Protection efficiency of zinc chromate-based historical primers: A multiscale investigation by electrochemical and spectroscopy methods

Primers are the first protective layers deposited on metallic parts, which make them crucial in strategies employed by the aeronautical industry to prevent corrosion. Studying archaeological parts collected on crashed or collection aircraft is a good way to extract historical information on this specific technology. In this work, two types of primers were analysed: the first primer was sampled on a Dewoitine D.520 aircraft (crashed in 1940 and excavated in the 2000s) and contains zinc yellow (4ZnCrO4· K2O· 3H2O) as an anti-corrosion pigment. The second is a wash primer, sampled from a Morane Saulnier MS733 aircraft (1951, part of a collection). Wash primers, obtained by mixing a zinc tetraoxychromate (ZTO) with phosphoric acid and a polymer, were specifically developed to provide a higher protective efficiency than classical primers. Historical primers were analysed in the as-received conditions and after artificial aging to assess their protection efficiency thanks to electrochemical characterization at macro-scale. Then synchrotron radiation (SR)-based X-ray techniques: high angular resolution X-ray Powder Diffraction (HR-XRPD), micro X-ray fluorescence (µ-XRF), micro X-ray absorption near edge structure (µ-XANES) spectroscopy and laboratory scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) were used in a complementary manner to probe the speciation of key elements, including Cr, Zn, O and P, at both the micro- and nanoscale lengths within these layers. The pigments present in both primers were compared against reference pigments, either commercial or synthesized following historical recipes. The µ-XANES results highlight clear differences in the oxidation states of Cr of the zinc yellow and the ZTO pigments in the wash primer, while the STEM-EELS analysis additionally reveals the presence of chromium phosphate in the wash primer and zinc phosphate, with the latter being more abundant in the artificially aged samples. The results provide new insights into the fabrication of historical primers for aeronautical applications and the long-term stability of zinc chromate-based pigments under severe weathering conditions.