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Originally Posted by airguy
Because they've already corroded.
Pure aluminum is very reactive - within seconds it will react with oxygen in the air to form aluminum oxide on the surface of the metal. This aluminum oxide layer is very tightly bonded to the surface and quite effectively blocks any further oxygen from reacting with the metal underneath. This aluminum oxide layer serves as a barrier, just as effectively as a primer coat. [snip]
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Greg,
I'd have to disagree with you. First of all, let me premise my response by the fact that I'm an engineer who designs heat exchangers for a living. Also, the last 2 years I've been working on a project which involves corrosion resistant aluminum materials.
Although it is true that aluminum oxide very quickly forms on the surface and that it is a protective barrier, it is not a perfect barrier. It will corrode under the right conditions. Mating copper and aluminum in an electrolyte will certainly do the trick.
To answer L.Adamson?s question about why aluminum fins don?t corrode in the presence of copper tubing in a refrigeration coil?well, they do corrode. It just takes a long time for it to happen. That?s a big part of an Engineer?s job; to design a product that will last the design life of the product. If the fins slowly corrode, the performance of the coil is gradually reduced, but still functional. If the tubing were to corrode, it would eventually leak refrigerant and the unit would not work at all. The tube does not corrode because the fins are sacrificial. This is a good thing.
The main reason that the fins take time to corrode is because there isn?t much surface area for corrosion to occur. The tube is expanded into the fins so tightly that no moisture can get in there. The only place for the galvanic corrosion to start is adjacent to the connection.
Many air conditioning coils are moving towards all aluminum brazed heat exchangers with parallel tubes (like a radiator). This is accomplished by using fins that are slightly sacrificial to the tube material. They are both aluminum, but different alloys. Aluminum radiators and charge air coolers have always been designed this way to resist salt spray corrosion.
I should also note that mating different metals together will have a different corrosion rates. For example aluminum corrodes very quickly when mated to copper but much less so with most other metals. Again, the dissimilar metals must be connected by an electrolyte.
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