Science of Water : Concepts and Applications

Water Treatment 305
Factors Affecting Corrosion
The primary factors affecting corrosion are pH, alkalinity, hardness (calcium), dissolved oxygen,
and total dissolved solids. Secondary factors include temperature, velocity of water in pipes, and
carbon dioxide (CO2 ).
Determination of Corrosion Problems
To determine if corrosion is taking place in water mains, materials removed from the distribution
system should be examined for signs of corrosion damage. A primary indicator of corrosion damage
is pitting. (Note: Measure depth of pits to gauge the extent of damage.) Another common method
used to determine if corrosion or scaling is taking place in distribution lines is by inserting special
steel specimens of known weight (called coupons) in the pipe and examining them for corrosion
after a period of time.
Evidence of leaks, conducting fl ow tests and chemical tests for dissolved oxygen and toxic metals
as well as customer complains (red or black water or laundry and fi xture stains) are also used to
indicate corrosion problems.
Formulas can also be used to determine corrosion (to an extent). The Langlier saturation index
(L.I.) and aggressive index (A.I.) are two of the commonly used indices. The L.I. is a method used to
determine if water is corrosive. A.I. refers to waters that have low natural pH, are high in dissolved
oxygen, are low in total dissolved solids, and have low alkalinity and low hardness. These waters
are very aggressive and can be corrosive. Both L.I. and A.I. are typically used as starting points in
determining the adjustments required to produce a fi lm.
1. L.I. approximately 0.5
2. A.I. value of 12 or higher
√ Note: L.I. and A.I. are based on the dissolving of and precipitation of calcium carbonate; therefore,
the respective indices may not actually refl ect the corrosive nature of the particular water
for a specifi c pipe material. However, they can be useful tools in selecting materials or treatment
options for corrosion control.
Corrosion Control
As mentioned, one method used to reduce the corrosive nature of water is chemical addition.
Selection of chemicals depends on the characteristics of the water where the chemicals can be
applied, how they can be applied and mixed with water, and the cost of the chemicals.
Another corrosion control method is aeration. Aeration works to remove carbon dioxide (CO2 );
it can be reduced to about 5 mg/L.
Cathodic protection, often employed to control corrosion, is achieved by applying an outside
electric current to the metal to reverse the electromechanical corrosion process. The application of
D-C current prevents normal electron fl ow. Cathodic protection uses a sacrifi cial metal electrode
(a magnesium anode) that corrodes instead of the pipe or tank.
Linings, coatings, and paints can also be used in corrosion control. Slip-line with plastic liner,
cement mortar, zinc or magnesium, polyethylene, epoxy, and coal tar enamels are some of the materials
that can be used.
√ Caution: Consult the district engineer before using any protective coatings!
Several corrosive resistant pipe materials are used to prevent corrosion, including:
1. PVC plastic pipe
2. Aluminum