Comparative studies of chemical and electrochemical preparation of artificial bronze patinas and their protection by corrosion inhibitor

Abstract

The bronze artefacts of cultural heritage are often covered with patina, a layer of corrosion products, which confers their aesthetic and also protects the substrate bronze. Due to the increasing atmospheric pollution these layers are often dissolving when exposed in urban environment. In this work we propose the use of an innoxious imidazole compound as a corrosion inhibitor for patinated bronze. On a Cu–6Sn (wt%) bronze, three types of patinas were synthesized: two by chemical methods (in a sulphate solution and a chloride one) and one by an electrochemical process (in a sulphate/carbonate solution). A blue-green patina was obtained in all three cases, and their morphological and structural characterization was performed by SEM, EDS and Raman spectroscopy. It was found that the sulphate patina is composed essentially of brochantite, the chloride patina of atacamite, and the electrochemical patina of malachite. All three patinas have also a smooth part of surface consisted of cuprite. As corrosion inhibitor 4-methyl-1-(p-tolyl) imidazole was used on all patinas, in a solution of 0.2 g L⁻¹ Na₂SO₄ + 0.2 g L⁻¹ NaHCO₃ acidified to pH 5 which simulates acid rain in urban environment. The results have shown that the inhibitor improves the stability of all three kinds of patinas and can be recommended for protection of works of art.

Introduction

When exposed to atmosphere, copper and its alloys form a thin layer of corrosion products, called patina. A patina on a bronze sculpture not only protects the substrate metal, but also enhances the aesthetic of art objects [1]. Coloured patinas form spontaneously on copper alloys by very slow controlled corrosion either in the presence of moisture, carbon dioxide, and oxygen, or in sea-water. The colour depends on the corrosion products formed, which depend partly on the alloy and partly on the environment. Particularly copper or bronzes exposed to urban atmospheres for various decades exhibit a greenish patina, because of the copper carbonate or sulphate crystals, containing several constituents that had been largely studied [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. The tone of green is darkened by the presence of copper sulphides or lead and lightened by lead carbonate or tin-oxide. The patina may be reddened by an underlying layer of copper (I) oxide, cuprite [14].

Patinas are often deliberately added by artists and metalworkers. Patinas may be used to ‘antique’ objects, as a part of the design or decoration of art and furniture. A wide range of chemicals, both household and commercial, can give a variety of patinas [15] and hundreds of recipes for patina formation are known [16]. Artists often use patination as creative act (which unite skill and spirit) on surface improvement either for colour, texture, or both, but also aesthetic dimension and artistic expression. Patina composition varies with the reacted elements and these will determine the colour of the patina. Exposure to chlorides leads to green, while sulphur compounds tend to brown. For artworks, patination is deliberately accelerated by heat. Colours range from matte sandstone yellow to deep blues, reds and various blacks, sometimes with the surface sheen enhanced by waxing for artwork displayed indoors [15].

Patinas, alongside their aesthetic dimension, also have precious protective role but can also be ignoble and cause destructive phenomena on bronze, on its surface or within for example chloride in patina cause well-known “bronze cancer”. Therefore chloride patinas are one of the objects of this study [17], [18].

Because of the markedly increasing air pollution, additional protection for patina exposed in urban environment is needed. The use of corrosion inhibitors is one of the methods for protecting patina. Since the inhibitors that are used for protection are often toxic, new non-toxic inhibitors are being developed and studied nowadays. In our previous work, innoxious imidazole inhibitors were studied on copper and patinated bronze [19], [20], [21], [22], [23], [24], [25]. Aside from being environment friendly [19], imidazole derivatives were found to exhibit high inhibiting efficiency in protection of copper in sulphuric acid [19], [22] and near neutral chloride solutions [23]. Previous studies have also shown that some of the imidazole derivatives give efficient long term protection to patinated bronze in simulated acid rain [25]. One of them was 4-methyl-1-(p-tolyl) imidazole (TMI) whose protective effect is examined in this work.

The present studies were performed on three types of artificial patinas: two chemical patinas (a sulphate patina and a chloride patina), and one electrochemical patina (made in a sulphate/carbonate solution). After analyzing their structure and composition, the possibility of their protection by an imidazole-based corrosion inhibitor was examined.