A. Hi Darren. If you have been successful priming zinc plated bolts (which include a clear chromate), you should be equally successful priming zinc plated bolts that have received zinc plating and a black chromate instead. These 'black zinc' bolts should be readily available.
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A. Hi Jared. Black oxide is an extremely thin (millionths of an inch) oxidation product created on the surface of a steel part; it has almost no corrosion resistance but it is attractive and does not affect the dimensions of parts; so it's used on things like bored sprockets, gears, and couplings, where dimensions are critical, and on things that are lovingly maintained and regularly oiled like rifles and shotguns.

Black zinc is zinc plating (say .0002" thick or more) followed by a black chromate conversion coating. You get the sacrificial corrosion protection of zinc, plus a nice appearance, but it does affect the dimensions enough to be a problem on very tight fitting parts. Luck and regards,

Ted Mooney, P.E.
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A. Hello Timothy. Black zinc plating is significantly more corrosion resistant but it does add thickness. The minimum thickness would be 2 ten thousandths of an inch and, due to the irregularity of the thickness, it could be 5 or 6 ten thousandths in some areas depending on the shape of the part. But zinc plating is done even on small fine-thread components, and a tolerance of .001" doesn't sound like a problem pending a review of the specific part. Good luck.

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A. Black oxide has little if any corrosion resistance and the only protection is provided by the oil that is on it. By "black trivalent chrome" what is actually meant is zinc plating followed by black chromating so you have lots more corrosion protection and very little chance for reaction with plastic. If anything to watch for it would be the build-up of the zinc plating - it's substantially more than black oxide which is nearly nil. Also, my experience with black trivalent products is that by themselves they are blotchy and not very eye appealing. To overcome this applicators of black trivalent typically apply an organic sealer - wax or lacquer based - which if you have electrical requirements (not likely if this is going into plastic) you've now lost conductivity. Bottom line - black trivalent is better and you should charge more! At least make your customer aware of upgrading their value without a cost impact.

Milt Stevenson, Jr.
Plating shop technical manager - Syracuse, New York
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A. Hi Troy. Although "the government" probably doesn't want excessive phosphates in the effluent water from metal finishing shops, it sounds like an exaggeration for your supplier to be claiming that "the government does not support phosphate coating". Phosphate coating is just about the only acceptable pretreatment for painting ... and for powder coating ... and for applying secondary coatings on top of galvanizing ... and as a break-in coating for the countless moving parts inside automobile engines ... and as a rust converter ⇦ on eBay or Amazon [affil link] so that consumers can maintain old equipment rather than throw it away. A government which "doesn't support phosphating" is a government with no respect for sustainability, and which prefers endless re-mining, re-smelting, re-fabricating, re-finishing, and re-landfilling to building stuff that lasts  🙂

But depending on the function of the screws in question, zinc plating may be perfectly satisfactory, and might well be better than phosphate and oil! Probably as a general rule, if part of the screw is painted or it sits in oil or grease, the phosphate and oil is better, whereas if the screw is exposed to the air, the zinc plating may be better. Again, only a very general guideline. Good luck.

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A. Hi Troy,

We do phosphate coatings on screws, and all the products we use are RoHS compliant. If he does not do phosphate because of environmental concerns, the sealer he can apply may have hexavalent chrome. There are many sealers without hexavalent chrome, environmentally friendly, but they are expensive (but not as expensive as a black zinc trivalent conversion coating as he applies).

If you ask me, if you ask for a zinc phosphate coating, you can have that without any environmental problem. There are many vendors offering these products. Black zinc is (if the conversion coating does not have hexavalent chrome) also environmentally friendly, but also more expensive.

Hope it does answer your question! Regards!
Daniel Montañés
Cañuelas, Buenos Aires, Argentina
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A. Hi Daniel. Your findings verify exactly what this thread tells you. Zinc plating offers sacrificial protection against corrosion and black oxide does not. Black oxide has no rust resistance except its wax or oil. In a somewhat corrosive shop environment I have seen it rust horribly within 45 minutes when the oil was stripped and not replaced within that time period.

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A. Hi, Vimal. People are forever hard at work trying to make things look like something else. If you were to review this whole forum you'd find hundreds of threads of this form: "I want my thingamajig out of Yyyyyy but I want it to look like it's made of Xxxxxx ... how do I make Yyyyyy look like Xxxxxx?".

Although the finishes you named might tend to look different, and some experienced platers could probably get the identification right 80 or even 95% of the time, after 50+ years in the finishing industry I personally have zero faith in visual identification of finishes.

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Ted Mooney, P.E.
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A. Hi Farid. Unfortunately, we can't take a loose, slang term like "plain finished" or "black finished" and tighten it up to something highly precise. Sorry.

I believe that ASTM A490M says that these high strength structural bolts cannot be galvanized, electroplated or mechanically plated, but you must check the spec. to make sure. That probably limits you to a zinc-rich dip-spin coating. I believe TYPE 3 is Weathering Steel (Corten), which might mean that unfinished is the suggested finish anyway.

Please try your best to explain your situation, rather than keeping it abstract. Thanks!

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A. Hi Andy. What does the spec referenced in the purchase order call for in this regard? The plating spec is your primary guideline for incoming inspection, and it often even instructs in fine detail about lot sizes & sample sizes, etc. Trying to inspect from a different starting point will only lead to contract disputes  🙂
Good luck!

