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 :-)

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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