HISTORICAL PRIMERS ON HERITAGE AIRCRAFT:
ANALYSIS OF THE ZINC-CHROMATE
ANTICORROSION PIGMENTS

Magali Brunet, Clément Holé, Andrea Balbo, C. Faulmann, Letizia Monico,
Philippe Sciau

► To cite this version:

Magali Brunet, Clément Holé, Andrea Balbo, C. Faulmann, Letizia Monico, et al.. HISTORICAL PRIMERS ON HERITAGE AIRCRAFT: ANALYSIS OF THE ZINC-CHROMATE ANTICORROSION PIGMENTS. Technart 2025 - International Conference on Analytical Techniques for Heritage Studies and Conservation, May 2025, Perugia (Italy), Italy. hal-05352148

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Submitted on 6 Nov 2025

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# HISTORICAL PRIMERS ON HERITAGE AIRCRAFT: ANALYSIS OF THE ZINC-CHROMATE ANTICORROSION PIGMENTS

**Magali Brunet**<fnref n="1" />, **Clément Holé**<fnref n="2" />, **Andrea Balbo**<fnref n="3" />, **Christophe Faulmann**<fnref n="1" />, **Letizia Monico**<fnref n="4" />, **Philippe Sciau**<fnref n="1" />

Although chromate-based pigments are known to be used in easel paintings as yellow pigments [1], they were also largely employed for their anti-corrosion properties in the protective layer (primer) of aeronautical metallic parts [2]. In this work, two types of primers collected on heritage aircraft are studied. The first primer was sampled on a Dewoitine D.520 aircraft (1940) and contains zinc yellow (ZnCrO₄·K₂O·H₂O) as an anti-corrosion pigment. The second is a wash primer developed after WWII [3], sampled from a Morane Saulnier MS733 aircraft (1951). This primer, obtained by mixing a zinc tetraoxychromate (ZTO, a chromate with formula ZnCrO₄·nZn(OH)₂) with phosphoric acid in alcoholic solution and vinyl butyral polymer, was specifically developed to provide a higher protective efficiency than classical primers. The two primers were fully characterized with laboratory and synchrotron-based techniques (XRD at ID22 beamline and µ-XRF and µ-XANES at ID21 beamline of ESRF). In particular µ-XANES technique was used to probe the speciation of key elements (Cr and Zn) at the microscale in these layers and to provide information on the chemical reactions that took place between the zinc chromate and phosphoric acid at the interface with the aluminium alloy substrate. The incorporated pigments were compared with reference pigments, either commercial or synthesized from historical published articles. Although ZTO presents a high variability of stoichiometries depending on the ratios of ZnO/CrO₃ used as reagent, it is possible to reproduce a similar primer as the historical one. The XANES results highlight then that the ZTO pigment inside the MS733 primer has already been reduced during fabrication, i.e. the Cr(VI) content is lower than for the non-reacted synthesized pigment (see Figure 1). Historical primers, artificially aged by immersion 15 days in acidic rain (pH=4.3), were also studied to assess their protection efficiency. These results contribute in gaining knowledge on the fabrication of historical primers for aeronautical applications and on the long-term ageing of the chromate pigments with time and/or under severe weathering conditions.

<figure><img src="image_2.png" /><figcaption>**Figure 1:** a) µ-XRF map (cross-section); b-c) XANES spectra at Cr K-edge and Zn-edge (ID21, ESRF) for the MS733 primer compared with synthesized ZTO pigment and synthesized primer (ZTO + H₃PO₄ + PVB).</figcaption></figure>

<footnote n="1">¹CEMES, CNRS, University of Toulouse, France</footnote>

<footnote n="2">²ESRF, France</footnote>

<footnote n="3">³University of Ferrara, Corrosion and Metallurgy Centre “A. Daccò” Ferrara, Italy</footnote>

<footnote n="4">⁴SCITEC, CNR, Perugia, Italy</footnote>

<footnote>magali.brunet@cemes.fr</footnote>

<footnote>[1] L. Monico et al., *Angewandte Chemie International Edition*, **2015**, *54*(47), 13923-13927.</footnote>

<footnote>[2] M. Brunet et al., *Studies in Conservation*, **2022**, *69*(1), 26-34.</footnote>
[3] L.R. Whiting, P.F. Wangner, *U.S.Patent Office (Ed.) United States, 1950*, n° 2,525,107.