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.

In recent years, with the excavation of an increasing amount of gold and silver artifacts, there has been an urgent need to optimize the formulations and methods of metallographic etching. Herein, a kinetic control study is performed to investigate the mechanisms leading to poor results when etching ancient gold materials with aqua regia, i.e., when secondary AgCl impurities form during the etching of the sample surface. To this end, a concentrated ammonia and sodium thiosulfate solution is used to dissolve AgCl impurities and obtain high-quality metallographic images of ancient gold materials using a coordination reaction to generate stable free-state coordination ions from Ag+. On this basis, a ferric chloride + sodium thiosulfate method is proposed to optimize the formulation of the etchant for ancient silver materials. The formulation is efficient, safe and easy to handle, and solves the problems of the easy failure of the commonly used etchant of ammonia + hydrogen peroxide and the complicated preparation process of acidified potassium dichromate while maintaining the long-term stability of the etched Ag–Cu alloy samples.

Thousands of archaeological irons were excavated from the Nanhai I ship of the Southern Song Dynasty sunk in South China Sea, most of which were severely corroded and fragmented. To understand the current corrosion state and guide the restoration and protection, one piece of these iron objects was selected for analysis. Using optical microscope, scanning electron microscope, micro-laser Raman spectroscopy, infrared spectroscopy, and X-ray diffraction, it was clear that the archaeological iron was hypereutectic white iron with carbon content of about 4.3-6.69% and had experienced low melt undercooling. There were many internal cracks formed by general corrosion extending to the iron core which was a tendency of instability. At the interface between the iron and rust, there was a black dense layer enriched with chlorine and a loose outer layer in yellow. The dense layer was mainly composed of magnetite, akaganeite and maghemite, while the rust of the loose layer was about lepidocrocite, goethite, feroxyhite, maghemite and hematite. Besides, the major phases of all corrosion products were akaganeite and lepidocrocite. Numerous holes and cracks in the rust layer had no barrier ability to the outside electrolyte, so that the iron core formed many redox electrochemical sites to be general corrosion with the rust. Meanwhile, the dense rust close to the iron core was broken locally by enriched chlorine layer, which was extremely detrimental to the stability of the archaeological iron. By electrochemical impedance spectroscopy, it could be determined that the rust layers had no protective effect on the internal iron core indeed under the condition of simulating seawater, even accelerating corrosion. The mechanism of the rust growth was proposed from the shipwreck sinking to the laboratory testing to explain the entire corrosion process. Based on the concept of authenticity on the preservation, the archaeological rusted iron of the Nanhai I ship excavated from marine environment should be properly dechlorinated and necessarily stabilized, such as corrosion inhibition and reinforcement, for the rust structure and the internal iron core well-retained together.