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Metal finishing Q&As since 1989


Does black oxide process cause hydrogen embrittlement and require baking?

Q. We manufacture a flat spring for the caster industry and have had a problem with the spring cracking. Could we have a hydrogen embrittlement problem because of the black oxide process? Will baking these parts remove the coating.

Mel Shearer
tech design - Springfield, Ohio, USA

Q. Can a black oxide finish induce Hydrogen embrittlement on a steel part? I understand that zinc plating can be a cause, but question the possibility of black oxide.

Claudia Dougherty
- Sarasota, Florida, USA

"Hydrogen Embrittlement of Steel in Metal Finishing Processes of Black Oxide and zinc Phosphatize"
by R. H. Wolff

on Amazon

(affil links)


A. I think that the black oxide step per se would probably not cause hydrogen embrittlement, but any acid immersion in the preparatory steps well might. Baking should not remove the black oxide finish but might effect the wax or oil on it.

For a more authoritative insight, please see if your library can get you a copy of "HYDROGEN EMBRITTLEMENT OF STEEL IN METAL FINISHING PROCESSES OF BLACK OXIDE AND ZINC PHOSPHATIZE" .
Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. We're trying to apply a black oxide coating to 9254 spring steel. This is a high-silicon manganese steel. The coating produces a mottled brown color. Our finisher attributes this to the chemical composition of the steel. Do you have any suggestions for obtaining a blue-black finish?

Mike Frank
Automotive parts - New York, New York, US

This page contains a lot of interesting perspectives but is probably not a good tutorial for the semantic reason that the term 'black oxide' means different things to different people :-)

- To gunsmiths and metal finishers it means a hot (±260 °F) aqueous treatment for ferrous metals in caustic soda [affil links] & nitrates, sometimes alternately called "bluing", for the purpose of imparting an attractive dark blue (if the parts are polished) or black (if the parts are matte) oil-retentive finish.
- To metal finishers it can also mean colder, sometimes room-temperature decorative blackening processes, sometimes called "cold blackening" that rely on other mechanisms such as the deposition of selenium compounds for the same decorative reasons.
- To people in the steam boiler field it means a specific corrosion product in the boilers. Thread 7330 is a quick into.
- To those in the printed circuit field, "black oxide" is a roughening process done on the copper to improve lamination. Thread 26016 is an intro.

And all of them seem to have entered this thread :-)




A. I have seen and experienced in our failure assessments that the Hydrogen, generated from the Schikorr reaction in boilers (which is responsible for the black oxide [magnetite]) resulted in hydrogen embrittlement if the local hardness in the area of welds or internals is above 250 hardness Vickers.

Harry Schrijen
- Hoensbroek, Netherlands

thumbs up sign Hi Harry. Thanks for the intriguing reply. Mel and Claudia are talking about the black oxide process as a metal finish, and I believe that the nitrates providing the oxidizing power comprise a different situation which doesn't produce the nascent hydrogen of the Schikorr reaction. I certainly don't insist, however, and would appreciate clarification on this from anyone.


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

A. Dear Harry Schrijen
There are several things that can lead to Hydrogen Embrittlement. The common reason is heat treatment, especially "Salt Bath" process. The parts should be acid washed in a proper acid plus corrosion inhibitors and detergents. This pickling process will remove hydrogen completely. Another cause of Hydrogen Embrittlement is bad pre-treatment of blackening or any other electroplating process. It also refers to bad pickling after alkaline degreasing. In every acid washing process you have to have Corrosion Inhibitor (Organic Ammonium compounds) and Detergents (Like Sodium Lauryl Ether sulphate or SLS). And as Ted said heating can remove the Hydrogen also but as I know the temp. should not exceed 200 °C.
Good Luck

alireza reyhan
Alireza Reyhan
- Istanbul, Turkey
April 1, 2014



A. If the black oxide is formed by the hot alkaline process there is no hydrogen entrapment. Hydrogen is formed in an electrolytic deposition system at the cathode in acid solutions, e.g., Nickel plating, etc., so you have no worries with the black oxide.

Allan Bennett
- Hobart, Tasmania, Australia


A. Hydrogen embrittlement perhaps not, but most steel must see some sort of pretreatment prior to black oxide. If the pretreat includes cathodic cleaning or acid pickling, hydrogen embrittlement might be a concern.

