Question

Why metal anode has to be replaced periodically?

Answer 1

During the electrolysis process, aluminium is deposited at the cathode and oxygen is liberated at the anode. Some of this oxygen reacts with the carbon in the graphite to form carbon-dioxide, thus slowly burning away the anodes.

Answer 2

During electrolysis, the metal at anode gets deposited on the cathode. Since it is gradually decreasing and gets deposited, a time comes when there is no flow of electricity as the anode is completely deposited on cathode.

So for the process to continue the anode is replaced periodically.

HOPE IT HELPS....