Optimization of Pickling Solution for Improving the Phosphatability of Advanced High-Strength Steels

This study investigated the optimum pickling conditions for improving the phosphatability of advanced high-strength steel (AHSS) using surface analysis and electrochemical measurements. To remove the SiO₂ that forms on the surface of AHSS, 30 wt.% NH₄HF₂ was added to the pickling solution, resulting in a significant reduction in the amount of SiO₂ remaining on the surface of the AHSS. The phosphatability was improved remarkably using HNO₃ concentrations higher than 13% in the pickling solution. Furthermore, phosphate crystals became finer after pickling with a HNO₃-based solution rather than a HCl-based solution. Electrochemical impedance spectroscopy (EIS) data indicated that the corrosion resistance of AHSS subjected to HNO₃-based pickling was higher than that of AHSS subjected to HCl-based pickling. Fluorine compounds, which were involved in the phosphate treatment process, were only formed on the surface of steel in HNO₃-based solutions. The F compounds reacted with the phosphate solution to increase the pH of the bulk solution, which greatly improved the phosphatability. The phosphatability was better under HNO₃-based conditions than a HCl-based condition due to the fineness of the phosphate structure and the increased surface roughness.