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Black zinc plating vs. Black oxiding


(2003)

Q. WHAT IS THE MAJOR DIFFERENCE BETWEEN BLACK OXIDE AND BLACK ZINC?

THANK YOU,

JARED MORIN
MACHINE SHOP ENGINEER - LIMERICK , MAINE, USA


(2003)

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, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



(2006)

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


(2006)

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, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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)

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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?

VIMAL RAJU
- INDIA , TAMILNADU, COIMBATORE


October 2014

A. Hi, Vimal. I seriously doubt it. If you were to review this whole 60,000-thread forum you would find hundreds if not thousands of threads of the form: "I want my thingamajig to look like it's made of xxxxxx but I actually want to use yyyyyy -- how can I make yyyyyy look like xxxxxx?". People are constantly busy trying to make things look like something else. Although the finishes you named might tend to look slightly different, and some experienced platers could probably get the identification right 90% of the time, after almost 50 years in this industry I personally have zero faith in visual identification of finishes.

Luck and Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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!

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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 does the purchase order and its related specs call for. That's the starting point for incoming inspection. Trying to inspect to anything else is only the starting point for a contract dispute :-)
Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
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?

FARHAN SIDDIQUI
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 our ability to help with an abstract question is very limited :-)

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
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 (desperately).

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
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.

Regards
Deepak

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 / dla.mil] . 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.

Regards,

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

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