How to prevent the molybdenum support rods from being eroded and avoiding glass color contamination?

1. Physical Isolation: Prohibit the molybdenum rods from entering the molten glass

◦ The molybdenum support rods of the electrode support are arranged in the gas phase area above the glass liquid surface, not extending into the molten glass liquid surface; they are only used to support the upper electrode structure and do not come into direct contact with the glass liquid.
 

◦ If the electrode base is close to the liquid surface: Use high-purity alumina/zirconia/silicon nitride insulation protection sleeves to wrap the root of the electrode to block the splashing of the glass liquid and the condensation of glass vapor from dripping onto the molybdenum support rods.
 

◦ Diverting plates / flow-blocking screens: Add high-purity ceramic screens below the molybdenum rods to prevent the splashing of glass droplets and the condensation of alkali metal coolant from flying onto the surface of the molybdenum support rods.
 

2. Reduce the exposure of molybdenum parts to alkali vapor-rich areas

The sodium-calcium glass melting kiln contains a large amount of Na and K vapor, which reacts with molybdenum to form low-melting-point molybdate (sodium molybdate / potassium molybdate). However, molybdate has a low melting point and will flow, flake off, and fall into the glass, causing black stones and coloration.
 

Therefore, we need to place the MoLa molybdenum support rods as much as possible in the position with higher kiln temperature and lower alkali vapor concentration;

utilize the kiln air flow design to avoid alkali vapor condensation on the molybdenum rods; the lower temperature area is more prone to condensation of alkali vapor, and corrosion will accelerate sharply.

1. Prefer MoLa (molybdenum-lanthanum alloy), reject Mo1; do not use TZM in sodium-calcium kilns

◦ MoLa has the strongest resistance to alkali metal vapor corrosion, with a corrosion rate much lower than pure molybdenum Mo1 and TZM; it produces fewer corrosion products and reduces the risk of contamination.

◦ The raw material billets must be selected with high purity molybdenum (Mo ≥ 99.95%), with low impurities: Fe, Ni, Cr, Cu are preferred. Iron and nickel are important sources of glass coloration.
 

2. Surface Treatment Selection

Recommendation: Stress relief annealing + fine polishing

◦ Polishing: Reduce surface roughness. Rough surfaces are more likely to adsorb alkali vapor and accumulate deposits, accelerating corrosion; Ra 0.8~3.2μm is suitable for glass kiln conditions.

◦ Annealing to relieve stress: Eliminate machining stress to avoid cracks; Cracks are more likely to accumulate alkali vapor and cause local corrosion and peeling.
 

Not recommended to apply coatings on the surface of molybdenum rods (such as Al₂O₃, ZrO₂ coatings)

In a high-temperature + alkali vapor environment, ceramic coatings are prone to thermal cycling cracking and peeling off. The fallen coating particles directly fall into the glass melt, causing stone contamination, with greater risks.

 

1. Strictly control oxygen content to inhibit oxidation

＞At 600°C with oxygen, molybdenum forms MoO₃ (easily volatile); MoO₃ reacts with alkali vapor directly to form molybdate.

◦ Kiln body sealed, strictly control air leakage, maintain the residual oxygen in the kiln at an extremely low level;

◦ The heating / cooling stages are particularly crucial: In the low-temperature section (＜600°C), a small amount of air can be allowed; above 600°C, a protective atmosphere (N₂/ inert / reducing atmosphere) must be maintained, and air must not enter;

◦ Avoid local negative pressure to draw in external air.
 

2. Stable kiln temperature, reduce temperature fluctuations.

Temperature cycling will: accelerate molybdenum recrystallization, causing intergranular corrosion; alkali metals repeatedly condense - evaporate, repeatedly erode the molybdenum surface.

◦ Try to achieve continuous and stable production, reduce frequent shutdowns and cold-hot cycles;

◦ Avoid local hotspots on molybdenum support components, local overheating will significantly increase the corrosion rate.
 

3. Control of electrode corrosion

Molybdenum supports cannot be directly adhered to nickel-based alloys or stainless steel electrode bases; contact between dissimilar metals at high temperatures will cause electrochemical reactions, accelerating molybdenum corrosion and generating corrosion debris that contaminates the glass.

The solution is to use molybdenum gaskets for isolation, and all contact surfaces should use molybdenum transition gaskets.

1. Reserve thermal expansion gap during assembly

Rigid fixation can cause thermal stress, resulting in cracking of the molybdenum rod and fragments falling into the glass.
 

2. Regular on-site inspection (infrared temperature measurement + visual maintenance)

Regularly monitor the deformation of the molybdenum rod and surface pitting:

◦ If obvious spots, pits, or local thinning appear on the surface, replace it proactively instead of waiting for it to break.

◦ During maintenance, clean the accumulated alkali salt deposits on the molybdenum rod surface to prevent continuous corrosion of the base material.
 

3. Spare parts cleaning management

Before installing the new molybdenum rod, perform degreasing and cleaning to remove machining oil stains and metal dust; external dust brought into the kiln is also a source of glass defects.