Corrosion of archaeological ferrous artefacts

Since the 19th century, the corrosion of iron and other metals has been extensively and finely studied in a large number of environments, but only for processes developing over relatively short periods compared with the durations involved in the alteration of archaeological objects. Moreover, the thick layers of corrosion products formed after several centuries of exposure to different archaeological environments have been described by restorers and conservation scientists in some detail, but often with limited analytical resources, giving a good grasp of the heterogeneities and variability that archaeological corrosion systems can present on a macroscopic scale, but offering few elements for understanding the mechanisms, particularly on micro and nanometric scales. However, it is clear that only a well-considered combination of these two approaches can provide a clear picture of such corrosion systems, which are sometimes highly complex and whose alteration kinetics are controlled by mechanisms operating at different scales, the synergies of which need to be understood.

In recent years, mainly for ferrous metals but also for other materials, the need has arisen to attempt a more detailed approach to the corrosion of archaeological objects, based on an understanding of the mechanisms involved. In conservation/restoration circles, this has been motivated by the search for specific information, and the need for reliable diagnostics and optimization of object stabilization treatments. In particular, the location in corrosion products of the object's former surface, on which primordial archaeological information can be found, is the subject of extensive research. What's more, the conservation of ferrous objects in museums or historical monuments requires a thorough understanding of the corrosion products formed in previous centuries and their evolution in relation to that of the environment. Finally, for ferrous objects, particularly those stored in marine environments, it is essential to carry out a stabilization treatment to remove chloride ions from the system prior to any museum display or storage, in order to avoid any dramatic effect of renewed corrosion, which could lead to the ruin of objects removed from their environment in just a few years. These sometimes lengthy treatments need to be optimized, and the empirical approach alone is not enough. To these motivations linked exclusively to the heritage field, a more recent problem has been added, which has been at the origin of significant advances in recent years in the field of understanding the nature of corrosion products developed on archaeological objects. Indeed, in contexts linked to nuclear engineering, the storage and disposal of long-lived radioactive waste is becoming a crucial issue. In France (law no. 91-1381) and other...