Galvanic corrosion = Bi-metallic corrosion on Aluminum

One of my most read articles on Anodizing World is the one about galvanic corrosion, therefore I have decided to dwell a little bit more about galvanic corrosion on aluminum.

When two dissimilar metals are in direct contact in a conducting liquid, experience shows that one of the two will corrode - hereby the name - Bi-metallic Corrosion or also called Galvanic Corrosion.

If two metals, e.g. aluminum and copper, are placed in sea water both metals will start to corrode. If the two metals are connect with a wire a current will flow, and aluminum will corrode faster and the copper will stop corroding. This is like a sacrificial anode of zinc on a boat, but here the anode is the aluminum.

Aluminum is a reactive (un-noble) metal compared to most of the metals used. Aluminum will therefore almost always be the anode, the part which corrodes, in contact with other metals.

The two main factors are the severity of the environment and the potential difference between the two materials, e.g. aluminum and copper.

The more aggressive the environment and the larger the difference in potential between the two metals are the more sever is the corrosion attack.

The bible "Surface Treatment and Finishing of Aluminum and its Alloys" by S.Wernick, R. Pinner and P.G. Sheasby shows the following table.

General guide to galvanic influence of various metals on aluminum.

Metal
Comment
Cadmium
Slight effect
Chromium
Small to negligible
Copper and copper alloys 
Severe
Graphite
Severe
Lead
Negligible except in severe marine environments
Stainless steel
Negligible except in severe marine environments
Steel and Iron                               
Slight except in severe environments
Tin
Negligible except in salt solutions
Titanium
Negligible except in severe marine environments
Galvanised steel
No effect until zinc coating is destroyed

If you find this article useful and you would like to know more please contact me [email protected]
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