Serious Education ... plus the most fun you can have in metal finishing.
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
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, P.E. RET
Pine Beach, New Jersey
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 VickersHarry Schrijen
- Hoensbroek, Netherlands
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, P.E. RET
Pine Beach, New Jersey
April 1, 2014
A. Dear Harry Schrijen
- Yerevan, Armenia
(2004) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. I HAVE BEEN KICKING THIS AROUND FOR QUITE A LONG TIME NOW AND CANNOT SEEM TO GET A VERY CLEAR ANSWER TO MY QUESTION. WHEN BLACK OXIDE COATING HIGHER ALLOY STRENGTH FASTENERS THAT ARE PLAIN AND ALSO ZINC PLATED IS A BAKE REQUIRED TO RELIEVE HYDROGEN?IHOR RAJCA
FASTENER DISTRIBUTOR - MIDDLEBURG HTS, OHIO, USA
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 [link is to free spec at Defense Logistics Agency / dla.mil] 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.
Syracuse, New York
Embrittlement of AISI 1074 due to black oxide?(2005)
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).
engines - Jenbach, Austria
Black Oxide 'bringing out' cracks?July 13, 2009
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...
Level II MPI Technician - Springfield Massachusetts, USA
July 25, 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 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 penetrate test, the kits are cheep and they work quite well. If cracks do indeed show up then it's a simple matter of back tracking your procedures and making change to the process or the materials. But as stated I have never experienced any cracking with caustic bluing.
Edmonton, Alberta, Canada
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.
Aerospace - Yeovil, Somerset, UK
Caustic embrittlement from black oxide processApril 15, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. Is caustic or hydrogen embrittlement possible in black oxide coating? What is the remedy to stop it ?Akhil Shah
bearing industry - Gujarat, India
May 24, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
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 AMS 2759/9 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?
Plating plant - Guadalajara, Jalisco, Mexico
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.
aerospace - Yeovil, Somerset, United Kingdom
Black oxide and hydrogen embrittlement relief bakeDecember 30, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
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.
plating shop employee - SANTA FE SPRINGS, California USA
January 5, 2015
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.
Plating Systems & Technologies, Inc.
Jackson, Michigan, USA
January 7, 2015 -- apologies, finishing.com misfiled this reply, and didn't post it until January 16
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.
Aerospace - Yeovil, Somerset, UK