Investigation of Pretreatment Effects on the Formation of Lanthanum based Conversion Coating on AA2024-T3¹

Abstract—The effects of a pretreatment process on the formation and properties of lanthanum based conversion coatings on AA2024-T3 was investigated using optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and open circuit potential (OCP) measurements. The results indicate that the conventional alkaline activation pretreatment with acidic desmutting could not create a suitable condition to produce lanthanum based conversion coating on AA2024-T3. Therefore, in this study, a new approach to produce lanthanum based conversion coating on AA2024-T3 by acidic pretreatment is proposed. The pretreatment of the alloy in acidic and chloride environments (90 s at 30 vol % HCl) creates more cathodic sites on AA2024-T3 compared to alkaline activation with acidic desmutting. Additionally, it was found that the formation stages of lanthanum based conversion coating is similar to those of cerium based conversion coating on AA2024-T3 except lanthanum ions require more localized pH increase to deposit on the alloy.