Electrochemical characterisation of novel γ/γ′-strengthened Co-base superalloys

Abstract

Electrochemical behaviour of a new γ/γ′-strengthened Co-base superalloy with the composition Co–9Al–9W–0.12B (in at%) was studied in comparison with pure Co at room temperature in 1M Na2SO4 (pH 5.9) and 0.1M NaOH (pH 12.8) aqueous solutions. Pure Co and the Co-base superalloy show passive behaviour in both solutions. Impedance spectra measured at the corrosion potential indicate comparable impedance values for the alloy and pure Co in neutral solution, but in alkaline solution the alloy shows a lower corrosion rate. Polarisation curves indicate a limited range of passivity for both materials in neutral solution. In alkaline solution, the alloy and pure Co show similar (albeit not identical) primary and secondary ranges of passivity, as previously described in the literature for pure Co. Furthermore, lower passive current densities are observed for the superalloy in the primary range of passivity. As the superalloy studied in this work is a promising material for high temperature applications, we moreover compared the corrosion behaviour of oxidised alloy surfaces with the bare alloy surface. For this, isothermal oxidation of the γ/γ′-strengthened Co-base superalloy was carried out in air at 800°C and 900°C for 24h and 500h. Compared to unoxidised Co–Al–W–B alloys and pure Co samples, the high temperature oxide scales provide orders of magnitude higher protection with good barrier properties in a potential range between −1 and +2V (vs. Ag/AgCl). In general, longer oxidation times and a higher oxidation temperature result in the formation of more protective oxide layers. The protective effect of the high temperature oxide layers is discussed in view of their thickness and chemical composition.