The effect of halide and metal ions on the corrosion of aluminium in ethylene glycol solutions

Prigmore, Robert Marshall (1985). The effect of halide and metal ions on the corrosion of aluminium in ethylene glycol solutions. PhD thesis The Open University.



Aluminium components attached to an engine cooling system, will be exposed to 50% ethanediol coolant contaminated with metal cations and halide anions.

Since copper cations enhance the corrosion rate of aluminium, the effect of other metal cations of metals commonly found in the cooling system on the corrosion rate were evaluated. Aluminium samples were exposed to 50% ethanediol solutions containing various metals and metal cations.

Previous work has shown that halide anion size and concentration have an effect on the corrosion potential of aluminium, but the effects of metal cations have not. been evaluated. A new corrosion cell was designed and built to evaluate these effects.

The nobility of the pitting potential of aluminium has been observed to decrease with chloride anion concentration and temperature increase in double distilled water. Potentiodynamic and potentiostatic experiments were therefore carried out to determine whether similar effects are observed with fluoride, bromide and iodide. Theoretically, if the penetration theory of pitting initiation is correct, then the nobility of Ep should increase with halide anion size.

It was concluded from the experimental work that:-
(1) Lead cations in the presence of chloride anions initiate corrosion of aluminium.
(2) Lead and copper cations in halide solutions shift the corrosion potential of aluminium to more noble values before deposition. Nobility continued to increase with metal cation concentration until the pitting potential was achieved and then levelled off. It is proposed that this effect is due to metal cations disrupting the outer Helmholtz plane.
(3) Ep values of aluminium in double distilled water and 50% ethanediol decreased in nobility with increase in halide concentration and solution temperature for all four halides.
(4) The fluoride Ep values were out of sequence, these values being as noble as those of iodide. A film was observed to form on aluminium exposed to the fluoride solution. It is proposed that this film increases the nobility of the pitting potential.

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