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Fundamentals of Corrosion
Water storage tanks are most commonly lined to protect the inner tank
walls from corrosion. Common water tank linings include coal-tar enamels
and paints, vinyl, and epoxy.
Although coal-tar-based products have been widely used in the past for
contact with drinking water, currently there is concern at the EPA about their
use because of the presence of polynuclear aromatic hydrocarbons and other
hazardous compounds in coal tar and the potential for their migration in
water. Table 11.3 summarizes the most commonly used pipe linings and lists
their advantages and disadvantages. Common water tank linings are summarized in Table 11.4.
Proper selection of system materials and adequate system design. In many cases,
corrosion can be reduced by properly selecting system materials and having good engineering design. Some pipe materials are more corrosion resistant than others in a specific environment. In general, the less reactive the
material is with its environment, the more resistant it is to corrosion. When
selecting materials to replace old lines or putting new ones in service, the
utility should select a material that will not corrode in the water it contacts.
Admittedly, this provides a limited solution because few utilities can select
materials based on corrosion resistance alone. Usually, some alternative
material must be compared and evaluated based on cost, availability, ease of
installation, and maintenance, as well as resistance to corrosion. In addition,
the utility owner may not have control over the selection and installation of
the materials for household plumbing. There are, however, several guidelines that can be used in selecting materials.
First, some materials are known to be more corrosion resistant than others
in a given environment. For example, a low-pH water that contains high DO
levels will cause more corrosion damage in a copper pipe than in a concrete
or cement-lined pipe. Other guidelines relating water quality to materials
selection are given in Table 11.5.
A good description of the proper selection of materials can be found in
The Prevention and Control of Water Caused Problems in Building Water Systems,
published by NACE.
Second, compatible materials should be used throughout the system. Two
metal pipes having different activities, such as copper and galvanized iron,
that come in direct contact with others can set up a galvanic cell and cause
corrosion. The causes and mechanisms of galvanic corrosion are discussed in
other sections of this book. As much as possible, systems should be designed
to use the same metal throughout, or to use metals having similar positions
in the galvanic series. Galvanic corrosion can be avoided by placing dielectric (insulating) couplings between dissimilar metals.
Use of inhibitors. There are several different types of phosphates used for
corrosion control, including polyphosphates, glassy polyphosphates, and
bimetallic polyphosphates. Recent developments in corrosion control include
the use of zinc with a polyphosphate or orthophosphate.
Fundamentals of Corrosion
Water storage tanks are most commonly lined to protect the inner tank
walls from corrosion. Common water tank linings include coal-tar enamels
and paints, vinyl, and epoxy.
Although coal-tar-based products have been widely used in the past for
contact with drinking water, currently there is concern at the EPA about their
use because of the presence of polynuclear aromatic hydrocarbons and other
hazardous compounds in coal tar and the potential for their migration in
water. Table 11.3 summarizes the most commonly used pipe linings and lists
their advantages and disadvantages. Common water tank linings are summarized in Table 11.4.
Proper selection of system materials and adequate system design. In many cases,
corrosion can be reduced by properly selecting system materials and having good engineering design. Some pipe materials are more corrosion resistant than others in a specific environment. In general, the less reactive the
material is with its environment, the more resistant it is to corrosion. When
selecting materials to replace old lines or putting new ones in service, the
utility should select a material that will not corrode in the water it contacts.
Admittedly, this provides a limited solution because few utilities can select
materials based on corrosion resistance alone. Usually, some alternative
material must be compared and evaluated based on cost, availability, ease of
installation, and maintenance, as well as resistance to corrosion. In addition,
the utility owner may not have control over the selection and installation of
the materials for household plumbing. There are, however, several guidelines that can be used in selecting materials.
First, some materials are known to be more corrosion resistant than others
in a given environment. For example, a low-pH water that contains high DO
levels will cause more corrosion damage in a copper pipe than in a concrete
or cement-lined pipe. Other guidelines relating water quality to materials
selection are given in Table 11.5.
A good description of the proper selection of materials can be found in
The Prevention and Control of Water Caused Problems in Building Water Systems,
published by NACE.
Second, compatible materials should be used throughout the system. Two
metal pipes having different activities, such as copper and galvanized iron,
that come in direct contact with others can set up a galvanic cell and cause
corrosion. The causes and mechanisms of galvanic corrosion are discussed in
other sections of this book. As much as possible, systems should be designed
to use the same metal throughout, or to use metals having similar positions
in the galvanic series. Galvanic corrosion can be avoided by placing dielectric (insulating) couplings between dissimilar metals.
Use of inhibitors. There are several different types of phosphates used for
corrosion control, including polyphosphates, glassy polyphosphates, and
bimetallic polyphosphates. Recent developments in corrosion control include
the use of zinc with a polyphosphate or orthophosphate.