HAKKO Soldering iron tips are comprised of a copper core for good thermal conductivity, plated with iron which brings less corrosion. And, the soldering part of the tips is plated with solder. The other area is plated with chrome in order to prevent them from being wet with solder. (Applies to all soldering irons except for RED, U, PORTABLE, MATCHLESS, JUNIOR)
When the tip life is over:
Point 1 Solder corrodes the iron-plated part and makes a hole.
Point 2 The solder-plated part is oxidized in black and the tip is no longer wet with solder.
Point 3 The surface of the heating element insertion part oxidizes and the heat conduction becomes poor.
(*Except for tips in composite type which has different structure)