Dissolution behavior of 304 stainless steel in HNO3/HF mixtures |
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Authors: | B S Covino J V Scalera T J Driscoll J P Carter |
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Affiliation: | (1) Avondale Research Center, 4900 LaSalle Road, 20782 Avondale, MD |
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Abstract: | The dissolution behavior of type 304 stainless steel was studied in typical pickling environments in an effort to reduce the
unnecessary loss of the critical metals nickel and chromium 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°, 50°, 70°, and 90 °C and containing 0 to 0.21 M dissolved Fe, Cr, or Ni. Dissolution rates increased
as a function of increasing HNO3 concentration from 0.4 M to 1.5 M HNO3, decreased at higher HNO3 concentrations, and increased with increasing HF concentration. Experiments to determine the effect of temperature on the
dissolution reaction gave a lower activation energy for solutions with higher HNO3 concentrations. The dissolved Fe and Cr decreased the dissolution rate of 304 stainless steel, while dissolved Ni had essentially
no effect. The Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) studies of films resulting from
exposure to several HNO3/HF solutions indicated that the fluoride did not penetrate the thin nonprotective films but remained on the outer surface
of the film. The scanning Auger microscopy (SAM) studies indicated that the fluoride was uniformly distributed over the surface.
Both the dissolution and surface studies are consistent with a dissolution process which is controlled by reactions occurring
either in solution or at the film-solution interface. |
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