Properly done, hot-process black oxide finish on plain steel with a wax
sealer can be quite corrosion resistance, there is even a MIL spec for
that (well, there is for everything including waffles, I suspect!). I
would think that accounts for the reports of the plain steel screws not
corroding.
Interestingly enough a reason a black oxide or especially black zinc
finish is applied to stainless steel is not related to appearance nor
corrosion resistance, but to forestall galling. The combination of the
intervening layer (>0.0002" of black-zinc conversion coating, 0.000005"
for hot-process black-oxide) and the wax or other sealer aids in
lowering interface friction. And although the wax acts a an EP
lubricant while tightening, it's stiction characteristic can aid in
fastener retention.
Howie - WA4PSC
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