The dissolution behavior of type 304 stainless steel was studied in typical pickling environments in an effort to reduce the loss of the critical metals Ni and Cr during the pickling process. Dissolution rates were determined for a 90-minute exposure to HNO3/HF solutions ranging from 0.8 M to 3.5 M HNO3 and 0.5 M to 2.6 M HF at 30 deg, 50 deg, 70 deg, and 90 deg C and containing 0 to 0.21 M dissolved Fe, Cr, or Ni. Dissolution rates increased with increasing HNO3 concentration from 0.4 M to 1.5 M HNO3, decreased at higher HNO3 concentrations, and increased with increasing HF concentration. A lower activation energy was found for solutions with higher HNO3 concentrations. Dissolved Fe and Cr decreased the dissolution rate of 304 stainless steel while Ni had essentially no effect. Auger electron spectroscopy and x-ray photoelectron spectroscopy studies of films resulting from exposure to HNO3/HF solutions indicated that the fluoride did not penetrate the thin nonprotective films but remained on the outer surface of the film. Scanning auger microscopy studies indicated that the fluoride was uniformly distributed over the surface. Both the dissolution and surface studies are consistent with a dissolution process that is controlled by reactions occurring either in solution or at the film-solution interface.
OP; Journal Article
Metallurgical Transactions. A. Physical Metallurgy and Materials Science
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