Page 292 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

270 CORROSION CONTROL AND PREVENTION

strength. Chloride ions migrate to the steel surface and cause corrosion. The corro-
sion products take up more volume than the original steel, causing cracking of the
concrete, further accelerating corrosion.
In asbestos cement pipes, asbestos fibers are used as reinforcement for tensile
strength. The main concern with these pipes is the release of asbestos fibers into
the drinking water. Other effects of cement-based material degradation are increased
hardness because of calcium dissolution (increased water hardness), increased pH
values, increased alkalinity, and migration of aluminum into the drinking water. A
common corrosion control method for concrete pipe is the application of internal pro-
tection using a cement mortar lining. The quality of the lining is measured in terms
of calcium oxide leaching resistance, which is a function of the mortar density.

4.18.5.3 Cement Mortar Linings New iron and steel pipelines are commonly
lined with cement mortar. The cement mortar linings are also used in rehabilitation
of older ductile iron, cast-iron, and steel water pipeline networks. The linings can
eliminate limited leaks of pipes and pipe connections as a result of the high resistance
of cement mortar to pressure, enhance the hydraulic characteristics of the mains,
and prevent further internal corrosion damage. The estimated costs for water pipe
rehabilitation by cement mortar lining as a percentage of pipe replacement costs are
as follows: (i) cleaning and cement mortar lining; (ii) excavation, pipe fitting, and
restoration of the road surface; (iii) materials; and (iv) labor costs (Table 4.32).
External corrosion modes of transmission water piping are general or localized
corrosion because of corrosive soils, galvanic corrosion through connections to other
utilities and structures, microbiological corrosion, ac stray current from power lines,
and dc stray current corrosion from CP systems on nearby structures. Corrosion con-
trol to mitigate these forms of corrosion consists of application of coatings and CP
by installation of impressed current or sacrificial anode systems. External coatings
on older water pipes are asphalt coatings and coal tar enamel coatings, while external
coatings on new pipes are coal tar enamel coatings, polyethylene base coatings and
fusion-bonded epoxy coatings.

4.18.5.4 CP and Coatings CP design, installation, and regular inspection of the
system should be done by specialists. The pipe must be electrically continuous for
the application of CP. CP can be applied on welded pipes with ease as these pipes
are electrically continuous. CP is more effective when used supplementally to the

TABLE 4.32 Estimated Costs for Water Pipe Rehabilitation by Cement Mortar Lining
Pipe Diameter (cm) Cost for Rehabilitation in Relation to Pipe Replacement (%)
8–15 39.5
20–30 41.0
50 33.2
60–80 19.4
100–120 13.0