Normally aluminium is protected from corrosion by formation of a thin
passive oxide film which is stable and prevents further corrosion.
This behavior is like that of stainless steels.
If the oxide film breaks down locally, then corrosion occurs via
A problem with aluminum is galvanic corrosion. Aluminium is more
active than other metals such as iron and copper. If you put your
small part in a moist outside environment and connect it to steel or
copper, then it will corrode (become an anode) in preference to the
adjacent steel or copper (cathode) which will be prevented from
corroding. The worst situation for galvanic corrosion is having a
small area of aluminum connected to a large area of copper.
You also need to beware of a pathological form of attack called
deposition corrosion. If you have other metals like copper, lead,
nickel, and tin located "upstream" of the part, then it is possible
for water passing over them to dissolve those metals. Then those ions
can plate out on the surface of your aluminum and set up an galvanic
cell with a large surface area of the other metal. The typical recipe
for disaster is a combination of a copper roof with runoff into
aluminum gutters. Just 0.05 ppm of dissolved copper can start pitting.
If you have these other metals "upstream", then you better think about
a different approach.
This is briefly mentioned in the Alubook "How to avoid or minimize
galvanic corrosion", Topic 11065. See:
http://www.alu-info.dk/Html/alulib/modul/A00109.htm Click the arrows<<
or >> to go either back or forwards in the topic.
In the UK the National Physical Lab (NPL) has a "Guide to the
Avoidance of Corrosion in Metal Guttering Systems"
It mentions that if you put aluminium downspouts on a building with a
copper roof then you will have big problems.
For a more detailed discussion, go to a good library and look in the
ASM (Metals) Handbook, Volume 13 on Corrosion (1987). Read the article
on "corrosion of aluminum and aluminum alloys" on pages 583-609. You
will find deposition corrosion mentioned on page 589.
There is also a good general discussion of galvanic (bimetallic)
corrosion in the Guides to Good Practice in Corrosion Control series
in a big 579K file at
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