Corrosion of Underground Pipelines

The corrosivity of soils also depends upon the oxidation-reduction potential as classified by Booth et al.15 The classification scheme of the corrosivity of soils is given in Table 4.4b. Macrogalvanic cells are formed in underground pipelines due to foreign structure the combination of new and old pipe dissimilar metals (stainless steel and carbon steel) differential aeration dissimilar soils and stray currents. All these lead to localized corrosion of underground pipelines. 

Another use of voltaic cells is to control the corrosion of underground pipelines and tanks. Such pipelines and tanks are usually made of steel, an alloy of iron, and their corrosion or rusting is an electrochemical process. 

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

Apart from corrosion due to differential aeration, corrosion of underground metal structures and pipelines may also arise from stray currents. How this comes about can be seen in the accompanying diagram (Fig. 12.32). The presence of a current-carrying cable in conducting soil results in stray currents passing through the soil. These stray currents may set up a potential difference between two portions of a pipeline, which then develops electron-source (cathodic) and -sink (anodic) areas. Thus, pipelines tend to corrode when they pass near electric lines. 

Stress Corrosion Cracking One of the serious forms of pipeline corrosion is see. This form of corrosion consists of brittle fracture of a normally ductile metal by the conjoint action of a specific corrosive environment and a tensile stress. In the case of underground pipelines, see affects the external surface of the pipe, which is exposed to soil/ground water at locations where the coating is disbonded. 

The most aggressive corrosive attacks occur in the presence of microbial communities that contain a variety of bacteria. In these communities, the bacteria act cooperatively to produce favorable conditions for the growth of each species. For example, obligate anaerobic bacteria can thrive in aerobic environments when present beneath biofilms/deposits in which aerobic bacteria consume the oxygen. In the case of underground pipelines, the severe attack has been associated with acid-producing bacteria in such bacterial communities (Fig. 4.9). 

G. Sabde, F. Gan, D.T. Chin, Cathodic protection of underground pipeline against crevice corrosion a review,). Chin. Inst. Chem. Eng. 24 (1993) 417—429. 

MIC has been estimated to be responsible for 10% of corrosion cases in the United Kingdom. jj has caused a lifetime reduction of flow lines in Western AnstraUa from the designed 20-plus years to less than 3 years. Also, microbial corrosion has been addressed as one of the major causes of corrosion problems of underground pipelines. " ... 

Amirat, A., Chateauneuf A. Chaoui, K., 2006. Reliability assessment of underground pipelines under the combined effect of active corrosion and residual stress. Int J of Pressure Vessels Piping. 83(2) 107-117. 

Internal corrosion of product pipelines can be controlled with linings and inhibitors (a few parts per milhon) such as amines and nitrites. Ingenious methods for lining pipehnes in place underground have also been developed. 

Ahmmad, M., Melchers, R.E., 1994. Probabilistic analysis of underground pipelines subject to combined stresses and corrosion. Engineering Structures 19 (12), 988—994. 

Most fuel system storage tanks, transfer lines, and underground pipelines are composed of 1018/1020 carbon steel. These system components are all susceptible to internal corrosion whenever fuel containing water is introduced. Other factors which can enhance fuel storage and transportation system corrosion include ... 

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