Abstract: Duplex stainless steel (DSS) has been applied to marine industries, chemical plants, and nuclear facilities because of its high mechanical strength, good weldability, good resistance to pitting corrosion in various aggressive environments, and relatively low cost than similar-performance materials. However, with the decline of iron and steel industries, the competition among stainless steel products is becoming fierce. To reduce the costs of stainless steel, enterprises have attempted to replace high-priced alloy elements with low-priced alloy elements. Thus, the project of substituting manganese for nickel has attracted considerable attention. In this study, effect of manganese addition in the range of 0. 93%-1. 26% on the resistance to pitting corrosion of duplex stainless steel in chloride medium was investigated. Chemical corrosion and potentiodynamic anodic polarization tests were conducted to assess the resistance to pitting corrosion. Categories and number of inclusions were analyzed using inclusion automatic analysis technology. Changes of inclusions before or after chemical and electrochemical corrosion were observed using in situ observation, scanning electron microscope and energy dispersive spectrometer analysis. Inductively coupled plasma test was conducted to analyze compositions of corrosion products. The results indicate that different inclusions have different effects on pitting corrosion. Types of (Mn, Si) oxides and complex inclusions of (Mn, Si, and Cr) oxysulfides are dissolved, which accelerates the corrosion process. However, the types of (Cr, Mn, and Al) oxides are stable in similar environment. The addition of manganese leads to the deterioration of the resistance of dplex stainless steel to pitting corrosion because manganese accelerates the formation of (Mn, Si) oxides that act as initial locations of pitting corrosion in chlorine ion corrosion environment. Dissolved inclusions help to expose fresh matrix to etchant solution, accelerating the corrosion ofthe matrix. Thus, the corrosion of the duplex stainless steel matrix is more serious than that of the inclusions.