base metal

Short definition from Wiktionary

Noun

1. 1
   Any metal at the lower end of the electrochemical series that oxidizes readily.
2. 2
   The metal to be welded (rather than that used to weld).
3. 3
   A common or inexpensive metal.
4. 4
   A common or inexpensive metal: one that was hoped to be transmuted into a precious metal.