Black Oxide vs Phosphate

A. Hi Abdulla. No it's not. We've now added a FAQs & TUTORIAL to this page which will answer you.

Luck & Regards,

A. Hi JP. If you are certain that it's an either/or situation, I think a quick test can be devised ... maybe as simple as a sample board for visual identification.

But if they are a mishmash and might also be black zinc, black nickel, black electroless nickel, black chrome, PVD, autophoretic, cathodic electrodeposition, etc., sorting them could be a totally impractical job.

Luck & Regards,

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"Optimization of Phosphating Process" by Vinod Prasad Sharma (2022)
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"Phosphating of Metals" by Werner Rausch (1991)
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"Phosphating & Metal Pretreatment" by D.B. Freeman (1986)
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A. Hello Cintia:

Below is a quick list of differences and likenesses.

1. Slight dimensional change with Phosphate vs none with black oxide.
2. Phosphate and oil affords more corrosion protection than black oxide with same oil.
3. Castings can be difficult to black oxide. Sometimes they can turn reddish to mahogany color in the black oxide process.
4. Phosphate runs at roughly 180 degrees F while black oxide runs at 290F.
5. Both processes have similar pre-treatment steps.
6. Black oxide gives a deep black color while phosphate coatings are grayish. Black oxide is more abrasion resistant than phosphate.

A. Hi Steven. It can't be said with 100 percent confidence, but I'd give it about 95 percent that the matte finish is phosphate and the glossy finish is black oxide. Good luck.

A. Hi Al. Hot black oxide with wax doesn't offer much corrosion resistance, but should be okay for indoor use if waxed. It will be cost competitive with high quality pretreatment and painting, but not with some of the junk that is out there. Black chrome plating would be a nicer and more durable finish, but much more expensive. Good luck.

Regards,

A. Hi Pradeep. First, you need to have some sample parts run and put into use before you think about changing production systems. Regardless of what you read, actually working with the parts will be critically informative. As the first quick observation, for example, I think you will find that cold blackened parts will be quite smutty; that might be no big deal or it could be completely unacceptable.

Please note that cold blackening is a completely different process than hot black oxide.

What are the parts made of, what is their function, and what environment will they see? Are they immersed in oil when used? Are they painted before use? What kind of phosphatization is currently done on them (iron, zinc, manganese)?

Finally, why might you want to do this? With full details I am confident that we can give you a good answer. Thanks.

Regards,

Q. Sir, Thanks for reply. We are Transpower coupling manufacturer. Our 70% components are made of Cast iron and rest of Mild steel (EN 8, 19, 24 etc.)

We currently having Zn Phosphating process along with Oil Sealing. Our couplings are exported by sea.

Our main purpose to switch over is visual look & cost. Please reply.

A. Hi Pradeep. Both hot black oxide and cold blackening have been used on such components. They will probably not have the corrosion resistance of your current components, so it's hard to predict without investigation whether they will survive sea transport.

Regards,

Q. Sir,
We are here to reply with clarification.
Also, we would like know shade difference than normal, i.e., greyish to black. In same basket some components are showing black shade while some greyish shade.
Is this shade difference going to change salt spray life?

Pradeep

A. Hi Pradeep. Sorry, but I am getting more confused. Have you switched from zinc phosphating to cold blackening or have you not switched?

May we assume that the components in any given basket are always the same part number, size and alloy? Is the problem as follows:

If this is the case, there is obviously something different in the alloy of the parts or their history.

Your question of whether the gray parts or black parts have a different salt spray life is backwards: the purpose of the salt spray test is to assist in process control. i.e., if there seems to be no reason for the color variation, and the variation does not represent a problem for you in some other way, you salt spray test the light grey parts vs. the blackish parts, so you now know whether the color variation is important. If there is no difference in their salt spray life, darkness becomes a factor that you can ignore.

I suspect that the process is not yet actually under control and as you study it you will find that what you have been told is incomplete. Good luck.

Regards,

A. Hi Geeta. "Blackodizing" is a term often used in India to mean the same thing as the USA term "Black Oxiding". We appended your inquiry to a thread which compares blackodizing and phosphating.

Regards,

A. Hi Paul,

Since the two coatings are so different in form and function, it's best to compare the pro's and con's of them. Since you stated that you're coating punches and chisels, neither offer much in the way of abrasion resistance, so after a few uses both coatings would be worn. The black oxide coating offers almost no corrosion protection on its own and relies on a top coat of wax or oil. The phosphate coating, either in manganese or zinc doesn't offer much more, except that the crystalline structure of the coating tends to hold more of the top coat than the black oxide. Also, the black oxide has no discernible change in dimension while the phosphate coating will slightly.

A. Hi Mike. It's generally a truism that if you are relying on several strengths of a particular finish or coating, it's probably the right one; but if you pick a coating which is not known for strengths in the qualities you seek, or the finish exhibits strengths you don't need, it may not be the ideal.

Black oxide offers nowhere near the corrosion resistance of powder coating, but it will exhibit mild corrosion resistance if it's waxed or oiled. Wax or oil are not a problem for you? On the other hand black oxide causes no dimension change unlike the couple of thousandths of an inch which thermoset powder coatings require ... is that important to you? Abrasion resistance is fair; Most gun barrels are black oxide to give you a qualitative idea of it's abrasion resistance.

Other choices might be black nickel, black chromium, black zinc ... or salt bath nitriding for great corrosion resistance and abrasion resistance. Good luck.

Regards,

A. Hi Mike. I don't think anyone will actually "recommend" a specific coating for a specific application which they are not familiar with, any more than an engineer would recommend a specific wide flange beam size based on rough internet information. But if wax or light oil sounds okay, then black oxide doesn't sound inappropriate offhand; try to get a sample piece from a black oxide shop for some tests. Many will do a free sample for industrial customers. Good luck.

Regards,