Corrosion of Aluminum:

The corrosion of aluminum is dependent upon a vast number of variables.  Those variables involve environment, temperature, alloy in question, flow velocities, impurities present within the environment, and chemistry conditions to that it is exposed. A further factor which affects corrosion is pretreatment.

Several of the factors are controlled through design and construction, like as alloy type, flow velocities, and temperature.  Pretreatment, soluble and solid impurities and chemistry are inside the control of the operator and will be elaborates in this text.

Experiments have displays which prefilming limits corrosion on aluminum-clad fuel assemblies. Within the tests conducted, the ratios of oxide film thickness for nonprefilmed and prefilmed elements were on the sequence of 2 to 3 and in a few cases even greater.

Impurities are main contributors to the corrosion of aluminum. Within most cases studied, the main source of contaminants has been makeup water systems.  From other plant materials corrosion products also contribute to the whole inventory of ionic and solid impurities. At last, organic impurities from the resin used in ion exchangers have been detected.  That will occur in a few installations since of the category of resin used and the particle filters generally installed on the ion exchanger effluents.  This problem has been decreased through improved resins and installing filters capable of erasing smaller particles.

Chemistry controls, involving pH, dissolved oxygen, and conductivity, greatly influence the configuration and propagation of the oxide film on aluminum surfaces.  Dissolved oxygen is controlled for fundamentally the similar purpose as for the corrosion of iron.  Conductivity is a quantitative indication of the impurities present within the system, and pH theoretically dictates the value of conductivity.

For those reactor plants in that aluminum is used for cladding and other structural components, pH is controlled within an acidic condition since of the corrosion properties of aluminum. The Plant pH has a marked effect on the rate of chemical reaction among the coolant water and aluminum. Within the area of the cladding, corrosion decreases the thickness and forms an oxide film which is a thermal barrier.  Broad tests carried out in support of DOE test reactors have revealed which minimum aluminum corrosion results along with a pH of 5.0 at normal operating temperatures. Further, studies have displays in which the aluminum corrosion products also exhibit a minimum solubility at a pH near 5.5 at 25ºC.  The aluminum corrosion products tend to decrease the substrate (base) aluminum metal corrosion rates.  Since, it is desirable to manage dissolved aluminum within the reactor coolant at the lowest practicable level that is desirable to manage the system pH level in the range of minimum oxide solubility. Below figure display the effect of pH on aluminum oxide solubilities for several forms of oxide and the effect of pH on corrosion rates. It should be remember that the values at that minimum corrosion and solubility are found shift to a lower pH as the temperature is raised.  For instance, at 300ºC, the value for minimum aluminum corrosion is near pH 3.0. Thus, the optimum pH for operation is determined through the operating temperature.