A special type of internal corrosion in partially filled sewage collectors made of cement binder-based materials is biogenic sulfuric acid corrosion (BSC), also called sulfide corrosion. If, under the direct influence of aggressive wastewater (although this is an exception, usually domestic wastewater with a pH = 7 is neutral), only the wetted zone of the collector is subject to corrosion, then the action of biogenic sulfuric acid corrosion (biogenic) sulfuric acid, which occurs from volatile sulfur compounds contained in wastewater or directly entering it, is directed exclusively at the gas zone.
Proteins (protein substances) contained in wastewater and sediments (in the part of the collector with reducing conditions) are broken down into volatile sulfur compounds, mainly hydrogen sulfide, as a result of microbiological processes under anaerobic or aerobic conditions. Additionally, due to bacterial metabolism under anaerobic conditions, sulfates can be reduced to hydrogen sulfide (desulfurization). Important parameters influencing the formation of volatile sulfur compounds are the prevailing operating conditions, in particular the composition of the wall waters, temperature, air humidity, flow time and the presence of sediments. Particularly in the case of long and partially filled collectors with minimal partial filling, the formation of volatile sulfur compounds is generally expected. Since these compounds are not oxidized quickly enough in wastewater, they diffuse and swirl from the wastewater into the collector atmosphere – the part of the collector cross-section with oxidizing conditions – and thus also onto the collector walls. There, the oxidation of the compounds to elemental sulfur continues, which, in turn, under the influence of various thiobacilli inhabiting the collector walls, is oxidized in the process of metabolism in sufficient humidity to sulfuric acid, which has an aggressive effect on concrete.
Particularly susceptible to biogenic sulfuric acid corrosion are the following sections of channels:
- behind pumping stations at the entrance to pressure pipelines – at the point of supply of wastewater from settling tanks, industrial production;
- at the point of supply of wastewater from pressure drainage systems;
- with closed drops and other structures that create eddies, where, in addition, is a sufficient amount of oxygen.
Depending on the formation of hydrogen sulfide (H2S), the forms of BSC are divided into autogenous, allogeneic and exogenous: autogenous is the form when H2S is formed within the section of the wastewater collector; allogeneic – when wastewater is supplied to the collector from the outside, as a result of which H2S is formed, and BSC occur in the collector, for example, at the entrance to a pressure pipeline; exogenous form – when wastewater containing H2S is fed into a sewage collector, for example, through a small sewerage treatment plants, which can lead to the formation of a BSC.
The formation of H2S in wastewater, the release of H2S from wastewater and the effect of a BSC can be calculated using formulas. According to the current state of knowledge, maximum values of the total amount of sulfides in wastewater > 1.0 mg/l or the concentration of H2S in the collector atmosphere > 0.5 ppm indicate a probable high degree of aggressiveness and, therefore, the need for passive corrosion protection (lining) of all structural parts made of non-acid-resistant materials (pH < 4.5) or active corrosion protection by chemically binding sulfides in wastewater, for example, with the addition of iron chloride (III) in an amount of Fe 1.5–2 times greater than the amount of sulfides, for example, 1 mg/l. The resulting difficult to settle sludge should be removed by preliminary clarification in a treatment plant to avoid increased chloride pollution of water bodies. However, an alternative to this in many cases is the use of corrosion-resistant materials.
Based on materials from European specialized periodicals, 2016.