Effect of microstructure on the corrosion resistance of 2205 duplex stainless steel. Part 1: Microstructure evolution during isothermal aging at 850 °C and evaluation of anticorrosion properties by methods of cyclic potentiodynamic polarization and electrochemical impedance tests

Abstract

Some detrimental phases, such as nitrides, carbides and intermetallics may precipitate due to improper manufacturing or fusion welding process and affect the mechanical properties and corrosion resistance of duplex stainless steels (DSSs) dramatically. In this work, the solution-annealed 2205 DSS samples were aged at 850 °C for different time to stimulate typical welding microstructures at different cooling rates and their corrosion resistance were assessed by methods of cyclic potentiodynamic polarization and electrochemical impedance tests. Microstructural characterization indicates that chromium nitrides and carbides lamellas precipitate first in the initial several minutes and chi phase nucleates and grows at ferrite grain boundaries or austenite/ferrite phase boundaries then. With aging time prolonged to 60 min plentiful sigma phase together with the secondary austenite precipitates alongside prior austenite/ferrite phase boundaries and propagates into ferrite. Electrochemical tests demonstrate that chromium nitrides and carbides precipitates accelerate the activated corrosion of matrix and have less impact on its pitting potential. Precipitation of chi and sigma can cause chromium and molybdenum depletion in surrounding areas and increase the susceptibility dramatically. So, it is necessary to optimize the welding specifications in practice to obtain microstructure that combines mechanical strength and corrosion resistance excellently.