Pickling

Before cold rolling,the surfaces of the coils must be cleaned of oxide or black scale formed during the hot rolling process and which would otherwise ruin the surface texture. This is done by pickling the coils in either dilute hydrochloric acid or dilute sulphuric acid and then washing them in hot water to remove the acid. The acid removes both the oxide scale and any dirt or grit which might also be sticking to the surface of the coil.Car Repair Talk's forum.