GENERAL CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 18For those reactor plants in which aluminum is used for cladding and other structuralcomponents, pH is controlled in an acidic condition because of the corrosion properties ofaluminum. Plant pH has a marked effect on the rate of chemical reaction between the coolantwater and aluminum. In the area of the cladding, the corrosion reduces the thickness and formsan oxide film that is a thermal barrier. Extensive tests carried out in support of DOE testreactors have revealed that minimum aluminum corrosion results with a pH of 5.0 at normaloperating temperatures. Additionally, studies have shown that the aluminum corrosionproducts also exhibit a minimum solubility at a pH near 5.5 at 25?C. The aluminum corrosionproducts tend to reduce the substrate (base) aluminum metal corrosion rates. Because it isdesirable 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. Figure9 shows the effect of pH on aluminum oxide solubilities for various forms of oxide, and theeffect of pH on corrosion rates. It should be noted that the values at which minimum corrosionand solubility are found shift to a lower pH as the temperature is increased. For example, at300?C, the value for minimum aluminum corrosion is near pH 3.0. Therefore, the optimum pHfor operation is determined by the operating temperature.The conditions that have proven to be most effective in limiting corrosion of aluminum are asfollows.Maintaining pH slightly acidic with the value of the pH depending largely uponoperating temperatureElimination of dissolved oxygenElimination of soluble and solid impuritiesPrevention of the introduction of organic impuritiesPretreatment (or pickling)Maintaining water purity
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