Removing Corrosive Agents
Chemistry control in the form of removal of corrosive agents from a system is a widely
used method. One method is using deaerators to remove dissolved oxygen and to a
lesser extent carbon dioxide. Treating the water by softening and demineralization
removes the dissolved solids and reduces the conductivity.
Chemical additions to a system that alter the chemical reaction or tie up a particular
corrodant is a common method of control. Filming amines (organic compounds that are
derivatives of ammonia) accomplish protection by forming adhering organic films on
metal surfaces to prevent contact between corrosive species in the condensate and the
metal surface. Phosphates and sodium hydroxide are used to adjust the system pH and
Corrosion of Aluminum
The corrosion of aluminum is dependent upon a vast number of variables. 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.