Winky - I think what you see is the bare steel that has had no oxidization and the 'black iron' [still steel actually
black iron being a different material] where the oxydizing process had started corrosion, and the electrolysis 'fixed' the corrosion to iron oxide and sent it to the anode, changing the mineral makeup of the remaining steel - I've found either electrolysis or phosphoric acid both leave previously rusted areas black, unrusted areas 'shiney' steel color, whatever the actual mechanism!
sodium carbonate is the right stuff, and steel for the anode, tho stainless will last longer
Bill
