Jia Minghao, Hu Pei, Hu Gang
School of Archaeology and Museology, Peking University, Beijing 100871, China.
Materials (Basel). 2022 Jul 18;15(14):4980. doi: 10.3390/ma15144980.
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.
从南宋时期沉没于南海的“南海Ⅰ号”沉船中发掘出了考古铁质文物。这些文物大多严重腐蚀且破碎。为了解其当前的腐蚀状态并指导修复和保护工作，选用了光学显微镜、扫描电子显微镜、显微激光拉曼光谱、红外光谱和X射线衍射进行分析。结果表明，该考古铁质材料为过共晶白口铁，碳含量约为4.3 - 6.69%，且经历了低熔点过冷。因全面腐蚀形成了许多内部裂纹并延伸至铁芯，这往往使材料不稳定。在铁与锈层的界面处，有一层富含氯的黑色致密层和一层疏松的黄色外层。致密层主要由磁铁矿、针铁矿和磁赤铁矿组成，而疏松层的锈则由纤铁矿、针铁矿、羟铁矿、磁赤铁矿和赤铁矿组成。所有腐蚀产物的主要相为针铁矿和纤铁矿。锈层中大量的孔洞和裂纹对外部电解质没有阻挡能力，因此铁芯与锈形成了许多用于全面腐蚀的氧化还原电化学位点。同时，靠近铁芯的致密锈层被富含氯的层局部破坏，这对考古铁质的稳定性极为不利。通过电化学阻抗谱可以确定，在模拟海水条件下，锈层对内部铁芯没有保护作用，甚至加速了腐蚀。通过实验室测试提出了锈生长的机制来解释整个腐蚀过程。鉴于已进行了十年的脱盐保护处理，不建议维持单一的脱盐操作。从海洋环境中发掘出的“南海Ⅰ号”沉船考古锈蚀铁应采用抑制腐蚀和将氢氧化铁转化为锈的方法进行适当的稳定和保护，因为锈结构和内部铁芯结合良好。
