These variables include environment, temperature, alloy in question, flow velocities, impurities present in the environment, and chemistry conditions to which it is exposed.
An additional factor that affects corrosion is pretreatment. Many of the factors are controlled by design and construction, such as alloy type, temperature, and flow velocities. Pretreatment, soluble and solid impurities, and chemistry are within the control of the operator and will be discussed in this text.
Experiments have shown that prefilming limits corrosion on aluminum-clad fuel assemblies. In the tests conducted, the ratios of oxide film thickness for nonprefilmed and prefilmed elements were on the order of 2 to 3 and in some cases even greater. Impurities are major contributors to the corrosion of aluminum.
In most cases studied, the major source of contaminants has been makeup water systems. Corrosion products from other plant materials also contribute to the overall inventory of ionic and solid impurities.
Finally, organic impurities from the resin used in ion exchangers have been detected. These occur in some installations because of the type of resin used and the particle filters normally installed on the ion exchanger effluents. This problem has been reduced by improved resins and installing filters capable of removing smaller particles.
Chemistry controls, including pH, dissolved oxygen, and conductivity, greatly influence the formation and propagation of the oxide film on aluminum surfaces.
Dissolved oxygen is controlled for essentially the same reasons as for the corrosion of iron. Conductivity is a quantitative indication of the impurities present in the system, and pH theoretically dictates the value of conductivity.
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.
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
this article is taken from: DOE FUNDAMENTALS HANDBOOK CHEMISTRY Volume 1 of 2 - DOE-HDBK-1015/1-93 JANUARY 1993