Corrosion Mechanisms and Conservation Strategies for Marine Recovered Bronze Artifacts: Insights from the Nanhai No. 1 Shipwreck

This paper presents a comprehensive analysis of three bronze ring fragments with distinct corrosion patterns from the Nanhai No.1 shipwreck, employing optical microscopy, SEM–EDS, µ-RS, and µ-EDXRF techniques. The results exhibit that each bronze ring fragment displays unique corrosion characteristics but shares similarities in their patina structures: an outer layer composed of CuFeS₂₎ and malachite, a subsurface layer of CuS) and/or Copper Tin Sulfide, and an inner copper-depleted layer rich in tin and lead. These layers indicate the artifacts experienced three distinct corrosion stages corresponding to aerobic, anaerobic, and re-aerobic conditions, aligned with the ship's sinking, sediment coverage, and post-recovery scenarios. The study reveals that the pre-anaerobic corrosion products significantly influenced the corrosion kinetics in subsequent environments, affecting the final composition and structure of the patina. Notably, the outer CuS layer, regardless of its compactness, is unable to prevent the selective dissolution of copper from the alloy. The formation of a cuprite layer beneath the copper-depleted layer post-recovery suggests a corrosion resumption due to changes in environmental oxygen levels. These insights emphasize the need for targeted conservation strategies for marine-recovered bronze artifacts, considering their complex corrosion history and environmental interactions.