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topic 40943

Black zinc plating vs. Black oxiding

A discussion started in 2003 but continuing through 2019






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, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha


Q. I have a plain carbon steel part that is exposed to a mild corrosive climate. It needs to retain its magnetic properties and must keep the post machining tolerance (0.001 thou). I know that black oxide coating will work well for this application but can I also use a black zinc coating?
I need help fast, thanks

Timothy Vandermeiden
Dynastream innovations - Calgary, AB, Canada


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.

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

November 21, 2012

Q. Hi. As a mechatronics engineer, mainly with electrical background rather than mechanical, I am stuck over a problem of coating. The engineering standards of my company support phosphate & oil coating on screws, but the supplier has been using zinc coating for the same part, saying phosphate coating is not environmentally friendly and the government does not support it. Now, I want to have comparisons drawn round off on a conclusion.

Troy Costa
- Pune, Maharstra, India

November 24, 2012

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 to claim that "the government does not support phosphate coating". Phosphate coating is just about the only acceptable pretreatment for painting, for powder coating, for applying secondary coatings on top of galvanizing, as a break-in coating for countless moving parts inside automobile engines, etc. A government which doesn't support phosphating would be a government with no respect for manufacturing at all.

But depending on the function of the screws in question, zinc plating may be perfectly satisfactory, and might even be better than phosphate and oil. Probably as a general rule, if part of the screw is painted or 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.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

November 22, 2012

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 Montanes
- Cañuelas, Buenos Aires, Argentina

sidebar2 November 24, 2012

thumbs up signThanks. That was indeed much helpful. Just for the record, the gov't here is the Chinese gov't in picture. Although nothing official against Phosphate usage in coatings, the engineering team has has been asked officially to smother the usage of phosphates.

Your suggestion solves my problem at large.

Troy Costa
- Pune, Maharstra, India

April 29, 2013

opinion!  Hi Troy. That Chinese shortsightedness is very depressing, but it probably helps explain the shiploads of worthless painted Chinese steel crap for sale in every big box store :-(

The world is unsustainably drowning in corroding worthless patio sets, lawn furniture, garden accessories, outdoor equipment, kitchen & bath accessories -- an endless list of millions of tons of rusting garbage that only lasts one to three seasons when it should and easily could last 15-20 years and more if simply properly phosphate pretreated before painting. All of this continuous re-mining, re-smelting, re-manufacturing, re-painting, re-packaging, re-shipping, re-stocking, re-landfilling -- needlessly replacing stuff every three years or even less -- and the Chinese government is focused on the milligrams of phosphate instead of the tons of steel? :-) Geez! Reminds me of our EPA.

I support many environmental groups & efforts, but sometimes the mind-numbing ignorance & stupidity seems beyond all hope. If we actually had any concern for "sustainability", we would be demanding that such things last a minimum of 15 to 20 years -- the finishing industry already knows exactly how to do it. But instead, governments are harassing to death the few manufacturers who actually do the proper phosphate pretreatment rather than the manufacturers who skip it and thereby ship a ton of soon-to-be-garbage steel products :-)   

AAAAAARRRGH!!! <-- (primal scream)


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

September 21, 2013

Q. Hi,

I have several questions I want to verify regarding the black oxide. I have a set of front rotors and currently using black oxide. After several days, it starts rusting in comparison to black zinc coating.

I'm testing which coating can last longer. Can you give me any advise for the black oxide if possible? Is there anyway that we can hold up the black oxide to prevent the rust?

Daniel Ip
- Toronto, Canada

September 26, 2013

A. Hi Daniel. Your findings verified what this thread tells you. Zinc plating offers sacrificial protection against corrosion and black oxide does not. Black oxide has no rust resistance without wax or oil.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

October 19, 2014

Q. Hi,
Is it possible to identify the difference among black oxide, black zinc and phosphate coated bolts just by visual inspection?


October 2014

A. Hi, Vimal. I seriously doubt it! People are forever busy trying to make things look like something else. If you were to review this whole 60,000-thread forum you'd find hundreds, maybe thousands, of threads of this form: "I want my thingamajig to look like it's made of Xxxxxx but I actually want to make it out of Yyyyyy -- how do I make Yyyyyy look like Xxxxxx?". Although the finishes you named might tend to look slightly different, and some experienced platers could probably get the identification right 75 or even 90% of the time, after 50 years in this industry I personally have zero faith in visual identification of finishes.

Luck and Regards,

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

Recommended finish for A490M Type 3 bolts?

November 20, 2014

Q. What is the accurate difference between plain finished and black finished? And what is the recommended finish especially for bolts A490M TYPE 3?

Farid Nagy
- Cairo, Egypt

November 2014

A. Hi Farid. Unfortunately, we can't take a loose, slang-y, term like "plain finished"" or "black finished" and tighten it up to anything precise. If someone claims that a rock is heavy, we can't ask a third party to tell us how many kilograms the claimant meant.

