An Electrochemical Impedance Study on Cermet Anodes in Alumina‐Saturated Molten Cryolite

Abstract

Electrochemical impedance spectra of NiO-NiFe204-Cu cermet anodes in alumina-saturated molten cryolite at anodic potentials above the decomposition potential of alumina exhibited a loop with a characteristic frequency of about 1 Hz. A similar feature was observed using platinum anodes under the same experimental conditions. Analysis of these data suggests the loop was due to gas bubbling. Features associated with charge-transfer processes were not sufficiently resolved to determine the corrosion properties of the cermet anode.Cermet anodes are currently being evaluated as candidate nonconsumable substitutes for carbon in HallHeroult cells for the commercial production of aluminum metal. These cermet anodes must corrode or wear at a low rate under electrolysis conditions to be acceptable. In this work, electrochemical impedance spectroscopy (EIS) was used to compare the electrochemical reactions occurring on a cermet anode with those occurring on platinum metal during electrolysis in alumina-saturated molten cryolite. The electrolytic production of oxygen gas is known to be the principal reaction for a platinum anode under these conditions. Thus it was hoped that significant differences between EIS data for platinum and the cermet anodes would suggest undesirable corrosion reactions for the cermet anode, while lack of significant differences between the two materials would indicate that the cermet was largely "inert." Unfortunately, from the aspect of corrosion analysis, the electrochemical impedance spectra for both the cermet anode and the platinum anode were dominated by a loop that appears to be due to gas bubbling. The loop obscured the characteristics of the charge-transfer process. Nevertheless, the EIS spectra are significant in that they are some of the first obtained on the behavior of gas-generating electrodes in a molten salt. ExperimentalCermet anodes of the type NiO-NiFe204-Cu were prepared from a spray-dried oxide powder blend fabricated by Stackpole, Inc., Pittsburgh, Pennsylvania (synthesized from 51.7 weight percent [w/o] NiO and 48.3 w/o Fe203) and Cu metal at 17 w/o of the cermet product, using methods reported previously (1). The cylindrical anodes were sheathed with boron nitride to expose only the bottom 1 cm 2 of surface area to the electrolyte. Platinum anodes were made from 1/8 in. diam rod obtained from Johnson Matthey, Inc. (Seabrook, New Hampshire). The electrolyte was composed of natural Greenland cryolite and excess A1F3 to give a bath ratio (NaF/A1F3 w/o) equal to 1.15, 5.5 w/o CaF2, 1.0 w/o MgF2, and A1203 at 8.0 w/o (saturation). The electrolyte composition is representative of the bath compositions used in commercial aluminum production facilities. The bath temperature was 983~ Electrochemical impedance spectra were obtained using a Solartron 1286 Electrochemical Interface and Solartron 1250 Frequency Response Analyzer. A three-electrode system was used with the anode as the working electrode, the graphite crucible as the counterelectrode, and an AYA1203 reference electrode. The ...