Effect of Nb alloying additions on the characteristics of anodic oxide films on zirconium and their stability in NaOH solution

The characteristics of oxide films on Zr and Zr– Nb alloys (with Nb content of 2.5, 5, and 10 at.%) galvanostatically formed (at a current density of 100 μA cm−2) in 0.5 M H2SO4 solution were investigated by means of electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy spectra were interpreted in terms of an “equivalent circuit” with the circuit elements representing the electrochemical properties of a single layer oxide. The resistance of the oxide films was found to increase with increased Nb content in the alloy while the capacitance showed an opposite trend. The stability of the anodic oxide films grown in the sulfuric acid solution on Zr and Zr–Nb alloys was investigated by simultaneously measuring the electrode capacitance and resistance at a working frequency of 1 kHz as a function of exposure time to naturally aerated 3 M NaOH solution. Analyses of the electrode capacitance and resistance values indicated a decrease in chemical dissolution rate of the oxide films with the increase of Nb content in the alloy.