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 SiO(2) that forms on the surface of AHSS, 30 wt.% NH(4)HF(2) was added to the pickling solution, resulting in a significant reduction in the amount of SiO(2) remaining on the surface of the AHSS. The phosphatability was improved remarkably using HNO(3) concentrations higher than 13% in the pickling solution. Furthermore, phosphate crystals became finer after pickling with a HNO(3)-based solution rather than a HCl-based solution. Electrochemical impedance spectroscopy (EIS) data indicated that the corrosion resistance of AHSS subjected to HNO(3)-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 HNO(3)-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 HNO(3)-based conditions than a HCl-based condition due to the fineness of the phosphate structure and the increased surface roughness.