| Abstract: | Corrosion resistance of metallic materials in medium concentrated hot sulfuric acid For the newly developed chromium-base material Alloy 33 (German material No. 1.4591) the corrosion rate has been gravimetrically determined in non aerated high purity grade sulfuric acid in the concentration range from 15 to 80 mass-% at temperatures from 50 to 90°C. This way critical limits of corrosion resistance behaviour under very unfavourable conditions of exposure result. An erratic increase of the corrosion rate by a few orders of magnitude takes place with increasing temperature according to the transition from the passive into the active state. This transition into the active state is shifted towards lower temperatures with increasing concentrations of sulfuric acid. Alloy 33 (1.4591) in the passive state possesses clear advantages against the reference materials for use in sulfuric acid: Alloy 825 (2.4858) and Alloy 20 (2.4660). Polarization measurements in agitated oxygen containing and deaerated sulfuric acid solutions show a higher tendency for passivation and a more stable passive state in comparision with the reference materials. The presence of small amounts of oxidants such as ferric sulfate, nitric acid or potassium dichromate results in a widening of the range of corrosion resistance of Alloy 33 in the studied temperature-concentration field. In addition the minimum concentrations of oxidants necessary for this behaviour have been determined by investigation of the measured corrosion rates of Alloy 33 (1.4591). Polarization tests as well as potential measurements of rest potential and redox potential serve to further explain the mode of operation of these oxidants. |
