Cerium-based conversion coatings were deposited on a Zn-5%Al alloy by immersing the alloy in cerium nitrate aqueous solutions with various immersion times. The growth behaviour of the cerium-based conversion coating on the Zn-5%Al alloy was investigated by the electrochemical impedance spectroscopy (EIS), SEM, energy dispersive spectroscopy (EDS), and XPS techniques. The results reveal that the coating mainly consists of ZnO, Zn(OH) 2 , Ce(OH) 4 , Ce(OH) 3 , CeO 2 , and Ce 2 O 3 . The growth of the cerium-based conversion coating is accompanied by metal dissolution. The dissolution mainly occurs on the η-Zn surface of the phase boundary and continues to extend to the Zn-rich phase as the coating grows. EIS results show that with increasing immersion time, the corrosion resistance of the Ce conversion coating gradually increases in the early growth stage and then decreases when the cracks appear.

Several silica-based solutions with 50 g/l of SiO 2 were prepared from sodium silicate solutions and silica sol; the silicate conversion coatings were obtained by immersing hot-dip galvanized steel sheets in these solutions. These solutions were characterized using high-resolution transmission electron microscopy and 29 Si nuclear magnetic resonance; the morphology of the coatings was observed by SEM and atomic force microscopy while the corrosion resistance was evaluated by electrochemical measurements as well as neutral salt spray tests. The results show that the coatings obtained from the single silica sol solution had poor adhesion and the coating obtained from the sodium silicate solution with low SiO 2 /Na 2 O molar ratio was uneven. By adding the silica sol to the silicate solution with low molar ratio, uniform coatings with better protection property were obtained. According to the results of 29 Si nuclear magnetic resonance spectra, the effects of the distribution of silicate anions with various polymerization degrees in the silica-based solutions on the microstructure and corrosion resistance of the silicate coatings are discussed.

The effect of NaF concentration on the formation of a phosphate film on a hot-dipped Zn-5Al coating, and consequently, the adhesion of the epoxy/Zn-5Al composite coating, has been investigated in this paper. The results showed that the morphology of the phosphate film and the adhesion, as measured by the tape test according to ASTEM D3359, strongly depend on the concentration of the fluoride ions derived from NaF in the phosphate solution. Adhesion failure of an electrostatically sprayed epoxy on a bare hot-dipped Zn-5Al coating can be prevented with the optimal concentration of NaF (0.4 g/L) in a phosphate solution used in this study. At the same time, phosphating improved the corrosion resistance of the Zn-5Al coating in a NaCl solution.

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The effects of HF pre-treatment on the cerium-based conversion coating on a Zn-5% Al alloy were investigated using a combination of scanning electron microscopy, energy-dispersive Xray spectroscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy. The results revealed that a porous structure appeared in the Al-rich phase of Zn-5% Al alloy after HF pre-treatment, and the cerium conversion coating with pretreatment was preferentially deposited on the surface of Al-rich phase and grew fast. Moreover, the corrosion resistance of the cerium conversion coating was improved by HF pre-treatment.