Corrosion Behavior Of Copper And Bronze Alloys Under Static And Dynamic Thin Electrolyte Layers

Copper and bronze alloys, due to their good mechanic properties, electronic and heat conductivity, and good anti-corrosion resistance property, have long been used as constructive materials such as sculptures, building roof, household utensils and work of art. Copper and bronze alloys tend to be passivated in humid atmospheres or in light corrosive environments. However, in aggressive environments, especially in chloride containing environments, they suffered severe corrosion, which is called "bronze disease" or "bronze cancer". Up to now, bronze disease is still a challenge for the protection of cultural relic, due to the complication of the corrosion mechanism in chloride containing environment. Atmospheric corrosion can be regarded as wet corrosion of material under the thin electrolyte layer (TEL) formed on the surface of the material. This kind of corrosion is usually considered to be of an electrochemical nature. The past studies dealt with atmospheric corrosion of copper and bronze alloys were focused on the characterization of the patina, which caused much debate for the corrosion mechanisms because the physical characterization is unable to get the direct electrochemical information. In this dissertation, the copper and bronze corrosion behavior under TELs was successfully in-situ monitored via electrochemical methods with the aid of modified equipment. The main contents are listed as follows:1. The corrosion behavior of pure copper under chloride-containing TEL with various thicknesses was investigated using cathodic polarization and EIS. The results show that at the initial immersion stage (in2h), copper corrosion rate under TEL increases with the thinning of the thickness of TEL because the corrosion is under the control of O2diffusion process during the first2h. As the immersion time increases to192h, the corrosion rate ranks as300μm>402μm>199μm> bulk solution>101μm due to the fact that the corrosion is under the control of cathodic process when the thickness of TEL is larger than300μm, while less than300μm, the corrosion is under the control of anodic process. The morphology study performed by SEM reveals that uniform corrosion is the main corrosion behavior for the copper under chloride-containing TEL. This corrosion behavior was significantly different from that of copper in bulk solution. In the bulk solution, pitting was the main corrosion behavior. At initial immersion time (in2h), the corrosion product on the copper surface was mainly Cu2O, and its amount increases with the decreasing of the covered electrolyte layers. After long time immersion the protective layer Cu2O suffers destruction and transferred into Cu2(OH)3Cl and Cu2(OH)2CO3in the presence of chloride ions. The formed corrosion layer can be defined with a duplex structure, i.e., an inner layer adhere to the substrate corresponding to cuprite (Cu2O) and an outer layer a mixture of atacamite (Cu2(OH)3Cl) and malachite (Cu2(OH)2CO3).2. The corrosion behavior of single-phase α-bronze under chloride-containing static TEL with various thicknesses was investigated using electrochemical methods including OCP, EIS and cathodic polarization. The results indicate that the corrosion behavior of a-bronze is similar, but not identical to that of copper. The corrosion behavior of a-bronze proceeds through a preferential dissolution of Cu and a concentration of corrosion products containing Sn, further, a localized corrosion process is occurred on its surface. Anodic polarization curve shows that the corrosion products of Sn present protection to the substrate.3. The effect of microstructure and composition of bronze alloys on the bronze corrosion behavior under TEL was also investigated. The results indicate that the cathodic polarization behavior and corrosion rate are dependent on the phase microstruture. The corrosion rate of76Cu with three phases is higher than that of86Cu and85Cu with two phases at the same thickness of TEL during the initial stage, and the corrosion rate of two-phase86Cu containing Pb is higher than that of two-phase85Cu without Pb. However, in the bulk solution, the corrosion rates rank as76Cu>86Cu>85Cu。 The potential maps of bronze alloys with different phase structures was investigated by SKPFM, and the corrosion behavior was further determined.4. The synthetic effect of Na2SO4and NaCl on the bronze atmospheric corrosion was investigated during the initial stage and corrosion products were characterized using XRD、Raman spectroscopy and SEM/EDS. The results indicate that the addition of a small amount of Na2SO4into NaCl solution reduces the corrosion rate of bronze under chloride-containing dynamic TEL, representing a inverse synthetic effect during the initial stage, whereas, after long time of exposure, the corrosion rate of bronze under NaCl containing TEL is lower than that of under NaCl and Na2SO4TEL, which is manifested as synthetic effect. The synthetic effect can be ascribed to the preferential dissolution of Pb caused by the NaaSO4, while in the only NaCl-containing dynamic TEL, a passivative film PbCl(OH) is formed on the bronze surface.