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.
A Chemical Reaction
The combination of oxygen and water provides an excellent environment for corrosion to occur. A steel fence post, for example, will usually rust off right at the surface of the ground. Below ground there may be moisture, but relatively little oxygen. Above ground there is continuous oxygen from the air, but only occasional moisture. Only at the surface of the ground are both moisture and oxygen common, and here is where corrosion is most severe.
Wherever water is exposed to air, some of the oxygen in the air will be absorbed by the water. As water falls to the earth as rain, or flows across the surface of the land in lakes and streams, it quickly becomes saturated with oxygen. On the other hand, oxygen in water is consumed as the water seeps into the ground through layers of decaying organic matter. Thus, deep well waters are usually free of dissolved oxygen. Exceptions occur, of course, in areas where the ground contains little organic matter. Further, the air cushion in a pneumatic tank can contribute air to a deep well water supply.
When the chemical reaction of corrosion occurs, a very thin film of hydrogen forms at the surface of the corroded metal. If this film could be retained, it would serve as a barrier to protect the metal from contact with the water, and the corrosion reactions would stop. However, when oxygen is present in the water, it combines with the hydrogen film and removes the film from the metal surface. Thus, corrosion will continue, eating deeper and deeper into the metal. Some experts are convinced that at least some oxygen must be present for any significant corrosion to occur.
Corrosion is a chemical reaction, and it has long been known that most chemical reactions proceed faster at higher temperatures. Thus, the temperature of water is an important factor in the rate of corrosion. One study indicated that the corrosion rate of steel increased three to four times when the water temperature was increased from 60°F to 140°F. Above 140°F, the rate of corrosion doubled with every 20° increase in temperature.
The purpose of galvanizing steel is to deliberately establish a galvanic cell. At normal temperatures, the zinc coating tends to go into solution and then deposit on any spots of exposed steel, thus protecting the steel against continued corrosion. However, in the range of 140°F to 160°F, this action tends to reverse itself. At temperatures above this range, it is the iron in the steel which tends to go into solution in a vain effort to protect the zinc. Since the areas of exposed steel are usually small, the rate of steel corrosion is concentrated in those areas, and the pipe wall is soon perforated.
From the above, it is apparent that corrosion is not a simple problem, and in most systems, is related to more than one factor. Further, it is impossible to completely halt corrosion where water contacts metals, but we do have some methods which will reduce corrosion.
Where the prime cause of corrosion is acidity as indicated by low pH values, the obvious answer is to neutralize that acidity. One of the simplest ways is to install a neutralizing filter, which contains materials such as calcite, (calcium carbonate) or magnesia (magnesium oxide). As the water passes through a bed of such materials, the carbonic acid is neutralized and a small amount of the bed is dissolved. This is essentially the same reaction which occurs when acidic waters trickle through limestone strata in the ground, and small amounts of hardness are added to the water. “Overfeeding” does not occur, because as the water is neutralized, the dissolving action automatically stops.
Neutralizing filters must be backwashed periodically because they do serve as mechanical filters to remove solid particles from the water. Further, particles of material at the top of the bed ultimately become so small that they tend to clog the bed. Thus, they must be removed by backwashing. From time to time additional material must be added to the bed to replace that which is dissolved.
An alternative method of neutralizing acid water is to feed a solution of soda ash (sodium carbonate) to the water supply with a chemical feed pump. The chemical feed pump may be wired to operate in conjunction with the well pump, and thus good proportioning of the soda ash to the water flow is obtained. By introducing the soda ash solution ahead of the pressure tank, good mixing and neutralization are obtained. The feed rate of the feed pump is usually adjusted to produce a treated water pH of 7.5 to 8.0.
Where acidity is the only problem, the neutralizing filters are usually the best approach. However, where the water contains much iron, or disinfection of the water is desired, the chemical feed pump is often used since hypochlorite bleach and soda ash may be mixed in a single solution and fed into the water system with the same pump unit. When the prime causes of corrosion are high concentrations of dissolved minerals, which increase its electrical conductivity, or dissolved oxygen, there are no feasible and economical methods of removing these materials from small private water systems. The pickup of dissolved oxygen from the air in the pressure tank can be reduced by the use of one of the pressure tank designs which incorporate either a flexible “membrane” or a floating disc to minimize the water are exposed to the air.
Although it is not feasible to remove dissolved minerals and oxygen from the water, two types of materials are available to control their corrosive actions. Several types of food grade polyphosphate compounds and silicate compounds are available which can be fed into the water system for corrosion control. In action, these materials lay down very thin films on the interior metal surfaces, thus minimizing the water to metal contact and reducing the rate of corrosion. Since the films do slowly re-dissolve, the feeding of the materials should be maintained at the proper levels. At the beginning of a feeding program, old corrosion deposits may be loosened and flushed through the system, and this often appears to make a “red water” problem worse, and higher than normal feed rates may be required until the system is reasonably clean and the film established. Then the feed rate can usually be reduced just to maintain the protective film.
Polyphosphate compounds can be added to the water either as dry soluble crystals or as a solution. The compound is fed into the water either as dry soluble crystals or as a solution. The compound is fed into the water either by a chemical feeder or a chemical feed pump. The chemical feeder is a tank-type unit which is installed so that a portion of the water flow passes through the tank and, in a manner determined by the technical design of the feeder, the solution formed by the compound is added to the water. Chemical feeders utilize one or both of the feed-rate control factors of solubility of the chemical compound and flow rate through the dispenser. For a greater degree of feed-rate control, some chemical feeders incorporate pressure differential devices and/or precision orifices within the feeder itself. Others are designed to be used with a valve in the main water line to create slight resistance to flow, to force some water flow through the feeder tank. Sometimes this valve is incorporated directly into the feeder design.
All chemical feeders must be refilled with the chemical compound periodically to replace that which has been dissolved.
The polyphosphate compound in liquid solution can also be added to the water by means of a chemical feed pump, which can be set to inject a specific amount of the solution from the chemical feeder tank into the water at regular intervals. Highly soluble silicate compounds in solution used for corrosion control can also be fed into the water by means of chemical feed pumps.
Thus, several approaches may be used to control corrosion in household water systems. Acid waters may be neutralized with filters designed for that purpose, or soda ash may be fed with chemical solution feeders. Chemicals such as polyphosphates and silicates may be fed to form protective films in the water systems. For further corrosion control, water heaters should be set only as high as necessary, and temperatures above 140°F avoided. Even when modifying plumbing systems, avoid the use of dissimilar metals where possible.
NOTE: The determination of which treatment method is best should be made only after careful consideration of many factors such as economics, water quality characteristic, the end use to which the water is to be put, temperature variances of the water to be treated, the inherent limitations of the available treatment technology, and others. This determination can best be made by your local water treatment representatives and they should be consulted prior to the purchase and installation of any water treatment equipment.