There are two cases of electric soldering iron tin:
The first is the temperature of iron tip is too high and the burning time is too long, the tin can not stick lead to iron tip oxidation.
The second is to use iron tip to burn plastic, film and the other hot melt stuff, so that the original hanging on the tip of the tin are all squeezed and oxidation.
 Re-tin need to clean the iron tip, remove the debris, in the case of cooling with alcohol and then heated while immersed in rosin, and immediately let it on the tin. Be careful not to use sandpaper or blade to wipe and scraping iron tip. After wearing the head of the protective layer in iron tip, it will soon be shorten service life.