The role of iron in sulfide induced corrosion of sewer concrete

• H2S exposure chambers were used to simulate real sewer corrosion.
• Mineral liberation analysis quantified minerals in corrosion layer and concrete.
• Corrosion progresses uniformly with gypsum/ettringite as main corrosion products.
• Iron rust instead of sulfur compounds is related to micro-cracking in concrete.
• A conceptual model is established to account for all corrosion processes.

The sulfide-induced corrosion of concrete sewer is a widespread and expensive problem for water utilities worldwide. Fundamental knowledge of the initiation and propagation of sewer corrosion, especially the interactions between chemical reactions and physical structure changes, is still largely unknown. Advanced mineral analytical techniques were applied to identify the distribution of corrosion products and the micro-cracking that developed along the corrosion boundary. It was found that sewer concrete corrosion caused by reactions with sulfuric acid progressed uniformly in the cement of concrete. In contrast to conventional knowledge, iron rust rather than gypsum and ettringite was likely the factor responsible for cracking ahead of the corrosion front. The analysis also allowed quantitative determination of the major corrosion products, i.e., gypsum and ettringite, with the latter found closer to the corrosion front. The conceptual model based on these findings clearly demonstrated the complex interactions among different chemical reactions, diffusion, and micro-structure changes.

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