Maybe its the water? Seems to be evidence that soft water can cause aluminium corrosion?
Deposition Corrosion
Direct quote
Quote:
Deposition Corrosion
Deposition corrosion is a subtle form of galvanic cell that can produce pitting in a liquid environment when a more cathodic metal is plated out of solution onto a metal surface. For example, soft water passing through a copper water pipe will accumulate some copper ions. If water is then admitted to a galvanized or aluminum vessel, particles of metallic copper will plate out, i.e. deposit on the surface and stimulate pitting by local cell action.
This plating out action, or deposition corrosion, may be an important factor in the corrosion of the more reactive metals near the top of the series, e.g. magnesium, zinc, and aluminum, when these latter metals come into contact with solutions containing ions of metals (particularly copper) lower in the series. Copper ions in concentrations less than one part per million have been observed to have a significant effect on the corrosion of aluminum by water. Metals, such as copper, that can aggravate corrosion of aluminum are sometimes referred to as "heavy metals in solution. The fact that they are heavier than aluminum is less significant than that they occupy a position lowers than aluminum in the electromotive or galvanic series.
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Copper plates out onto aluminium, causes pitting that gives the corrrosion a place to start. Most people probably don't get distilled water for coolant changes and might add water from the tap just to top off or something? I don't know.
Looks like there's a big debate as to whether softened water is safe to use in aluminium boilers? Maybe a link there?