Analysis of Chromate-based Primers for Protection of Aluminium Alloys on Historical Aircraft

Abstract

ABSTRACT Immediately following the use of aluminium alloys, particularly Duralumin, in the construction of aircraft, protective coatings with anti-corrosion properties were developed. Among the numerous solutions to prevent corrosion, the most widely employed materials were organic primers containing chromates. This paper reports the study of the corrosion inhibitive compounds (chromates) used by aircraft manufacturers during World War II. More specifically, we have identified the nature of the compounds and assessed their current efficiency. Our analyses on samples collected from three different wrecks (French and German) excavated on terrestrial sites reveal that the primer contained either zinc tetroxychromate or lead chromate. Using XANES to study the chromium oxidation states elucidated the different phenomena taking place during the aging of the paint primer. We found that the reduction of Cr(VI) to Cr(III) species occurred either within the primer or at the interface of the primer with the Al alloy. This is believed to be due to alteration of the paint layer and its long exposure to the natural environment. However, even after 80 years, Cr(VI) is still present in the primers of these archaeological parts. Although a little altered, the chromate compound can still provide a source of inhibitor, thus maintaining the protection of the alloys.