3 Fe2O2Fe3O44Fe3 O22Fe2O3Reactor Water ChemistryDOE-HDBK-1015/2-93CHEMISTRY PARAMETERSRev. 0CH-03Page 15In facilities using aluminum components, pH is maintained on the acidic side of the scalebecause of the corrosion characteristics of aluminum discussed in Module 2. In these facilitiespH may be controlled by the addition of a dilute nitric acid (HNO ) solution to the reactor3coolant system in conjunction with an ion exchange system of some type.Regardless of the pH range maintained, most facilities use an ion exchange process (describedin Module 4) to help control pH. For the high pH facilities, the most common means of controlis the use of a lithium or an ammonium form cation and a hydroxyl form anion. When lithiumis used, it must be Li because other lithium isotopes produce tritium, which represents a7significant biological hazard to personnel. In facilities that employ high pH chemistry controland do not use chemical shim reactivity control, it is sometimes necessary to add a strong basesolution such as ammonium or lithium hydroxide. When chemical additions are used for pHcontrol, facility design and operating procedures are utilized to preclude overconcentration atany point in the system, which may lead to caustic stress corrosion conditions. Many reactionsthat take place in the reactor coolant system can affect pH; therefore chemistry control must beconsidered carefully to preclude upsetting the pH balance provided by the ion exchanger.DissolvedOxygenControl of the dissolved oxygen content in the reactor facility system is of paramountimportance because of its contribution to increased corrosion. The base reactions of concernregarding high concentrations of dissolved oxygen are the following.(3-19)(3-20)They are dependent on both the concentration of oxygen and temperature. Reaction (3-19) ispredominant at high temperatures (>400 F) in the presence of lower oxygen concentrations.This corrosion film, ferrous oxide, is also known as magnetite and is a black, generallytightly-adherent film that provides a protective function to surfaces within the facility.Reaction (3-20) occurs at temperatures below about 400 F in the presence of higher oxygenconcentrations. Ferric oxide (Fe O ) is more commonly known as rust and is generally a23reddish color. This corrosion product adheres loosely to surfaces and is therefore easilyremoved and transported throughout the system for subsequent deposition and possibleirradiation. In either of the reactions, the corrosion rate is accelerated by increasedconcentrations of dissolved O and can be aggravated further by the presence of other2substances that may be present in the system.
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