Stainless steel pickling sludge consists largely of surface scale from stainless steel strips, being composed mainly of Cr oxides. Moreover, the present work shows that the Cr concentration of the sludge is approximately twice that of ordinary stainless steels. The particle size of sulfuric acid sludge is 0.325 mu m, while that of nitrichydrofluoric acid sludge is 9.2 mu m Nitric-hydrofluoric acid sludge tends to coagulate in a relatively short time, while sulfuric acid sludge essentially does not tend to coagulate. The main components of sulfuric acid sludge and nitric-hydrofluoric acid sludge are FeCr2O4 and Cr2O3. In the mechanism of sludge precipitation in the pickling process, acid penetrates cracks in the scale and dissolves part of the stale and the substrate. The undissolved scale then loses the ability to bond with other scale and the substrate and settles out in the pickling tanks.