Microstructural and Compositional Characterisation of Chromate Pretreatment on Aluminium

Chromate conversion coating developed on aluminium has been examined using SEM/EDX and CTEM/EDX with a view to having further knowledge of its intrinsic surface, plan, and sectional morphologies which will aid the understanding of their roles in improved corrosion and adhesion properties of the underlying substrate. The surface consists of spherical clusters of particulate materials. The sections, however, reveal approximately parallel, linear features which terminate at or close to the metal/coating interface, while plan views show cell-like particulate features. The coating is composed of chromium and aluminium compounds, both, probably hydrated. For a conversion coating to fully replace its chromate counterpart, most of these features may have to be replicated in the nonchromium coating material which should contain nontoxic, leachable corrosion inhibiting species. 1. Introduction Unfortunately, hexavalent chromium present in chromate conversion coating (CCC) formulations as well as in the dried coatings [1–6] has been listed as a possible carcinogen. Consequently, active investigations [1, 7–14] are on processes and formulations to replace chromate’s corrosion protective properties and improved adhesion of subsequently applied finishes on aluminium and other alloys. Such formulations include those based on noncarcinogens as Zr [7], Zr/titanium/phosphonic acid [10] and cerium [13, 14] based conversion coatings. Some of these have been patented and are being used in mild environments. Although these and a number of alternative processes and formulations based on sol-gel chemistry are probably in the forefront as replacements, their performances lag [11] behind those of chromate conversion coatings. In view of these shortcomings, the present study seeks to know more about the changes in microstructure and composition imparted on aluminium by a typical chromate conversion coating formulation for its outstanding performances in terms of improved corrosion resistance and adhesion. This knowledge will enhance the pursuit of more favourable results in advances from the search for suitable nonchromium replacements. 2. Materials and Methods 2.1. Materials Aluminium specimens, of nominal composition, 0.002？wt.% Fe, 0.002？wt.% Cu, and 0.003？wt.% Si, were prepared by cutting up as-received materials into spade-like electrodes of dimensions 10？mm × 5？mm × 0.5？mm. The electrodes were electropolished according to the standard guide ASTM E1558-09 [15] rinsed in distilled water and dried in air. These electrodes were immersed for various periods in a conversion