Why Lead anode is used in electrowinning?

Why Lead anodes is used in electrowinning?

Electrowinning anodes are almost always cold rolled lead-tin-calcium (Pb-Sn-Ca) alloys containing about 98.4% lead (oxygen scavenged prior to alloying), 1.5% tin and 0.1% calcium. Tin provides corrosion resistance and corrosion layer conductivity while calcium and cold rolling add strength.

This Pb-SnCa blades are soldered onto slotted copper hanger bars for support in the electrolytic cells. Lead is then electrodeposited around the joints to protect them from corrosion.

The Pb-Sn-Ca alloy forms an adherent corrosion layer which minimises lead contamination of the cathode copper and extends anode life. Other lead alloy anodes also used in electrowinning contain silver or antimony additions. 

However, since lead alloys are not dimensionally stable, since they slowly dissolve in electrolytes, leading to problems such as changes in the gap between the anode and cathode and product contamination by lead.