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I'm looking into possible failures of a cast zinc part. I'm wondering if the Zinc-oxide that develops after a time would have a thickness great enough to cause dimensional tolerancing issues?

Tolerances on my parts are plus-minus 0.1 mm. Assuming the part is made within spec could an oxide (or other zinc corrosion) cause the part to fall out of spec?

Environmental conditions are standard air pressure, room temperature and normal air composition.

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  • $\begingroup$ What tolerance do you mean? Do you mean the length and width of a cast box? I worked for a year in a place that made gearboxes with cast zinc cases. There were gear pin holes drilled in the cases and even after lengthy storage in Florida climate the drilled holes would still be within tolerance. We didn't measure the outside dimension other than complete assembly (case and cover) thickness but I don't recall the tolerance; there were no problems with using old stock castings. $\endgroup$ – Gwydionforge Apr 3 '17 at 16:56
  • $\begingroup$ Pin holes would likely be the closest comparison to the part I'm working on. Do you recall what your clearances were on those holes? (I'm guessing the oxide layers or corrosion of cast zinc is probably measured in nanometers not 10^-2 milimeters) $\endgroup$ – Diesel Apr 3 '17 at 17:13
  • $\begingroup$ A bit of googling suggests that zinc is very good for anti-corrosive properties (which may explain why we galvanize so much). Seems like it doesn't really react actively with common compounds. If I had to guess I'd say it's like aluminium which forms a very thin layer of oxide that protects interior layers; but perhaps it's even more resistant than that. $\endgroup$ – JMac Apr 3 '17 at 17:22
  • $\begingroup$ I don't know what the tolerances were on the locations for the pin holes, I worked in the production and development lab doing testing on durability and developmental configurations. The only thing I did with quality was ensure the operators filled out and signed the inspection report log. As long as you keep it away from any sort of electrolytic cell you should be fine. $\endgroup$ – Gwydionforge Apr 3 '17 at 18:49
  • $\begingroup$ The thickness of the oxide layer is typically a few nanometers, so it is irrelevant for dimensional tolerances unless the environmental conditions are continually removing the old layer and allow a new one to form. That is very unlikely to happen for a part in "normal" air at room temperature. $\endgroup$ – alephzero Apr 4 '17 at 4:35
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One possible issue is galvanic corrosion. In certain conditions, if dissimilar metals are in contact with each other the more electronegative metal will corrode at a greatly accelerated rate, effectively the same situation a a metal/metal battery. This can be a particular issue if you have steel studs or other interests in a non-ferrous casting.

Indeed sacrificial zinc anodes are used to protect thing like boat hulls and submerged piles from corrosion and this is why galvanising is such effective corrosion protection, even if the zinc surface layer is scratched through to the base metal the galvanic effect will continue to provide protection.

Equally zinc is a fairly reactive metal which relies on a passive oxide layer for corrosion resistance so anything which disrupts this layer can cause very rapid corrosion. Even quite weak acid can strip galvanising in a few hours.

Also with any cast part you should consider shrinkage as a potential source of tolerating issues although this should be apparent from new. Equally inadequate design of a casting can generate high residual stresses and other defects which can cause failure over time.

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  • $\begingroup$ "You should consider shrinkage" - the OP's 0.1mm is a sensible practical tolerance for a small die-cast metal part (maximum dimension less than 100mm, say). $\endgroup$ – alephzero Apr 4 '17 at 15:06
  • $\begingroup$ @ChrisJohns I'm thinking that the issue I'm experiencing isn't coming from shrinkage just due to the low recurrence of the problem. Any other case though this could definitely be an issue but our parts have been made for a while now and they're coming from the manufacturer within spec. $\endgroup$ – Diesel Apr 4 '17 at 16:42
  • $\begingroup$ @Chris Johns , Good to know about corrosion, though I doubt that's the case for my parts (cast zinc in a dry plastic housing (no electrolytic solution for current flow). Out of curiosity, do you know how much the corrosion could change the part dimensions? $\endgroup$ – Diesel Apr 4 '17 at 16:47

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