Steel Pickling for Hot-Dip Galvanizing: Effects of Zinc and Iron on Pickling Rates for Hydrochloric and Sulfuric Acids

Abstract

Hot-dip galvanizing (HDG) is an industrial process that provides corrosion resistance to steel members by dipping and coating them in liquid zinc. In the preparation of the steel members for HDG, diluted strong acids are used to remove surface level imperfections to ensure the zinc coating adheres to the base metal of the steel in a process known as pickling. These acid solutions containing metal are commonly referred to as pickling liquors. Improvements in the longevity of the acid solutions can be achieved through a practice referred to as acid regeneration, for which guidance exists in the context of hydrochloric acid pickling, but less so for sulfuric acid pickling. The purpose of this study was three-fold: first to validate the process and methods found in existing literature describing regeneration for hydrochloric acid solutions; second to characterize regeneration behavior for sulfuric acid solutions; and third to explore the effects of zinc on the efficacy and efficiency of both acid solutions. The study was conducted using small-scale tests via benchtop experimentation utilizing both hydrochloric and sulfuric acid, elemental iron and zinc powders, and steel coupons with mill scale to determine the pickling rates for the acid solutions. Through testing, the existing procedure for hydrochloric acid regeneration was recreated, a regeneration procedure was developed for sulfuric acid, and the effect of zinc on the pickling reaction was characterized for both acids. It was found that the regeneration process was able to reduce pickling times up to 50%. The presence of zinc was found to impede pickling at low acid concentrations.