CHAPTER 14: Outdoor Bronze and Its Protection

Copper and copper alloys, such as bronze, undergo spontaneous oxidation in humid air in a process called natural patination. In an environment containing aggressive ions, such as chlorides, sulphides and nitrates, secondary corrosion takes place. Patination can be also achieved by artificial application of different chemicals achieving different textures and colours. The main aim of this chapter is to present two different statues in marine and urban environments and the corrosion products that form during exposure to the atmosphere. Raman analysis included detailed analysis of different parts of the statues revealing different minerals that have formed during exposure. In the marine environment atacamite was found, as well as non-crystalline cuprite and cuprous sulphide, next to PM and other pollutants. In the urban atmosphere brochantite and antlerite were found, with cuprous oxide and the remains of brown artist's patination. By Raman analysis it was also shown that a BTA (Benzotriazole) and MBI (2-mercaptobenzimidazole) inhibitor formed a chemical bond with the bronze and the oxidised bronze surface, whereas TMI (4-methyl-1-(p-tolyl) imidazole) did not form a chemical bond. These observations were confirmed by electrochemical tests, which showed that the formation of chemical bonds at the surface of bronze offers better protection than a physical bond, as observed from the change in corrosion current densities.