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Metal finishing Q&As since 1989


Black Oxide on stainless steel: problems, corrosion properties, RoHS, etc.


Q. How to turn 18-8 fasteners black?
I need some black fasteners for a vintage car restoration (fastening interior defroster ducts) that I can only find in 18-8 SS. I have a hobbyist zinc plating kit. My plan was to zinc plate them and then black chromate the zinc. I have read that I need a nickel strike prior to the zinc plating. Is that really necessary? What happens if I don't do the nickel strike? My other option is to just spray paint them black.


Dave Christensen
- Tucson, Arizona
March 13, 2023

A. Hi Dave. You posted with a bad e-mail address so you may never see this, but for the benefit of other readers whose curiosity has been aroused ...

• Black oxided plain steel fasteners are probably available from specialty fastener suppliers even if local stores don't carry them.
• Stainless steel can be black-oxided, which is probably the best answer if these fasteners truly are available only in stainless steel.
• The reason that stainless steel must receive a nickel strike before zinc plating is that plating must take place on raw metal for a proper metallurgical bond, but stainless steel instantly oxidizes. The nickel strike is designed to simultaneously dissolve any oxides, and keep them dissolved, while applying a very thin layer of fresh nickel which you will be able yo plate upon. If you deposit zinc directly on stainless fasteners, it will chip off immediately because it has no metallurgical adhesion. Good luck! Regards,

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

Q. All,
I am having problems getting a consistant black oxide finish on 303 stainless steel. Any thoughts would be appreciated. My process is as follows:
Clean: 5 to 10 minutes
Pickle/25% HCL
Black Oxide-10 to 20 minutes using PX-5 solution
Spray Rinse
Oil-Water Displace

James Berry
- Colorado Springs
March 22, 2024

Ed. note -- Readers: Please offer generic answers only, we can't publicly compare proprietary products like Heatbath's PX-5 to Birchwood Technologies [a supporting advertiser] or EPI / Electrochemical Products Inc. [a supporting advertiser] or other proprietaries ( huh? why?)

⇩ Related postings, oldest first ⇩

Q. I am a Buyer for a company that manufactures gauges for high performance race and show cars. We have been having problems with the plating on some of the Stainless Steel screws we use. The screws are made from 18-8 Stainless and we want to use a Black Oxide finish.

Because these screws are sometimes used on white face gauges, we like to have them as dry as possible to eliminate any oil marks on the dial. When we have them plated using no wax or oil, they don't look very black.

There is another part we use that is made from 430 Stainless with a Black Oxide finish. Again, we specify no wax or oil in the plating process, but we never have any problems with these. Both are done by the same plater.

The manufacturer will not make the screw from 430 because it is too soft to use in a screw machine, but has offered to use 410. Is 410 easier to plate than 18-8? I know both 410 and 430 are magnetic, but 18-8 is not. Does that affect the plating process?

I am open to any suggestions you might have on this subject.

Thank You,

Scott Eakens
- Pleasant Grove, Utah

A. Hi Scott. 18-8 stainless means 18% chrome & 8% nickel. The most common 18-8 stainlesses are probably type 303, 304 and 316. They are non magnetic because of the crystal structure generated by this composition whereas 4XX stainless steels contain no nickel and are magnetic. The 3XX stainless steels are also more corrosion resistant than the 4XX.

I don't think their magnetism is the direct cause of them being more difficult to black oxide, but the nickel content probably is. I guess an easy way to look at it is that if a material is more corrosion resistant, i.e., less willing to participate in chemical reactions, it can be harder to perform chemical reactions like black oxiding on it.

But one more thing that may enter the equation though: on a rough matte surface, black oxide tends to look black; on a highly polished surface it tends to look more of a dark jewel blue color. Luck & Regards,
Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Need black oxide for large stainless steel panels -- treatment or or tanks

Q. China associate needs black oxide for large st. steel panels -- treatment or tanks
1. Can anyone provide information regarding black oxide treatment for large stainless steel panels 3'-4' x 6'-9'?
2. Can anyone provide information regarding the possibility of hot black oxide treatment without tanks -- i.e. heating the stainless steel directly and applying the solution?
3. Can anyone provide information regarding (polypropylene? mild steel?) tanks suitable for black oxide treatment -- 6 or 7 tanks to accommodate large stainless steel panels 3'-4' x 6'-9'.

