Investigation on the corrosion behavior of lanthanum phosphate coatings on AZ31 Mg alloy obtained through chemical conversion technique

A lanthanum based phosphate coating on magnesium AZ31 alloy was prepared through chemical conversion coating technique and characterized their corrosion performance. The coating was obtained in a two stage procedure using two different solvents, water or ethanol for preparing the coating nitrate bath. The chemical compositions and structures were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Corrosion performance of the developed coatings in 3.5 wt% NaCl was evaluated using electrochemical polarization and immersion tests. The results indicated that lanthanum hydroxide formation rather than lanthanum phosphate was predominant in the coating prepared using water as solvent for the nitrate bath whereas using ethanol as solvent produced lanthanum phosphate rich and defect less coating. An excellent corrosion resistance (in immersion test) was noticed with coatings prepared using ethanol as solvent (0.046 mm/y) over the bare sample (1.30 mm/y) as well as samples coated using water as solvent (0.278 mm/y).