Microbial corrosion cells

The anaerobic corrosion of iron was noted in the nineteenth century and many theories were proposed about its mechanism. Decades of scientific research projects and investigations on the complex influence of microbes on increasing or decreasing corrosion rates have provided a much deeper insight in the role microorganisms play on the life of systems exposed to waters and grounds where they proliferate (see Chap. 10 for more details). 

Microorganisms tend to attach themselves to solid surfaces, colonize, proliferate, and form biofilms which may in turn produce an environment at the biofilm/metal interface radically different from the bulk environment in terms of pH, dissolved oxygen, organic and inorganic species. Since the biofilm tends to create nonuniform surface conditions, localized attack might start at some points on the surface leading to localized corrosion, usually in the form of pitting [22]. 

When microorganisms are involved in the corrosion of metals, the situation is more complicated than for an abiotic environment, because microorganisms not only modify the near-surface environmental chemistry via microbial metabolism but also may interfere with the electrochemical processes occurring at the metal-environment interface. Many industrial systems are likely to contain various structures where MIC and biofouling may cause serious problems open or closed cooling systems, water injection lines, storage tanks, residual water treatment systems, filtration systems, different types of pipes, reverse osmosis membranes, potable water distribution systems and most areas where water can stagnate. 

Such failures can take two forms. First is the failure of a system to hold water. This is most often seen in the development of the pinholesized leaks, often considered to be a sure sign of MIC infection. This is also t5rpically the only concern in many treatment investigations. 

Second, and more concerning, is the failure of a system to achieve its designed purpose that of fire control. Several systems with MIC 

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Can SRB Counts Affect Corrosion Rate One of the measures, or rather, rule of thumb, that is stiU in use in some industries is classification of the corrosivity of water environments based on the number of SRB cells, shown as cells per milliliter. According to this classification, if the SRB amount is 1000 cells/mL or less, the environment is considered as low corrosive. If the SRB amount is between 10 and 10 cells/mL, then the environment is considered as being mild corrosive. Therefore, if the SRB level is more than 10 ceUs/mL, the environment is labeled as highly corrosive. Likewise, there are also general criteria for evaluation of soil microbial corrosivity based on SRB counts alone. ... 

The combination of favorable properties of PANI and TiO opens the possibility for various applications of PANI/TiO nanocomposite materials, such as piezoresistivity devices [41], electrochromic devices [99,118], photoelectrochemical devices [43,76], photovoltaic devices/solar cells [44,50,60,61,93,119], optoelectronic devices/UV detectors [115], catalysts [80], photocatalysts [52,63,74,75,78,84,87,97,104,107,121,122,125], photoelectrocatalysts [122,123], sensors [56,61,65,69,85,86,95,120,124], photoelectrochemical [110] and microbial fuel cells [71], supercapacitors [90,92,100,109,111], anode materials for lithium-ion batteries [101,102], materials for corrosion protection [82,113], microwave absorption materials [77,87,89], and electrorheological fluids [105,106]. In comparison with PANI, the covalently bonded PANI/TiO hybrids showed significant enhancement in optical contrast and coloration efficiency [99]. It was observed that the TiO nanodomains covalently bonded to PANI can act as electron acceptors, reducing the oxidation potential and band gap of PANI, thus improving the long-term electrochromic stability [99]. Colloidal... 

Microbial layers (sludge) on metal surfaces can cause metal pitting or corrosion due to differing charge potentials between the covered and uncovered areas. Biopolymers in the biofilm trap ions creating a concentration of ions in the covered area. This will shift the potential of the metal surfaces to create localized corrosion cells. The area of lower concentration will be attacked. 

In previous chapters, the importance of biocorrosion and its possible mechanisms were discussed. We also looked at some crucial factors that could increase the likelihood of MIC in a given system The particular focus of the chapter was the avoidance of microbial corrosion. However, almost all the time, what happeais in real life is that the system of concern has already been contaminated and the outstanding question is no longer how to prevent, but rather how to estimate the severity and extent of MIC. For instance, while for SRB-induced MIC, some investigators believe that no relationship exists between the corrosion rate and the number of the bacteria cells [1] the number of acid-producing bacteria in a system has a profound effect on the corrosion rate. 

It will now be evident, from consideration of possible cathodic reactions, that corrosion cells may arise because of localized compositional differences in the electrolyte with respect to, for example, pH, dissolved O2, metal ions, oxyanions and the metabolism of microbial infections. 

Manganese and iron oxidation are coupled to cell growth and metabolism of organic carbon. Microbially deposited manganese oxide on stainless and mild steel alters electrochemical properties related to the potential for corrosion. Iron-oxidizing bacteria produce tubercles of iron oxides and hydroxides, creating oxygen-concentration cells that initiate a series of events that individually or collectively are very corrosive. 

Duncan KE, Perez-Ibarra BM, Jenneman G, Harris JB, Webb R, Sublette K (2014) The effect of corrosion inhibitors on microbial communities associated with corrosion in a model flow cell system. Appl Microbiol Biotechnol 98 907-918... 

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