Composition and Protective Properties of Weathering Steel Artificial Patinas for the Conservation of Contemporary Outdoor Sculpture B. Ramírez Barat
D. de la Fuente
T. Palomar
E. Cano*
[email protected] [email protected]
[email protected] [email protected]
B. García
Centro Nacional de Investigaciones Metalúrgicas (CENIM) Consejo Superior de Investigaciones Científicas (CSIC) Avda. Gregorio del Amo 8 28040 Madrid, Spain
[email protected] * Author for correspondence
Abstract Weathering steels (such as Cor-Ten®) are known for their ability to produce beautiful and protective oxide layers when exposed to a mild urban atmosphere. For this reason, they have been extensively used in contemporary art and architecture. Nevertheless, the natural patina formation is a slow process, requiring many years and specific environmental conditions. For aesthetic purposes, artificial patinas are created by treatment with different oxidants. The composition, structure and properties of these patinas have not been yet investigated, so it is not known how future conservation will deal with this type of patination. The aim of this work is to evaluate the characteristics and properties of these artificial patinas compared to natural patina and follow their evolution over time, in order to make recommendations for contemporary art conservation. Four artificial patinas obtained by treatment with different
oxidants (hydrogen peroxide, sodium bisulfite, hydrochloric acid), as well as a commercial product (Metal Effects Rust Activator Solution) and a well-formed five year natural patina have been studied by colorimetry, optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Corrosion behavior has been evaluated by Electrochemical Impedance Spectroscopy (EIS) with a gel polymer electrolyte (G-PE) cell specifically developed for in-situ measurements of cultural heritage. Results show that artificial treatments produce thin irregular patinas, mainly composed of lepidocrocite, with non-protective properties, that show less corrosion resistance than the bare metal.
Keywords patina, corrosion, weathering steel, contemporary sculpture, Electrochemical Impedance Spectroscopy
Introduction Weathering steels (such as Cor-Ten®) are types of steel alloys capable of acquiring a natural protective oxide layer when exposed to a mild urban atmosphere with wetting and drying cycles for a certain amount of time. It is known that the formation of a good quality patina on these steels takes some years under specific conditions (Morcillo 2013). Besides its protective properties, this patina has an attractive range of orange to purple colors, making it widely used in art and architecture in the last century. When used for artistic purposes, the quick acquisition of a rusty appearance is demanded, so artificial patinas are created by treatment with different oxidants such as hydrogen peroxide, hydrochloric acid, or commercial products specifically developed for this purpose. This acceleration of the process may produce a different composition and/or microstructure of the patinas, rendering them less protective and posing a problem
for the conservation of these works. If these patinas lack the protective properties expected for this material, the corrosion rate will increase and may compromise their conservation. In that case, additional protection strategies might need to be considered, such as the employment of corrosion inhibitors or protective coatings. The purpose of this paper is to evaluate the protective properties and the composition of artificial patinas produced on weathering steels with different accelerated treatments, and to compare them with natural patina, in order to propose recommendations for contemporary art conservation.
Materials and methods Weathering steel (Arcelor S355J2W, EN 10025–5-2004) coupons were prepared by sandblasting, the usual procedure in modern sculpture. The composition of
2 ICOM-CC | METAL 2016 | NEW DELHI, INDIA
SCULPTURE
the alloy is shown in Table 1. Natural patina (patina N) was produced by exposing the steel coupons to a mild urban atmosphere for five years, in an atmospheric corrosion station (Madrid, Spain). Artificial patinas were produced by brushing a 1M aqueous solution of different oxidants: hydrogen peroxide (H2O2, patina A1), sodium bisulfite (NaHSO3, patina A2) and hydrochloric acid (HCl, patina A3), and a commercial product called “Metal Effects Rust Activator Solution” (PA904) by Modern Masters (patina A4), which is an acidic aqueous solution of copper salts. Loose oxide was removed with a soft brush and wetting and drying cycles were applied for a week, spraying the coupons twice a day with demineralized water to avoid introducing additional ions. Table 1. Weathering steel composition (% weight) C
Si
Mn
P
S
Cr
Ni
Cu
Al
Nb
0.057
