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Archaeological iron objects were excavated from the Nanhai I ship from the Southern Song Dynasty that sunk in the South China Sea. Most of these artifacts were severely corroded and fragmented. In order to understand their current corrosion state and guide their restoration and protection, optical microscopy, scanning electron microscopy, micro-laser Raman spectroscopy, infrared spectroscopy and X-ray diffraction were all selected for analysis. It was clear that the archaeological iron material was hypereutectic white iron with a carbon content of about 4.3–6.69%, and had experienced low-melt undercooling. There were many internal cracks formed by general corrosion that extended to the iron core, which tended to make the material unstable. At the interface between the iron and rust, there was a black dense layer enriched with chlorine, and a loose yellow outer layer. The dense layer was mainly composed of magnetite, akaganeite and maghemite, while the rust of the loose layer was composed of lepidocrocite, goethite, feroxyhite, maghemite and hematite. The major phases of all corrosion products were akaganeite and lepidocrocite. Numerous holes and cracks in the rust layer exhibited no barrier ability to the outside electrolyte, hence the iron core formed many redox electrochemical sites for general corrosion with the rust. Meanwhile, the dense rust located close to the iron core was broken locally by an enriched chlorine layer that was extremely detrimental to the stability of the archaeological iron. Using electrochemical impedance spectroscopy, it could be determined that the rust layers had no protective effect on the internal iron core under conditions of simulated seawater, and these rust layers even accelerated the corrosion. A mechanism for the rust growth as a result of laboratory testing was proposed to explain the entire corrosion process. In view of the desalination preservation treatment that had been applied for ten years, it was not recommended to maintain a single desalination operation. The archaeological rusted iron of the Nanhai I ship that was excavated from the marine environment should be properly stabilized and protected using corrosion inhibition and rust transformation for iron oxyhydroxides, since the rust structure and the internal iron core retain well together. [ABSTRACT FROM AUTHOR]

A large number of iron pot artifacts were excavated from the "Nanhai I" shipwreck. The rapid embrittlement and pulverization of these objects (that have a good matrix) in the atmospheric environment is worthy of attention. Metallographic analysis showed that the material of the iron pots was hypereutectic white cast iron. Micro-CT observations revealed that there were numerous microcracks and fissures within the seemingly well-preserved iron. These fissures increased and enlarged with prolonged exposure to air. Scanning electron microscopy exhibited that the microcracks were initially created at the "concave surface" formed by the ferrite corrosion, and the cementite was gradually torn by the local accumulation of large internal stresses. Micro-Raman proved that the structure of rust was varied and complex during the generation and development of fissures. The dimension of corrosion products could expand and contract in mutual transformation, forming local internal stress and promoting the expansion of microcracks. This work proposed a reasonable mechanism for the rapid embrittlement and pulverization in the atmospheric environment of such hypereutectic white cast iron artifacts out of the sea, which provided a scientific reference for future protection. [ABSTRACT FROM AUTHOR]

This work took the iron objects from the Nanhai No. 1 shipwreck in the Southern Song Dynasty of China as the sample to test and analyze the application potential of catechin, an environmentally friendly corrosion inhibitor and rust converter. The article used metallographic microscopy to clarify that the structure of the iron artifact was hypereutectic white iron. By means of micro-Raman, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), potentiodynamic polarization and electrochemical impedance spectroscopy, catechin had the ability to react with iron oxyhydroxides such as goethite, akaganeite and lepidocrocite in the rust, forming an amorphous substance with a marked signal about 1380 cm−1 as phenolic-Fe in infrared properties. The new products could make the original rust layer form a laminated dense structure. After the archaeological iron was soaked in 3.0 g/L catechin, the corrosion current density decreased by 37.13% and the corrosion potential shifted positively by 32.67 mV. The anode reaction was more inhibited than the cathode in the polarization curve. The rust resistance in electrochemical impedance increased to 3.75 times and the ion diffusion resistance increased to 6.33 times. The corrosion inhibition efficiency was 21.75% and the rust conversion efficiency was 73.26%. After 36 h of accelerated corrosion, the protection effect of the newly transformed rust layer was still better than that of the original state. Catechin was a mild protection material which showed satisfactory performance for archaeological iron and has a good application prospect. [ABSTRACT FROM AUTHOR]