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

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 WWII. 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 of the chromium oxidation states allowed to highlight the different phenomena taking place during the aging of the paint primer. We found that the reduction of Cr(VI) into 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 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.

Historical primers and paints used for aeronautical protection and colouring during WWII: A multi-techniques approach on archaeological parts

“…The white pigment, anatase, is the same as in the DE520. On both aircraft, the primer, already studied in [20], contains hematite (for the red colour) and zinc, potassium tetroxychromate (4ZnCrO4.K2O.3H2O) as an inhibitive pigment. It is similar to the primer found in the HE111 sample (from the German Heinkel 111 aircraft).…”