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topic 13935

Black oxide is rubbing off

A discussion started in 2002 but continuing through 2019


Q. We are have been using a stainless steel handle w. black oxide finish on our electronics assemblies. Recently we have had batches of these handles come in with the black oxide finish rubbing off and the handle looking splotch-y. Based in what I know about black oxide finishes they should be more robust than this however I have recently heard that this problem can happen with cold-process black oxide.

Is this normal?


Steve Ferranti
- Dallas, Texas

simultaneous 2002

A. It's only a guess, but it does sound like your plater has switched from a hot process to a cold blackening process, since these can lead to smutty, marginally adherent coatings. These are your parts, and your plater should not have carte blanche to change the process applied to your parts at his whim. Find out what he was doing, and what he is doing, and write a spec that makes him do it the old way.

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


A. This is common with cold blacks. It can also happen with hot blacks for stainless, especially if surface is not properly activated.

Steve Loudermilk
- Springdale, Arkansas

June 13, 2013

Q. For about the past month the product we put through our black oxide tanks, the black rubs off but it's just the surface film. Dry touch or regular oil, it still does it. Any ideas?

Mike Find
- Buffalo, New York, US

Black process leaves color on hands

October 19, 2015

Q. We are manufacturer of fasteners. I am working at plating shop and facing problem regarding black oxidizing process on nuts bolts. Solution contents are:
Tank volume 400 ltr
Sodium hydroxide (caustic Soda) 75%
Sodium nitrate 25%
Solution temperature 110°C

Process we are doing:
Acid pickling for 20 min
Water rinsing
Process for 10 min
Hot water (not boiling) rinsing
Coating of rust protective oil
In our process the material leaves black color on hands. After cleaning with cloth the material's color comes perfectly OK.

Please guide me why the color leaves in unloading?

arshad masood
plating shop employee - karachi, sindh, pakistan

Hot Black oxide on Stainless Steel is rubbing off

August 23, 2017

Q. Hello, my name is Dan. I am a shop/Fabrication manger for a company that makes mounting hardware for glass. I have been doing Cold Black oxide in my shop for a few years now and I can get a satisfactory product NOW (a lot of time and work put in). That being said, it is very time consuming and labor intensive. So I decided to outsource to a company that does HOT Oxide as my company is expanding.

They were able to give me a sample and it came out great. I have now sent them a larger order and they are having issues with the oxide taking to the pieces. Now I know the first question is going to be about the processes they are using and I have seen them and am confident they are doing it correctly with the process; I cant comment on the the times, temps, or formula they are using but they have been open and oxidizing for 40 years ... I think they should know by now. My question is that I have done as much research as I can and have found a few possible problems but none have been the issue yet. They (my plater) tell me that the Stainless steel must be contaminated but I have the results from when it was made and tested was all fine. They are Dipping 4 pieces that assemble into one hardware at the same time in the same tank and 1 of 4 are coming out flawlessly the other 3 just rub off the black when touched. I'm hoping someone has some insight on my problem. I am willing to answer any questions to clarify; just hoping I have provided enough info. Thanks. Hope to hear from someone soon.

Parts are,
304 stainless all parts
High speed machined
using proper cutting oils (water soluble)
approx. round 1 1/2" (dia) x varying lengths all under 3"
Through holes on 2
Blind holes on 1
Screw is the part that is coming out great
Some holes are threaded

Dan B [last name deleted for privacy by Editor]
Product Designer - Etobicoke ON, CANADA

August 24, 2017

A. Hi Dan,

You are correct that the first question will be about the process. Black oxide over stainless is an art that typically requires some effort in tweaking parameters when starting a new job. I'm guessing that the coating is very smutty and leaves the part only matte gray when wiped off? If that's the case, what are they using to pickle, what's the concentration, and what's the immersion time? I would look into cutting the immersion time a bit as a first step, and even play around with cutting back your process time in the black oxide bath. When it comes to blackening stainless, usually less is more.

Jameson Grout
- Springfield, Massachusetts USA

September 5, 2017

A. Dan,

Jameson is correct, excessive time in the black can cause the finish to rub off. We no longer perform this process (mainly due to all the similar issues we faced as a job shop), but one other possibility comes to mind. Since you have already stated that all of these parts are made from the same material, is your plater using different types of tooling for these various parts (hooks, baskets, wire, etc)? Dissimilar metals or a mix of metals can sometimes cause adverse reactions in these baths.

Bruce Brady
- Lincoln, Nebraska USA

October 12, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

I deal with hot bluing process, so after blasting specially in case of rework, I carried out the bluing process, it gets blued good finishing but color removes didn't stop, as much you apply the oil and just keep on cleaning, color still removes, so how can I avoid this.

Thanks & Regards,

Aijazulla Tajir
- Abu dhabi UAE

November 3, 2017

A. Aijazulla,
When hot blackening steel there are three common causes of rub off: boiling temperature is too high, slow transfer time between black oxide bath and the rinse, and high colloidal iron built up. As I mentioned in a previous post, blackening stainless exhibits smutty rub off more often but this is usually related to pre-treatment or process time.
The topcoat oil has no effect on this and you can't clean it off.
In my experience rework parts don't exhibit rub off more often than virgin parts. Are you adequately removing the topcoat oil before blasting off the oxide? Its a stretch but if you're embedding oils during your stripping process that could interfere with the black activation. I've seen cases where a lubricant your degreaser can't remove can lead to rub off.

Jameson Grout
- Indian Orchard, Massachusetts

May 24, 2019

Q. I'm having some parts black oxide coating. The same place has been doing them for about 10 years now, and I've never experienced this 'issue' before. Twice now it has happened. The last time, they said there was an error in the curing process, redid them, and they looked just like always. This last time they are telling me there is no issue, and they are within spec.

The parts are not oily, and there is a black powdery substance on each part that wipes off. The material is grade 50 steel, which is already black, so I'm not 100% if these are even coated once you wipe the black soot looking material off.

Any thoughts?


Wayne Hartwig
- Spokane, Washington, USA

May 2019

A. Hi Wayne. I suggested earlier on this page the possibility that a black oxiding shop may have switched from hot black oxide to cold black oxide because cold blackened parts are often or usually smutty, with some rub off because the process involves the application of a black selenium compound, and actual oxidation of the steel is not the only (and possibly not the main) source of the black color.

When you say your supplying shop never gave you smutty parts for 10 years but now insists that smutty parts are normal, I wonder about the same thing again. But I have no actual hands-on experience with these processes, so hopefully a reader who does can suggest whether you need a heart-to-heart with your applicator about what changes he's been up to, or whether Mooney is just a broken record and there are better explanations for the blackening rubbing off :-)


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

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