Elimination of sources causing biocorrosion

Here we have investigated different approaches on how to identify the important causes of microbial growth and MIC and to mitigate the problems. Microbes as any other living organisms need food and energy. In a complex system such as a district heating system these needs can be met be a rather large array of sources including feed water, chemical water treatment, intruding drinking water and dirt. The growth limiting factor in a system can be identified and by creating a mass balance for the limiting factor a strategy for the elimination of the most important nutrient and/or energy sources can be developed and implemented. An important factor in determining the severity of MIC is the presence of oxygen, because the presence of both results in more severe corrosion attacks than either one of them alone. A widely used approach to minimise the extent of oxygen corrosion is addition of oxygen reducing chemicals to the system water. A high reaction rate of these chemicals with intruding oxygen is important to minimise the risk of oxygen reacting with the mild steel surfaces of the system. Acidification of microenvironments e.g. underneath a biofilm can be an important mechanism of MIC. This was investigated by measuring pH-microprofiles through tubercles developed on mild steel coupons that had been exposed in a district heating system with deionised, pH-adjusted water. pH-values as low as 5 were found at the bottom of the pits underneath the tubercles while the overlaying system water held a pH of around 9. Using sidestream-mounted annular reactors it was shown that increasing the buffering capacity of deionised district heating water succeeded in substantially reducing local corrosion rates. The influence of PEX-piping on microbial growth and corrosion was investigated in a test system. Under the given test conditions localised corrosion and biofilm growth occurred regardless the type of PEX (B or C) used. Using advanced microbiological methods based on incorporation of substrates labelled with radioactive isotopes in bacterial cells (MicroAutoRadiography, MAR) it is shown that bacteria can take actively part in cathodic depolarisation by utilising H2 produced in the cathodic corrosion reaction. Furthermore, along with the expected alkalophile bacteria also bacteria which could only live and multiply at a neutral pH were isolated from samples from district heating systems. This finding points towards the existence of microenvironments with a considerably lower pH than found in the bulk water where pH-values of 9.5 to 10.0 are obtained. Finally, the obtained knowledge on corrosion processes whether chemical or microbial influenced are condensed in a Corrosion Key. This Key should allow personel at district heating plants to perform an initial analysis of corrosion phenomena they might observe in district heating systems. The Key points out possible causes and solutions to these corrosion phenomena. And it is our hope that it will become usefull as a practical tool in the day to day work at district heating plants