How to move nut that is corroded onto tap?
You can purchase a variety of thread-loosening lubricants to dissolve rust, or you can lubricate with plumber's wax. In the end, however, it requires heavy wrenches with long handles and a certain amount of brute force to make headway when separating the most tightly-locked joints.
Spray lubricant on the outside of a corroded joint and give it time to seep into the joint and dissolve rust on the threads, which usually takes about five to 10 minutes.
Grip the fitting you are trying to separate with one pipe wrench; grip the pipe to which it is attached with another. Hold the pipe steady while you turn the fitting counterclockwise with respect to the pipe. Spray more lubricant and wait an additional five to 10 minutes if the fitting won't turn, then try again.
Tap the fitting with a hammer as an additional way to loosen the threads. Create more torque with the wrenches by lengthening the handles with 2-foot lengths of 1-inch metal pipe.
Lubricate the joint with a plumber's candle as an alternative to using spray lubricant, or as a supplemental measure. Heat the joint with a propane torch until it is hot enough to melt wax. Hold the tip of the candle against the joint and move it around the pipe while the wax wicks into the joint. Try turning the fitting when the wax cools. Repeat the procedure if the fitting still is locked.
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