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Corrosion

A Chemical Reaction
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Dissolved Oxygen

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.

Water Temperature

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 60F to 140F. Above 140F, 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 140F to 160F, 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.

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