Corrosion of aircraft heritage: a comparison between modern and historic Duralumin alloys

The development of innovative solutions for the conservation and the protection of historic aircraft is a recent and important issue raised in the cultural heritage field and represents the goal of the PROCRAFT Protection and Conservation of Heritage Aircraft) project. Assessing and documenting the constituent materials as well as their degradation state is necessary to determine and understand factors inducing aluminium alloy corrosion, in order to develop tailored conservation treatments and identify effective protective coatings. The first step of the project was the identification of the constituent materials of aircraft wrecks. Al-Cu-Mg alloys—Duralumin and Super Duralumin, with a higher content of Mg—were the most employed alloys for structural and non-structural parts. These materials undergo a wide range of alterations, amongst which pitting, exfoliation and galvanic corrosion. In the present work, results of the characterisation of wrecks from a North American Republic P-47D Thunderbolt, crashed in Italy in 1945, are reported and compared to wrecks from a French Breguet 765 Sahara n°504 64-PH, built in 1958. The constituent alloys resulted to be similar to the modern 2024 and 2017A alloy. In order to assess the representativeness of these modern alloys in simulating the corrosion behaviour of the historic ones, exfoliation susceptibility (ASTM G34) as well as non-destructive and destructive electrochemical tests in 0.1 M NaCl were carried out on both historic and modern alloys. These results contributed to the selection of representative substrates for the development of protective coatings as well as to the expansion of the dataset on composition and microstructure of historic Al alloys for aircraft.