Page 293 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

DRINKING WATER AND SEWER SYSTEMS 271

coating system. CP system in the absence of a coating is economically unfavorable for
water pipes. For prestressed concrete pipe, CP can be used to augment the protection
provided by standard mortar coating in aggressive soil environments (50). Care must
be taken not to overprotect the prestressing steel.
The two types of CP systems are: (i) impressed-current systems that require recti-
fiers necessitating periodic inspection, monitoring, and adjustment by trained opera-
tors, and (ii) sacrificial systems that require less attention. Thus a sacrificial anode
system consisting of buried zinc or magnesium is generally preferred for welded
steel pipe.

4.18.6 Corrosion Control in Water Distribution Systems

Distribution water pipes are of smaller diameter than larger transmission pipes and
are made of ductile iron, PVC, and copper. Corrosion problems and corrosion control
methods for ductile iron and the deterioration of PVC are similar in pipes of both
small and large diameters.
Use of chloramines for disinfection, as a means to reduce trihalomethane, accel-
erates corrosion and degradation of metals and elastomers common to distribution
plumbing. The oxidation effects of free and combined chlorine species on mild
steel, copper, bronze, Pb/Sn solder, Sn–Sb solder, Sn–Ag solder, elastomers such
as natural rubber, acrylonitrilebutadiene, styrene–butadiene, chloroprene, silicone,
ethylene–propylene, fluorocarbon, and three thermoplastics were studied. The
results showed with few exceptions that solutions of chloroamines produced material
swelling, deeper and denser surface cracking, rapid loss in elasticity, greater loss of
tensile strength than with an equivalent amount of chlorine. Only newly engineered
synthetic polymers performed well in chloroamine exposure. All the tested chlorine
disinfectants accelerated corrosion of copper and its alloys. Unlike elastomers, free
chlorine exerted greater oxidizing effect than chloroamines. There was no significant
effect on the galvanic corrosion of solder while lead-free and tin-based solders were
immune to chlorine attack.
In addition to ductile iron and PVC, copper and lead are used in pipes, and
brass in fixtures and connections. Lead is released because of uniform corrosion.
Copper is also released because of uniform corrosion, localized-attack cold water
pitting, hot water pitting, MIC, corrosion fatigue, and erosion–corrosion. Lead
pipes and lead-tin solder exhibit uniform corrosion. Brass corrosion includes
erosion–corrosion, impingement corrosion, dezincification, and SCC. The direct
health impacts are because of increased copper, lead, and zinc concentrations in
the drinking water. Mechanical problems because of corrosion include leaks from
perforated pipes, rupture of pipes, and the loss of water pressure because of blockage
of pipes by corrosion products.

4.18.6.1 Corrosion of Lead Pipes and Solders Lead is generally not present in
domestic water supplies such as rivers and lakes. The lead content of drinking water
is generally below detection limits; however, lead can enter water because of the
corrosion or wear of brass fixtures, lead pipes, or solders. Stagnant water in pipes can