Pickling of Process-Oxidised Austenitic Stainless Steels in HNO_3-HF Mixed Acid

摘要

Oxide formation during production annealing and the subsequent pickling response in mixed acid have been studied. The aims were tocharacterise the oxides formed and to understand how the pickling mechanism and kinetics are affected by the nature of the oxide. Totally, eightdifferent versions of the austenitic stainless steel grades AISI 301, 304L and 309L were studied, all annealed in production lines. Cold rolledoxides (formed during annealing) are thin (< 1μm), dense and formed in a multilayered manner. Hot rolled oxides (formed during reheating, hotrolling and annealing) are thicker (>1 μm) and more heterogeneous in thickness and composition. The dissolution rate of the chromium depletedlayer (CDL) under the oxide is the most important factor for the overall pickling rate. The permeability of acid through the oxide and the tendencyof the oxide to spall are also important factors affecting the pickling kinetics. The dense oxide formed on cold rolled materials can to some extenthinder the acid to reach the CDL. The oxides on hot rolled materials are porous and do not provide such a barrier but they are thicker and therebymore difficult to remove. Shot-blasting prior to pickling of the hot rolled materials improves the pickling performance because it thins the oxide,improves the permeability and increases the tendency of the oxide to spall during the pickling step.