Soils, corrosion distribution systems

A large percentage (57%) of mains and services (46%) is metal (steel, cast iron or copper), and corrosion is a major issue. For distribution pipe, external corrosion is of primary importance, although internal corrosion has been noted in some cases. The methods of monitoring corrosion on cathodically protected pipe are similar to those in the transmission pipeline sector, including pipe-to-soil potential and coating surveys. One difference is that in distribution systems, leak detection is an acceptable method of monitoring for these pipelines without CP (nearly 15% of the steel mains). 

Corrosion in public water piping systems is responsible for large economic loss. In pipes of cast iron, steel and other metallic materials, corrosion may be prevented by use of coatings (Section 10.6) or by water treatment (addition of calcium compounds, alkalization or carbonation). Water distribution systems are further dealt with in Section 8.4, Corrosion in Soils. 

System reliability is of the utmost importance to water suppliers and their customers. However, corrosion problems can vary greatly within a single system because many variables affect corrosion, for example, pipe material, pipe age, pipe wall thickness, water additives, corrosion inhibitor treatment, soil chemistry, soil moisture content and/or local groundwater level, and stray currents [2]. Table 8.2 summarizes some of the physical, environmental, and operational factors that can affect the deterioration rate of water distribution systems and lead to their failrue [4]. 

Corrosion is a threat to the environment. For instance, water can become contaminated by corrosion products and unsuitable for consumption. Corrosion prevention is integral to stop contamination of air, rater and soil. The American Water Works Association needs US 325 billion in the next twenty years to upgrade the water distribution system. 

Stray currents are currents flowing in the electrolyte from external sources, not directly associated with the cathodic protection system. Any metallic structure, for example, a pipeline, buried in soil represents a low-resistance current path and is therefore fundamentally vulnerable to the effects of stray currents. Stray current tends to enter a buried structure in a certain location and leave it in another. It is where the current leaves the structure that severe corrosion can be expected. Corrosion damage induced by stray current effects has also been referred to as electrolysis or interference. For the study and understanding of stray current effects it is important to bear in mind that current flow in a system will not only be restricted to the lowest-resistance path but will be distributed between paths of varying resistance, as predicted by elementary circuit theory. Naturally, the current levels will tend to be highest in the paths of least resistance. 

When a very long, unprotected or poorly protected embedded structure such as a pipeline crosses different types of soil, the dissolution potentials of the metal with respect to the soil are not consistently the same. This leads to the circulation of currents, which results in localised corrosion at the exit zones into the soil. This is also observed with immersed or semi-immersed structures such as ship hulls. For this reason, the return current should not flow through the hull, as one would be inclined to do on a small craft with battery-powered electric equipment. One conductor for each polarity is required if the system is distributing direct current, and one conductor per phase (plus one for the neutral, if required) for alternating current. 

---