Handling and Downgraded Use of Oxidized Molybdenum Electrodes

Molybdenum begins to oxidize appreciably above 400 degrees; the oxide formed is pale yellow to white, bulkier than the metal itself, and builds a loose layer on the surface. Electrodes withdrawn at shutdown and left in air for a while will often show discoloration, which is not surprising. The point is to judge how far the oxidation has gone and whether the electrode can still be used.

First separate surface oxidation from internal damage

A surface oxide layer is usually uniform in colour and limited in thickness; it can be removed easily with a scraper, exposing a silver-grey metallic lustre. If the layer is blotchy, with local spalling pits, or if the electrode rings dull when struck, oxidation may already have progressed inward. Another case that must be distinguished is in-service erosion: erosion edges are rounded and the bottom is dense, unlike the loose structure of oxide, and the response is different.

Oxidation usually starts at the end face and the melt line, because both run hot and see frequent atmosphere changes. Inspection can proceed zone by zone rather than judging only the overall appearance of the electrode. Differences between zones on the same electrode say more about service conditions than the average thinning does.

How to assess oxidation depth

In the field, the diameter can be measured with a vernier caliper at several cross-sections and compared with the original size to calculate the reduction per side. Measuring points should include the end, the middle and the vicinity of the melt line, since temperature and exposure differ at each. On threaded or connecting sections, check in particular that the thread form is intact. Where conditions allow, ultrasonic thickness measurement gives a more continuous thickness profile, but it requires coupling and a flat surface.

• Small, evenly distributed thinning: grind and continue in service.
• Marked local thinning with pitting at the end: assess for downgraded use.
• Through-wall cracks or damaged threads: consider scrapping.

Grinding and re-inspection

The purpose of grinding is to remove the loose layer and surface defects, not to reduce the whole electrode. Fine abrasive paper or a dedicated tool should be used with light strokes along the axis, watching the colour change until dense metal appears. After grinding, rinse with clean water and dry thoroughly so that no residue generates gas on heat-up. Re-inspection covers dimensions, straightness, surface cracks and the fit of connecting sections; the results should be recorded as the baseline for the next assessment.

Where the boundary for downgraded use lies

Downgraded use means placing the electrode in a lighter-duty position, for example a zone of lower power density, or a furnace that does not demand such strict temperature uniformity. Before downgrading, three things must be confirmed: whether the remaining cross-section can carry the current density at that position, whether the connection remains reliable, and whether oxidation has actually stopped. If the electrode keeps oxidizing while in storage, the moisture protection is inadequate, and it should not be used further.

1. Check the relationship between remaining diameter and permissible current.
2. Confirm the mechanical loads at the installation position and the water-cooling fit.
3. Shorten the inspection interval after first operation and watch for abnormal temperature rise.

Downgraded use is not a compromise; it places an electrode with limited remaining life where the risk is controllable.

Throughout this assessment, records matter more than conclusions. Putting the oxidation condition, service hours and post-downgrade performance of the same batch together gradually builds judgment experience suited to the plant. Inspection items and acceptance criteria are given in the quality inspection notes, and product specifications with permissible deviations are in the product data.