Water, corrosion piping deterioration

Distribution Systems. A substantial amount of contamination of drinking water can occur while the water is in transit to the consumer after treatment. Pipes are made of copper, galvanized iron, asbestos-cement, lead, or plastic, and often polymeric or coal tar coatings are used. All of these are capable of contributing contaminants to the water, especially if the water is corrosive. Lead, copper, cadmium, and polynuclear aromatic hydrocarbons in finished water are primarily problems of water distribution and not source water contamination. Physical deterioration of the distribution system can also permit biological contamination to occur during transit. 

Plastic piping [polyvinyl chloride (PVC)] does not show corrosion as in the case of metal piping, but the properties of plastic piping deteriorate over time. In severely corrosive soils PVC piping may be selected rather than a metallic piping because it is inert to the chemical conditions. PVC has a lower density than steel and iron and hence it is relatively easy to handle in the field. However, PVC has lower strength and traditional welding is not possible. PVC has been used for a relatively short time, compared with steel and iron water lines. Thus, there is limited data on the expected service life of PVC pipelines, and calculations of comparative total life-cycle costs are not possible. 

Lead pipes The corrosion resistance of lead is generally excellent, but it is attacked by certain waters. This is usually of little significance so far as deterioration of the pipe is concerned, but is important because of danger to health, since lead is a cumulative poison even very small doses taken over long periods can produce lead poisoning. It is for this reason that its use for carrying potable water has been discontinued. 

Deterioration of water pipe fittings such as service saddles contributes to water main failures. This type of failure may be due to galvanic corrosion since the pipe is cast or ductile iron, saddle is either bronze, steel or cast iron and the service line is made of copper. 

Corrosion is the deterioration of a material due to interaction with its environment It is the process in which metallic atoms leave the metal or form compounds in the presence of water and gases. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All metals and alloys are subject to corrosion. Even the noble metals, such as gold, are subject to corrosive attack in some environments. 

Distribution water pipes are of smaller diameter than larger transmission pipes and are made of ductile iron, PVC, and copper. Corrosion problems and corrosion control methods for ductile iron and the deterioration of PVC are similar in pipes of both small and large diameters. 

Water containing 0.01 wt% sand is flowing through a pipe at a flow velocity of 60 m/s. Describe how the annual erosion depth increment in the pipe wall can be calculated on the basis of short duration tests in a similar pipe where the sand concentration in the water is varied in the range 0.1-10 wt% and the flow velocity in the range 20—40 m/s. Assume in the first instance that the deterioration process is mainly erosion and only little corrosion. 

Carbon and low-alloy steels are probably the most commonly used materials for pipes handling water, petroleum products, and some chemicals. Reference 1 provides a summaiy of the different specifications used for pipelines. Steel tends to corrode by both pitting and uniform surface deterioration [2]. Steel must usually be protected from corrosion both on the inside from the material being carried and on the outside from corrosion by the atmosphere, soil, or water that surrounds the pipe. External corrosion protection is provided by material selection, selective backfill, barrier coatings, stray current control, and cathodic protection. Internal corrosion protection can be provided by inhibitors, coatings, design process control, and materials selection. 

The deterioration of metals, commonly referred to as corrosion, is a critical factor affecting the useful life of metals in facilities. Most importantly, the rate of corrosion affects how long a particular metal component wiU function in its intended use. Some metals corrode at a very slow rate, which makes them good candidates for certain applications. For example, aluminum has been found to be an excellent material for hatch covers in atmospheric exposures. On the other hand, the use of 304 stainless steel for pipe hangers in piers over salt water has resulted in failures within a year. The application, the environment, and the intended service are critical to proper material selection. 

Cathodic protection A means of applying an external electric current to reduce corrosion virtually to zero. A metal surface can be maintained in a corrosive environment without deterioration as long as the cathodic protection system provides the external current. Cathodic protection can be galvanic, such as that on galvanized pipe, or it can be impressed current, such as that usually found on large potable water tanks and pipelines. 

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]. 

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