Science of Water : Concepts and Applications

304 The Science of Water: Concepts and Applications
dissimilar metals; or (3) differential concentration cells. Corrosion starts at the surface of a material
and moves inward.
The adverse effects of corrosion can be categorized according to health, aesthetics, economic
effects, and other effects.
The corrosion of toxic metal pipe made from lead creates a serious health hazard. Lead tends to
accumulate in the bones of humans and animals. Signs of lead intoxication include gastrointestinal
disturbances, fatigue, anemia, and muscular paralysis. Lead is not a natural contaminant in either
surface waters or groundwaters, and the MCL of 0.005 mg/L in source waters is rarely exceeded. It
is a corrosion by-product from high lead solder joints in copper and lead piping. Small dosages of
lead can lead to developmental problems in children. The USEPA’s Lead and Copper Rule addresses
the matter of lead in drinking water exceeding specifi ed action levels.
√ Note: The USEPA’s Lead and Copper Rule requires that a treatment facility achieve optimum
corrosion control.
Cadmium is the only other toxic metal found in samples from plumbing systems. Cadmium is
a contaminant found in zinc. Its adverse health effects are best known for being associated with
severe cone and kidney syndrome in Japan. The primary maximum contaminant level (PMCL) for
cadmium is 0.01 mg/L.
Aesthetic effects that are a result of corrosion of iron are characterized by “pitting” and are a
consequence of the deposition of ferric hydroxide and other products and the solution of iron; this is
known as tuberculation. Tuberculation reduces the hydraulic capacity of the pipe. Corrosion of iron
can cause customer complaints of reddish or reddish-brown staining of plumbing fi xtures and laundry.
Corrosion of copper lines can cause customer complaints of bluish or blue-green stains on plumbing
fi xtures. Sulfi de corrosion of copper and iron lines can cause a blackish color in the water. The byproducts
of microbial activity (especially iron bacteria) can cause foul tastes and odors in the water.
The economic effects of corrosion may include the need for water main replacement, especially
when tuberculation reduces the fl ow capacity of the main. Tuberculation increases pipe roughness,
causing an increase in pumping costs and reducing distribution system pressure. Tuberculation and
corrosion can cause leaks in distribution mains and household plumbing. Corrosion of household
plumbing may require extensive treatment, public education, and other actions under the Lead and
Copper Rule.
Other effects of corrosion include short service life of household plumbing caused by pitting.
Buildup of mineral deposits in the hot water system may eventually restrict hot water fl ow. The
structural integrity of steel water storage tanks may deteriorate, causing structural failures. Steel
ladders in clear wells or water storage tanks may corrode, introducing iron into the fi nished water.
Steel parts in fl occulation tanks, sedimentation basins, clarifi ers, and fi lters may also corrode.
Types of Corrosion
Three types of corrosion occur in water mains: galvanic, tuberculation, and pitting.
1. Galvanic—When two dissimilar metals are in contact and are exposed to a conductive
environment, a potential exists between them and current fl ows. This type of corrosion
is the result of an electrochemical reaction when the fl ow of electric current itself is an
essential part of the reaction.
2. Tuberculation—This refers to the formation of localized corrosion products scattered
over the surface in the form of knob-like mounds. These mounds increase the roughness
of the inside of the pipe, increasing resistance to water fl ow, and decreasing the C-factor
of the pipe.
3. Pitting—Localized corrosion is generally classifi ed as pitting when the diameter of the
cavity at the metal surface is the same as or less than the depth.