Corrosion is a phenomenon associated with the behavior of a metal in its environment, which in this discussion refers to the behavior of metals used in plumbing systems in freshwater suitable for household use. Corrosion has also been properly described as the tendency of a metal to revert to its natural stable state as an ore. The process may involve a second step in which an oxide, hydroxide, or carbonate of the metal may form and deposit at the corrosion site. Thus corrosion takes place when a metal dissolves in or is disintegrated by water. In certain cases, the second step corrosion products or deposits formed may be protective against further corrosion.
Further complexity is added by the fact that different metals have different tendencies to corrode or not to corrode in the same water. A certain type of water may corrode one metal and not another, but the reverse may be true for another type of water.
Water itself can present many different environments. Surface water is quite different from groundwater. Both vary geographically and surface water also varies seasonally. Many communities take their water supply from multiple sources and certain communities even supply mixed surface and groundwater, and so much for the variable water environment.
Household plumbing constitutes a second variable environment. The fact that a number of metals, alloys, and even nonmetals may commonly be used in a single household plumbing system or water-using appliance adds further variation and complexity.
Corrosion is a phenomenon associated with the behavior of a metal in its environment, which in this discussion refers to the behavior of metals used in plumbing systems in freshwater suitable for household use. Corrosion has also been properly described as the tendency of a metal to revert to its natural stable state as an ore. The process may involve a second step in which an oxide, hydroxide, or carbonate of the metal may form and deposit at the corrosion site. Thus corrosion takes place when a metal dissolves in or is disintegrated by water. In certain cases, the second step corrosion products or deposits formed may be protective against further corrosion.
Further complexity is added by the fact that different metals have different tendencies to corrode or not to corrode in the same water. A certain type of water may corrode one metal and not another, but the reverse may be true for another type of water.
Water itself can present many different environments. Surface water is quite different from groundwater. Both vary geographically and surface water also varies seasonally. Many communities take their water supply from multiple sources and certain communities even supply mixed surface and groundwater, and so much for the variable water environment.
Household plumbing constitutes a second variable environment. The fact that a number of metals, alloys, and even nonmetals may commonly be used in a single household plumbing system or water-using appliance adds further variation and complexity.
Unfortunately, therefore, domestic water supplies vary in quality and in their tendency to corrode, and the materials used to construct plumbing systems differ in their tendency to be corroded.
Up to this point we have viewed corrosion in water as a chemical phenomenon, subject to variations in chemical impurities (gases and minerals) present in the water and variations in the chemical composition of plumbing materials. We have not yet considered the physical factors of water temperature and water flow velocity both of which can strongly influence corrosion rates.
Lastly, it is generally accepted that corrosion of metals is electrochemical, resulting from the flow of electric current between electrodes, which may be between different metals, or between anodic and cathodic areas on the surface of a single metal. The disintegration of the metal occurs in the anode areas. This brings our discussion of corrosion to a third variable or complexity: the fact that metal surfaces themselves are not homogeneous in their composition and therefore a number of anodic and cathodic areas may be present on the surface of the metal. Metallic impurities, accumulations of sediment or corrosion products, even adherent biological deposits may be directly or indirectly related to the development of electrical corrosion circuits or galvanic cells.
An excellent method for corrosion control in water heaters is cathodic protection which involves the use of a sacrificial anode, usually composed of magnesium or aluminum. Chemical control of corrosion attempts to retard electrode reactions. Ever since the problems connected with corrosion of municipal distribution systems and household plumbing systems in the United States began to be recognized early in this century, achievement of a cure has continued to remain elusive, as has the determination of exact causes of corrosion and the ability to predict or anticipate rates of corrosion and useful life expectancy of distribution and plumbing systems. Unfortunately, therefore, domestic water supplies vary in quality and in their tendency to corrode, and the materials used to construct plumbing systems differ in their tendency to be corroded.
Up to this point we have viewed corrosion in water as a chemical phenomenon, subject to variations in chemical impurities (gases and minerals) present in the water and variations in the chemical composition of plumbing materials. We have not yet considered the physical factors of water temperature and water flow velocity both of which can strongly influence corrosion rates.
Lastly, it is generally accepted that corrosion of metals is electrochemical, resulting from the flow of electric current between electrodes, which may be between different metals, or between anodic and cathodic areas on the surface of a single metal. The disintegration of the metal occurs in the anode areas. This brings our discussion of corrosion to a third variable or complexity: the fact that metal surfaces themselves are not homogeneous in their composition and therefore a number of anodic and cathodic areas may be present on the surface of the metal. Metallic impurities, accumulations of sediment or corrosion products, even adherent biological deposits may be directly or indirectly related to the development of electrical corrosion circuits or galvanic cells.
An excellent method for corrosion control in water heaters is cathodic protection which involves the use of a sacrificial anode, usually composed of magnesium or aluminum. Chemical control of corrosion attempts to retard electrode reactions. Ever since the problems connected with corrosion of municipal distribution systems and household plumbing systems in the United States began to be recognized early in this century, achievement of a cure has continued to remain elusive, as has the determination of exact causes of corrosion and the ability to predict or anticipate rates of corrosion and useful life expectancy of distribution and plumbing systems.
While great strides have been made, chiefly in the use of corrosion-resistant materials of construction and through the use of cathodic protection for water heaters, each solution has encountered a few problems in universal countrywide application. In fact, the use of each material or system has uncovered new types of corrosion problems which were largely unpredictable and unanticipated.
Corrosion of plumbing systems in water supplies depends on so many interdependent variables that no simple equation or corrosion index is capable of predicting the corrosion potential of a water supply, and no generally applicable recipe for appropriate corrective treatment is available.
The corrosion index
In 1936, W. F. Langelier pointed out in a discussion of the saturation index (corrosion index) which bears his name ". . . the Langelier saturation index is an indication of directional tendency and of driving force but it is in no way a measure of capacity. The capacity to coat the pipe will depend on the property of the water to resist change in the value of the index following attack." Since the Langelier index relates only to the state of saturation of calcium carbonate at any given time, it is incorrect to classify water with a negative Langelier index as corrosive. It would be more accurate to state that a water sample exhibiting a positive Langelier value will deposit calcium carbonate and that a water sample exhibiting a negative value will not deposit, but will dissolve calcium carbonate. Thus, these indices, based on calcium carbonate solubility equations are approximate measures of solubility, no corrosivity. This is verified by many water supplies that do not conform to the predictions of these indices. For example, there are many water supplies exhibiting high positive index values that are corrosive and many water supplies with negative values that are not corrosive; facts which are in direct contradiction to the original hypothesis that positive index values signify deposition of calcium carbonate in pipes and thus protection, whereas negative index values signify lack of deposition, or dissolving, of calcium carbonate from pipes and thus corrosion.
Even though no simple equation or corrosion index could predict the corrosive potential of a water supply accurately, and though no universally applicable method for corrosion control was available, a great deal of highly creditable work in determining the causes of corrosion in water supplies and the development of methods for corrosion control has been done over a period of more than forty years by water chemists and engineers. Such studies have resulted in the development of a number of sound methods for the control of specific corrosion problems at the point of use, even though these methods cannot be applied universally for all types of corrosion in all water supplies. However, with proper knowledge of specific water characteristics, actual plumbing systems, and other pertinent existing conditions, the point-of-use water conditioning professional can successfully employ specific corrosion control methods to correct specific corrosion problems.