134
Fundamentals of Corrosion
Elastomers in outdoor use are exposed to the deteriorative agents mostly
considered in the category, which are sunlight, heat, oxygen, stress with
atmospheric ozone, atmospheric moisture, and atmospheric nitrous oxide.
Table 4.13 provides the resistance of elastomers to atmospheric pollution.
Crosslinkage or chain growth usually decreases elongation and increases
hardness and tensile strength, whereas chain rupture will have the opposite
effect. Some elastomers will continue to harden and some to soften, whereas
others will show an initial hardening followed by softening. All are irreversible reactions.
The operating temperature ranges of the common elastomers are shown in
Table 4.14. The low-temperature properties of an elastomer must also be taken
into account when used in outdoor service. With many elastomers, crystallization takes place, at which time the elastomer is brittle and will fracture easily.
Table 4.15 gives the relative low-temperature flexibility of the more common
elastomers. Table 4.16 gives the brittle points of the common elastomers.
4.8.3 Masonry Materials
Concrete and cement-based products are widely used construction materials. The primary ingredient is Portland cement, which is also used to produce building block. Although concrete and all cement-based products are
inherently weak in tension, they are strong in compression. To overcome the
weakness in tension, steel rods (reinforcing rods) are placed in the uncured
mix. The reinforcing steel can be plain, prestressed, or poststressed. Stressed
steel places the concrete in compression (its strong point). Any tensile load
placed on the structure must overcome the compressive load due to the prestressed steel before the concrete will be placed in the tensile load mode.
Micro- and macrocracking of the concrete results from the weakness in tensile loading ability, which reduces the life expectancy in a corrosive environment. Corrosion gains access to the interior of the concrete through these
cracks. If this results in the rusting of the embedded steel, then the volume
of the excess iron oxide cannot be accommodated. Because of the poor tensile
strength of the concrete, spalling of the cement mass will take place.
The resistance of concrete to corrosion is the result of its nonoxidizable structure (resistance to water and hydrogen). Steel that has been embedded in the
mix is passivated as a result of the hardened materials’ pH of 12.5. Concrete
will resist degradation as long as nothing in the environment dissolves the
cement matrix or reduces its ability to passivate the embedded steel.
The alkalinity of the concrete is provided by the presence of calcium oxide
(lime). Any material that will cause the calcium oxide or hydroxide to be
removed, thereby lowering the pH, will prove detrimental and cause solution of the cement hydrates. Organic acids can be generated when organic
materials ferment.
When carbon dioxide dissolves in water that may be present on the concrete, weak carbonic acid is produced. The weak carbonic acid lowers the pH
Fundamentals of Corrosion
Elastomers in outdoor use are exposed to the deteriorative agents mostly
considered in the category, which are sunlight, heat, oxygen, stress with
atmospheric ozone, atmospheric moisture, and atmospheric nitrous oxide.
Table 4.13 provides the resistance of elastomers to atmospheric pollution.
Crosslinkage or chain growth usually decreases elongation and increases
hardness and tensile strength, whereas chain rupture will have the opposite
effect. Some elastomers will continue to harden and some to soften, whereas
others will show an initial hardening followed by softening. All are irreversible reactions.
The operating temperature ranges of the common elastomers are shown in
Table 4.14. The low-temperature properties of an elastomer must also be taken
into account when used in outdoor service. With many elastomers, crystallization takes place, at which time the elastomer is brittle and will fracture easily.
Table 4.15 gives the relative low-temperature flexibility of the more common
elastomers. Table 4.16 gives the brittle points of the common elastomers.
4.8.3 Masonry Materials
Concrete and cement-based products are widely used construction materials. The primary ingredient is Portland cement, which is also used to produce building block. Although concrete and all cement-based products are
inherently weak in tension, they are strong in compression. To overcome the
weakness in tension, steel rods (reinforcing rods) are placed in the uncured
mix. The reinforcing steel can be plain, prestressed, or poststressed. Stressed
steel places the concrete in compression (its strong point). Any tensile load
placed on the structure must overcome the compressive load due to the prestressed steel before the concrete will be placed in the tensile load mode.
Micro- and macrocracking of the concrete results from the weakness in tensile loading ability, which reduces the life expectancy in a corrosive environment. Corrosion gains access to the interior of the concrete through these
cracks. If this results in the rusting of the embedded steel, then the volume
of the excess iron oxide cannot be accommodated. Because of the poor tensile
strength of the concrete, spalling of the cement mass will take place.
The resistance of concrete to corrosion is the result of its nonoxidizable structure (resistance to water and hydrogen). Steel that has been embedded in the
mix is passivated as a result of the hardened materials’ pH of 12.5. Concrete
will resist degradation as long as nothing in the environment dissolves the
cement matrix or reduces its ability to passivate the embedded steel.
The alkalinity of the concrete is provided by the presence of calcium oxide
(lime). Any material that will cause the calcium oxide or hydroxide to be
removed, thereby lowering the pH, will prove detrimental and cause solution of the cement hydrates. Organic acids can be generated when organic
materials ferment.
When carbon dioxide dissolves in water that may be present on the concrete, weak carbonic acid is produced. The weak carbonic acid lowers the pH