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Ted Mooney, P.E. RET
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A. Hi Farhan. The overwhelming majority of nuts & bolts in the world are zinc plated. The adhesion is fine for this tough service. Further, zinc is cathodic to steel, so it can largely continue to serve its corrosion protection mission even if scratched.

But the black coloration is chromate conversion, which is thinner and less rugged than the zinc plating. I would expect the black finish to be marred from certain heavy things repeatedly sliding over it. Please describe your situation as much as you can because our ability to help with abstract questions is limited  🙂

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A. Hi Sam. Most plating specifications are concerned with the functionality of the plating more than the aesthetic requirements, but check the spec first. If you want consistent appearance from vendor to vendor, the only practical way to do it that I am aware of is with sample boards: "this component is ideal" "this component is satisfactory", "too shiny", "too matte", "too light", "too dark".
But sample boards are something that a lot of professionals scoff at for a few good reasons --
- The samples will age over time, and few buyers will undertake the continuous effort to keep them current.
- How do you get Shop B to agree to let you use Shop A's samples in the sample board instead of theirs?
- Different inspectors will still pass/fail different parts; it's still not 100% objective.
I think I understand what you want, but it honestly doesn't work quite that way  🙂
Certainly you can tighten up your purchasing a little by finding ASTM, ISO or other specs, but in the end either you rely on the technical expertise of your plating shop or your own (or your consultant's).
Consistency is possible, Apple achieves it on millions of iPhones. It may be that the best path forward is to pick your favored vendor, work with them on the sample board, and only accept work from other vendors if they agree to live by the sample board. Second opinions welcomed!
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Ted Mooney, P.E. RET
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A. Hi Deepak. Black oxide has very poor corrosion resistance, but if you're sure you want black oxide rather than black zinc plating or black phosphating or black chrome, there are many threads on this site which are more informative about the process details than this one. But you don't specify thickness for black oxide coatings (they are almost indetectably thin, although salt spray testing might be appropriate. Here are some typical specs: ISO 11408, AMS2485, MIL-DTL-13924 [⇦ this spec on DLA]. Please pick one and see if it covers your needs.
Are there reasons which make you feel black oxide is appropriate for these parts and this application? Please explain the parts and your situation because in that way can people best help you, and your inquiry will attract the most interest.
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Ted Mooney, P.E. RET
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A. Hi Sergio,

You don't control the gloss of the zinc plating process, as the only way to get good thickness and adherence is having a glossy zinc layer. The chromate could modify this but not so much to take into account.

If you want a semi-matte finish you could do some dip-spin paint over zinc, a little more expensive but I think far more superior.

Hope you get what you want! Best regards,
Daniel Montañés
Cañuelas, Buenos Aires, Argentina
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A. Hi Tom. Hardware stores and the labeling on screw packages don't generally give you enough information to determine the thickness & type of plating and coating, and it's not customary to try to buy hardware for a small home project to your own engineering specifications as the time and cost involved are out of proportion. I'd probably just look for deck screws in a satisfactory shade. I've seen them in olive green, gray, and black -- and they'll be plenty corrosion resistant. Good luck.
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Ted Mooney, P.E. RET
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A. Black zinc is almost always zinc electroplate followed by a black conversion coating. In the old days this was a hexavalent chromate conversion coating incorporating a small amount of silver. Today, with platers leaning toward trivalent passivates, there are a variety of formulations. The word on the street I hear is that they are not as black or reliable as prior art technology. Black conversion coatings over zinc-iron, though, have been successful. Since the underlying coating is zinc, sacrificial protection is provided. And the passivate provides inhibitive protection.
Black oxide is a conversion treatment for iron. (It converts the iron to magnetite (Fe3O4)). It is almost always done by immersing and iron or steel article in a very hot, very alkaline, very strong oxidizing agent. It looks great, but provides little corrosion protection of any kind. Post treatment with a mineral oil (or the equivalent) provides a modicum of corrosion protection and a little gloss. The bottom line is this: if it's going outdoors, go with black zinc; if it's going indoors, black oxide should be okay.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.


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A. Hi Carson. I've never heard of such an issue but, sorry, I've never researched it either.
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A. Hi Robert. In my limited experience cold blackening (selenium sulfide based processes) are usually smutty, but real hot black oxide significantly less so. "Small mild steel plate components that will be handled by customers" might be better with black zinc plating though, because it is more corrosion resistant.
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A. Hi Jake. Some black chromates are silver based and will indeed turn green or other colors over time and after UV exposure. It is my understanding, but you would need to back it up by talking to a supplier, that if you do zinc-iron plating and then black chromate conversion coat it, that the iron content makes stable black chromating possible.
Even still, black zinc or black zinc-iron may not be quite what you're looking for if you want a truly decorative long term finish; painting the heads after black chromating would be ideal. Black chrome plating is a fully decorative finish but may be prohibitively expensive.
When I picture what "off road" means to me, black zinc-iron sounds fully acceptable, but to some people the tiniest paint scratch on their "Trail Rated" Grand Cherokee would be considered a tragedy, so it may not be decorative enough for them  🙂
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Ted Mooney, P.E. RET
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A. Hi Chris. Yes it is possible to strip all of the existing plating, and yes it is possible to zinc-iron plate it and black chromate it. And if the hardware is properly baked for hydrogen relief at the necessary stages it is possible to do most of it without compromising its strength. But there are some specs for very high strength fasteners (see the reference to ASTM A490M above, for example) that might forbid stripping or plating.
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