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

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.