Microbial corrosion of sewer pipe in Australia - initial field results

Microbial induced corrosion (MIC) of concrete sewers pipes is a significant global problem incurring losses in the order of billions of dollars per year. While the basic theory of the MIC process is understood management of sewer corrosion is hindered by a limited understanding of several key in-sewer processes. This paper describes the initial findings of a program of fieldwork in which newly cast and pre-corroded concrete samples have been installed in sewers around Australia and allowed to corrode for a period of up to 12 months under a range of environmental conditions. The first 12 months exposure to the sewer environment has produced noticeable changes in virtually all coupons recovered. Surface pH has been lowered from a starting level of 10.3 down to as low as pH=6 over the year on new coupons and from pH=8.1 down to as low as 3.5 for pre-corroded coupons. Generally the rate of acidification has been a strong function of H₂S concentration in the sewer headspace with pre-corroded coupons experiencing more rapid decreases for a given H₂S concentration. Extensive corrosion of sound concrete has also been observed on pre-corroded samples over this period while minor losses are becoming apparent on new coupons. The combination of pH change, concrete losses and appearance of biological activity on the coupon surface all point to a rapid transition through an initial abiotic corrosion phase to the next stage in which corrosion is the result of neutrophilic microbial activity. A comparison of the early corrosion activity of new and pre-corroded coupons and anomalous readings at one high H2S location also indicate that the corrosion rate will most likely vary in a complex fashion over time and will be influenced by a variety of environmental factors in addition to H₂S levels. As more field data is collected and complementary laboratory and biological data become available a realistic, phenomenological model of this process will be developed allowing the service life of sewer pipes in Australia to be realistically predicted.