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Corrosion is a natural process involving chemical or electrical degradation of metals in contact with water. The rate of corrosion will vary depending on the acidity of the water, its electrical conductivity, oxygen concentration, and temperature. Acidic water with pH values in the range of 6 to 7 is more corrosive to the metals used in plumbing systems than alkaline water. Both ground and surface water can be acidic.

Common causes for acidic surface water are acid rainfall due to atmospheric carbon dioxide and other airborne pollutants, runoff from mining spoils, and decomposition of plant materials. Acidic ground water can also be caused by the above factors but is mostly controlled naturally by the equilibrium relationship with surrounding minerals.

Alkaline water does not eliminate corrosion if it has high electrical conductivity. When two different metals such as steel and brass are in contact with a solution which will conduct electricity, a galvanic cell is established. One of the metals will corrode in proportion to the electricity generated. If plumbing is installed using different metals (copper, steel, brass, zinc, and various alloys) corrosion will occur.

Oxygen dissolved in water will also enhance the process of corrosion. Deep well water is usually free of dissolved oxygen, but it is present in surface water. The temperature of water is a significant factor in the rate of corrosion.