Thank you.

JP Proskauer
Architectural - Hicksville, New York

A. Hello, JP. True black oxide is a process conducted at about 290 °F (the process solution is so salty that this is the boiling point). I have never heard of it being applied in wiping-style, only by immersion, and the temperature precludes polypropylene tanks. The tanks are usually steel with gas-fired burners underneath them.

There are "cold blackening" processes which we introduce in our FAQ about Black Oxide and Cold Blackening. Please make sure that cold black oxide is the process you want to do before investing too much effort in figuring out how to do it though -- it is a process of limited capability.

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

Q. Can anyone provide me with information on Black Oxide? I was told that this finish is actually a controlled rust finish which is controlled with Chemicals like oil for example. Would black oxide on stainless rivets not being oiled but rather submerged in water cause a problem? They have not been submerged yet, but are experiencing rusting? They have not been oiled since plater. Thanks so much!

Brandon Moran
Fastenal Company - Baton Rouge, Louisiana, USA

simultaneous replies

A. Hello.

Black oxide on SST would not normally get an oil treatment. Black oxide is a cosmetic finish and for the most part offers little corrosion resistance.

For better corrosion resistance parts should be passivated. Many military jobs require passivation prior to black oxide.

Choice of SST alloy also plays an important role as each alloy has unique properties.

Bill Grayson
- San Jose, California

A. Black oxide without oil or wax is a poor corrosion preventer that is not up to water submersion even with oil. Since it is on stainless, the SS will have a good corrosion resistance in plain water. The SS parts are possibly rusting because the plater did not adequately rinse them, or did not blow the parts dry immediately or stored them in an iron pan or they had iron imbedded in them from the header or other operation.

James Watts
- Navarre, Florida

White Residue Problem in Black Oxided Stainless

Q. I have a product that is made from 303 steel with a black oxide coating. The customer is placing them over a salt solution (not dipping just hanging) and we are getting severe corrosion of the coating and a white residue. The parts need to survive sitting on a boat at sea for some time. The steel seems unaffected. Is this a normal characteristic of the black oxide coating. I keep reading it is only mildly resistant to corrosion under mild environments but I have not seen anyone really characterize this with any detail.

Another question, would this problem be any different whether or not I use a hot or cold process. Any help would be greatly appreciated.

We are now considering using black chrome instead. Would this be a better coating to apply for the type of environment the part will be in. Thanks

Armando Figueroa
Mechanical Engineer - Carlsbad, California
October 8, 2008

A. Hi, Armando. Hot black oxide is a slightly modified form of rust, so it doesn't take much in terms of a corrosive environment for it to prove unsatisfactory. Hot black oxide is better than cold black oxide, but not by enough to be considered corrosion resistant. Still, if this is a hot black oxide process, it may need a "boil out" which is lacking. Whether hot or cold, it's the oil on it that offers corrosion resistance.
Black chrome is much better but requires a substantial underlayer of nickel plating. The best black coating in your instance might be salt bath nitriding..


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

Q. Our customer has raised an issue relative to a black oxide requirement on a 304 stainless steel spring holder per MIL-DTL-13924 [on DLA], Class 2 or Class 3.

Since the baths contain hexavalent chromium compounds as well as the 0.06% chromic acid dip, will we have some percentage of hexavalent chromium within the iron oxide layer?

The oxide layer generally has a uniform thickness of about 0.000020 inch. The program requirement is the RoHS 0.1% maximum criteria for hexavalent chromium.

Ronald Mazzarella
Buyer - Wayne, New Jersey, USA
March 12, 2009

A. Hi Ronald. It sounds like a tough calculation, but the last step in the process, deliberately drying hexavalent chrome onto the surface of the parts (even though dilute), sounds like direct violation of the spirit of the RoHS law even if it complies with the letter of the law.

Even if you measured the chrome content, and it was below the RoHS limit, all it would take is for one part to be tested and found not in compliance for you to have an awful lot of 'splainin to do. I think you should suggest to your customer that this really doesn't sound like a good finish to use if you must have RoHS compliance :-)
Luck & Regards,
Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Need black oxide on stainless steel wire

RFQ: I need to black oxide to MIL-DTL-13924 [on DLA] CLASS 4 on 0.020" diameter 304 stainless steel wire in large quantities. I am assuming that this will have to be a reel to reel process to get adequate coverage.

