Structure of Corrosion Layers on Archaeological Iron Artifacts From Nanhai I

Abstract 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.