Homebrew PCBs

hallo!
first of all, let me say i'm not an english speaker so forgive me if 
i have mistakes.
i'm soldering a lot, using a cheap 5$ iron. i'm some problems:
after a short while soldering, the the metal part on the iron get 
covered with a black crust and after anoter while the metal starts 
to literary disappear- and finaly the whole "head" of the iron is 
like eaten slowly. what sould i do? sould i use a wet sponge? should 
i use a soldering paste? and if i should, how do i use it? i even 
tried grinding the black crust with my grinder.is that ok? should i 
buy a soldering station? hope you understand what i wrote, and i'm 
sure there is a name to the black crust on the iron, and to 
the "disappearence" of my iron.
thanks a lot,
mumin.