Does anyone know someone with capability to do this?

Steve B [last name deleted for privacy by Editor]
Manufacturing - Prairie Grove, Arkansas, USA
December 8, 2009

Ed. note: Sorry, this RFQ is old & outdated, so contact info is no longer available. However, if you feel that something technical should be said in reply, please post it; no public commercial suggestions please ( huh? why?)

Q. Does anyone know if black oxide coating alters the corrosion resistance properties of 316 (or 316L, or any other) stainless steel? I'm curious as black oxide contains iron compounds (and iron is not corrosion resistant).

Alvin Vue
Engineering - Minneapolis, Minnesota, USA
February 18, 2010

A. Well you know what that is something that no one has ever asked me and for that matter I have never thought about. The only reason I ever applied black oxide to stainless was to make it look black. I had always assumed that the black finish formed on stainless was the same as that formed on chrome molybdenum steel, magnetite or Fe3O4, but after reading your question and thinking about it I am not even sure if this is the case. My education in this particular case is simply inadequate. I can tell you for a fact that if you took a stainless object that is blued by conventional methods and an un-blued object made of the same material and exposed them to the elements for a period of time (I mean the normal climate found in Alberta Canada), the black finish on the blued object would degrade, dull and in some case even pit slightly. The un-blued object would for the most part remain unchanged. In an ocean front environment both objects would suffer although the blued example would suffer much worse. I'm just a gun plumber and I'm drawing on experience here not education. I think we need a chemist !

rod henrickson
Rod Henrickson
gunsmith - Edmonton, Alberta, Canada

A. If the coating is properly applied, it shouldn't decrease the corrosion resistance of the stainless. However, I have seen many cases at a particular shop where significant red rust and pitting on the stainless occurred during a 2-hour salt spray exposure. This obviously puzzled us, but after a lengthy investigation, we discovered that the problem came from the use of aluminum oxide blast media that had been contaminated with iron through use. The blast media apparently embedded this contamination which ended up corroding. When we switched to a no-blast pretreatment procedure or to a virgin media pretreatment procedure, the problem went away.

Jon Barrows
Jon Barrows, MSF, EHSSC
GOAD Company
supporting advertiser
Independence, Missouri

Stainless steel blackening at room temperature

Q. We are looking for stainless steel black oxide in room temperature process by meaning of normal temp 25-30 °C.
At present we are using the blacking solution form Europe, Taiwan but the finishing components still not good at all.

Jaroon Ton
Director,Engineer - Thailand
August 11, 2010

A. Hi, Jaroon.

Cold blackening is less robust and reliable than hot blackening, and blackening of stainless is more difficult than blackening of plain steel, so it's not too surprising that you are encountering some problems. But we have dozens of threads on line about blackening of stainless steel. We'll need to start with what alloy you are you working, what heat treatment it has received if any, what is its surface condition, what wear and corrosive conditions it will see, and (perhaps most importantly) why you are blackening it. It is possible that there is a cold dip that will satisfy your needs, or that anodizing of stainless steel, or plating with a blacken-able metal may be more appropriate. But you'll need to define those needs for us please. Thanks. Good luck.


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

Q. We are having a hell of a time getting consistent results with black oxide (MIL-DTL-13924 [on DLA], ty 4) on 300 SS fasteners. Lack of adhesion and inconsistent color, brown to grey. Using a well respected company. I was told years ago that some heats of 300 SS just do not react well to the black ox process. Attempts to re-black them only makes things worse. Anyone familiar with this and the suspected reason about different heats of the material? More importantly, any solution?

Thanks folks!

Al Mueller
- New York, New York
December 14, 2022

? Which 300? They're all different beasts. 303 has high sulfur content and often is an issue, for example.

You need to really break down your issue into sources such as:
- grade of raw material
- batch of raw material (and/or supplier of raw material)
- batch of black oxide processing

For example, are some parts in a batch blotchy and others are fine? Or is the batch run on Tuesday fine and the one run on Wednesday all terrible? Or do you have good parts for a week but when you get a new batch of raw material it all goes to mud?

Get whoever is in charge of supplier quality to push back hard on your supplier, too. A lot of this information should be coming from them.

Jenna Tong
- Melbourne, Australia

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