A comparative study of the structure and corrosion resistance of ZnAl hydrotalcite conversion layers at different Al3+/Zn2+ ratios on electrogalvanized steel

Abstract

The dissimilarities of the composition, crystal structure, and corrosion resistance of ZnAl hydrotalcite (ZnAl-HT) conversion films grown “in situ” on electrogalvanized (EG) steel substrate were investigated for different Al3+/Zn2+ ratios at pH 12. The corrosion behavior of all conversion films in 0.1 M NaCl was compared through electrochemical techniques. The composition, morphology, structure, and thickness of conversion films with different Al3+/Zn2+ ratios were characterized by Fourier-transform infrared spectroscopy (FT-IR), scanning electronic microscopy (SEM), and electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), and X-ray diffraction (XRD). The polarization curves indicated that the ZnAl-HT films provided anodic inhibition for EG steel and the inhibition efficiency of the ZnAl-HT conversion films increases sharply with decreasing the Al3+/Zn2+ ratios from 5/1 to 5/3, while, for the 5/4 and 5/5 ratios, the corrosion resistance of ZnAl-HT conversion films showed a downward trend after two immersion hours. However, the corrosion behavior of HT conversion films with the Al3+/Zn2+ ratios from 5/3 to 5/5 remained stable for a longer immersion time. The protection performance of the ZnAl-HT films can be attributed to the barrier effects, ion-exchange competitive adsorption of chloride ions, and protective deposition of ZnO on the EG steel surface.