The protection of metallic archaeological objects using plasma polymer coatings

One major concern of conservators and archaeologists is to limit the alteration and the corrosion of metallic archaeological objects. Organic coatings are one of the most important ways of protecting metal against corrosive agents, thanks to their diffusion barrier properties. The aim of this work was to develop effective protective coatings fitting with the specific requirements of heritage preservation (in particular the questions of reversibility and aesthetic appearance). Ultrathin (5-50 nm) polymer-like films have been deposited by plasma polymerisation in a methane or propane discharge on various metallic samples (stainless steel, Fe, Cu, Ag and Roman iron nails). Some important results are: 1. on tested substrates, except Ag, adhesion is very good; 2. concerning the reversibility of such layers, they can be totally removed in H-2 plasma; 3. the coatings are slightly coloured, which seems linked to the amount of unsaturated C=C bonds in the C layer and to a finite electrical conductivity; 4. they show interesting barrier diffusion properties to H2S, O-2 and water vapour molecules; and 5. on silver substrates, some problems of adhesion are observed due to the diffusion of Ag ions through the organic films. (C) 2000 Elsevier Science S.A. All rights reserved.