Corrosion of Aluminum or Aluminium
Aluminum Corrosion
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 of aluminum 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
useful source of information:
AluMatter - a freely-accessible website that provides innovative and interactive e-learning resources for aluminium science and technology
AluSelect - a computer-based reference database containing technical information on the most widely used aluminium alloys. This freely accessible web-site allows the user to obtain information about the mechanical, physical and chemical properties of aluminium alloys. At present, 35 wrought alloys and 12 casting alloys are indexed.
useful papers and docs:
Corrosion Resistance of Aluminum
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