Surface neutralization and H2S oxidation at early stages of sewer corrosion: Influence of temperature, relative humidity and H2S concentration

While the involvement of a range of environmental factors in sewer corrosion is known, a comprehensive understanding of the processes involved and the exact role of individual environmental factors in sewer corrosion is still lacking. The corrosion of concrete in sewer systems is reported to be initiated through chemical reactions (involving H2S and CO2) that lower the surface pH to a level then conducive for biological activity. However, the specific influence of environmental variables, such as H2S level, temperature, and relative humidity etc. remains unclear; although, they are expected to control these initial surface reactions of the concrete sewer pipe. We examined changes in the surface chemistry of concrete during the early stages of corrosion by exposing concrete coupons to thirty-six independent conditions in well-controlled laboratory chambers that simulated conditions typically found in various sewer environments across Australia. The conditions employed were combinations of six H2S levels, three gas-phase temperatures and two relative humidity levels. Our results indicate that the role of CO2 on initial surface pH reduction is insignificant when compared to the influence of H2S. Within the first 12 months, a decrease in surface pH by 4.8 units was observed for coupons exposed to 30 °C and 50 ppm H2S, while significantly lower pH reductions of 3.5 and 1.8 units were detected for coupons exposed to 25 °C and 18 °C respectively, and 50 ppm H2S. Elemental sulphur was found to be the major oxidation product of H2S and elevated concentrations were detected at the higher levels of H2S, temperature and relative humidity. More significantly, the data obtained from the controlled chamber experiments correlated with those obtained from the field-exposed coupons. Hence, these findings can be extended to real sewer corrosion processes.

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► Sewer conditions were mimicked in controlled chambers to study the corrosion processes.
► Concrete surface pH decreases as a linear function of increase in H2S level.
► H2S oxidizes predominantly to sulphur due to O2 limitation during corrosion initiation.
►  The pH reduction below 10.3 is driven by H2S and effect of CO2 is insignificant.
► Corrosion reactions are enhanced by increase in temperature and relative humidity.