Corrosion Indicators

The most common indicators of corrosion in the distribution system are red water complaints and leaks. If the incidence of these problems increases in a certain area of the distribution system, then some sort of corrosion control may need to be undertaken. Red water is usually caused by tuberculation and iron bacteria while leaks are caused by the pitting below tubercles. However, one should be aware of other possible causes of these problems. High iron concentrations in the source water can cause red water problems while leaks can be caused by corrosive soil acting on the outside of the pipes as well as by corrosive water acting on the inside of the pipes.

During routine maintenance of the distribution system, the operator should watch out for signs of corrosion and scale. When pipes are removed and replaced, the old pipes should be visually examined for signs of tubercles, pitting, or uniform corrosion, and for excessive scaling.

Dissolved oxygen and toxic heavy metals in the distribution system can be used as indicators of corrosion over a much shorter time frame. Dissolved oxygen is tested at various points in the distribution system at the same time. If the dissolved oxygen concentration becomes lower further from the treatment plant, then the oxygen is probably being used up by corrosion. However, the operator should be aware of the possibility that D.O. is being used to oxidize organic matter.

Toxic heavy metals, such as copper and lead, are tested at the consumer’s tap. High concentrations of these metals in the water indicate corrosion in the distribution system, although in a few cases the metals may have originated in the source water.

Both the Langelier Index and the Marble Test are laboratory tests which can determine the degree of calcium carbonate saturation in the water at the treatment plant. Water which is just saturated with calcium carbonate or which is slightly supersaturated with calcium carbonate is considered stable and safe to release into the distribution system.