Aluminum coil  is actually a very active metal, in the sense that it oxidizes very quickly. This feature, while a weakness for most metals, is actually the key to its ability to resist corrosion. When oxygen is present (in the air, soil, or water), aluminum instantly reacts to form aluminum oxide. This aluminum oxide layer is chemically bound to the surface, and it seals the core aluminum from any further reaction. This is quite different from oxidation (corrosion) in steel or other metals, where rust puffs up and flakes off, constantly exposing new metal to corrosion. Aluminum’s oxide film is tenacious, hard, and instantly self-renewing.

Therefore, as long as this oxide layer is not damaged, aluminum coil will remain resistant to corrosion. What factors can affect this oxide layer? Basically, extreme pH levels. Very high or very basic environments can destroy this protective layer and it might not renew itself as fast as needed to to keep protecting the alumnium core. Normally, aluminum´s protective oxide layer is generally stable in the pH range of 4,5 to 8,5.

Salt water DOES NOT corrode aluminum coil !

There is a traditional fear in the AC industry about heat exchangers exposed to marine conditions. For sure, aluminum does not corrode in lakes and pools. But, what about seawater? Well, surprising as it may be, seawater does not corrode aluminium, simply because of its neutral pH. Then, why traditional copper coils made of copper and aluminum corrode so much near the ocean, up to the point where the aluminum fins disappear? Will not this happen even more likely to aluminum coils being aluminum a softer metal than copper?

The reason is a more complex kind of corrosion called galvanic corrosion and saltwater can be a major facilitator for this.

microchannel coils are more corrosion resistant than traditional copper and aluminum coils because

Aluminum is a very stable, low corrosion material due to its natural protective oxide layer.
This protective oxide layer is very stable as long as you keep the surrounding pH between 4,5 and 8,5.
The most frequent cause of corrosion in aluminum is due to galvanic corrosion.
The good news is that if you properly understand how galvanic corrosion works, it is very easy to avoid or at least, minimize.
Galvanic corrosion takes place when:
We have two metals in contact
They are inmersed or surrounded (totally or partially) by an electrolyte (in AC applications the most common would be humidity and salt).
We also know:
The further these metals are in the galvanic series, the stronger the reaction
The anode will be the sacrifice material whereas the cathode will get stronger.