Dissolution and electrochemical properties of molybdenum carbide (Mo2C) in basic solutions

The dissolution and electrochemical properties of molybdenum carbide were investigated using a number of electrochemical techniques, including general corrosion tests (GCT), linear voltammetry (LV), potential controlled electrolysis (PCE) and Scanning Electrochemical Microscopy (SECM), in basic media (0.5-4 M NaOH and 1 M NaHCO 3 /Na 2 CO 3 , pH 9.1-11.3). It was shown that the Mo 2 C corrosion potential (E corr ) shifted towards negative values from -0.39 to -0.96 V/SCE with an increase of the OH - concentration and did not depend on the CO 3 2- concentration in the electrolyte. LV measurements in carbonate buffer (pH 9.2), evidenced three potential regions: passivation from E corr to -0.2 V/SCE, pseudopassivation from -0.25 to 0 V/SCE and anodic dissolution (transpassivation) at E > 0.1 V/SCE. The potentials delimiting the mentioned regions decreased with the increase of the OH - concentration, becoming undistinguished in 2M NaOH. The Mo 2 C dissolution rate (k diss ) in the transpassive state was estimated using LV data. In 4M NaOH at -0.1 V/SCE, k diss reached 430 mg cm -2 h -1 and decreased with the decrease of the OH - concentration and the electrolysis potential. The Mo 2 C dissolution current efficiency varied between 12 and 13 F mol -1 Mo 2 C, proving the formation of the intermediate products of a carbon oxidative degradation during PCE. The presence of C 2 O 4 2- and non-identified aromatic compounds in the electrolytes after Mo 2 C dissolution was ascertained, using capillary zone electrophoresis. Scanning electrochemical microscopy (SECM) confirmed the formation of a pseudopassive film and demonstrated the increase of its thickness with the increase of the applied potential. The effect of the presence of Mo 2 C on the irradiated UC fuel dissolution rate and the mechanisms involved are discussed.