Chapter 7: Corrosion in Mineral Acids: Sulfuric, Nitric & Hydrochloric

Sulfuric acid, the most widely produced chemical, is manufactured primarily through the contact process and lead-chamber process, and its corrosive impact on materials varies significantly based on concentration and temperature. While ordinary carbon steel is effectively used for concentrations (greater than) 70 percent at lower temperatures, its behavior is complicated by specific corrosion spikes near 101 percent concentration, requiring precise analysis via isocorrosion charts. Other materials like chemical lead are preferred for dilute sulfuric acid (lesser than) 70 percent, while high-silicon cast irons and specialized alloys such as Durimet 20 offer broader resistance across varying strengths. Nitric acid, a powerful oxidizing agent, necessitates the use of materials that develop protective passive films, often relying on high chromium content; consequently, stainless steels and high-silicon irons are the primary choices for this service. In contrast, hydrochloric acid is identified as one of the most difficult substances to contain due to its aggressive nature and the destructive influence of aeration and oxidizing contaminants like ferric chloride. While expensive reactive metals like tantalum provide near-total resistance to hydrochloric acid, industries often utilize nickel-molybdenum alloys or high-silicon irons containing molybdenum for cost-effective handling. Hydrofluoric acid presents unique challenges by attacking traditionally resistant materials like glass and silicon-based irons, making Monel and magnesium particularly valuable due to the formation of protective fluoride films. The chapter also addresses the role of nonmetallic materials, such as Teflon, glass-lined steel, and various polymers, which offer immunity to specific chemical attacks but are limited by thermal and mechanical constraints. Ultimately, selecting the appropriate construction material requires a detailed understanding of how temperature, acid purity, velocity, and environmental aeration interact to dictate the long-term durability of industrial equipment.