Page 289 - Corrosion Engineering Principles and Practice

262 C h a p t e r 8 C o r r o s i o n b y W a t e r 263

that the corrosion occurs only internally. Although the cost per
system can be calculated reasonably accurately using this method,
interactions with other systems are difficult to evaluate. The system
size, location, population served, materials used, water quality, and
soil conditions all significantly influence corrosion susceptibility.

Short-Term Corrosion Management
Short-term corrosion problems are often indicated by customer
complaints, such as the occurrence of red or yellow “rusty” water or
a sudden decrease in water pressure. A reason for rust-colored water
is generally the presence of corrosion products that have flaked off of
the internal pipe walls, while a water pressure drop may be caused
by a leak in the transmission or distribution system.
Finding a leak in an underground pipe system is often difficult
because the leak may start small and go undetected for a period of
time. Once the leak is so severe that water is literally coming from the
ground, it may cause a local flood. In addition to the lost water, the
damage can be significant and the repair work is more than what
would have been needed to fix a small leak [2].

Long-Term Corrosion Management
Long-term corrosion impact is generally indicated by system integrity
studies in which maintenance and inspection teams are tasked to find
leaks and failures. Some large utilities have a specialized corrosion
team to monitor the water quality, using corrosion loops in which
treated water circulates over weight-loss coupons. These coupons are
often made from different materials and exposed to various water
flow rates. The coupons would be periodically measured to determine
average corrosion rates.
Water samples would also be routinely tested to ensure that
the water quality is acceptable. The test results are used to make
assessments about corrosion as well. For example, the water pH is
important both for water consumers and for system integrity. The
pH is kept within a predetermined range by adding pH adjusters to
the treatment process.

Necessity of Long-Term Corrosion Planning
Because of the long life expectancy of water systems, a long-term vision
for corrosion management is required. Unfortunately, some managers
give in to short-term cost savings over long-term investments. As an
example, the average thickness of cast iron and ductile iron pipe has
been continuously decreased over the last 100 years because thinner,
higher strength pipe has become available [2]. Unfortunately, corrosion
rates are not significantly dependent on the strength of ductile iron or
steel as was demonstrated by extensive corrosion studies [5]. As a
result, thinner wall pipe will have a smaller corrosion tolerance than
thicker wall pipe and will show more frequent failures. The time to