Sequence of Descaling Methods

Usually, more than one method of cleaning is used to remove scale from stainless steel. For instance, in mill processing sheet, salt bath descaling and acid pickling often are used for a cold-rolled product and abrasive blast cleaning and acid pickling for a hot-rolled product.

In a continuous annealing and pickling line, multistep cleaning usually occurs twice in producing stainless steel sheet. The sequence of operations is: hot roll, anneal, descale, pickle, cold reduce 30 to 50%, anneal, descale, and pickle. As shown in Fig. 2, salt descaling is used for cold-rolled sheet, whereas the salt is bypassed and abrasive blast cleaning is used when a hot-rolled product is being cleaned. Salt also can be eliminated on low-chromium cold-rolled sheet.

Fig. 2 Sequence of annealing and scale-removing operations in mill processing of stainless steel sheet

Cold-rolled sheet is left in the salt bath 10 to 15 min. Oxides are not removed from the surface. The rack of hot sheet is transferred to a water bath, and the violent reaction that results on immersion blasts much of the scale from the steel. The surface is essentially free of scale, but quite dull and dirty. To remove the last remnants of oxide, the rack is transferred to a 10 to 15% sulfuric acid (H2SO4) solution at 70 °C (160 °F) for 3 to 5 min. After removal from the H2SO4 tank, the rack of sheet steel is rinsed and then immersed for 1 to 2 min in HNO3-HF (8 to 10% HNO3, 1 to 2% HF) at 60 °C (140 °F). This removes all smut and residue and produces a white finish. The sheet is finally rinsed, removed from the rack, scrubbed, and dried with an air blast.

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