The process of pickling carbon steel with hydrochloric acid has established itself world-wide over the past few years as an effective pickling process and is used in virtually all countries and steel works. In stainless steel pickling, however, there was always the technical belief that conventional steel pickling methods could not be used for stainless steel because of the different levels and type of scale formation on stainless steel. Surprisingly, however, it was shown that a pickling effect most certainly could be achieved with hydrochloric acid. However, the pickling times required to obtain a scale-free surface on stainless steel using hydrochloric acid are very long compared with carbon steel. A steel strip of grade St 12, for example, is pickled over a maximum of 30 seconds at 80.degree. C., whereas a comparable strip of grade AISI 409 stainless steel takes up to 10 minutes at 90.degree. C. These extremely long pickling times for stainless steel using hydrochloric acid are unsuitable for practical operations because the number and length of tanks needed for pickling would be too great.
During the formation of stainless steel sheet products, a layer of scale is typically produced on the surface of the steel sheet. The scale must be removed, typically by pickling, before the steel can be used or further processed. Many stainless steel pickling baths use a mixture of hydrofluoric acid and nitric acid. The mixture of these acids causes numerous problems associated by the sludge formation of these acids in the tanks. In addition, acid pickling of stainless steel by the conventional process is a very slow process and requires complex treating baths to provide adequate retention time of the stainless steel in the acid bath. The sequential pickling steps result in increased operating costs.
Accordingly, there is a continuing need in the industry for improved methods of pickling stainless steel.