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 or vacuum plating, or possibly black oxiding. I believe TYPE 3 is Weathering Steel (Corten), which might mean that unfinished is the suggested finish anyway.

Please try your best to supply us with the context, i.e., your own situation, rather than keeping it vague and abstract. Thanks!


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

Inspecting selective black zinc plating?

September 30, 2015

Q. Hi,

Is there a way to visually check or guidelines for partial plating on steel for incoming inspection on black zinc coating?

Thank you

Andy Rada
- Ottawa Ontario

A. Hi Andy. What exactly does the purchase order and the specs it references call for in this regard? Those specs are the guideline for incoming inspection. Trying to inspect to anything else is the starting point for a contract dispute and little else :-)
Good luck.


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

December 16, 2016

Q. I want to ask about the abrasion resistance of Zn Plating? I mean if some heavy thing slides over it, can the protective layer be damaged or otherwise?

Global Engg - Islamabad Pakistan

December 2016

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. I would expect the black finish to be marred from certain heavy things repeatedly sliding over it. Again, please describe your situation as much as you can because our ability to help with an abstract question is very limited :-)


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

Aesthetic standards to get matching parts from multiple vendors

March 30, 2017

Q. Hi, I am working in a small company and we usually ask our vendors to make specific coatings and finishes for the parts that they make for us. For example we just say "Black Zinc" or "Black Oxide". But since we have different vendors the products come in different colors and shapes. So I want to make a standard way to communicate with vendors to have identical finishes. Our parts are usually made of steel or aluminum and right now aesthetic is more important than anything for us.
What sources should I use to make such an internal standard? ASTM or ISO standard or anything else? Is there any general code for colors of these coatings and finishes?
Thanks for your help.

Sam bahremand
- Irvine, California, United States

March 2017

A. Hi Sam. Most plating specifications are concerned with the functionality of the plating more than the aesthetic requirements. 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", "this component is too shiny", "this component is too matte", "this component is too light", "this component is too dark".

But sample boards are something that a lot of professionals scoff at for a few good reasons --
- The samples age over time, and few buyers will undertake the major 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 just not 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, but they are perhaps the world's biggest company, not a small company. 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!


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

June 13, 2017

Q. Hi,

I need to specify black oxide coating on a few of my parts but I don't know how much thickness and salt spray life need to be specified on it.

The machine has to run in a muddy and dusty field.

Please support.


Deepak Bathla
- Chandigarh,Punjab , india

June 2017

A. Hi Deepak. If you're sure you want black oxide rather than black zinc plating or black phosphating, 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, although salt spray testing might be appropriate. Here are some typical specs: ISO 11408 [link by ed. to spec at TechStreet], AMS2485 [link by ed. to spec at TechStreet], MIL-DTL-13924 [link is to free spec at Defense Logistics Agency /] . 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.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

November 27, 2017

Q. Hi All,

Considering Black Zing plating for a clamp mount design for a task light. Wondering how much control you get over gloss levels with this process? Is it possible to have a semi-matte finish, or are we limited to glossy?

Thanks in advance

sergio silva
- brooklyn, New York USA

November 28, 2017

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 Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

January 2, 2018

Q. We are building a wooden bench in our back yard and want a black finish on the bolts/screws we use to hold the bench together. What would you recommend for the type of bolts/screws with what type of black finish to get the longest life?

Tom Murray
- Portland, Oregon, USA

January 2018

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.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

February 13, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Can someone share with me the differences of zinc black and black oxide?

Catherine Ang
student - Singapore

February 15, 2018

(You might think this is TMI!)

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 no 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 fine.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
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October 12, 2018

Q. Hello all, I have a part that is currently black oxide for a finish against corrosion. Our supplier has indicted to change to zinc plate for better corrosion resistance, based on this thread that is the correct thing to do. However, some of our units are used in an oxygen environment, is there any concern with compatibility with black zinc plating and oxygen?

Carson Krajco
- Denver, Colorado, USA

April 15, 2019

Q. I require a black finish for some small mild steel plate components that will be handled by customers. I require mild corrosion resistance as well as a black finish. I have been recommended a Black oxide finish and a Zinc Blacking process. Black oxiding is cheaper, but does it leave a residue on fingers? This would be undesirable.

Robert Sedgeman
- Gloucester, Gloucestershire, UK

April 2019

A. Hi Robert. In my 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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Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

October 3, 2019

Q. Hello,
My company manufactures off-road truck bumpers and we're looking for a black plating process for ordinary nuts, bolts etc that will be a deep black that is corrosion and UV resistant. I've read that black zinc will change colors over time. Is there a good option for black hardware that is constantly exposed to the elements?

Thanks in advance,

Jake Holmes
Chandler Equipment, Inc. - Springdale Arkansas

October 2019

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. 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 :-)


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

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