Aluminum 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 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!
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.
What is galvanic corrosion?
Galvanic corrosion (or dissimilar metal reaction) is the basic principal behind the way batteries and fuel cells work. Galvanic corrosion is an electromechanical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte. It can be sinthesized in the schema below:
When two dissimilar metals are immersed in an electrolyte solution, a battery is created. If the two metals get in contact through a conductor or simply by physical proximity (like the copper tubes and the aluminum fins of a heat exchanger) the electrical loop is closed and the current is created. The metal with the highest potential will be the cathode and the one with the lowest potential will be the anode. Electrons start flowing from one metal to the other. The anode will start losing ions and the cathode gains them. This process is actually transferring mass from the anode to the cathode, making the anode weaker and the cathode more resistant.
Which metal will be the anode and which one the cathode?
Metals are ordered in what it is known as the galvanic series. Just “google” it and you will find several sources for this. Galvanic series are different for every electrolite. The further the metals are in the series, the higher the potential difference and, therefore, the higher their galvanic corrosion. The metal closer to the noble end will be the cathode and the one closer to the active end will be the anode. In the case of traditional copper aluminum coils, copper will be the cathode and aluminum will be the anode.
CLIMETAL 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.
How to avoid corrosion
So, once we understand the nature of aluminum and how galvanic corrosion works there are some simple rules to make your coils last for years:
- Avoid extreme pH environments
- Avoid contact between aluminum and other metals by using some plastic, rubber or polymeer washers. Be aware that there are some materials that have a high metal content like pressure treated wood (plywood) and concrete.
- If you cannot avoid contact between aluminum and another metal, try at least to isolate them from the electrolyte with some kind of polymeer sleeves, shrink wraps or paint. For example, Climetal supplies all alu-copper brazed connections with plastic shrik wraps.
- Avoid grounding electrical circuits to aluminum coils. Make sure your AC systems are completely ground isolated and that there is no electrical derivation going through the coil.
Despite these easy and simple recommendations, we at CLIMETAL are always ready to help. Send us your questions and doubts and we will be happy to give you our best advice.
Rubber washer Shrinkwrap