Effects of nitrogen oxides, sulfur dioxide, and ferric ions on the corrosion of mild steel in concentrated sulfuric acid
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MILD steel and cast iron are widely used as standard construction materials in systems for handling concentrated sulfuric acid (H2SO4). Corrosion has not been a concern for 80 to 98 pct H 2 S O 4 under usual conditions of transport and storage because iron sulfate forms a protective layer on the metal. Since most acid has been produced in very pure form in sulfur-burning plants, any impact of impurities on corrosivity has been negligible. However, as sulfide smelter emission controis have improved, by-product sulfuric acid has captured about 10 pct of the U.S. acid market. The smelting process may contribute impurities to the by-product acid which affect both product quality and corrosion during storage and shipment. Major impurities often include residual sulfur dioxide and iron from corrosion within the contact acid plant. Nitrate and nitrous acid are not frequently present in bothersome concentrations but may form when high-temperature burners are used. Another dozen impurities present at ppm and sub-ppm levels should not play a major role in corrosion, although they may have impact on quality. Although corrosion of iron and steel by H2SO 4 has been the subject of numerous investigations, effects of impurities at levels pertinent to smelter acid, i.e., up to several hundred ppm, have not received a great deal of attention. Effects of other parameters, which are often more important than impurities, have been established, however. These are summarized to put impurity effects into perspective. The significant effects of H2SO 4 concentration and temperature are well-documented in isocorrosion studies and charts TM and in reports of field tests and experiences. 5-8 Increasing temperature accelerates corrosion on the order of 10 pct deg-~. The dependence of corrosion o n H 2 S O 4 concentration is a complicated function with several maxima and minima. However, the weight loss vs concentration curve runs almost parallel to the solubility isotherm for iron sulfate at the same temperature: The general increase in corrosion TERRELL N. ANDERSEN is Senior Consulting Chemist, Kennet cott Minerals Company, Process Technology, Salt Lake City, UT 84147, NAOLA VANORDEN is Visiting Scientist, Sacramento City College, Sacramento, CA, and W. JOSEPH SCHLITT is Manager, Hydrometallurgical Department, Kennecott Minerals Company, Process Technology, Salt Lake City, UT 84147. Manuscript submitted November 30, 1979.
with dilution caused by the pickup of moisture from the atmosphere can be a serious problem at H2SO 4 concentrations less than approximately 70 pct, the practical lower limit of acid concentration for self-protection of carbon steel.l.5 Although moisture pickup by smelter acid does not lower the H2SO 4 concentration to such levels, dilution of acid strength from 96 to 93 pct approximately doubles the FeSO4 solubility 9 and increases the corrosion potential accordingly. Other parameters which have important accelerating effects on corrosion are agitation or motion of the acid past the steel 1,5,8and the presence of an air-li
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