364
Fundamentals of Corrosion
deposit this protective scale, the pH of the water must be slightly
above the pH of saturation for CaCO 3 , provided sufficient alkalinity
and calcium are present.
For systems that do not rely on CaCO 3 deposition for corrosion control, it is
more difficult to estimate the optimum pH. If lead and/or copper corrosion is
a problem, adjusting the pH values from 7.5 to 8.0 or higher may be required.
Practical minimum lead solubility occurs at a pH of about 8.5 in the presence
of 30 to 40 mg/L of alkalinity. Adjustment of pH coupled with carbonate
supplementing may be required to minimize lead corrosion problems.
Phosphates and other corrosion inhibitors often require a narrow pH range
for maximum effectiveness. If such an inhibitor is used, consideration must
be given to adjusting the pH range to within the recommended value.
It is recommended that the effects of pH change over time be monitored.
Evaluating the performance of chemical feed systems for pH adjustment is
the key to an effective corrosion control program. Addition of lime, soda ash,
or other chemicals for pH control can be evaluated by a continuous-readout
method for successful corrosion control in this system. Because of the specific water quality and pipe materials in a system, a particular method may
be successful in one system but not in another.
There are three basic approaches to corrosion control of pipe material in
direct contact with the water:
1. Modify the water quality so that it is less corrosive to the piping by
pH adjustment or oxygen removal.
2. Place a protective barrier or lining between the water and the pipe.
3. Select a pipe material and design the system so that it is not corroded
or is less corrosive to a given water.
Other common ways of achieving corrosion control include the use of inhibitors and cathodic protection.
Recorders can be used to monitor the pH of the water as it leaves the utility and can be wired to send a signal to the feed mechanisms to add more or
fewer chemicals as necessary. The pH levels at the outer reaches of the distribution system should be checked periodically for any indications of changes
occurring within the system that might be due to corrosion.
Keep in mind that although pH adjustment can aid in reducing corrosion
it cannot eliminate corrosion in every case. However, pH adjustment is the
least costly and most easily implemented method for achieving some corrosion control, and utilities should use it if at all possible.
Oxygen removal is another method used to inhibit corrosion. Oxygen is an
important corrosive agent for the following reasons:
Fundamentals of Corrosion
deposit this protective scale, the pH of the water must be slightly
above the pH of saturation for CaCO 3 , provided sufficient alkalinity
and calcium are present.
For systems that do not rely on CaCO 3 deposition for corrosion control, it is
more difficult to estimate the optimum pH. If lead and/or copper corrosion is
a problem, adjusting the pH values from 7.5 to 8.0 or higher may be required.
Practical minimum lead solubility occurs at a pH of about 8.5 in the presence
of 30 to 40 mg/L of alkalinity. Adjustment of pH coupled with carbonate
supplementing may be required to minimize lead corrosion problems.
Phosphates and other corrosion inhibitors often require a narrow pH range
for maximum effectiveness. If such an inhibitor is used, consideration must
be given to adjusting the pH range to within the recommended value.
It is recommended that the effects of pH change over time be monitored.
Evaluating the performance of chemical feed systems for pH adjustment is
the key to an effective corrosion control program. Addition of lime, soda ash,
or other chemicals for pH control can be evaluated by a continuous-readout
method for successful corrosion control in this system. Because of the specific water quality and pipe materials in a system, a particular method may
be successful in one system but not in another.
There are three basic approaches to corrosion control of pipe material in
direct contact with the water:
1. Modify the water quality so that it is less corrosive to the piping by
pH adjustment or oxygen removal.
2. Place a protective barrier or lining between the water and the pipe.
3. Select a pipe material and design the system so that it is not corroded
or is less corrosive to a given water.
Other common ways of achieving corrosion control include the use of inhibitors and cathodic protection.
Recorders can be used to monitor the pH of the water as it leaves the utility and can be wired to send a signal to the feed mechanisms to add more or
fewer chemicals as necessary. The pH levels at the outer reaches of the distribution system should be checked periodically for any indications of changes
occurring within the system that might be due to corrosion.
Keep in mind that although pH adjustment can aid in reducing corrosion
it cannot eliminate corrosion in every case. However, pH adjustment is the
least costly and most easily implemented method for achieving some corrosion control, and utilities should use it if at all possible.
Oxygen removal is another method used to inhibit corrosion. Oxygen is an
important corrosive agent for the following reasons: