Corrosion microbial activities

Microbiauy induced corrosion (MIC) probes. Devices are available to measure the amount of microbial activity in some environments. MicrobiaUy induced corrosion is known to be an actor in many corrosion-related problems in processing plants. The monitoring devices for MIC are limited in their range and, at present, are available only for a few specific environments. This is an exciting area for development of corrosion probes and monitoring systems. 

As mentioned earlier, microorganisms can attack drilling fluid additives and introduce corrosive agents to the system. Therefore, it is very important to monitor their activity and detect any source of problem as early as possible. API RP 38 is probably the most widely used testing procedure in the industry [201]. The methods that can be used to monitor the microbial activity can include the following [201,208] ... 

Soil resistivity The role of soil in the electrical circuitry of corrosion is now apparent. Thus the conductivity of the soil represents an important parameter. Soil resistivity has probably been more widely used than any other test procedure. Opinions of experts vary somewhat as to the actual values in terms of ohm centimetres which relate to metal-loss rates. The extended study of the US Bureau of Standards presents a mass of data with soil-resistivity values given. A weakness of the resistivity procedure is that it neither indicates variations in aeration and pH of the soil, nor microbial activity in terms of coating deterioration or corrosion under anaerobic conditions. Furthermore, as shown by Costanzo rainfall fluctuations markedly affect readings. Despite its short comings, however, this procedure represents a valuable survey method. Scott points out the value of multiple data and the statistical nature of the resistivity readings as related to corrosion rates (see also Chapter 10). 

The finish on metallic materials such as stainless steel, whether it is a refined mill finish, polished to specific grit, or an electro-polished treatment, should complement system design and provide satisfactory corrosion and microbial activity resistance. 

Microbial activity is a major concern in systems in which water-based fluids are used. Particularly, glycol fluids provide a good source of nutrition to some types of biological species. In salt-based brines, however, microbes do not survive because of high osmotic pressure. When microbes start to grow inside a system, they create a layer known as biofilm on the walls of the pipes and heat exchangers. This reduces the heat transfer rate. Some microbes are capable of creating acids and hence cause a substantial amount of corrosion in the system. 

Aluminum alloys can also be attacked by microorganisms. For example, there have been MIC problems with aluminum fuel tanks and transfer lines. In this case, microorganisms grow in the water layer under the fuel to produce volcano-shaped tubercles, frequently evolving gas. Pitting occurs under the tubercles. MIC attack on copper alloys is usually insignificant. However, corrosion of copper condenser tubes by microbially produced ammonia has been reported. In addition, sulfuric acid has been produced by microbial activity by corrosion of underground copper pipes. 

It is extremely diflScult to estimate the eosts related with corrosive processes attributed to the activity of microorganisms (SRB and other bacteria) in the oil industry. In recent years, the costs involving the control of the activity of SRB were significant with annual values estimated at approximately 150,000 per platform when only biocides are used to control microbial activity [2],... 

Microbial activities that produce sulfides, organic, or inorganic acids causing direct metal oxidation are major driving forces in biocorrosion. Biochemical corrosion is enhanced by stagnant water, soil, and organic products. 

F. Kajiyama, K. Okamura. Evaluating cathodic protection reliability on steel pipes in microbially active soils. Corrosion, Vol. 55, No. 1, pp. 74—80, 1999. 

Biofilm formation at the air-water interface. The bathtub ring often formed at the air-water interface around the sides of the basin is likely to be a biofilm due to microbial activity. This film acts like a trap and is known to concentrate caesium and other radioactive isotopes contained in the basin water. This biofilm should be removed mechanically by wet brushing, using water to hold down any airborne activity. A 35% solution of hydrogen peroxide has proven effective in suppressing microbial activity and could be used to assist in biofilm removal without corrosive attack on aluminium alloys. 

Various microelectrode techniques can be powerful tools for evaluating effects of microbial activities on corrosion behavior at metal surfaces. Membrane tyjje microelectrodes have been developed for measuring microprofiles of dissolved O2, N2O, H2S, and pH across the thickness of microbial mats and biofilms [101-106], These have been used successfully to determine oxygen diffusion fluxes [101,102], microbial photosynthetic and respiration rates [103], nitrogen cycling [104], and to locate the oxic/anoxic boundary [105,106]. 

Crevice or pitting corrosion, which often results from microbial activity. 

Chemical compounds were applied for the inhibition of metal corrosion in aqueous environments as ee ly as the 1930s. The choice of inhibitor is greatly influenced by the pH of the water. Deposits on metal surfaces arising from scale formation, fouling, and microbial activity cause accelerated corrosion as a result of differential aeration. Fouling of surfaces may lead to overheating, unscheduled shutdown time, and loss in efficiency. 

The martensitic chromium steels can be used in natural water at ambient temperature up to a maximum chloride content of 200 to 300 mg L (Fot and Heitz, 1967 Effertz and Forchhammer, 1977). No general limits for the maximum chloride level can be given if additional corrosive mechanisms, e.g. microbial corrosion, are active. 

Fixing of anodic reaction sites, whereby microbiological surface colonies lead to the formation of corrosion pits, driven by microbial activity and associated with the location of these colonies... 

The same debatable effects might have also affected the results in the work by Obuekwe. It seems that applying a voltage to the medium (as was done in Obuekwe s works on corrosion of mild steel by IRB) may not resemble MIC properly because there is no way to know how the microbial activity has been affected by the applied voltage and how this would affect the outcome of the experiments. 

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