Corrosion of Cathodes in Anodic Protection Systems in Hot Concentrated Sulfuric Acid-Part 2: Alternative Cathde Alloys

摘要

The accelerated corrosion of cathodes commonly used in anodically protected H_2SO_4 coolers (N06200 and N10665), as well as several alternatives, can occur under cathodic polarization. The electrolytic formation of elemental sulfur may be responsible in part for the fouling of cathodes under cathodic polarization. Depending upon the polarized cathode potential, sulfur may be formed as a stable cathode reduction product or it may be formed as a product from the reaction of H_2S with H_2SO_4. Fouling may be reduced either by selecting a cathode material that has a low overpotential, forming SO_2 as the stable reduction product associated with establishing and maintaining protection, or by removing the protective perforated PTFE sheath commonly used. In comparing the possible fouling and corrosion tendencies of selected alloys, N06200 was the most suitable cathode for drying acid (93 to 94 wt percent H_2SO_4), and N10665 was the most suitable for absorbing acid (98 to 99 wt percent H_2SO_4). Both alloys contain a relatively high molybdenum content, which suggests molybdenum has a beneficial influence on cathode performance. Performance will likely be enhanced by adopting a recent innovation of using larger-diameter bare cathodes. Tungsten was found to be a promising replacement cathode material in the absorbing acid.