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Black zinc plating vs. Black oxiding

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

THANK YOU,

JARED MORIN
MACHINE SHOP ENGINEER - LIMERICK , MAINE, USA


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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.

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 nice appearance, but it does affect the dimensions enough to be a problem on very tight fitting parts. Luck and regards,

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


++++++appended

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


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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 at least 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 even with this minimal thickness spec.

pic of Ted Mooney 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 a large 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 doesn't support 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 sits in oil or grease, the phosphate and oil may be better, whereas if the screw is exposed to the air, the zinc plating may be better... again only a very general guideline.

Regards,

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

First of two simultaneous responses -- 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


Second of two simultaneous responses -- November 24, 2012

thumbsup2Thanks. 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

Hi Troy. That Chinese shortsightedness is very depressing, but may explain the worthless painted 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 -- an endless list of millions of tons of rusting garbage that only lasts a year or two when it should and easily could last 15 years if properly phosphate pretreated before painting. All of this continuous mining, smelting, manufacturing, painting, packaging, shipping, landfilling -- needlessly replacing this stuff every two years or less -- and they are focused on the milligrams of phosphate instead of the tons of steel? :-)

I support many environmental groups and efforts, but it seems almost hopeless when we should be focusing on making things last 15 years instead of 2 years, and we already know exactly how to do it -- but we harass the few manufacturers who use the necessary pound of phosphate and do the proper pretreatment instead of the manufacturers who skip the pound of phosphate and thereby ship 100 tons of soon-to-be-garbage steel products :-)  

AAAAAARRRGH!!! <-- (primal scream)

Regards,

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



September 21, 2013

Hi,

I have several question 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.

Regards,

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