Chi Phase after Short-term Aging and Corrosion Behavior in 2205 Duplex Stainless Steel

Correlation between pitting corrosion behavior and chi (X) phase formed after a short-term aging (5, 10 and 15 min) at 850 °C of 2205 duplex stainless steel (DSS) was investigated using potentiodynamic polarization tests, optical microscopy, and scanning electron microscopy equipped with energy-dispersive spectrum system. Results showed that after aging for 5 min, the X phase initially precipitated at ferrite grain boundaries, developed and then became linked with prolonging aging time. The X phase was rich in Cr and Mo, resulting in formation of depleted zones nearby. The X phase could reduce corrosion resistance of OSS and slightly influence its stability, but the specimens still displayed the capacity for repassivation. Some lines of evidence showed that stable pitting corrosion initiated at the boundaries of precipitates. The X phase was selectively corroded during the first stage of corrosion and then the depleted zones nearby were attacked. In addition, the grain size and volume of precipitates also affected pit nucleation and progress, and suitable size and distribution of X phase could aggravate pit initiation at precipitate boundaries. The X phase with considerably low volume fraction and small size was not sensitive position for pit initiation.