A Study On The Soil Corrosion Mechanism And Protection Methods Of Archaeological Iron

The chemical, electrochemical methods and various surface analytical techniques were employed to investigate different factors affecting the soil corrosion of archaeological iron, such as the soil environmental factors, microstructures, and corrosion products. The local corrosion mechanism of archaeological iron was discussed in detail. Chemical stablization method was attempted to apply to the harmful rusts. A new fluorocarbon modification agent was developed to protect iron artifacts from the deleterious environmental factors. The experimental results will provide academic supports for the preservation of iron relics.The influence of the soil environmental factors such as the concentration of anions(Cl-、SO42-、HCO3-), water content, and pH value on the corrosion behaviors of simulated archaeological iron was studied by the methods of potentialdynamic polarization, electrochemical impedance, weight loss, and potential monitoring.The corrosion behaviors of simulated archaeological iron with four different microstructures in soil media were investigated through weight loss method, potentialdynamic polarization, simulated occluded cell (O.C.), electrochemical impedance spectroscopy(EIS), optical microscope(OM), as well as scanning electron microscope(SEM). The influence of different microstructures on the corrosion mechanism was studied in detail by comparing the chemical, electrochemical behaviors and the corrosion morphology of four types of cast iron. The results showed that the anti-corrosion performance of simulated archaeological cast iron decreased in the order:white cast iron, mottled cast iron, pearlite-ferrite gray cast iron, and pearlite gray cast iron.The immersion corrosion of archaeological iron in solution simulating soil water was presented. The evolution of archaeological iron from iron to iron oxide and to iron oxy-hydroxides compounds was investigated by scanning electron microscope and X-ray diffraction (XRD) analysis. Four oxyhydroxides like goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and amphorous iron oxyhydroxide (8-FeOOH) were synthesized and characterized. The corrosion of cast iron covered by lepidocrocite and akaganeite with the reaction of NO3-, Cl", HSO4-, and Cl-+HSO4-was investigated. According to the morphology, phase composition, and transformation process, the contributions of each corrosion product to archaeological iron were discussed.The potentialdynamic polarization and simulated occluded cell galvanostatic tests were employed to study the local corrosion behaviors of simulated archaeological iron. X-ray diffraction, energy dispersive spectrometry (EDS), and scanning electron microscope were used to characterize the corrosion morphology and the corrosion product. Moreover, the microstructures, slag inclusions, rust morphology and composition of ancient Chinese iron coins were investigated. The coins were exhumed from Emei Mountain and Baoji after818-966years imbedded underground. Metallographic technique, scanning electron microscopy, energy dispersive spectrometry, X-ray photoelectron spectrometry (XPS) and Fourier transform infrared spectroscopy (FTIR) were employed. The possible reasons for the presence of the identified phases on the iron coins deeply buried underground with long time were discussed.The reactions of tannic acid/phosphoric acid to γ-FeOOH and β-FeOOH were investigated by X-ray diffraction and X-ray photoelectron spectrometry. The suitable concentration of tannic acid/phosphoric acid was filtered for the development of rust convertor. A fluorocarbon modification agent for iron artifacts was researched. The fluorocarbon emulsion in which nano-materials dispersed were used as top-coat, and fluorocarbon emulsion added with corrosion inhibitors was used as bottom coat. The main performances of this composite protective coating were tested under the requirement of the conservation for archaeological iron. It is found that this water base preservative was not only organic solvent free, innoxious, but also colorless, transparent. It is promising in the conservation of iron antiquities.