Effect of HNO3 and FeCl3 Additives on the Pickling Ability of Fayalite-Containing Oxide Film from Si-Containing Steels

Abstract

The present work discusses the morphology and pickling behavior of tertiary oxide scale formed on 4 types of hot-rolled steels (0.008 Si-0.019 Cr, 0.43 Si-0.017 Cr, 0.35 Si-0.525 Cr and 0.034 Si-0.72 Cr (wt.% basis)) in four successive tanks of acid containing 4, 6, 8 and 12% HCl solution, respectively, with the addition of strong oxidizer like FeCl3 and HNO3 along with inhibitor. The presence of fayalite (Fe2SiO4) in oxide scale of 0.43 Si-0.017 Cr and 0.35 Si-0.525 Cr steels was confirmed. This fayalite layer is difficult to be removed by pickling. Therefore, pickling was done at room temperature and at 80 ± 3 °C with different strong oxidants, such as FeCl3 and HNO3 added to the pickling liquor and it was found that even at room temperature, the addition of 0.1N FeCl3 and 0.1N HNO3 could remove the oxide scale completely for the high Si-containing steel too. However, only the addition of 0.1N FeCl3 was sufficient to remove the fayalite layer when pickled at 80 ± 3 °C. Additionally, an inhibitor helped to restrict the over-pickling.