coatings and increasing the corrosion resistance ability. The Ce modified Al-Li HT coatings show better corrosion protection than the La modified coatings, due to a behaviour of “active corrosion protection” that enables precipitation of insoluble Ce(IV) oxides to reinforce the existed HT coatings.