A couple of other interesting thoughts on this are --
(1) the new MIL-DTL-13924 [on DLA] for mil spec black oxide requires post coating baking presumably for hydrogen embrittlement relief. Black oxide has such poor corrosion resistance by itself, we often end up with rusted parts right out of the oven! Oiling the parts prior to baking is a stinky, smoky mess that I wouldn't suggest.
(2) Helicopter manufacturer Sikorsky no longer uses black oxide due to what they claim was CAUSTIC EMBRITTLEMENT of high strength steel. I'm not positive that the two had anything to do with the other but it's interesting that the new mil spec and Sikorsky's "no black oxide" edict coincided within months of one another! I think there's a conspiracy here and it begs the question: is CAUSTIC EMBRITTLEMENT removed by baking like HYDROGEN? Any insight into either the mil spec rationale for baking or the phenomenon of caustic embrittlement would be appreciated.

milt stevenson jr.
Milt Stevenson, Jr.
Syracuse, New York

Embrittlement of AISI 1074 due to black oxide?

Q. Our supplier delivered us with electronic components, where spring clips (AISI 1074 annealed steel with 44-47 Rockwell hardness and black oxide surface treatment) have been used, which where not correctly baked. Now some of them are breaking due to hydrogen embrittlement.

We know that the initial strength capacity is reduced, but is there a long term effect of hydrogen embrittlement which would reduce the tensile strength and ultimate strength in the future (fatigue life capability).

Hubert Hilscher
engines - Jenbach, Austria

Black Oxide 'bringing out' cracks?

Q. Recently, the company I work for got a job where the customer asks for MPI (magnetic particle inspection) after a black oxide process. The engineers here decided that since we were not an approved source for the MPI for this particular customer, we would do an in-process check before the black oxide to see if we cracked them (very hard, thin walled part had been through copper, carburization, and I believe a couple of stress relieves).

At MPI, we found nothing. So we sent them along to black oxide, and then to our own outside vendor for MPI. The parts came back with cracks on a variety of different surfaces. The vendor told us they were 'grinding cracks', so the grinding department has been experiencing a little abuse. Everything they have tried yields a good product before black oxide, but we have about a 35% failure rate after the black oxide is done. Is it possible I'm missing something, or could it just not be there until after the black oxide? I'm trying to explain to the engineers about this caustic embrittlement idea, but since the outside vendor instantly said they were grinding cracks, they are having a hard time opening their minds to other options. If it matters, we are using forgings rather than a piece of bar stock, AMS 6274. When we review them after the outside MPI, they don't look like grinding cracks to me...

John Lapierre
Level II MPI Technician - Springfield Massachusetts, USA
July 13, 2009

A. Well its kinda hard to say what you are seeing. I suspect they probably are polishing or grinding marks. I have been doing professional bluing for 25 years and I must admit I have never had anything crack. I have blued some pretty weird alloys over the year too. Everything from super hard gun parts to drills, springs, taps, dies chrome moly, chrome vanadium and HSS steels. Mind you I am using sodium hydroxide and Sodium Nitrate [affil links] and the hottest the solution ever gets is 310 °F. A lot of machine shops or production shops blacken with molten potassium and I'm not certain how hot that gets but I once watched an idiot spit into one of the tanks and the reaction was quite spectacular! I suppose the simple thing would be to do a dye penetrant test, the kits are cheep and they work quite well. If cracks do indeed show up then it's a simple matter of backtracking your procedures and making changes to the process or the materials. But as stated I have never experienced any cracking with caustic bluing.

rod henrickson
Rod Henrickson
gunsmith - Edmonton, Alberta, Canada
July 25, 2009

July 28, 2009

A. Has the steel been carburised before you black oxided the parts? I had a quick look at the AMS as my initial thought was that maybe the steel would require a stress relief after grinding and found it to be a low alloy steel suitable for carburising.

If it has been carburised then you have a very hard casing around a soft core (in relative terms). Placing the parts into a hot caustic solution after grinding may well induce cracks in the surface.

I would suggest a low temperature stress relief after grinding, 275±10 °F for about six hours may help to mitigate some of the cracking you are seeing.

If the parts have not been carburised then I am not certain what is happening. The steel is of a low enough strength/hardness that you would not expect stress to build up in the surface and I can't imagine that you would be abusing the surface that badly during the grinding operation to cause actual cracking. You may be leaving striations, which may look like cracks, but these would be pretty well visible to the naked eye, so if you are inspecting these parts before you send them out you would probably catch them.

Here's an idea for you, if you have enough time to do it. Set off a couple of batches. MPI after each stage of the manufacturing process, e.g. after machining, grinding, black oxide. One batch try stress relieving as suggested above after grinding and then compare the results. Well, it's an idea to get you going.

Another suggestion is that you walk the process and see if there is something obvious amiss with your process route. Cheap and easy to do, but very effective at picking up some quite simple errors that you may well not have thought of.

