Page 176 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

154 CORROSION CAUSES

corrosion can be a problem in corrosive soil, in particular, when the soil is of low
resistivity, high moisture content, and has corrosive chemical species. When piping
is electrically continuous (welded steel piping), CP can be applied; however, this is
not generally applicable in the case of discontinuous pipe made of ductile iron or
cast iron.
Plastic piping [polyvinyl chloride (PVC)] does not show corrosion as in the case
of metal piping, but the properties of plastic piping deteriorate over time. In severely
corrosive soils PVC piping may be selected rather than a metallic piping because it
is inert to the chemical conditions. PVC has a lower density than steel and iron and
hence it is relatively easy to handle in the field. However, PVC has lower strength
and traditional welding is not possible. PVC has been used for a relatively short time,
compared with steel and iron water lines. Thus, there is limited data on the expected
service life of PVC pipelines, and calculations of comparative total life-cycle costs
are not possible.
Cement-based piping deteriorates by corrosion of the reinforcement steel, which
is accelerated by chloride from salt-treated icy roads during winter. Corrosion
occurs when the passive surface film that naturally forms on the steel in high-pH
concrete/cement breaks down in the presence of chloride. The corrosion product has
greater volume than the original steel, creating internal stresses that cause cracking
and spalling of the concrete/cement pipes.

3.15 ELECTRICAL UTILITIES

Electricity generation plants can be divided into seven generic types: fossil fuel,
nuclear, hydroelectric, cogeneration, geothermal, solar, and wind. The majority of
electric power in the United States is generated by fossil and nuclear steam supply
systems.
Two types of light water reactors, namely, the boiling water reactor (BWR) and the
pressurized water reactor (PWR) are in use in the United States of America. The fuel
for these reactors consists of long bundles of 2–4 wt% of enriched uranium dioxide
fuel pellets stacked in zirconium–alloy cladding tubes.
The BWR design (Fig. 3.19) consists of a single loop in which the entering water is
turned directly into steam for the production of electricity. As operating temperatures
must remain below the critical temperature for water, steam separators and dryers are
used with a “wet-steam” turbine.
The PWR design (Fig. 3.20) is a two-loop system that uses high pressure to main-
tain an all-liquid-water primary loop. Energy is transferred to the secondary steam
loop through two to four steam generators. The PWR design also uses a wet-steam
turbine.

3.15.1 Fossil Fuel Steam Supply Systems
The electric power industry uses three types of fossil fuel power plants: coal-fired
steam, gas turbine, and combined cycle power plants. The most common and widely
used is the pulverized coal-fired steam power plant. Fuel oil can be used in place of