General Corrosion Summary (Cont.)
During the initial fill of a reactor system, great care is taken to promote the
formation of the best possible protective corrosion film. The protective film acts
as a barrier and stops further uncontrolled corrosion from taking place. This
process, called pretreatment or pickling, involves careful control of reactor
coolant water chemistry and temperature during the pretreatment period.
Four methods used to chemically control general plant corrosion are:
Passivators (inhibitors) consist of a naturally active metal that corrodes
at a very low rate. A common passivator is potassium chromate.
Cathodic protection is the external application of an external electric
current to the iron so that it acts as a cathode and has no anodic areas.
This is accomplished by using either an external electrical source or the
use of a sacrificial anode to provide the electrical current.
Removing corrodants to reduce the dissolved oxygen and lesser extent
carbon dioxide. Treating the water in this manner reduces the dissolved
solids and reduces the conductivity.
Chemical addition to alter the chemical reaction or tie up a particular
corrodant. Filming amines, phosphates, and sodium hydroxide are used
to provide organic films and adjust the system pH.
The six water chemistry conditions that limit corrosion of aluminum are:
Maintaining pH slightly acidic with the value of the pH depending largely
on operating temperature.
Elimination of dissolved oxygen
Elimination of soluble and solid impurities
Prevention of the introduction of organic impurities
Pretreatment (or pickling)
Maintaining water purity