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I have a heavy-duty air cleaner whose metal bottom plate is fastened to the metal casing via 4 sheet metal screws. They go through holes at the edge of the bottom plate and anchor onto metal tabs projecting from the bottom edge of the casing sidewall.

I sent the unit for repair by the manufacturer, but it came back with one of the screws loose because of stripped threads and a missing star lock washer (they have external teeth). It was easy enough to find a sheet metal screw that was one size up (#10) but when I looked for a star lock washer, I had to decide between zinc plated and stainless steel. I find different information about which is more resistant against galvanic corrosion.

I don't know what the metals are for (i) the casing, (ii) the bottom plate, or (iii) the #10 screw. For the latter, the packaging says "para metal"; Google doesn't find anything for that, but "para" refers to mercury, which the screw certainly is not. I want to err on the side of caution by getting a more corrosion resistant lock washer.

As I mentioned, there doesn't seem to be consensus in the online information as to whether zinc plating is better than stainless steel. It seems to be dependent on the moisture condition. The air cleaning unit won't be immersed in water nor exposed to high humidity, but impeller does induce some vibration. Maybe this means that there is greater contact between metals over time and/or greater chafing away of zinc coating (if that is the chosen washer). For the latter, I don't know how robust the coating is. Under these conditions of non-extraordinary humidity and moderate vibration, which is better?

P.S. Given the different possible metals out there, how do sheet metal screw manufacturers ensure against galvanic corrosion?

Notes

Stainless steel external star lock washer (#10) from Ottawa Fasteners, zinc-plated from Elmvale Home Hardware.

The air cleaner is the Austin HealthMat.

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    Good chance all the metal is a type of steel, so galvanic corrosion would be very low on the list to worry about. Choice of washer will probably come down to which store is closer. Mixing Aluminum/copper/steel is where you worry.
    – crip659
    Commented Aug 26 at 21:51
  • I understand what you're saying about the likelihood of all parts being metal, but if I wanted to minimize the probability of corrosion anyway (just for piece of mind), which of the two fastener options would be best? I suspect that the washers come in quantities of much more than 1, so taking that single precaution now would lower the risk for all future needs for such a washer (though frankly, not being a handyman, I've never had such a need before, so might never have again).
    – user2153235
    Commented Aug 26 at 22:02
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    Zinc is a coating/protection. If the the coating/protection is scratched or damaged, then the base steel can rust. Stainless steel is stainless steel all through, so will not rust/corrode .
    – crip659
    Commented Aug 26 at 22:10
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    A magnet will stick to steel but not to aluminium. It is unlikely that stainless steel would have been used for the bottom plate or casing - if it was, it would probably be unpainted and you could tell from the colour that it wasn't aluminium.
    – Andrew Morton
    Commented Dec 1 at 17:24
  • Interesting way to troubleshoot! I've already replaced the washer, and I don't remember what I did. But I'll keep this trick in mind! Thanks!
    – user2153235
    Commented Dec 2 at 18:57

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Most of the metal should be a type of steel, so no galvanic corrosion to worry about.

Either washer type will work.

Zinc is a coating/protection. If the the coating/protection is scratched or damaged, then the base steel can rust. Stainless steel is stainless steel all through, so will not rust/corrode.

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  • He only stated heavy duty air cleaner metal bottom plate. The make/model was not stated so how can you be so sure most of the metal should be a type of steel ? And that it is not aluminum?
    – ron
    Commented Aug 27 at 13:04
  • @ron Probably for the reason OP asked this question. Mixing metals is usually not a good idea unless for special needs. Steel can usually be made thinner and formed than what aluminum can for the same strength. Not saying there are not many examples that prove this thinking wrong, but usually you would find something between the mixed metals.
    – crip659
    Commented Aug 27 at 13:27
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    I added the detail of what the air cleaner is (Austin HealthMate). But the metal isn't specified, at least not that I could find).
    – user2153235
    Commented Aug 28 at 22:13

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