Molybdenum Electrodes for Rare Earth Molten-Salt Electrolysis: Fluoride Salt Corrosion and Conductor Bar Life Management

Rare earth metals are mostly produced by fluoride molten-salt electrolysis: rare earth oxide is dissolved into an NdF₃-LiF system and electrolyzed at 1,000-1,100℃, metal deposits at the cathode, and oxygen released at the anode reacts with graphite to form CO and CO₂. This duty combines high temperature, aggressive corrosion and high current, with cell currents from several thousand to tens of thousands of amperes. Molybdenum electrodes here serve both as cathode conductors and as metal receivers, and the material logic differs completely from glass furnaces - glass electrodes fear oxidation, while here the fear is fluoride salt penetrating along grain boundaries.

Four Molybdenum Parts, Each Carrying a Different Risk

The first is the cathode conductor bar and electrode rod, which need high-temperature strength and resistance to molten-salt wash; diameters commonly run 30-60 mm, with the lower end often tapered or threaded for extension to make reliable contact with the salt zone. The second is the metal receiving crucible and liner: the deposited liquid rare earth metal is also a strong corrosive to most crucible materials, and molybdenum is one of the few choices that balances temperature resistance, corrosion resistance and cost, commonly with an outer diameter of 300-600 mm, depth of 400-800 mm and wall thickness of 8-15 mm. The third covers structural parts such as feed pipes and thermocouple wells. The fourth is the cell's conductive connection hardware. High-current cells require flatness and roughness at contact faces close to busbar engineering practice, and any point of high contact resistance causes local overheating.

Basis for testing and judging the above: electrode products per YS/T 1312-2019 Molybdenum Electrodes; molybdenum rods per ASTM B387 / GB/T 17792; molybdenum plates and discs per ASTM B386 / GB/T 3876; a certificate of quality accompanies each shipment and batches are traceable.

Three Failure Channels of Fluoride Salt Corrosion

Uniform thinning with grain-boundary penetration is the main form: fluoride molten salt enters the molybdenum body along grain boundaries and causes embrittlement without obvious thinning. Thermal-cycle cracking from frequent cell start-up and shutdown is more common in plants with irregular schedules; interfacial corrosion at the gas-liquid-solid triple junction on the crucible inner wall directly determines the crucible's campaign life. Purity and densification are therefore the core variables, and hot isostatically pressed crucibles have far better density and microstructural uniformity than ordinary pressed-and-sintered parts.

TZM and MoLa have advantages in high-temperature strength, but the stability of their titanium and zirconium content in a given fluoride salt must be verified for that salt system; glass furnace experience cannot be transferred directly. Before changing material, run a coupon test, record thinning rate, grain-boundary morphology and mechanical property changes, and only then decide whether to scale up. For continuously operating cells, material qualification usually takes months, and a short trial should not be used to draw conclusions.

Current Distribution and Contact Management

Current distribution in a large cell determines the temperature field and the local load on conductor bars. When several bars are connected in parallel, small differences in contact resistance cause current bias; the biased bar runs hotter and wears faster, creating a vicious cycle. At installation, ensure contact-face flatness and cleanliness, apply the design tightening torque, and retighten once during the early operating period. During operation, infrared temperature measurement or loop-current monitoring can reveal bias for timely correction.

Bars connect to the busbar by clamping, threading or welding. Clamping is easy to replace but the contact face oxidizes readily; threading is reliable but tends to seize on removal; welding gives low contact resistance but is hard to repair on site. Choose according to maintenance habits and cell structure, not on initial resistance alone. Whichever method is used, leave temperature measurement points and maintenance access.

Start-Stop Regime and Thermal Cycling

Frequent start-stop is a hidden killer of molybdenum part life. With every heat-up and cool-down, the crucible and conductor bar undergo expansion and contraction, stress concentration at grain boundaries accumulates, and cracks form. Production planning should minimize unnecessary starts and stops; when a cell must be stopped, follow a staged cooling curve and avoid direct cold-air blowing or rapid quenching. Keep the cell dry during shutdown to prevent residual salt from absorbing moisture and corroding surfaces.

New cell start-up also needs a controlled rhythm. Heating straight to full power from cold makes the temperature difference across the crucible wall too large and generates excessive thermal stress. The reasonable approach is staged heating and staged holding, then gradual current loading once the temperature is uniform. The current ramp rate during start-up should be written into the operating procedure rather than left to shift experience.

Supporting Changes as Cells Grow Larger

Rare earth cells are moving toward higher current; as cell current rises, the cross-section, number and cooling method of conductor bars must be rematched. Insufficient cross-section raises current density and end temperature; insufficient number overloads each bar; insufficient cooling accelerates oxidation and corrosion. Large cells also demand higher installation accuracy: the parallelism between bars, insertion depth and their relationship to the cell centerline all affect current distribution.

The other side of larger cells is a bigger loss per failure. One failed conductor bar can shut down the whole cell, so the spare-parts strategy should shift from "buy when broken" to "stock per cell". A common practice is to hold 20% of the in-service quantity in standard sizes and agree an expedited delivery cycle with the supplier. For critical cells, keep a service file for each conductor bar recording cumulative operating hours and failure modes, as a basis for the replacement cycle.

Cell Voltage and Energy Consumption

Cell voltage is the sum of the theoretical decomposition voltage, the inter-electrode resistance drop and the contact drop. Too large an electrode gap raises melt resistance and cell voltage, increasing power consumption; too small a gap intensifies gas and metal misting near the cathode and lowers current efficiency. The contact drop between bar and busbar is only part of the total but easily runs out of control, because it rises gradually with oxidation and loosening during operation.

The way to cut energy is not simply to lower voltage but to manage each component separately. Melt temperature and salt composition set conductivity and belong to process; gap and anode structure belong to design; contact-face condition belongs to maintenance. Of the three, maintenance is easily neglected and easily improved through inspection. Put contact-point temperature, loop current and cell voltage on the same record sheet; when cell voltage rises while process parameters are unchanged, check the contact faces first.

Material Verification and Contract Terms

Rare earth electrolysis duties vary widely, and salt composition, current density and start-stop regime all affect molybdenum part life, so material cannot be judged by grade name alone. The technical agreement should define acceptance criteria for purity, density, grain size and mechanical properties, and specify the evaluation method for a coupon test or first-batch trial period. For crucibles, wall thickness tolerance, bottom structure and flaw-detection requirements should be stated; for conductor bars, diameter tolerance, thread accuracy and end treatment.

The contract should also cover transport and packaging. Molybdenum parts that get damp or take impact during transport show oxidation or hidden cracks on the surface, which may surface during heat-up after installation. Use moisture-barrier packaging and dedicated supports, and inspect appearance and dimensions piece by piece on arrival.

Current, diameter and crucible dimensions in this article are common ranges; the actual configuration should follow the cell design, salt conditions and the technical agreement between the parties.