Concrete is the most commonly used material in sewers, especially for large sewer pipes, which can be attributed to its strength, durability and relatively low maintenance costs. However, due to the nature of concrete and the unique sewer atmosphere, corrosion of concrete pipes and associated infrastructure (e.g. manholes and pumping stations) has been widely recognised as a significant challenge for the utility industry.
Concrete corrosion in sewers is caused by a series of abiotic and biotic corrosion processes. Acidification of concrete caused by carbonation and gaseous hydrogen sulfide initiates the first stages of corrosion, before developing into the highly destructive, microbially induced corrosion process.
The chemical reactions of carbon dioxide and gaseous hydrogen sulfide with the concrete lower the surface pH of the concrete to about eight, enabling the growth of sulfide oxidising bacteria.As the surface pH gradually decreases, the sulfide oxidising bacteria colonise the concrete surface and produce large amounts of biogenic sulfuric acid, which can directly attack the cementitious materials in concrete.
Collectively, gaseous hydrogen sulfide and microbial activities are the major causes of concrete corrosion in sewers.Professor Zhiguo Yuan, Director of The University of Queensland’s Advanced Water Management Centre, said that concrete sewer corrosion is a long-standing and costly problem for the water industry.
Deterioration of a material, usually a metal, that results from a chemical reaction with its environment.Corrosion occurs when the protective layer in concrete steel reinforcement is destroyed.
Steel in concrete is usually protected against corrosion by the high pH of the surrounding portland-cement paste. Cement paste has a minimum pH of 12.5, and steel will not corrode at that pH. If the pH is lowered (for example, to pH 10 or less), corrosion may occur if moisture, oxygen, and chloride ions are present.
Chloride ions destroy the protective layer on the steel reinforcement, making it prone to corrosion. The corrosion product (rust) occupies a greater volume than the steel and exerts destructive stresses on the surrounding concrete.
Intro to Corrosion in Concrete
H2S Sewer Corrosion Effects Sewer Failure