PICKLING, DESCALING, AND DERUSTING OF STEELS

Abstract

When mild steel is heated in air between 575° and 1370°C., an oxide layer or scale forms on the steel surface. This scale consists of well-defined layers, the nature and thickness of which depend on the time and temperature of heating. In general, the layers from the steel base comprise a relatively thick layer of wüstite, which approximates to the formula FeO, a layer of magnetite (Fe3O4), and a thin layer of ferric oxide (Fe2O4), If the scaled steel is cooled rapidly, the wüstite layer remains more or less unchanged, and it is found that the scale is difficult to pickle. If the steel is cooled slowly in air, however, the scale becomes enriched in oxygen, and the remaining wüstite layer decomposes into magnetite and finely divided iron. On pickling this steel, acid penetrates through cracks in the insoluble magnetite and ferric oxide layers and then rapidly attacks the decomposed wüstite layer. As a result of this action, the outer scale is loosened and removed from the surface. The rapid dissolution of the decomposed wüstite layer is due to the setting up of small electrolytic cells between the iron particles, acid, and magnetite; the iron particles, being anodic, dissolve to form ferrous ions, and the magnetite, being cathodic, is reduced and dissolves to form more ferrous ions. Since the iron, acid, and magnetite are good electrical conductors, the resistance of the cells is small and so, in the end, the rate of dissolution of this layer is largely governedby the rate at which acid diffuses through the cracks in the upper layers. A similar but slower action takes place between the exposed base metal and the upper layers of magnetite and ferric oxide. If the scaled steel is held between 400° and 570°C. for a longtime, the iron particles in the wüstite layer coagulate; consequently the resistance of the cells increases and pickling speed decreases.