Abstract: S31803 duplex stainless steel tubes in heat exchanger of a chemical factory failed by corrosion perforation after using for 14 months. The reason for corrosion perforation was investigated by chemical and phase composition analysis, microstructure and micromorphology observation, hardness testing and microarea composition measurement. The results show that chromium-depleted σ phase precipitated in the steel tube during cold drawing, leading to poor chromium around, and therefore reducd the corrosion resistance of the steel tube. Corrosion thinning or perforation occurred on the outer-wall of the steel tube under the action of corrosion of H₂S, NH₄⁺ and other media in the shell gas. The pipe-circulating water entered the shell side through the perforation, and a loose structure of CaCO₃ was deposited on the outer surface of the steel tube, causing the enrichment of corrosive mediums, thereby accelerating and forming a comprehensive corrosion.