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Corrosion is a chemical process in which the metals commonly used in plumbing systems are eaten away and ultimately fail. Some types of corrosion cause a fairly uniform attack of metals, gradually thinning the entire metal surface, often causing "red water" from iron or steel water systems, or blue stains from copper or brass systems. Another type of corrosion concentrates its attack in small areas, developing deep pits which can penetrate the wall of a pipe or tank. This type of corrosion usually does not contribute iron or copper to the water, but even a single hole in a length of pipe or a tank can destroy its usefulness.
Corrosion is a perfectly natural process. Man has learned how to convert the naturally occurring ores into useful metals, but all of these metals have a tendency to revert back to their natural, stable ore forms. Some metals are highly resistant to corrosion, but these are usually too costly to be used in plumbing systems.
Similarly, all waters are corrosive in at least some degree. However, the rates of corrosion produced by different waters vary widely, depending upon a number of factors. The major factors which govern the rate of corrosion include acidity, electrical conductivity, oxygen concentration, and temperature. Each of these factors is discussed in the sections below.
Acidity or Low pH
The pH scale is used by chemists to express the balance between the materials in water which on one hand tend to make it acid, and on the other hand tend to make it alkaline. On this scale, 7.0 is the neutral point, indicating a perfect balance between the acid and alkaline materials: pH values below 7.0 indicate an increasing overbalance of acid materials, and pH values above 7.0 indicate an increasing overbalance of alkaline materials.
As water falls to the earth in the form of rain, it picks up carbon dioxide from the air. As this same water trickles through the earth, decaying vegetation adds more carbon dioxide to the water. This carbon dioxide, the same gas which is present in carbonated beverages, combines with the water to form carbonic acid. In areas where the groundwater trickles through limestone (calcium and magnesium carbonates), the carbonic acid and limestone combine to form soluble bicarbonates, neutralizing the acid in the process. The resulting waters are usually hard, somewhat alkaline, have low carbon dioxide concentrations, and pH values in the range of 7to 8.
Where the underground strata does not contain limestone, the groundwaters retain their acidity, commonly have pH values in the range of 6 to 7, and are known to be corrosive to the metals used in plumbing systems.
When two different metals, such as steel and brass, and are in contact with each other and with a solution which will conduct electricity, a galvanic cell is established. In this cell, electricity is generated, and one of the metals will be dissolved, or corroded, in proportion to the electricity generated. This galvanic corrosion occurs very close to the joint between the dissimilar metals.
Absolutely pure water is a very poor conductor of electricity, but many dissolved minerals and gases separate into charged particles called ions which are capable of conducting electricity. Thus, water supplies which have few dissolved minerals are poor conductors, but waters containing high mineral concentrations are relatively good conductors.
It is rare that an entire water system is constructed of a single metal. Galvanized pipe systems often use brass valves, and the zinc (galvanizing) surface is broken at the ends of lengths of pipes and at threads. Copper plumbing systems often use solder at the joints, and the valves are made of a different alloy. Even if a single metal were used throughout a system, galvanic cells can exist due to spot impurities at the surfaces, and by differences between bare metal and metal covered with scale or other deposits from the water.
Under these circumstances, every water system has a number of potential galvanic cells and sites for possible severe corrosion. Where the water has low mineral concentrations, this type of corrosion does not present major problems. However, in some areas the mineral concentrations and conductivity of the water is so high that galvanic corrosion does create major problems.