Corrosion products and mechanism of the iron anchor artifacts from Pujindu, Shanxi Province

The iron anchor artifacts found at Pujindu in Shanxi Province are severely corroded. In order to provide a scientific basis for the protection and restoration of such iron cultural relics, the corrosion products and corrosion mechanisms of the iron anchor from this site have been investigated. The burial environment, surface morphology, rust products and rust layer structure of the iron anchor cultural relics were studied using stereomicroscopy, scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray fluorescence analysis (XRF), laser confocal Raman spectrometry, X-ray diffractometry (XRD) and ion chromatography, and the corrosion mechanism was analyzed. The results show that the anchor material is gray cast iron, and the cast iron structure includes ferrite, cementite and graphite. Graphite is uniformly distributed in an irregular flaky form in cementite and ferrite. The moisture content of the soil around the anchor is relatively high (7.21%), the pH is close to neutral (7.156), and the concentration of corrosive Cl^- reaches a moderately hazardous level. The layers of mud and rust on the surface of the anchor exhibit a distinct stratification phenomenon. The outer and middle layers of the mud layer are mainly composed of stable rust components such as α-FeOOH. The inner layer of the mud layer, as well as the outer and middle layers of the rust layer, are mainly composed of components such as α-FeOOH, Fe₃O₄, Fe₂O₃, and γ-FeOOH, indicating that the rusting reaction is still ongoing. The unstable γ-FeOOH continues to be oxidized into stable α-FeOOH and Fe₃O₄, confirming that a chloride-free mechanism is responsible for the observed corrosion. The inner layer of the rust layer is loose and mainly composed of α-FeOOH, γ-FeOOH, Fe₃O₄, and FeOCl. The harmful rust substance β-FeOOH was detected in the cracks, indicating that Cl^- can penetrate the interior through the cavities of β-FeOOH and the cracks in the rust layer and participate in corrosion, resulting in the formation of FeOCl. FeOCl further transforms into harmful rust β-FeOOH, confirming the presence of a chloride-containing corrosion cycle mechanism.