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Fundamentals of Corrosion
Sodium silicate (water glass) has been used for over 50 years to reduce
corrosivity. The way in which sodium silicate acts to form a protective film
is still not completely understood. However, it can effectively reduce corrosion and red water in galvanized iron, yellow brass, and copper plumbing
systems in both hot- and cold-water systems.
The effectiveness of sodium silicate as a corrosion inhibitor depends on
water qualities such as pH and bicarbonate concentration. As a general rule,
feed rates of 2 to 8 mg/L and possibly up to 12 mg/L sodium silicate are
sufficient to control corrosion in a system once a protective film is formed.
Silicate has been found to be particularly useful in waters having very low
hardness and alkalinity, and a pH of less than 8.4. It is also more effective
under higher velocity conditions. The equipment needed to feed sodium silicate is the same as needed to add phosphate.
Cathodic protection. Cathodic protection is an electrical method for preventing the corrosion of metallic structures. Metallic corrosion involves
contact between a metal and an electrically conductive solution that produces a flow of electrons or current from the metal to the solution. Cathodic
protection stops the current by overpowering it with a strong current from
some outside source. This forces the metal that is being protected to become
a cathode; that is, it has a large excess of electrons and cannot release any of
its own.
There are two basic methods of applying cathodic protection. One method
uses inert electrodes, such as a high-silicon cast iron or graphite, that are
powered by an external source of direct current. The current impressed on
the inert electrodes forces them to act as anodes, thus minimizing the possibility that the metal surface being protected will become an anode and
corrode. The second method uses a sacrificial galvanic anode. Magnesium
or zinc anodes produce a galvanic action with the iron such that they are
sacrificed (or corrode), while the iron structure to which they are connected
is protected from corrosion. This type of system is common in small water
heaters. Another form of sacrificial anode is galvanizing, where zinc is used
to coat iron or steel. The zinc becomes the anode and corrodes, protecting the
steel, which is forced to be the cathode.
The primary reason for applying cathodic protection in water utilities is
to prevent internal corrosion in water storage tanks. Because of the high
cost, cathodic protection is not a practical corrosion control method for use
throughout a distribution system. Another limitation of cathodic protection is
that it is almost impossible for it to reach holes, crevices, or internal corners.
11.3.3.4 Design Features
The design of the pipes and structures is as important for water supply
systems as the choice of construction materials. A faulty design may cause
severe corrosion, even in materials that may be highly corrosion resistant.
Some important design considerations include:
Fundamentals of Corrosion
Sodium silicate (water glass) has been used for over 50 years to reduce
corrosivity. The way in which sodium silicate acts to form a protective film
is still not completely understood. However, it can effectively reduce corrosion and red water in galvanized iron, yellow brass, and copper plumbing
systems in both hot- and cold-water systems.
The effectiveness of sodium silicate as a corrosion inhibitor depends on
water qualities such as pH and bicarbonate concentration. As a general rule,
feed rates of 2 to 8 mg/L and possibly up to 12 mg/L sodium silicate are
sufficient to control corrosion in a system once a protective film is formed.
Silicate has been found to be particularly useful in waters having very low
hardness and alkalinity, and a pH of less than 8.4. It is also more effective
under higher velocity conditions. The equipment needed to feed sodium silicate is the same as needed to add phosphate.
Cathodic protection. Cathodic protection is an electrical method for preventing the corrosion of metallic structures. Metallic corrosion involves
contact between a metal and an electrically conductive solution that produces a flow of electrons or current from the metal to the solution. Cathodic
protection stops the current by overpowering it with a strong current from
some outside source. This forces the metal that is being protected to become
a cathode; that is, it has a large excess of electrons and cannot release any of
its own.
There are two basic methods of applying cathodic protection. One method
uses inert electrodes, such as a high-silicon cast iron or graphite, that are
powered by an external source of direct current. The current impressed on
the inert electrodes forces them to act as anodes, thus minimizing the possibility that the metal surface being protected will become an anode and
corrode. The second method uses a sacrificial galvanic anode. Magnesium
or zinc anodes produce a galvanic action with the iron such that they are
sacrificed (or corrode), while the iron structure to which they are connected
is protected from corrosion. This type of system is common in small water
heaters. Another form of sacrificial anode is galvanizing, where zinc is used
to coat iron or steel. The zinc becomes the anode and corrodes, protecting the
steel, which is forced to be the cathode.
The primary reason for applying cathodic protection in water utilities is
to prevent internal corrosion in water storage tanks. Because of the high
cost, cathodic protection is not a practical corrosion control method for use
throughout a distribution system. Another limitation of cathodic protection is
that it is almost impossible for it to reach holes, crevices, or internal corners.
11.3.3.4 Design Features
The design of the pipes and structures is as important for water supply
systems as the choice of construction materials. A faulty design may cause
severe corrosion, even in materials that may be highly corrosion resistant.
Some important design considerations include: