Weathering steel (high strength low alloy steel also known as Cor-Ten® or U.S. Steel) has been widely used in architecture and engineering for its unique characteristics and resistance against atmospheric corrosion. This selfprotecting material has the quality of gradually forming a stable corrosion resistant oxide patina through outdoor weathering. However, specific problems from its use in art have not been so deeply investigated; there is still an insufficiently explored field for conservator-restorers dealing with this type of works. Investigations carried out in this work involved identification and characterization analysis of corrosion products from four public sculptures (pieces by E.Chillida, exposed to outdoor urban seacoast environment in Bilbao). Analytical diagnosis combined non-destructive techniques such as Raman and X-Ray fluorescence. The main compound identified was lepidocrocite (γ-FeOOH), instead of goethite (α-FeOOH) the compound claimed as that corresponding to a stable passivation layer. As all of the sculptures have already been exhibited to outdoor environment over 10 years, theoretically enough time for the stable polymorph of FeOOH to form, we must conclude the negative effect of the urban environment on the long-term stability of some of these artworks.
CorTen steel differs from other steel because it develops a protective layer composed of iron oxides that increases the resistance against atmospheric corrosion. The rust layer is produced by the particular distribution and concentration of alloying elements such as Cu, Co, Ni and Zn. The presence of these elements and the exposition of the steel to the weather (wet-dry cycles) accelerates the development of this protective patina. This layer makes the CorTen steel a very adequate material for works exposed outdoor. Besides, it does not need any kind of protective painting, thus, it suppose an important save of money. From an artistic point of view, its color-changing surface and its chemical characteristics make this material the preferred steel material by artists for artworks exposed outdoor. It is generally accepted that the protective layer of weathering is composed of goethite (α-FeO(OH)), lepidocrocite (γ-FeO(OH)), and another non-crystalline compounds. The fraction of these compounds present in the rust layer depends on the exposition time. At initial time of exposure the main mineral phase present in the layer is the lepidocrocite, and after decades of exposure this lepidocrocite will transform onto goethite (γ-FeO(OH)), the stable iron oxyhydroxide . Thus, the ratio α/γ increases proportionally to the exposure time. Furthermore, as this mineral phase change occurs, the corrosion rate decreases. Nowadays, this index is used as an evaluator of the protection provided by the rust layer. However, the composition of the layer can change due to some environmental aspects, such as the presence of salt airborne, acid gases, etc. Moreover, sometimes, these kinds of steels are subjected to some processes like sanding, that could affect to the development of the rust layer. It is very interesting to research on the protective layer and to study the factors that affect the formation of the rust layer because the durability of this material exposed outdoors is going to depend on it. The aim of this study was to compare CorTen steel sculptures in good state of conservation with ones in bad state in order to discover the differences among them. This information would define the factors that affect the durability of this material.
THE CONSERVATION OF WEATHERING STEEL SCULPTURES
Carbon Steel, Corten Steel, Stainless steel
Rust, Bare, Powder Coated, Polished, Black Heat-resisting Paint, Iron Oxide
Diameter : 320mm--1200mm
Thickness : 1.2mm--3.0mm
Inside : Anti-wear foam paper
Outside : Plywood box
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