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TIG welding experiment of Zr-702 zirconium plate was carried out with 99.999% high purity Ar as shielding gas. The Zr-702 welding joint was studied at 250%, 15%(ω) by static isothermal weight loss method. Influences of soaking time in sulphuric acid medium on its corrosion performance and explore its corrosion mechanism. The results show that the corrosion rate of welded joint is higher than that of base metal in the same temperature and concentration and different soaking time in sulfuric acid medium. When soaking time is 2 days, both welded joint and base metal have good corrosion resistance. The corrosion rate of welded joint and base metal increases significantly when the soaking time increases to 180 days, and the corrosion rate of welded joint and base metal is greatly affected by the soaking time. According to the characterization and analysis of composition, structure and properties of 180-day soaked samples, a large amount of zirconium hydride was precipitated in weld zone and heat-affected zone of weld joint, and the amount of zirconium hydride in base metal or base metal zone was relatively small. The corrosion of Zr-702 base metal in sulphuric acid medium is the combination of uniform corrosion and hydrogen corrosion. Potential difference exists in weld joint due to obvious structural gradient in each zone. Therefore, its corrosion is the local corrosion combined by galvanic corrosion and hydrogen brittleness.