Improvement of duplex stainless steels' pickling efficiency

There is an ongoing increasing demand in the steel industry to produce duplex stainless steels at a higher rate, consequently in order to do so it is necessary to have a more efficient pickling process. In this project, the annealing and pickling conditions of this family of grades have been studied in order to determine conditions that can increase their pickling efficiency. For cold rolled materials, electrolytic pre-pickling is shown to be a key process towards facilitating the pickling process for material both annealed in production line and in laboratory experiments. The mechanism for the neutral electrolytic process on duplex 1.4462 and austenitic 1.4401 stainless steels has been examined and the oxide scale found to become thinner as a function of electrolytic pickling time. Spallation or peeling of the oxide induced by gas evolution did not play a decisive role. A parametric study was therefore carried out to determine whether adjustment of process variables could improve the current utilisation. For hot rolled materials, chemical pre-pickling in HF gives a dramatic reduction in the subsequent mixed acid pickling time. On the other hand, it was observed that a porous or cracked scale and a large chromium depleted layer, associated with a low chromium concentration at the metallic surface, permit a decrease of the pickling duration. Neutral or reducing annealing atmospheres are recommended for the formation of oxide layer types easy to pickle. The study of new baths, free of nitric acid, points out the possibilities of using mixtures of hydrofluoric acid and hydrochloric acid to improve the pickling conditions.