Plate iron is an ancient salt-making tool and a typical representative of large-scale iron artifacts. To understand its corrosion characteristics and causes, this study employed various methods to analyze the composition, structure, and morphology of the matrix and corrosion products of seven Plate iron relics unearthed in Nantong. Finite element simulation analysis based on 3D scanning was also conducted in combination with the way of display. The results indicated that stress concentration, arising from the self-weight of the artifacts, in conjunction with a high-humidity, oxygen-rich environment, and the high-silicon grey cast iron material with multiple impurities, contributed to complex corrosion mechanisms. Additionally, hundreds of years of salt-making use exacerbated the deterioration, significantly affecting the artifacts’ stability. Therefore, conservation efforts should focus not only on the removal or suppression of active corrosion but also on stress relief and humidity control.