The metal ion (Fe") that is dissolved from the pickling reaction is oxidized using
the accelerator (e.g., nitrite/nitrate, chlorate, or peroxide) and will precipitate
out as sludge. In the case of zinc ions (Zn-') they are incorporated in the coating
reaction and will not form any sludge. Aluminum ions (AI'3), by the use of fluoride,
react to form a fluoroaluminate complex. This complex is precipitated as
kryolith.