Galvanic corrosion is a serious issue across the engineering industry and it has a variety of solutions, which all boil down to one technique: breaking the circuit. Anodising is one way to break the circuit. The anodic coating is made up of aluminium oxide, which is a good electrical insulator. The oxide layer is a thousand times thicker than the natural oxide that forms on aluminium, which offers essentially no galvanic protection.
It is sensible to use anodising because it has such good adhesion (owing to being a conversion coating) and so won't flake off like a paint might. Hard anodising can also provide protection in situations where higher wear is expected, which paint will not be able to withstand.
Galvanic corrosion occurs when two dissimilar metals come into contact. It is also necessary for there to be moisture, but it is rare to find an environment without moisture. Galvanic corrosion leads to the deterioration of one of the metals and can occur in environments that would otherwise not be considered corrosive to said metal. The reaction is very similar to the reaction that occurs in batteries.
While galvanic corrosion can be used in positive ways (batteries, sacrificial coatings e.g. zinc plating, etc.), most of the time it is a problem and it can cause failures that engineers had not predicted.
Precautions When Using Anodising for Galvanic Corrosion Protection
Anodised aluminium has an electrically insulative surface, just like any of the other barrier protection techniques and if this surface is damaged then the protection it provides will be reduced or eliminated. For this reason, anodising must be done after all machining, folding or welding has been completed and any work after anodising must take care not to damage the anodising (for example, pressing in inserts).
Depending on the application, it is advisable to use anodising and another technique so as not to have a single point of failure.
The beauty of anodising is that because it is an insulator and not a metal, it is compatible with all other common metals (from a galvanic perspective) by virtue of being non-conductive. This means, that it is acceptable to use stainless steel, zinc coated steel, titanium, nickel and others in contact with anodised components. Although care may be necessary to ensure that the harder metals do not scratch the anodising. Some metals such as mercury or gallium should never be used in contact with aluminium because they can diffuse into the aluminium and cause catastrophic failures. Some readers may be interested to see a video of gallium causing an aluminium drinks can to fail.
More Info and Specialist Advice
Please email us if you would like advice on galvanic corrosion prevention. This page just gives general suggestions and our specialists will be able to provide more in-depth information.
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