Visualization of the impact of fluorocarbon (CFₓ) passivation films and bubble layers on the anode effect

The Anode Effect (AE) in primary aluminum production is a critical issue for the industry, contributing significantly to greenhouse gas emissions and current efficiency. This study investigates the role of the bubble layer and fluorocarbon (CFₓ) passivation film on a 50 cm² industrial carbon anode during AE initiation, using in-situ video observation in a high-temperature transparent electrolytic cell at 940 °C, combined with non-in situ X-ray photoelectron spectroscopy (XPS) and gas chromatography techniques. Additionally, it analyzes the reasons for gas changes before and after AE. Video observations combined with XPS analysis confirm that the bubble-free region formed during the electrolysis stage contains a complex high-resistance and insulating CF x film, which is the root cause of AE. CF x cannot continuously generate and cover the electrode surface, nor can it quickly trigger new AE again. The bubble layer covering the anode is a secondary phenomenon resulting from the deterioration of wettability caused by CF x . CF x plays a central role in impeding current flow, increasing cell voltage, and halting electrochemical activity in the affected areas. The composition of CF x can be altered by AE, leading to variations in back electromotive force(BEMF). Gas chromatography detection confirms that even after AE, the partial CF x film remains on the anode surface, where it forms a "memory effect" and decomposes at high temperatures to produce PFC. In industrial testing, the changes in anode gas composition before and after AE can be attributed to the CF x film formed on the anode surface and its degree of fluorination. This study suggests that targeting CFₓ layer formation, rather than bubble layer dynamics, could be a more effective strategy for reducing AE frequency, enhancing energy efficiency, and minimizing emissions in aluminum smelting. These insights offer a refined understanding of AE mechanisms for improving the sustainability and stability of industrial aluminum production processes.