Electrochemical behavior of nickel-chromium alloys in a high-temperature aqueous electrolyte

This paper describes the electrochemical behavior of oxide films formed on pure Ni, pure Cr, and Ni-Cr alloys at elevated temperatures, 200degreesC and 300degreesC. The applicability of the mixed-conduction model (MCM) to explain the features observed on Ni-based materials and Cr was studied. The MCM emphasizes the coupling between the spatial and potential distribution of ionic point defects in the film and the electronic conductivity of the film. The experimental part of this work includes measurements with pure Ni, pure Cr, and three Ni-Cr alloys (10, 15, and 20 wt% Cr) in 0.1 M sodium tetraborate (Na2B4O7) at 200degreesC (pH(200) approximate to 8.7) and 300degreesC (pH(300) approximate to 9.1) using cyclic voltammetry, contact electric resistance (CER), and electrochemical impedance spectroscopic (EIS) techniques. The measurement results showed that the electrochemical behavior of Cr dominated the electrochemical response of the alloys in the passive region at 200degreesC, but in the transpassive region Ni dominated. The passive region was quite narrow at 300degreesC and the transpassive oxidation played a major role in the behavior of all studied materials in a wide potential region. The features of the model and the experimental results were found to be in qualitative agreement. A mass-transport impedance was found to have an important role in the behavior of the oxide films at 200degreesC, whereas interfacial charge-transfer processes seemed to dominate at 300degreesC.