Cathodic Wear by Delamination of the Al 4 C 3 Layer During Aluminium Electrolysis AI ¨ MEN E. GHERIBI, MOJTABA FALLAH FINI, LOIG RIVOALAND, DIDIER LOMBARD, GERVAIS SOUCY, and PATRICE CHARTRAND In aluminium reduction cells, an electrochemical reaction occurs between the molten electrolyte film below the aluminium pad and the carbon cathode blocks, leading to the formation of an Al 4 C 3 layer on the cathode blocks. The properties and role of this Al 4 C 3 layer are therefore important for the aluminium production industry, as they could help increase the life expectancy of electrolysis cells and impact the resistive voltage losses. The purpose of this study is to gain a better understanding of the formation, growth and mechanical stability of the aluminium carbide layer formed on top of the cathode block. A reliable scenario describing both the mechanical and electrochemical behaviours of the Al 4 C 3 layer is proposed. For different industrial graphitized cathode grades, a series of experiments were carried out in a bench-scale Hall-He´ roult electrolysis cell and the Al 4 C 3 layer formed on top of the cathode was characterized. Thereafter, the CALPHAD method was combined with density functional theory simulations to estimate the electrical and physical properties of Al 4 C 3 together with the phase equilibria occurring at the interface between the carbide layer and the aluminium pad and the cathode blocks respectively. From these calculations, a scenario for carbide layer growth and mechanical stability was established. https://doi.org/10.1007/s11663-019-01731-9 Ó The Minerals, Metals & Materials Society and ASM International 2019 I. INTRODUCTION WITH nearly 60 million tonnes per year, aluminium is currently the second most widely produced metal in the world. Aluminium production showed a very sub- stantial increase from 2000 to 2018. To adapt to the continuously rising demand, aluminium primary pro- ducers work on designing higher amperage electrolysis cells (‡ 400 kA) working at higher current to increase their production. However, increasing the amperage also means increasing the probability of a rapid dete- rioration of the cells. Indeed, life expectancy of elec- trolysis cells is too often limited by the cathode blocks; a higher cathode current density accelerates the wear of the graphitized cathode blocks, which may lead to premature death of the cells. The wear mechanism is said to be due to: particle detachment, physical abrasion by solid alumina particles and chemical wear due to the formation of an aluminium carbide (Al 4 C 3 ) layer at the interface between the cathode and metal pad. Alu- minium carbide can be formed either chemically (Chem.) by a direct reaction between the liquid metal and the cathode carbon according to: 4Al ðlÞ þ 3C ðsÞ ! Al 4 C 3ðsÞ Chem. ½1 or via an electrochemical (E. Chem.) reaction between the cathode carbon and the electrolyte or bath ( Na 3 AlF 6 , 11 wt pctAlF 3 , 3 wt pctAl 2 O 3 and 5 wt pctCaF 2 ) used to dissolve the alumina. A film of partially molten electrolyte is present in electrolysis cells between the liquid metal pad and the cathodic carbon blocks due to falling electrolyte particles or droplets (from alumina feeding process) and interfacial phenomena. The electro-chemical reaction at the sur- face of this film is usually reported as: 4AlF 3ðbathÞ þ 3C ðsÞ þ 12e  ! Al 4 C 3ðsÞ þ 12F  bath E. Chem. ½2 When formed, Al 4 C 3 is subsequently slowly dissolved in the electrolyte film to form the Al 3 CF 3  8 complex anions according to the reaction: AI ¨ MEN E. GHERIBI and PATRICE CHARTRAND are with the Department of Chemical Engineering, CRCT - Polytechnique Montre´al, Box 6079, Station Downtown, Montre´al, QC H3C 3A7, Canada. Contact e-mail: aimen.gheribi@polymtl.ca MOJTABA FALLAH FINI and GERVAIS SOUCY are with the Department of Chemical Engineering, Universite´ de Sherbrooke, Boulevard de l’Universite´, Sherbrooke, J1K 2R1, Canada. LOIG RIVOALAND is with the Carbone Savoie, 30 Rue Louis Jouvet, 69200, Ve´nissieux, France. DIDIER LOMBARD is with the Solutions Technologiques Aluminium - LRF, Rio Tinto, 73302, Saint Jean de Maurienne, France Manuscript submitted June 4, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B