-- The Home Page of the Finishing Industry
A website for Serious Education, promoting Aloha,
& the most FUN smiley you can have in metal finishing

on this site
current topics
topic 7605

Black Oxide vs Phosphate

A discussion started in 2001 & continuing through 2017


Q. I am bringing up a product line from Forest City, North Carolina to the plant here in Elyria, Ohio. In NC, they phosphate coat their bodies; and in OH, we black oxide our bodies. What are the pros and cons to using a black oxide finish vs. a phos finish on cast iron parts?

Cintia Ulloa
-Elyria, Ohio, USA


Phosphating Metal Pretreatment
by Freeman

Phosphating of Metals
by Rausch

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.

Heatbath Corporation has vast experience with both black oxide and phosphate coatings. Please contact us for further details.

Tom Walen
Heatbath Corporation
supporting advertiser
Springfield, Massachusetts
Heatbath banner

Friction characteristics of black phosphate vs. black oxide


Q. We use a spring clip which calls out "black phosphate and oil or 2nd choice - black oxide (bake to relieve hydrogen)". Sometimes the clips have a matte finish and sometimes they have a gloss finish.
The matte finish provides a little more friction which is desirable in our application.

Is the phosphate or the oxide providing the matte finish?

Steven E. Miller
medical equipment manufacturer - Indiana, United States


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.

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

April 16, 2015

Q. Hello, I am designing a small (indoor use) side table with a steel leg base composed of 1" tube stock steel welded in a 2' x 1' x 1' frame. I am interested in this black oxide treatment for my steel leg frame as it will resist rust but will retain a more "industrial" or "natural metal" look compared to a powder coat or other finishing methods I have entertained. Would this be an appropriate treatment? Also important, will this be cost effective?

Thank you very much for your time and expertise in advance.

Al Stewart
Designer/student - Lexington, Kentucky, USA

April 2015

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.


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

April 8, 2015

Q. Sir, We are using Hot phosphating process (class A1) for our finish products.

Now we are thinking to switch to Room temperature black oxide coating process.

Can you guide what may be impact if we switch to process.


Pradeep Tanksale
- Pune, Maharashtra, India

April 2015

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.


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

April 21, 2015

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.

Pradeep Tanksale [returning]
- Pune, Maharashtra, India

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.


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

July 17, 2015

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 Tanksale [returning]
- Pune, Maharashtra, India

Trouble in Your Tank
by Larry Durney

July 2015

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:

"a basket is filled with identical parts and carried through the zinc phosphating process. The parts continue to look identical until they enter the zinc phosphating tank. When they are removed from the zinc phosphating tank, some are greyish but some are very dark, almost black. There is no pattern to it, the black parts may be at the top, bottom, or middle of the basket?"

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.


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

Blackodizing vs. Phosphating?

December 4, 2015 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. What is the difference between Blackodizing & Phosphating?

Geeta Arvind Khandkar
- Pune, Pimpri-Chinchwad,

December 2015

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.


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

October 25, 2016

Q. My company is looking to optimize the design and finish of our automotive chisels and punches and we are trying to decide between a black oxide and black phosphate finish. I have seen a number of threads comparing the benefits of the two coatings, but not for this specific application. Our manufacturer would prefer to use phosphating, but black oxide seems to be much more common for automotive tools in North America. Thank you kindly for your insight.

Best regards,

Paul Thornton
Automotive Tool Company Associate/Engineer - Vancouver, BC, Canada

March 25, 2017

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.

jim conner
Jim Conner
Anoplex - Dallas, Texas USA

This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site


Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices

©1995-2018, Inc., Pine Beach, NJ   -   About   -  Privacy Policy
How Google uses data when you visit this site.