Civil Engineering Reference
In-Depth Information
of corrosion products, or other damages commonly associated with corrosion of con-
ventional metal systems. However, water distribution systems are old, complex, and
represent a costly investment for any utility. The protection of the current system with
its many different metal types is usually the most important corrosion problem facing
a utility. The strategy most commonly employed to control corrosion is to change the
quality of the distributed water either through pH and alkalinity adjustment, use of
chemical corrosion inhibitors, or the blending of different waters.
The corrosiveness of the water passing through the distribution system and resi-
dential plumbing can be due to the natural water quality, to treatment processes that
have altered the natural water quality, or to any of a number of different chemicals
added for treatment purposes. Some of the chemical factors that are important to
distribution system corrosion are listed in Table 21-7. Many of these are closely re-
lated, and a change in one often changes another. An example of this is the interrelation
between carbon dioxide, dissolved inorganic carbon (DIC), and alkalinity—three
closely related parameters that are profoundly influenced by each other, and in turn
are both influenced by and influence the pH of the water.
In practice, a distribution system consists of many different materials in contact
with a distributed water that varies somewhat in quality from one part of the distri-
bution system to another. Because of this, the nature of a corrosion problem is likely
to vary across a system, and it is never possible to achieve a control strategy that is
optimum for all materials. Most corrosion-control programs are an attempt at compro-
mise. They try to sustain a minimally suitable water quality for each pipe material
without exacerbating corrosion on any particular material type. Table 21-8 presents
some of the different corrosion concerns associated with the common distribution
system materials.
There is no universal consensus on how best to control corrosion on each of the
different material types. Because corrosion is often influenced by more than one chem-
ical parameter, a corrosion control strategy that works in one water quality may not
be entirely effective in a different water. While a consensus is not possible, some
general guidelines on a range of desirable chemical parameters to control corrosion
on the different distribution piping and residential plumbing systems have been de-
veloped; they are presented in Table 21-9.
Corrosion Inhibitors
Because of the serious economic consequences of corrosion in many industrial settings
(oil refineries, manufacturing, etc.), hundreds of different chemical formulations have
been developed to inhibit corrosion for site- and material-specific applications. These
include compounds such as chromates, molybdates, and phosphonates, all of which
can be extremely effective on a variety of metal types under adverse chemical con-
ditions. There are, in fact, few examples of corrosion that cannot be controlled chem-
ically. The more typical issue is what inhibitors are appropriate for the particular
environment in which the corrosion is occurring. This is obviously the concern in a
drinking water application, where because of public health concerns, the number of
chemical additives that can be used is limited to a few inorganic chemical formulations.
In the United States there are dozens of vendors marketing products designated as
corrosion inhibitors for drinking water systems. Because of toxicity concerns, all of
these products are based on a few phosphate and silicate formulations, and blends
