Empirical Modeling of Iron Oxide Dissolution in Sulphuric and Hydrochloric Acid
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RODUCTION
DURING hot rolling, oxide scales are formed on the steel surfaces. These scales consist primarily of three different iron oxides, namely Fe1yO (wustite), Fe3O4 (magnetite) and Fe2O3 (hematite). The composition and stability of these oxides are obvious in the Fe-O diagram.[1–4] In theory, the oxide with the highest oxygen content represents the upper layer (hematite), and the oxide with the highest metal content is close to the substrate (wustite). Less than 843.15 K (570 °C), the formation of a two-layer oxide is expected. Greater than 843.15 K (570 °C), three oxide layers, consisting of wustite (iron rich), magnetite in the middle, and hematite[5–7] occur. The composition also depends on the cooling speed after hot rolling. The transformation of wustite into magnetite and alpha-fe occurs most rapidly between 623.15 K and 823.15 K (350 °C and 550 °C).[1,4] To remove these adherent layers from the steel substrate, pickling is a commonly used technology.[8–10] Usually hydrochloric, sulfuric, nitrous, and hydrofluoric acids are used. To accelerate pickling, different predescaling processes are well known: for example, shot blasting or stretch levelling. Both processes lead to a reduction of the scale on the surface and so to a reduction in acid consumption and pickling time. A new approach is the technology of hydroabrasive pre-descaling with very small grain sizes. Owing to utilizing a hydroabrasive descaling method, the scale layer is
perforated. Especially for steel grades with very severely adherent scale layers, hydroabrasive pre-descaling is a promising alternative.[11] In future, the proposed model could also be useful to calculate pickling times as a function of pre-descaling parameters. In the literature, there are different approaches to calculate pickling times. In the past, pickling equations and pickling times often were based on empirical data generated by analyzing a complete oxide layer on different substrates.[12–15] These known studies do include neither the reaction kinetics and rate constants of single oxide dissolution nor the calculation of oxide dissolution of two-layer and three-layer oxide scales with different compositions. Thus, this article will present a new approach to calculate pickling times on the basis of the reaction kinetics for every single iron oxide. During pickling, the dissolutions of iron (Fe) and the three main oxides wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3) in hydrochloric acid (HCl) and sulfuric acid (H2SO4) are characterized by Fe2 O3 þ 6HCl ! 2FeCl3 þ 3H2 O Fe3 O4 þ 8HCl ! FeCl2 þ 4H2 O FeO þ 2HCl ! FeCl þ H2 O Fe þ 2HCl ! FeCl2 þ H2 FeO þ H2 SO4 ! FeSO4 þ 4H2 O Fe3 O4 þ 4H2 SO4 ! Fe2 ðSO4 Þ3 þ 4H2 O
JAN C. HEMMELMANN, Environmental Scientist, HAO XU, Student, and WOLFGANG KRUMM, Professor, are with the University of Siegen, Siegen, Germany. Contact e-mail: j.hemmelmann@ et.mb.uni-siegen.de Manuscript submitted May 7, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
½1
Fe2 O3 þ 3H2 SO4 ! Fe2 ðSO4 Þ3 þ 3H2 O Fe þ H2 SO4 ! FeSO4 þ H2 :
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The speed
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