The electrochemical behaviour of 2205 duplex stainless steel in alkaline solutions with different pH in the presence of chloride

The electrochemical behaviour of 2205 duplex stainless steel in alkaline solutions with different pH values in the presence of NaCl was evaluated by different techniques: potentiodynamic measurements, electrochemical impedance spectroscopy and capacitance measurements (Mott–Schottky approach). The chemical composition was studied by X-ray photoelectron spectroscopy (XPS). In addition, the site of pitting nucleated preferentially on the duplex stainless steel had been confirmed by scanning electron microscopy (SEM). The results indicated that both the pH and immersion time play important roles in the evolution of the film resistance, charge transfer processes and the occurrence of pitting. The composition of the surface film changes with the pH value. The film formed in the alkaline solution, presenting a bilayer structure, predominantly contained Fe species and Cr-oxide. The NH3/NH4+ species in the passive film are disappeared in the pH 10.5 solution. The pit initiation sites occurred in austenite phase near the γ/δ boundaries or at inclusions. The number and size of the pits increases with the decrease of pH value after long time immersion.

► The pH plays important roles in 2205 DSS passive film composition and occurrence of pitting.
► The passive film of 2205 DSS in the alkaline solution, presenting a bilayer structure, composed of an outer and inner layer.
► As the pH increases, there is a gradual enrichment in chromium and nitrogen in the surface film.
► The peak of NH3/NH4+ is disappeared in the pH 10.5 solutions.
► The pit initiated at inclusions or in austenite phase near the γ/δ boundaries.