Roestschaal

Roestschaal (Dutch: rust scale) is a term used to describe a crust of corrosion products that forms on iron- and steel-based surfaces as a result of oxidation in the presence of moisture. The layer can appear reddish-brown, orange, or black and may be powdery or adherent. It commonly develops on metal objects exposed to humidity, rain, or marine environments, including infrastructure, machinery, ships, and archaeological finds.

Composition and formation: The rust scale consists mainly of hydrated iron oxides and oxyhydroxides, such as goethite (FeO(OH)), hematite (Fe2O3), magnetite (Fe3O4), and related compounds. Its exact chemistry depends on factors such as oxygen availability, water content, salt presence, temperature, and pH. Formation is driven by electrochemical corrosion: localized anodic dissolution of iron and cathodic reduction of oxygen, with water acting as an electrolyte.

Characteristics and implications: The scale's structure can be flaky and porous, or dense and adherent. Dense scales may temporarily slow further corrosion, while loosely bound crusts can trap moisture and contaminants, accelerating deterioration. In conservation practice, assessing the rust scale helps determine corrosion rate and the appropriate stabilization or cleaning strategy.

Conservation and management: Treatments aim to minimize ongoing corrosion and preserve the object’s integrity. Approaches include humidity control, desalination for salt-laden artifacts, mechanical cleaning, and chemical inhibitors or chelating agents. After stabilization, protective coatings may be applied, and storage conditions adjusted to reduce future rust formation.