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The electrochemical behavior of 2205 duplex stainless steel in acidic, neutral and alkaline solutions with different pH values in the presence of 3.5% NaCl was evaluated by different techniques: cyclic potentiodynamic measurements, electrochemical impedance spectroscopy (EIS) and capacitance measurements (Mott–Schottky approach). In addition the surface morphology and the site of pitting nucleated preferentially of the duplex stainless steel was investigated by scanning electron microscopy (SEM). Mott–Schottky analysis revealed that passive films formed on duplex stainless steel, behaved as n-type and p type semiconductor (double structure). In neutral and acidic solutions, n-type semiconductor behavior was in the potential range of -0.4 up to 0.4V. Similar behavior in alkaline solutions in the potential ranges of 0.1 to 0.35V happened; but p-type semiconductor behavior for the potential range of around 0.4 to 0.7V in acidic and neutral solutions and of -0.4 to 0.1V and 0.35 to 0.7V for alkaline solutions happened. The results indicated that the pH play important roles in the evolution of the film resistance, charge transfer processes and the occurrence of pitting. In acidic solutions the decrease of pH value caused more weight loss and in alkaline solutions, decreasing the solution pH offered better conditions for forming passive films with higher protection behavior, due to the growth of a much thicker and less defective film. At pH 10, almost largest passive range, smallest passive current density and highest polarization resistance; the most stable passive film was produced.

Keywords: Duplex Stainless Steel, Passive, Mott-Schottky, EIS, Cyclic Polarization Curve

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