Threaded Connections on Molybdenum Electrodes in Rare Earth Electrolysis Cells

Connecting a molybdenum electrode in a rare earth molten salt electrolysis cell looks like nothing more than tightening a thread, but it actually involves contact resistance, preload, galling prevention and high-temperature relaxation. If assembly details are handled poorly, the joint develops heating, loosening or even fracture after a period of operation.

Before Installation: Cleaning and Inspection

Remove oil, oxide scale and metal chips from the thread and contact faces, and check that the thread form is complete and free from knocks. Molybdenum is hard and has limited toughness, so forcing the thread can easily damage it. Fix the electrode with a dedicated fixture so that clamping force is not concentrated on thin-wall sections.

Preload: Enough Is Enough

Insufficient preload raises contact resistance, while excessive preload may crack the thread or the connector. Specify a torque range according to the connector material and thread size, and tighten in stages with a torque wrench. Under high-temperature duty also allow for differential expansion and leave room for compensation.

Galling Prevention and Sealing

Molybdenum threads readily gall against dissimilar metals at high temperature. A suitable high-temperature lubricant or anti-galling coating may be used, provided it is confirmed not to contaminate the electrolyte. Sealing faces should be flat, and the gasket material must be compatible with the molten salt and the temperature so that it does not harden and fail in service.

Dismantling: Avoid Forcing a Cold Joint

If the connector is already oxidized or seized at temperature, cold forcing during maintenance will tear the thread. Cool down to the process requirement, clean the surface, then dismantle progressively using a penetrating agent and dedicated tools. Number the removed electrodes and connectors, record their service position and store them separately in a dry environment so that refitting alignment and life statistics remain possible.

Rechecks During Operation

After commissioning, check joint temperature, voltage drop and tightening condition at regular intervals. If local heating, falling current efficiency or abnormal vibration appears, stop the cell and inspect it rather than running it in a degraded state. For spare-parts management, threaded connectors and gaskets are best stocked as complete sets by maintenance batch.

Connection details determine how steadily the cell runs.