Application of Hydrogen Permeation Techniques to Assess the Effect of Inhibitors in Hydrochloric Steel Pickling of Low-C Steels and Interstitial-Free Steels

The acid pickling phase, generally entrusted to HCl-based solutions, plays a fundamental role in the production of low-C and Interstitial Free steel coils. Very often, however, pickling operations are challenging due to the possible onset of surface defects (under-pickling, over-pickling, appearance of surface defects, etc.) linked to complex phenomena involving numerous factors, such as acid concentration, Fe ions in solution, temperature, time, steel type and scale structure. Moreover, the use of inhibitors, which are essential for controlling the pickling process, represents one of the major challenges. Their dosage, however minimal, must be carefully chosen based on type of processed coils and other plant engineering variables. The flow of hydrogen that is generated on the surface of the steel during etching causes absorption of hydrogen by the metal and its measurement is strictly linked to the phenomena occurring during scale removal. The correlation of data obtained from a Devanahan double-cell hydrogen permeation system to results of other techniques, such as weight loss, surface analysis, metallography, electron microscopy show that this system can be effectively used to indirectly measure the solution aggressiveness. This experimental approach is applied to two industrial steels: a simple carbon base and an Interstitial Free Ti steel.