is aluminium stainless

Maike:
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 pitting.
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: Click the arrows> 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" at: 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
Pittsburgh Pete -------------------------------------------------------------------------------
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