Although galvanic cells are responsible for corrosion problems, by far the most common corrosion cell is the oxygen concentration cell. A dead end in the distribution system is the end of a water main that is not connected to other parts of the distribution system by means of a connection loop of pipe. An oxygen concentration cell can be started in this dead end of the water line. At this location, ferrous ions and oxygen ions react to form solid ferric hydroxide and hydrogen ions. The electrons from the anode reaction flow through the metallic pipe where they react with dissolved oxygen which is replenished by flowing water. The excess hydrogen ions lower the pH and make it more corrosive.

In the dead pipe, an absence of oxygen and low pH make the conditions ideal for the growth of anaerobic bacteria. An anaerobic condition is one in which atmospheric or dissolved oxygen is not present. The action of these bacterial populations on trace organic material and on reducing the sulfate ions to sulfide are responsible for foul odors usually found in the dead ends of water mains.

As ferric hydroxide ages, it forms other minerals such as ferric oxide or iron rust. Eventually, the crust from ferric oxide becomes so thick that negative ions cannot enter the pit where corrosion is occurring, and iron ions cannot escape. The corrosion in the area stops. At this point, the pit is inactive. The reaction of dissolved oxygen with ferrous ions is very slow at low pH values. When the pH is less than 7.0, the reaction is so slow that tubercles do not form and the pits are not self-perpetuating. New pits keep starting in different places, so that corrosion appears to be uniform over the surface of the pipe.

Calcium Carbonate Saturation

Internal corrosion of pipes can be detected through rusty water complaints and by examining the insides of pipes for pitting, tubercles, and other evidence of corrosion. Corrosion from unstabilized water can be predicted by the amount of calcium carbonate saturation. The methods that are used to predict the stabilization of water are the Marble Test, Enslow Column, and the Langelier Index. Water is considered to be stable when it is just saturated with calcium carbonate. In this condition, the water will not dissolve or deposit calcium carbonate. Water treatment plant operators use two approaches to determine the calcium carbonate saturation level of their water: the Calcium Carbonate Precipitation Potential (CCPP) and the Langelier Index.

Marble Test and Enslow Column

To conduct a Marble Test for calcium carbonate saturation, first measure the pH, alkalinity, and hardness of the water sample. Add a pinch of powdered calcium carbonate and then stir the water for at least five minutes. The water should be stirred in a nearly-full stoppered flask to avoid the introduction of carbon dioxide from the air. Also, the water being stirred should be at the same temperature as the water in the distribution system. If the pH, alkalinity, or calcium carbonate increase, the water was under saturated with respect to calcium carbonate. If they decrease, the water was supersaturated. The CCPP equals the quantitative change in alkalinity (or calcium carbonate) due to the water being exposed to the powdered calcium carbonate.

The Enslow column can be conveniently used to perform the Marble Test. This tube is a column packed with calcium carbonate granules. The pH, alkalinity, and calcium carbonate are measured on a sample stream of water before and after passing through the column. The results are interpreted in the same manner as the Marble Test.