Corrosion of iron from heritage buildings: Proposal for degradation indexes based on rust layer composition and electrochemical reactivity

In the present work, the authors tried to establish degradation indices for heritage ferrous artefacts, especially those used in ancient buildings and submitted to indoor atmospheric corrosion. The authors focused on the site of the Amiens Cathedral in the north of France. Samples coming from this reference site were carefully characterised in order to identify the different phases constituting the corrosion scale. The scale consists in a matrix of iron oxyhydroxide goethite embedded with several ferrihydrite marblings. Other phases such as lepidocrocite and akaganeite are scarcely present in the external part of the corrosion scale. Moreover, electrochemical measurements on both references and ancient samples enable to define the reduction reactive phases. From the nature of these phases and their localisation, two degradation indices were defined to evaluate rust reactivity. Finally, a curve that links these two factors is proposed as a first step towards a corrosion diagnosis.