Page 291 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

DRINKING WATER AND SEWER SYSTEMS 269

TABLE 4.31 Materials Used in Transmission Water Pipes
Material Percent
Cast iron 48
Ductile iron 19
Concrete and asbestos concrete 17
PVC 9
Steel 4
Other 2

4.18.5 Corrosion Control in Water Transmission Systems
Water is pumped from temporary storage tanks or from the treatment facilities
through large-diameter transmission water pipes. The transmission water piping
system contains large valves where the water amounts are measured using water
meters. The materials of construction of transmission pipe are cast iron, ductile iron,
prestressed concrete, asbestos concrete, PVC and welded steel piping. All of these
materials except PVC contain ferrous metal components, which need corrosion
protection. The different materials used in transmission are as shown in Table 4.31.
Ductile iron pipe is used extensively for drinking water and wastewater systems.
The pipes are made from the manufactured sections of pipe, with a bell-and-spigot
connection sealed with O-rings.
The most common failure modes of these pipes are uniform corrosion (both
external and internal), graphitization, and pitting under unprotected corrosion
scales. Loose tubercles may cause blockage of pipes. The corrosion control of loose
particles is by the addition of corrosion inhibitors, which protect the inside pipe
walls or internal lining of the pipe. Other protective linings are specialty cement
mortars, epoxies, polyethylene, and polyurethane.
In some cases, coal tar has been used, but these pose health hazards and oily
organic residues given off by coal tar coatings limit their use. Steel pipes are used for
about 4% of the US transmission water lines. The most common corrosion control
methods for external corrosion of steel pipes are coatings or coatings and CP.

4.18.5.1 Effect of Reduced Pipe Wall Thickness Significant problems occur in
older transmission pipes made from cast iron and ductile iron, as the wall thickness
is reduced by corrosion until a leak occurs. Problems in newer iron pipes are sim-
ilar to those found in older iron pipes, but occur after shorter time periods because
of decreased wall thickness. Therefore, an effective corrosion control method is the
selection of thicker wall pipe to provide a larger corrosion tolerance to wall thinning.
Expensive thicker wall pipe will be cost-effective because of its long life and low
need for maintenance.

4.18.5.2 Corrosion of Cement-Based Pipes Nearly 17% of US transmission water
lines are built from concrete and asbestos concrete materials. Internal steel reinforce-
ment wires and bars (rebar) steel mesh and steel plates are used to provide tensile