GENERAL CORROSION
DOE-HDBK-1015/1-93
Corrosion
CH-02
Rev. 0
Page 18
For those reactor plants in which aluminum is used for cladding and other structural
components, pH is controlled in an acidic condition because of the corrosion properties of
aluminum. Plant pH has a marked effect on the rate of chemical reaction between the coolant
water and aluminum. In the area of the cladding, the corrosion reduces the thickness and forms
an oxide film that is a thermal barrier. Extensive tests carried out in support of DOE test
reactors have revealed that minimum aluminum corrosion results with a pH of 5.0 at normal
operating temperatures. Additionally, studies have shown that the aluminum corrosion
products also exhibit a minimum solubility at a pH near 5.5 at 25?C. The aluminum corrosion
products tend to reduce the substrate (base) aluminum metal corrosion rates. Because it is
desirable to maintain dissolved aluminum in the reactor coolant at the lowest practicable level,
it is desirable to maintain the system pH level in the range of minimum oxide solubility. Figure
9 shows the effect of pH on aluminum oxide solubilities for various forms of oxide, and the
effect of pH on corrosion rates. It should be noted that the values at which minimum corrosion
and solubility are found shift to a lower pH as the temperature is increased. For example, at
300?C, the value for minimum aluminum corrosion is near pH 3.0. Therefore, the optimum pH
for operation is determined by the operating temperature.
The conditions that have proven to be most effective in limiting corrosion of aluminum are as
follows.
Maintaining pH slightly acidic with the value of the pH depending largely upon
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
