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  • Brass and aluminium plates, stainless steel screws = CORROSION?

    Discussion in 'The main mechanical design forum' started by GarethW, May 26, 2012.

    1. Mark Stapleton

      Mark Stapleton Active Member

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      Gareth, just putting my 2 cents in here, and that may be all that it's worth, but although I agree with the posts above regarding electrolytic action I also think that the amount of corrosion which is likely IF the assembly is not in a MARINE environment is not great. The aluminum becomes the "sacrificial" anode in the reaction and it is that which will corrode (oxidize). The unfortunate upshot of this is some loss of electrical conductivity over time, not so much structural weakening or nasty looking corrosion. Passivating the aluminum should help, of course. Cadmium plating is another method, but that's expensive because of cadmium's high toxicity. Your sketch (with o-rings) appears as if you're trying to seal something, but I'm not sure why you're using aluminum in the first place. If you need both thermal and electrical conductivity, why not use copper, or a copper alloy?
       
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    3. AndrewNew

      AndrewNew Well-Known Member

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      We had a similar dilema about a year ago, but our parts were exposed to much more aggressive conditions than yours (on the back of a sea-going boat). We went for A4 stainless steel screws. You may see some localised galvanic corrosion of the aluminium, since the aluminium, being more anodic than the stainless, acts as a sacrificial anode. Since you're fixing a large block of aluminium with small fasteners, this is the way round you want it - assuming a little bit of corrosion of the aluminium is tolerable - as it actually protects the fasteners. The corrosion is minimal even in our wet/salty conditions. A bit of retainer/thread seal (e.g. Loctite 511) doesn't hurt to help keep the threads from binding.
       

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