Electrochemical Impedance Study of Hot Corrosion of the Mo-Rich Ni_3Al-Base Alloy IC6 at 750-800 °C Beneath Solid Na_2SO_4 Deposits

摘要

A two-electrode probe was used for electrochemical impedance studies of hot corrosion kinetics of molybdenum-containing Ni_3Al-base alloy IC6 covered with a solid Na_2SO_4 film at 750 and 800 °C in air. The alloy was subject to catastrophic corrosion at both temperatures, forming a thick porous oxide layer, as a result of the formation of Na_2MoO_4-MoO_3-Na_2SO_4 melt. The experimental temperature affected the formation of the melt, the ionic conduction of the corrosion layer, and thus the impedance characteristics. For the corrosion at 800 °C the Nyquist plots were composed of two capacitive loops at high-mid frequency and a line at low frequency indicating a diffusion-controlled reaction. At 750 °C, however, the plots consisted of a single capacitive loop in the initial stage, followed by the same impedance features as at 800 °C. Two equivalent circuits were proposed to fit the impedance spectra at the two temperatures. Based on the precise measurements of diffusion impedance, the diffusion flux of oxygen through the salt layer was calculated, and the main reduction reaction was also discussed.