Metal finishing Q&As since 1989

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Reducing the Visibility of Black Spotting

Hi there,

My first question on this community, so please be gentle...

Our company recently rattled off a batch of a few hundred components for one of our aerospace customers who place great emphasis on aesthetics. This part in particular is to be used in a highly visible application, and so it was Murphy's law that this particular component batch is suffering heavily from black spotting, readily visible after the acid etch, before any other finishing process is undertaken.

Interestingly, and perhaps foolhardily, our line does not run a desmut stage before anodising/bright nickel. My question is; are there any solutions to removing or reducing the visual impact of the dark spotting? will a comprehensive desmut solve the issue? dry-milling the parts is our next step, to see if reducing the hydrocarbon exposure from the coolant will help matters, but in the interim we have 300 parts already machined to spec, with silver/grey camo pattern!

Thanks for any help.

James Harris
Manufacturing Engineer - Lancashire, UK
2007

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2007

James,

A little bit more detail if you could:

What is the alloy you are using? Is it a cast material, it sounds a bit like it?
Is your etch/deoxidise an alkaline etch or acid one? This would give us idea of what may be causing your black spotting. Once we know this we may be able to give you a clue as to what to do next.

Brian Terry
Aerospace - Yeovil, Somerset, UK