The Basic Type of Corrosion Explained

When water is acid, or even slightly alkaline, it has a tendency to be corrosive. No doubt, you have seen examples of how strong acids have rapidly dissolved metals. When water is low in pH, the same type of action occurs, although at a slower rate. Acid water may be traced to several different causes. For example, it may be acid due to the presence of certain dissolved gases, such as carbon dioxide, or hydrogen sulfide. The acidity may also be due to certain acid industrial wastes.

Water Faucet

My waters frequently contain high concentrations of strong acids and are probably the most corrosive of all "natural" water supplies. In addition, they are frequently heavily charged with iron. These mine waters can, however, be satisfactorily treated, providing their sulfate content is not too great. When acid water attacks the walls of a metal container, the entire metal surface usually corrodes rather evenly. An exception occurs where water flows in a steady, consistent pattern through a container. When this happens, the water is likely to eat deep grooves in the metal.

Curiously, byproducts of corrosion frequently act to protect metals from further attack. One common byproduct is hydrogen gas. If the water is quiescent, the hydrogen gas acts as a protective film to prevent further corrosion. Another byproduct is zinc carbonate. This is found when galvanized pipe corrodes. Other byproducts vary depending on the type of metal. In many cases, they tend to act as a protective film. If these byproducts are swept away by the flow of the water, there is nothing to protect against the damaging effects of continuing corrosion.

The electrical conductivity

The electrical conductivity of a water supply also affects its corrosive action. It is well known that an electrical current can be produced by immersing plates of dissimilar metals in a solution that conducts electricity. Under such conditions, a definite and measurable amount of electricity will flow through a connection between the plates. This connection may be an external wire, or it may be direct contact between the metal plates.

For example, if plates of zinc and copper are placed in a solution that conducts electricity, and a wire connected between the plates, the following occurs:

  • The zinc will pass into a solution as zinc ions.
  • A flow of current will run through the connecting wire.

A similar situation may occur in a household plumbing system. It sometimes happens that zinc galvanized pipe comes in contact with copper or brass (a copper alloy). Under these circumstances, that second condition direct contact between the metal plate exists. As the water is an electrolytic solution (that is, capable of carrying an electric current), the zinc in the galvanized pipe will pass into the solution as zinc ions. The speed of this reaction increases with the conductivity of the water. Over a period of time, this loss of zinc ions can be detected in the deterioration of the pipe.

We can say then: When dissimilar metals are in contact in a solution that can carry an electric current, two actions occur:

  • An electric current flows between the two metals.
  • One of the metals gradually dissolves.

In many home water systems galvanized pipe is used in conjunction with brass valves or other fittings. At every joint between the dissimilar metals, an electric current is generated, and this causes corrosion of one of the metals. The rate at which this reaction occurs is largely determined by the conductivity of the water and the amount of brass in relation to zinc. In turn, this conductivity is determined by the amounts of various minerals in the water. Fortunately, most water supplies have low conductivity. Thus, the corrosion which occurs does not cause major problems. However, with other water supplies, this type of corrosion increases the amount of iron in the water and leads to premature failure of pipelines and water heaters. Galvanic corrosion produces pitting or deep etching in the less noble* of the two metals.

corrosion

Further, this type of corrosion occurs close to the point where the less noble connects with the more noble metal. It would seem that the use of a single metal throughout a plumbing system might prevent this type of corrosion.

While this practice does help, it will not guarantee the prevention of corrosion. Why? Because local impurities may exist on the metallic surface. There may be impurities in the zinc used to galvanize steel pipes. Again there may be impurities due to either the metallic or the nonmetallic substances in the water itself. In any event, there is always the possibility of corrosion. The rate of corrosion due to electrolytic action depends on the dissolved mineral solids in the water. The more of these solids, the greater their ability to carry an electric current. Hence the greater its corrosiveness.

The Free Oxygen

Free oxygen in water can also cause corrosion. Just as metals are exposed to air rust or tarnish, metals in the plumbing system can be attacked by the oxygen in the water. The oxygen in water actually combines with the metal to form an "oxide." The chemical reaction is the same as occurs when moist air rusts unprotected metallic surfaces. In water or in moist air, a reaction with oxygen consumes a portion of the metal present. Corrosion in water systems due to dissolved oxygen commonly takes the form of deep pitting rather than a general attack of the entire surface. The end result may not look as bad as when the entire surface is attacked. But a single small hole can make a component of a plumbing system useless.

Temperature increases affect the rate of corrosion because an increase in temperature increases the rate of the electrochemical corrosion reaction. As a rule, an increase in temperature results in a stepping up of the corrosive action of the water. In household water supplies the use of greater volumes of hot water has intensified the corrosive action of water. Even where there is little or no corrosion in the cold water lines of a home, a high corrosion rate may exist in the hot water system. Certainly, corrosion is far more probable in the hot water lines than in the cold in almost any household installation. Studies show that the corrosion of steel may be stepped up three to four times the normal rate when the temperature of the water is increased from 60°F to 140°F. Over 140°F the rate may double with every 20-degree increase in temperature.

Due to the possible interplay of the three major causes of corrosion, it is not always sim­ple to pinpoint the source of corrosion trouble in any given instance. Yet you need to have some idea of what's causing the trouble in order to correct a corrosion problem.

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