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"Stainless or Zinc Plated Screws on Aluminum, how much/fast will corrosion occur?"





March 2, 2010

Hello folks,

I am converting a motorcycle from gas to electric. I am building (welding) 6061 aluminum angle (1" x 1" x 1/8") into trays that the batteries will sit in. There are nine batteries that weigh about 18 lbs each, and they are enclosed in aluminum housings.

In order to ensure that the batteries don't jump out of the frames while I'm driving, I was going to tap the frame and housings and use M6 machine screws to hold the battery/housings in place.

I'm trying to decide on screw material and it pretty much comes down to stainless or zinc plated machine screws due to general availability. I've been (through links posted on this site) about galvanic corrosion between aluminum and stainless or zinc.

In my situation, there will be far more aluminum angle/housing than screws, and I've read that it's good to pick the more voluminous material to be the sacrificial piece. What I'm concerned about here is that the aluminum is structural (the aluminum frame holds many batteries), and the screws are not structural (they hold the batteries in frame). There will be four or eight screws holding a battery single battery pack in place, so if I choose zinc plated screws, they should be the sacrificial piece (I think) and if one or more screws starts to fail, the battery may start rattling or jumping around in the frame and I can replace the screw. however if I use stainless screws, the aluminum frame will corrode, which could be more catastrophic and also is much harder to replace.

As for actual conditions, I live in Northern California, so the humidity level is relatively low most of the year, however this is a motorcycle, so it will be ridden on the road and may see some water due to puddles or light rain (I don't plan on riding in the rain)

So, questions I have are,

* should I use stainless or zinc screws? Based on my (very limited) understanding, I probably want zinc, however I'm not really sure here.

* How fast will noticeable/problematic corrosion occur? Given the conditions are relatively light in terms of exposure to moisture how concerned should I be?

* Is there a coating I can use to help? I've heard that an anti-seize compound would be useful, though I need to use locktite to make sure the screws don't come loose, I don't know if locktite and anti-seize are compatible.

If there's anything else I'm missing, please let me know!

Thanks!

--Tango

Alex Tang
Hobbyist/Product Prototype Maker - Mountain View, California, USA
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