Bi-Metallic corrosion is the additional corrosion that occurs when dissimilar metals are in contact in the presence of an electrolyte.
The corrosion of a metal, the anode, results from the positive current flowing from the anode to the less reactive (more noble) metal, the cathode, through the electrolyte.
This process is similar to the conventional corrosion of a single, uncoupled metal but generally proceeds at a higher rate depending on the difference in electrochemical reactivity of the anode and cathode metal.
The requirements for bi-metallic corrosion are as follows:
An electrolyte bridging the two metals
Electrical contact between the two metals.
A difference in potential between the metals to enable a significant galvanic current
A sustained cathodic reaction on the more noble of the two metals.
What do we mean by electrical contact? This is when two dissimilar metals make physical contact with one another, allowing ions to travel between the metals. To make them reactive with one another, you need an electrolyte.
Where do you get an electrolyte? Mother Nature supplies it in the form of rain and road splash. Man improves on it a little with the addition of de-icers or road salt, which provides an even better conductor for the ions to travel in.
The manifestation of this is a white powdery corrosion that forms on the surface of aluminium in contact with steel. This generally bubbles the paint and deteriorates the aluminium. Many other metals suffer galvanic corrosion, but in the automotive industry, the primary metals affected by this problem are aluminium, zinc and magnesium.
Cross Contamination
Beware of spreading aluminium sanding and grinding dust onto steel cars or parts and vice versa. This is easier than you think. All you need to do is pick up a sander that you just used on a steel car and use it with the same piece of sand paper on aluminium. The most likely result of this cross contamination and galvanic corrosion causing bubbles in the finish.
Take special care when working around bare aluminium, a little common sense and caution can prevent problems that might be costly to fix later:
A separate room is preferential for aluminium work.
A different set of tools may also be required.
Wipe power sanders down with a damp cloth before beginning work.
Don’t use compressed air to remove sanding dust, use vacuum.