Electrochemical Behaviour of Thick Rust Layers on Steel: Strategies for Protection of Cultural Heritage Artefacts
Résumé
For several decades, a large number of heritage collections are related to the technical and industrial field because society wishes to preserve those materials which constitute traces of our industrial development. Compared to the collection of archaeological artefacts, the industrial and technical heritage is composed of numerous objects in cast iron or steel as the collection of extraction machines of coal in the Mining History Centre of Lewarde (France), or has a huge size as U4 blast furnace in Uckange in France. The diagnosis of the conservation state and the choice of the restoration treatment are important questions that have to be solved by curators to avoid expensive restoration works.The purpose of this study is to characterize the key-parameters of the electrochemical behaviour of different thick rust layers, observed on these objects. Then, the objective is to propose a simple electrochemical strategy to block or to reduce the electrochemical activity of these complex systems in order to lower the degradation rate of structures, without applying thick organic coatings as paints or varnishes.Previous studies (1) have demonstrated that some iron(III) phases constituting the rust layer play a role in the corrosion processes during wet and dry periods, and can increase the corrosion rate of the underlying metal.On this kind of artefacts, two main rust layers were observed and characterized by combining metallographic observations and different techniques of materials analysis (X-ray diffraction, Raman spectroscopy): a ferrhydrite/goethite and magnetite/goethite rust systems. The electrochemical activity of the oxyhydroxide iron compounds was characterized by cyclic voltamperometry in a carbon paste electrode, and the total system “metal/rust layer” was described by electrochemical impedance spectroscopy in aqueous corrosive media in function of immersion time.Corrosion-inhibiting formulations in aqueous media based on sodium decanoate and decanoic acid solutions were set-up for the treatment of these rust systems, from previous studies (2, 3, 4). These compounds are non-toxic and fulfil the criteria (reversibility, no modification of the final visual appearance) for their application in the field of the cultural heritage conservation. The impedance measurements coupled with the analysis of the treated surface shows that the application of the inhibiting solution has a positive effect on both the chemical diffusion of iron species in the pores network of the layer and the electrical conductivity of the rust system. So, the electrochemical reactivity of several iron oxide phases is drastically reduced.From those experiments, it turns out that those inhibitor solutions represent an interesting alternative of new environmentally safe treatments for corroded iron artefacts.