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The straddle suspender of a suspension bridge is made up of wire ropes. The suspension bridges of cross-sea bridges are located in a marine atmospheric environment. Sulfur dioxide is an important component of this environment; it can increase the corrosion rate and result in suspender fracture. The corrosion mechanism of a steel wire is explained macroscopically and microscopically. As indicated in the digital images, scanning electron microscope, and X-ray diffractometer, the indoor experiment reacted faster in the first 6 days but slower after the 6th day. The experiment tended to stabilize from the 12th day to 15th day. The corrosion products in the low SO 2 concentration condition are γ-FeOOH (lepidocrocite) and Fe 3 O 4 (magnetite) while products in the high SO 2 concentration condition are γ-FeOOH (lepidocrocite), α-FeOOH (goethite), and Fe 3 O 4 (magnetite). The composition of the corrosion products did not change significantly from the 6th day to 12th day. The higher the SO 2 concentration is, the more severe corrosion reacted on Fe. However, due to the effect of α-FeOOH, the Fe substrate was protected in the high SO 2 environment. A high SO 2 concentration generates more α-FeOOH. The tendency that the formula is exponential at both high and low SO 2 concentration is similar.