Characterisation of various conversion coatings on 2024-T3

Several commercial and developmental non-chromate conversion coatings on 2024-T3 were studied using RBS, SEM, TEM and XPS. This paper examines coatings based on the chemistries of cobalt, permanganate and cerium, outlinesthe physical characteristics and corrosion/adhesion performance of each and compares them with the corresponding features of chromate. The cobalt coating appears to be composed of two layers, a lower dense layer and a top porous layer. The vanadate seal appears to deposit vanadium oxide flakes on the outer surface and also penetrates the oxide. The permanganate process deposits a mixed aluminium-manganese coating and requires a polymeric seal to provide adequate corrosion performance. The cerium-based conversion coating deposits first over intermetallic sites and then more generally across the surface. The silicate seal reacts with the surface to form a barrier layer. The cerium-based oxide thickening process results in a boehmite layer on the surface with incorporated cerium. A silicate seal is required for this process to achieve adequate corrosion resistance.