Investigation of the corrosion protection of chemically and electrochemically formed patinas on recent bronze

The impact of urban rain on the changes of bare and artificially patinated bronze during 9-year exposure

Atmospheric pollutants in the air form acid rain which interacts with bronze surfaces exposed in urban outdoor environment. In this study, different types of patinas on bronze were investigated during and after 9 years of exposure to urban environment in moderately polluted continental city. Natural bronze patina and artificial brown sulphide, green chloride, and green-blue nitrate patinas were investigated. Visual assessment was carried out at defined periods. After 9 years of exposure, an electrochemical study was performed to investigate the electrochemical activity of the patinas in artificial urban rain. Additionally, the patinas were characterised using a variety of techniques, including metallographic examination, scanning electron microscopy/energy dispersive X-ray spectroscopy, Raman spectroscopy, X-ray diffraction analysis, X-ray-photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry to analyse the surface morphology, chemical composition, and stratigraphic features of the patinas. Evolution of the patinas was shown to be a result of both, the composition of the acid rain and the hydrophobicity of the patinated surfaces.

Protection of Patinated Bronze with Long-Chain Phosphonic Acid/Organic Coating Combined System

Bronze cultural heritage in urban areas is susceptible to decay due to the significant amount of pollutants present in the air. This causes the dissolution of bronze and the patina on its surface. The efficient protection of outdoor bronze cultural heritage is still an unresolved problem. The aim of this work is to investigate 16-phosphonohexadecanoic acid as an environmentally friendly and non-toxic corrosion inhibitor for patinated bronze. The corrosion protection of sulphide-patinated bronze by phosphonic acid alone and in combination with acrylic coating Paraloid B-72 is examined. In order to achieve efficient corrosion protection, various parameters of the phosphonic acid application were studied. The efficiency of protection is examined by electrochemical impedance spectroscopy (EIS) during the immersion in simulated acid rain solution and after exposure to a corrosion chamber. It was found that the studied phosphonic acid provides corrosion protection to patinated bronze and significantly improves the protective properties of Paraloid B72. This was also confirmed by scanning electron microscopy (SEM) examination of the coating surface after exposure to a corrosive environment.

Spontaneous Symmetry-Breaking in the Corrosion Transformation of Ancient Bronzes

In general, during long-term museum conservation, ancient bronzes will generate new corrosion products also called the “secondary corrosion” on the surface of the unearthed “primary corrosion” products due to various environmental conditions. In this paper, the corrosion stages of several ancient Chinese bronzes are characterized by using optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD) and Raman spectrometer (Raman). It is found that there exist phase transformation relationships in between the “primary corrosion” and the “secondary corrosion” stages (i.e., (1) the crystal lattice type tends to transform from a high symmetry system to a low symmetry system; (2) in case of the same crystal lattice type, the corrosions exhibit an inter-transformation or symbiosis). It is interesting to note that these transformation rules are very consistent with the well-known physical law of “spontaneous symmetry-breaking”, which won the 2008 Nobel Prize in Physics and also has been considered to be general law of nature, in addition to the principle of Gibbs free energy reduction. The significance of this discovery allows us to achieve the predictability and controllability of the bronze corrosion products (i.e., to predict the corrosive trends in advance and control the “second corrosion” by adjusting the conservation conditions. This research provides a novel conservation concept of ancient bronzes.