Brian Terry
Aerospace - Yeovil, Somerset, UK

Caustic embrittlement from black oxide process

Q. Is caustic or hydrogen embrittlement possible in black oxide coating? What is the remedy to stop it ?

Akhil Shah
bearing industry - Gujarat, India
April 15, 2012

Q. Does Black Oxide, per Alkaline oxidizing process, impart hydrogen to the parts being processed?

Any help on this matter will be appreciated.

If we are not applying any acids prior to black Oxide, does the process generate Hydrogen on the part? and if so, is it dissipated thru Hydrogen embrittlement Relief or Baking?

We Zinc Phosphate and we have 2 options; either bake for 1 hour or keep the parts untouched for 48 hours, because the Hydrogen dissipates by itself, it's the same for Black Oxide? There is no metal finish on top of the part that inhibits the hydrogen from escaping, we don't see the need for baking.

Also, Spec AMS2759/9 in table 1 gives the baking times for various finishes, like Cadmium, zinc, Nickel, and chemical Treatment. Would Black Oxide be interpreted as chemical Treatment?

Sergio Hernandez
Plating plant - Guadalajara, Jalisco, Mexico
May 24, 2012

A. Hi Sergio,

There are effects caused by immersion in hot black oxide processes. Sometimes we are lazy and label it as embrittlement, giving the impression that hydrogen is involved. In the case of black oxide it has to do with the highly stressful nature of the treatment to the base material. The baking operation is more like a low temperature stress relief rather than a de-embrittlement operation.

As Milt has previously observed, Sikorsky dropped black oxide without replacement due to what they called "caustic embrittlement", probably as they had no better way of describing the failure. In my previous company we built up plenty of evidence that black oxide will drastically reduce fatigue life if the parts are not baked (can be as much as 80%, dependent on the operating conditions!)

Hydrogen embrittlement is a completely different story, is time dependent and I have never heard of allowing parts to just sit around for 48 hours to allow natural dispersion of hydrogen. If you are using a zinc phosphate on steels with a strength greater than 1100 MPa (160 ksi) or a hardness greater than 36HRC then you should be actively de-embrittling to prevent parts suffering from damage.

Brian Terry
Aerospace - Yeovil, Somerset, UK
May 30, 2012

Black oxide and hydrogen embrittlement relief bake

Q. Hello everyone,
I work for a company that provides metal finishing services in California, Our main product is Black Oxide process. My question is regarding black oxide per MIL-DTL-13924D AND HYDROGEN EMBRITTLEMENT BAKE PROCESS, We have a problem with black oxided parts coming out of the hydrogen embrittlement process turning RED, Is this red coloration acceptable for baked parts? It would be greatly appreciated if someone could answer my question. Thank you.

Basilio V.

plating shop employee - SANTA FE SPRINGS, California USA
December 30, 2014

A. Here is what is happening chemically: The black iron oxide, Fe3O4, is being oxidized (at least in part) to red iron oxide, Fe2O3. (This wouldn't happen in a reducing atmosphere). I wonder why the parts are being treated to reduce hydrogen embrittlement? First, black oxiding is not normally considered an embrittling process. Secondly, it is not typically used on high strength fasteners. Thirdly, are the parts over 32 Rockwell 'C' scale? If not, they are not subject to hydrogen embrittlement anyway.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.
supporting advertiser
plating systems & technologies banner ad
January 5, 2015

A. Hi Basilio,

Are you sure it is the coating turning red or is it corrosion of the base material, which is then consequently showing through the thin black oxide coating?

One common problem with stoving of black oxide is that if the parts are stoved with even a hint of moisture the base material will corrode, causing the reddening that you may be seeing. One way one of my previous companies got over it was to use a water displacing fluid after hot water rinsing and blow drying followed by a vapour degrease.

We have a habit of calling the stoving operation a hydrogen embrittlement relief operation. In the case of black oxide it is due to something that has been labeled as caustic embrittlement. Essentially the process is severely fatiguing (see previous posts on this thread) and the operation acts more like a stress relief operation than a hydrogen embrittlement relief operation.

Brian Terry
Aerospace - Yeovil, Somerset, UK
January 7, 2015

Can Spring steel (Hardened & Tempered) be blackodized?

Q. We have a flat press component made from spring steel (i.e., hardened & tempered). It's 1 mm thick. The customer expects it to be blackodized; however my vendor (i.e., who provides blackodising service) claims that spring steel components cannot be blackodised. Can anyone please let me know if it's true or not ?

Pravin Jain
- Mumbai, Maharashtra, India
May 29, 2020

A. Hi Pravin. Your metal finisher has perhaps seen the parts and I haven't, so I'm not anxious to disagree with him :-)

Instead we appended your inquiry to a thread on the subject from which you may be able to learn the answer, or or least the issues to consider. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
May 2020

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