Metal finishing Q&As since 1989
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Dark grey/black coating for metal hardware
Q. Hi,
For a product I'm designing, I am using anodized aluminum carabiners that clip into the eye portion of metal screw eyes. For my prototype, I'm using hardware-store-sourced zinc-plated screw eyes (silver in color), but I would like my finished product to use dark/black colored hardware. The product will be exposed to the outdoors (rain/snow, though not submerged or exposed to salt water, etc.), so I'm looking for something with decent corrosion resistance and that will maintain its finish/appearance if possible.
Any suggestions on suitable finishes you could offer would be greatly appreciated.
PS: If I end up using readily-available hardware (silver colored, instead of black), would nickel-plated screw eyes or zinc-plated be more suitable for my intended purpose?
Product designer - Edmonton, AB Canada
July 25, 2016
A. Hi Lee. I'd suggest a zinc alloy plating like zinc-iron, zinc-cobalt, or zinc-nickel, with a black chromate conversion coating. The alloys have more corrosion resistance than plain zinc plating and are widely available because they are used in the automotive industry.
Zinc plating and these alloys are "sacrificial" coatings which protect the underlying steel. Nickel plating is a "barrier layer" coating, and steel is sacrificial to nickel; so in the event of pinholes, porosity, or scratches the rusting of the underlying steel is accelerated by the nickel plating.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2016
Burnished Black Zinc Longevity on Interior Hardware?
Q. Hi Everybody,
We are trying to achieve a darkish/blackish finish on interior mild steel coat hooks that have a burnished or highlighted look. These are limited production coat hooks with matching screws. What we have experimented with is a zinc-black finish that is then very lightly abraded to highlight texture and edges. We are briefly wet tumbling the zinc-black plated hooks with small plastic media to burnish them.
The results look good and their is surprisingly little to no residue that comes off the hook on a white rag. After two months of sitting in the humid shop summer air, they have yet to red rust. A sign that we didn't abrade past the zinc?
My concerns of course are that the black top coating will continually wear away over time. Is there any other process like this that will give me a dark highlighted look that could be applied to a coat hook? Or anything I should be further concerned about that I missed with our test process?
Thanks-
Sam
owner, fabricator - Roanoke, Virginia USA
July 27, 2016
(To show dark relieved areas)
Courtesy: Balfour.com
A. Hi Sam. The black finish will not wear off of the recesses. That's what a burnished look is about: it looks real and natural when recesses are dark and high spots are bright, because it is .
To me the issue is that when you have removed the black chromate, I think you've significantly reduced the resistance to white rust. Fortunately, the interior of living rooms and hallways is a mild exposure, but it could be a problem if they are used in a bathroom or for wet snowy coats and hats. Against that possibility, have you considered clear coating them after the burnishing operation?
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Thanks for your response Ted. I have thought about clear coating them, but in my past experiences it always looks horrible when it's worn off and/or scratched. Of course, my experience is mostly with waxes, lacquers, and powder coatings. Can you catch me up to speed on more durable clear coats? In satin?
Sam Forest [returning]- Roanoke, Virginia USA
July 27, 2016
A. Hi again Sam. I would suggest additional corrosion testing. Your real-world corrosion testing beats any accelerated test, but a salt spray test might more quickly reveal if in fact there is reduced corrosion resistance in the areas where the black chromate has been worn off. Maybe for some reason the corrosion resistance is not reduced and everything is fine.
As for clearcoating, we don't compare brands here, but G.J. Nikolas [a finishing.com supporting advertiser] and Everbrite [a finishing.com supporting advertiser] are two suppliers you could talk to.
I would note, though, that when people say clearcoats look poor as they deteriorate, they are usually referring to their personal experience as homeowner/consumer/hobbyists where the clearcoats are applied poorly and under bad conditions -- parts not really clean, dusty environments, one sloppy thick layer instead of multiple coats, etc. Almost every automobile in the world has a clearcoat, exposed to extremely bad conditions, and how many look horrible? Good luck.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2016
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