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Letter 26050
Nickel plating vs zinc on air/oil
separators
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My company manufactures air/oil separators for the air compressor
market. All of the steel endcaps are zinc plated (commercial grade)
but we encounter customers who want more corrosion resistance and
request nickel plating instead. Why is nickel plating better and what
can I supply for technical data to support this extra expense. The
answer I keep hearing is that nickel is thicker than the zinc with a
clear chromate finish.
David P 
Filter manufacturer - Chicopee, Massachusetts, USA
+++
The nickel plating probably is thicker than the zinc plating, but
that doesn't have too much to do with anything. Nor is nickel
necessarily a more corrosion resistant finish. It's not really
particularly complex to explain, but it will take a lot of words and
the responders will probably run out of enthusiasm for typing before
you've got a clear understanding of what they would like to tell you
if they had the time :-)
But I'll start. Zinc is a sacrificial plating; it does its job by
corroding in lieu of the steel corroding. You've probably heard of
zinc anodes being used to sacrificially protect the steel hulls of
ships, etc. That makes it ideal for many applications because it
protects the steel even if the coating is scratched. But zinc has no
acid resistance at all and if the plant environment has acidic
vapors, as for example in a plating shop, the zinc will do no good at
all.
Nickel is a "barrier layer" coating. it protects steel by
enveloping it and keeping the environment away. But nickel is not
sacrificial to steel; in fact the opposite is the case. So if there
is a breach in the plating, from a scratch, there will be accelerated
corrosion at the breach. So nickel is probably only better where zinc
has proven itself ill suited.
Electroless nickel is another story, but I ran out of enthusiasm
for typing :-)
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Ted Mooney, P.E.
finishing.com
Brick, New Jersey
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