The stability of corrosion products on iron artifacts has significant effects on the preservation and conservation of the objects. This is a crucial factor in evaluating the overall conservation state of iron objects. The protective ability index (PAI), which primarily considers the ratio of stable phase to unstable phases, is applied to assess the protective ability of the corrosion layer and the corrosion tendency of iron objects. Recently, Raman spectrometry has been more often used as a semi-quantitative approach for quantifying various components of iron corrosion, making it a valuable tool for calculating the PAI value. In this study, Raman spectrometry, combined with macro X-ray florescence imaging and other techniques, was applied to quantify the composition of corrosion products on an iron bar excavated from the Nanhai No.1 shipwreck. Subsequently, the PAI values were calculated. The results indicate that the corrosion products predominantly comprise four types:α-FeOOH, γ-FeOOH, β-FeOOH and Fe₃O₄. These products present distinct distribution patterns in the inner and outer layers of the object. The PAI values ranged primarily from 1 to 10, with the lowest recorded at 0.34 and highest at 20.38. Notably, the PAI values for corrosion in outer layer are higher than those in the inner layer, suggesting greater stability in the outer rust. Corrosion present at the stripping surface, with a PAI value below 1, is indicative of significant instability. Similarly, instances of unstable corrosion were observed within the inner layer, as evidenced by a PAI value of 0.73. The application of PAI values in assessing iron corrosion provides a quantitative and reliable method for diagnosing the conservation state of iron objects.