Effect of different Mn contents on tensile and corrosion behavior of CD4MCU cast duplex stainless steels

Despite the significant use in the industry, the effect of Mn on the tensile and corrosion behavior of cast duplex stainless steels has not been well established. In the present study, the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mn contents of 0, 0.8 and 2%, respectively, was therefore examined. The polarization and the in situ slow strain rate tests were conducted in 3.5% NaCl + 5% H2SO4 aqueous solution to quantify the resistances to pitting corrosion and stress corrosion cracking with different Mn contents. The addition of Mn, which stabilized ferrite in the present study, affected the microstructure of the present alloy, and eventually the tensile and corrosion behaviors in a complex manner. Tensile properties of CD4MCU cast duplex stainless steel, for example, was found to be determined by the volume fraction of hard ferritic phase and the shape of austenitic phase. The addition of 0.8% Mn was detrimental to both pitting corrosion and stress corrosion cracking properties of CD4MCU cast duplex stainless steel due to the significant increase in contact area between the less-noble ferritic and the noble austenitic phases. With the addition of 2% Mn, the resistance to pitting corrosion and stress corrosion cracking in 3.5% NaCl + 5% H2SO4 aqueous solution is recovered. The resistance to stress corrosion cracking of the specimen with 2% Mn was still greatly inferior to that of the 0% Mn counterpart. The relationship between the microstructural evolution and the tensile and corrosion behavior of CD4MCU cast duplex stainless steels with different Mn contents was discussed based on the micrographic and fractographic observations.