Effects of chloride ion addition on the pickling of hot-rolled steels in fluoboric acid solution

Effects of chloride ion addition to, fluoboric acid (HBF4) solution on the pickling of a high-strength, hot-rolled steel were investigated by using open-circuit potential transient technique and alternating current (AC) impedance spectroscopy. The occurrence of an open-circuit potential plateau indicated dominant dissolution of one oxide phase in the scale. In contrast, the open-circuit potential decay was. caused by the simultaneous dissolution of two oxide phases in the scale. Thus, the shape of the open-circuit potential transient was determined crucially by the oxide scale structure. Time to reach a steady-state value of open-circuit potential decreased with increasing chloride ion concentration, indicating accelerated descaling. The decrease in the electrical resistance of the oxide scale estimated from AC impedance spectra also suggested that dissolved chloride ions accelerate descaling. From the experimental findings, it was concluded that the addition of chloride ions to HBF4 solution accelerates descaling and simultaneously impedes the subsequent dissolution of the bare steel in the pickling process.