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Zinc production in electrolytic cell?

In electrolytic cell, normally the metal in anode is oxidised to give out electrons and cathode takes electrons to from metal.
In zinc production, the anode is lead ( Pb) and the cathode is aluminium (Al) and the eletrolyte is zinc sulfate ( ZnSO4)
My question is why in the anode, the water is oxidised but not the lead metal???