Zelebium was a substance used for the construction of contact points for the guidance systems of class 9 cargo vessels. If these points were not properly maintained, over time, they could fuse together from the effects of corrosion. Under those conditions, the guidance system experienced a burn out.
In early 2365, Captain Thadiun Okona's ship, the Erstwhile, experienced such an event. For the repair of his guidance system, the chief engineer of the starship USS Enterprise-D, Lieutenant Geordi La Forge, replaced the zelebium with tricellite. (TNG: "The Outrageous Okona")