Improvement of the application of an electrochemical method for the determination of transport properties of an alumina scale .1. Alumina scale on a beta-NiAl alloy

An electrochemical method based on analyzing the potential-current curves, plotted at various applied Po,for an alumina scale developed at 1100 degrees C on a beta-NiAl alloy, was used to determine transport parameters in the scale. The variation of V-o vs P-O2, indicates that the scale consists of an inside zone characterized by an ionic-transport number t(i) similar to 0.4 and an outside part with t(i) similar to 0.1. This leads to a value of the oxygen pressure at the NiAl/Al2O3 interface similar to 2 x 10(-27) atm, not far from the equilibrium value. The variation of the oxygen chemical potential inside the scale was determined showing a steep variation of mu(O) in the middle of the scab. The calculated oxidation constant is close to the experimental one, which would indicate that mainly charged species are responsible for the alumina scale growth.