Sewer corrosion

Corrosive conditions - not only high or low pH, but also high levels of sulfate, which is corrosive to cement and therefore unacceptable in the public sewers. High temperatures can be unacceptable ... 

A simplification of the polarization resistance technique is the linear polarization technique in which it is assumed that the relationship between E and i is linear in a narrow range around E . Usually only two points ( , 0 are measured and B is assumed to have a constant value of about 20 mV. This approach is used in field tests and forms the basis of commercial corrosion rate monitors. Rp can also be determined as the dc limit of the electrochemical impedance. Mansfeld et al. used the linear polarization technique to determine Rp for mild steel sensors embedded in concrete exposed to a sewer environment for about 9 months. One sensor was periodically flushed with sewage in an attempt to remove the sulfuric acid produced by sulfur-oxidizing bacteria within a biofilm another sensor was used as a control. A data logging system collected Rp at 10-min intervals simultaneously for the two corrosion sensors and two pH electrodes placed at the concrete surface. Figure 2 shows the cumulative corrosion loss (Z INT) obtained by integration of the MRp time curves as ... 

Figure 2. Cumulative corrosion loss Y. INT for mild steel sensors embedded in concrete and exposed to a sewer bypass (a) control sample, (b) flushed sample. (Reprinted from Ref 6 with permission from NACE International.)...

Openings around water pipe entries and sewer exits that pass through concrete can be easily sealed using caulks. Many builders use plastic sleeves to protect metal pipes from corrosion when they pass through concrete. In this case, an effort can be made to leave a space around the pipe that can be sealed with caulk or backer rod and caulk. The same techniques can be used for pipes passing through block walls. 

In-sewer processes, e.g., hydrogen sulfide formation and fermentation, may cause corrosion, toxicity, and odor problems. 

Anaerobic conditions in wastewater of sewer systems may give rise to formation of volatile substances that are typically identified by a number of problems, like malodors, health risks and corrosion. The conditions and processes related to such problems are dealt with in Section 3.2.2. Further information on measurement, modeling and control of odors can be found in Stuetz and Frechen (2001). 

The transformation and transport rates that are involved in the sulfur cycle shown in Figure 4.4 determine to what extent the relevant components will exist in the different phases of the sewer system. As already shown — and further focused on when dealing with the concrete corrosion in Section 6.2.6 — the... 

TABLE 4.6. Levels of Total Hydrogen Sulfide Concentration in Wastewater of a Sewer System and Associated Problems in Terms of Malodors, Health and Corrosion. 

Anaerobic conditions are traditionally a major concern when dealing with microbial-induced transformations of wastewater in a collection system. The problem is primarily associated with the risk of hydrogen sulfide and odorous organic compounds. The corresponding problems appear as concrete and metal corrosion, health-related impacts and malodors. Such in-sewer process-related problems have been reported as early as over 50 years ago (Parker, 1945a, 1945b Pomeroy and Bowlus, 1946). 

Concrete corrosion is associated with the formation of hydrogen sulfide. Hydrogen sulfide-induced concrete corrosion has been a well-known major consequence of anaerobic conditions in sewers for many years (Parker, 1945a, 1945b, 1951 Fjerdingstad, 1969 Thistlethwayte, 1972 USEPA, 1974). Concrete corrosion is still a worldwide phenomenon that has great economic impact (Vincke et al., 2000). 

The principle of corrosion in a concrete sewer pipe is shown in Figure 6.5. 

FIGURE 6.5. Principle of concrete corrosion in a sewer pipe. 

A reliable concrete corrosion rate is difficult to predict. As already mentioned and also shown in Figure 4.4, it requires that several process and exchange rates in terms of primarily sulfide formation, emission to the sewer atmosphere, sulfide absorption and sulfide oxidation on the sewer walls can be determined. 

There are several examples from the literature that show that corrosion has seriously (and often quickly) deteriorated sewer networks (EWPCA, 1982 Aldred and Eagles, 1982 ASCE, 1989). Although corrosion is difficult to predict, the number of examples and extent of the problems observed have given a comprehensive knowledge of where and when concrete corrosion may exist. This knowledge can briefly be summarized as follows. 

Systems that are exposed to excessive turbulence of anaerobic wastewater and a potential increased release of hydrogen sulfide. Systems with a risk for increased turbulence are inlet structures, drops, cascades, sharp bends and inverted siphons. As an example, changes in the flow regime from a pressure pipe into a gravity sewer may give rise to the release of hydrogen sulfide. Corrosion of the sewer pipe wall is often pronounced near the daily water... 

Estimating an average corrosion rate, the sewer life expectancy can be calculated assuming the thickness of an allowable concrete loss. 

Metal corrosion has often been observed in pumping stations and sewer structures with electronic equipment. 

Examples of corrosion-resistant designs of sewer systems are found in Kienow and Pomeroy (1978) and ASCE (1989), for example. 

In addition to the active design principles for reducing the risk of sulfide problems, a number of more passive principles exist. The following design considerations are especially relevant for the reduction of corrosion selection of corrosion-resistant materials and design of a well-ventilated sewer system. 

The concrete material used for sewer construction is typically based on the use of Portland cement. Different types of Portland cement have not shown significant differences in the corrosion rate of concrete. However, an increase of the relative amount of cement used in the concrete reduces the corrosion rate— in units of mm y-1—according to the increase in the alkalinity per unit volume of the concrete material (Grennan et al 1980). The use of high-alkaline materials... 

Grennan, J.M., J. Simpson, and C.D. Parker (1980), Influence of cement composition on the resistance of asbestos cement sewer pipes to H2S corrosion, Corrosion Australasia, 5(1), 4—5. 

Kienow, K.K. and R.D. Pomeroy (1978), Corrosion resistant design of sanitary sewer pipe, ASCE (.American Society of Civil Engineers) Convention and Exposition, Chicago, IL, October 16-20,1978, p. 25. 

Meyer, W.J. and G.H. Hall (1979), Prediction of sulfide generation and corrosion in concrete gravity sewers A case study, J. B. Gilbert Associates, A Division of Brown and Caldwell, Sacramento, CA. 

Parker, C.D. (1951), Mechanics of corrosion of concrete sewers by hydrogen sulfide, Sewage Ind. Wastes, 23, 1477-1485. 

In-sewer processes may have adverse effects on the sewer itself and the surrounding environment. Well-known examples are the health, corrosion and odor problems that are related to sulfide formation. Other examples are... 

Examples of products fireplaces, vanities, plaques, shower stalls, playground equipment, bowling balls, sewer pipe, pistol grips, corrosion resistant tanks... 

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