Corrosion and passive behaviour of duplex stainless steel 2205 at different cooling rates in a simulated marine-environment solution

The corrosion and passive behaviour of duplex stainless steel 2205 at six cooling rates (1, 5, 10, 15, 20 °C s⁻¹ and water quenched) in a simulated marine-environment solution was investigated using electrochemical measurements of potentiostatic critical pitting temperature, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and Mott–Schottky curves. The microstructural evolution and pitting morphologies of the specimens were visualised using an optical microscope and scanning electron microscope. The electrochemical responses of the passive film show that passivity of the steel was enhanced as the cooling rate increased; however, the threshold cooling rate was 20 °C s⁻¹, beyond which pitting corrosion resistance remained stable. Based on the analyses of microstructural evolution and pit morphologies, the proportion of the ferrite phase increased with the cooling rate and the ratio of austenite and ferrite was close to 1:1. The pitting size decreased as the cooling rate increased, and most metastable pits on specimens were located in the ferrite phase and on the ferrite–austenite interface. Thus, pitting resistance of steel is governed by the phase that provides the lowest pitting resistance equivalent number. The optimised pitting corrosion resistance for steel 2205 was achieved when it was greater than or equal to 20 °C s⁻¹.