Mathematical modelling of the electrolytic pickling of steel
Michael Vynnycky, Department of Mechanics

Pickling is an industrial process used worldwide to remove surface oxide layers formed during earlier steps of the steelmaking process. Electrolytic pickling, in which an electrical potential is applied to the steel being pickled, allows the use of a relatively harmless neutral aqueous solution of sodium sulphate. This mitigates the various negative consequences of the commonly-used chemical pickling process, which uses a highly reactive mixture of nitric and hydrofluoric acid: the formation of NOx and acid fumes, metal oxides, and hazardous wastes, as well as large losses of unoxidized metals due to overpickling, all of which add significant manufacturing costs. In electrolytic pickling, the use of acids and the loss of unoxidized metals are reduced, and the original metal oxides are transformed to easily-removable metal hydroxides; in addition, the process generates both hydrogen and oxygen bubbles, whose effect on the process must be accounted for. The interest in having a better understanding of electrolytic pickling is further motivated by the fact that there is an increasing demand in the steel industry for higher production rates, as well as lower capital and operating costs; thus, the time to remove the surface oxide layer has to be shortened, and the pickling process has to be made more efficient; for example, the current efficiency of the electrolytic pickling process, where the steel band to be pickled is indirectly polarized, has been estimated to be only about 30%

Figure 1: Electrolytic pickling tank

A schematic of the industrial problem, which this project will address, is given in Fig. 1, which shows an electrolytic pickling tank through which an aqueous electrolyte is pumped and recirculated. A stainless steel band with oxidized surfaces moves between electrodes having an imposed difference in electrical potential. The electrode potential creates an electric current that flows through the electrolyte and the band. In addition to the passage of current through the band to promote the removal of the oxide layer, electrolysis of water occurs, which generates hydrogen and oxygen gas bubbles on all conducting surfaces. Gas evolution is thought to consume most of the current supplied for scale removal; in particular, some researchers claim that more than half of the current supplied is spent on oxygen gas evolution; others claim, without proof however, that gas evolution to some extent improves the pickling process, in that growing bubbles help to peel away the oxide scale. The project will involve the development of mathematical models that describe the complex interaction of electrochemistry and turbulent, multiphase hydrodynamics in the electrolytic pickling process. To obtain a better qualitative understanding, we will use applied mathematical techniques to derive and solve reduced versions of the models; to obtain quantitative results for the actual results, we use computational fluid dynamics. Furthermore, although the problem to be studied is quite specific, the methods and approach used will give the student the tools needed to solve a wide variety of industrially-relevant problems involving turbulence and multiphase flow.