hard corrosion to build up
It's not the solder the iron you have is a great iron it just gets to hot and corrodes the tip when setting I use tip cleaner on mine radio shack sales it for $9:00. But thats a lot of work so I got a dimmer from Lowes and made a box with dimmer and outlet in it.
Set that for about half heat till I need the iron and now she is great tip stays clean and if I need all that heat I still have it.
Spent about a hour playing with the dimmer to make setting I can set and just use the iron.