Metallic coatings pipelines

Soil resistivity measurements can be affected by uncoated metal objects in the soil. Values that are too low are occasionally obtained in built-up urban areas and in streets. Measurements parallel to a well-coated pipeline or to plastic-coated cables give no noticeable differences. With measurements in towns it is recommended, if... 

The potential attenuation (fluctuation of potential) in an impressed-current system has a detrimental effect on conx)sion prevention of a metallic structure containing defects. For instance, a coated pipeline buried in soil is subjected to experience current leakage due to coating defects known as holidays. 

Metal anodes using platinum and precious metal oxide coatings are also incorporated into a variety of designs of impressed current protection for pipeline and deep weU appHcations, as weU as for protection of condenser water boxes in power generating stations (see Pipelines Power generation). 

After 1860 in the United States, water mains were only occasionally given coatings of tar. About 18% the activities of Engii.sh undertakings were extended to America, where chiefly bare metal pipelines had previously been laid. Water supply pipes were coated internally with bitumen in America after 1912. Vical (1837) in France and J. Bull (1843) in America introduced the widely known cement mortar as a protective material for water pipes 16]. 

The electrical resistance of a coating on a pipeline will depend upon the effectiveness of its initial application to the metal and on its condition at the time of testing absorption of moisture may decrease the resistance by as little as 10% during a 2y-year period. The leakage resistance of a given section of pipeline can be determined by the following procedure ... 

The most usual forms of buried metal structures are pipelines, piles, tanks and power and telephone cables. Power cables must usually have some metal protection, covered by expensive continuous factory-applied sheathings of considerable thickness. Since water, gas and petroleum pipelines provide the greatest area of metal surfaces to be protected below ground, a detailed discussion of the protection given to them would appear to be the best means of dealing with coatings for underground use. 

The aim in applying a coating to a buried metal such as a pipeline is to prevent electrical contact with an electrolyte such as soil and/or water. The characteristics required are as follows ... 

It is seen that the organic molecule adsorbs on the Fe in the same way with respect to potential as on Pt, there being a shift in the potential of the maximum corresponding to the difference of the pzc for Pt and Fe. Thus, it is correct to consider inhibitor adsorption in pH 4 as occurring on the metal of the pipeline rather than on an oxide or carbonate coating. It is possible that some oxide or carbonate exists on the metal surface in these situations of pH 4 (C02 saturated), but that the corrosion and its inhibition occurs through cracks in the film, which expose bare Fe to the solution. 

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