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Hydrogen de-embrittlement cycle for fasteners



Q. We buy fasteners of 10.9 & 12.9 class from our vendors. These fasteners are zinc plated. We are facing a problem of delayed failure of these bolts. Our vendors are following hydrogen de-embrittlement cycle as per IS1367 Part 11. In spite of this we are facing bolt failures problem. We would like to know whether any other standard or alternate process is available to overcome this problem of hydrogen de-embrittlement. We also would like to know whether there is any kind of testing/inspection available to test fasteners for hydrogen embrittlement relief treatment.


- Bangalore, Karnataka, India


A. Typical approach of platers that do not know the difference or do not care is to pickle the parts in acid for far too long because it makes their life easier. If 1 minute in the acid will not work, then they need to be precleaned better. This is supposed to be an activation step, not a pickle. Next is the common practice of waiting until you have a full load for the oven before baking. They need to get into the oven (with the heat on) as soon as possible. The bake cycle will remove a bit of the hydrogen, but can not really get to deep attack from setting around all day.You will have far less problems if they will do the above. When a current bake cycle does not work, extend the bake time. Also thermocouple the middle of the load in the oven. It can take hours for the middle to reach full temp. There is equipment for testing hydrogen embrittlement in a bolt, but it is very expensive and requires tender loving care in its operation. Notch bar testing will help a lot, if it is handled exactly the same as the bolts.

James Watts
- Navarre, Florida


A. James Watts provided you with some excellent tips for better control of the plating process. I wanted to add that grade 12.9 fasteners are extremely susceptible to embrittlement and stress corrosion cracking, which is the reason that they are not used in the automotive industry. Also, there are a number of standards available on this subject-- how to reduce the susceptibility, embrittlement relief procedures, tests for embrittlement, etc. ASTM has a great publication with many of them compiled together with other industry standards. The publication is called Mechanical Hydrogen Embrittlement Methods for the EVALUATION and CONTROL of FASTENERS.

Toby Padfield
- Michigan


A. You might want to look into having your fasteners Mechanically plated. This process is free from hydrogen embrittlement. Another advantage of mechanical plating is that you can vary the thickness of the coating, any where from 5 micron to 50 microns.

Simon Cook
- Melbourne, Victoria, Australia

Mechanical Hydrogen Embrittlement Methods for Evaluation and Control of Fasteners 2001


Q. The original question states that there is a problem with a delayed failure of these bolts. Is it one or two bolts in a 'batch' or is it the whole batch that fails after some time? If the first, How can one test for HE if it's only one or two, that are going to fail? Also, What are the distinctive features of a HE fracture?

Bob Howell
- Toronto, Ontario, Canada


Q. We need a simple test for checking the components are hydrogen de-ebrittlement has been carried out or not.
Please reply.

R V Vijay Ganesan
manufacturing of auto components - Coimbatore, TamilNadu, India

July 23, 2008

A. Sustained load test as ASTM F606 [link by ed. to spec at TechStreet] is a good test to prove the proneness of hydrogen embrittlement failure at fastener level. Sample size to be determined based on the batch size undergoing the surface treatment. Also if the UTS is more than 1350 MPa it is better to go with vacuum deposition techniques rather that plating.

Thomas Kurian
- Trivandrum

October 3, 2008

Q. Similar problem is being traced out at our end.

We are doing Zinc (Green) passivation on the Class 12.9 bolts.

2 of such cases are found during in-house assembly.

My question is how we can trace that the bolts are properly de embrittled or not?

What is the easy method for ensuring this.

Deepak Mumbare
- Pune, India

June 23, 2012

A. Dear Ganesh!

Hydrogen embrittlement chances in >10.9 grade fasteners are very high due to acid treatment while plating process.
You can reduce the hydrogen embrittlement severity by processing the fasteners at 200 °C. for 4 hrs within 1 hr of plating process completion.

You can test the components by dipping in liquid paraffin at 180 °C. Bubbles coming out from components shows No/ineffective hydrogen de-embrittlement.

Amit Behl
- Hosur, Tamilnadu, India

Paraffin test for hydrogen embrittlement

August 17, 2014

Q. Dear Mr Amit, or all,

I am facing similar problem in 10.9 grade studs,

Paraffin test -- is it a proven one or is it said in any standards?

If so, is paraffin test to be done before plating, or after plating, or after passivation (adding brightness to the fastener?

Kindly explain.

C kesavaram
- chennai, tamilandu, INDIA

September 9, 2014

A. Greetings! You would find the thread starting at letter 1216 on this great website to be instructive. Regarding the paraffin test, I know of no standardized procedure for this test. In my humble opinion, the reason that I have never seen one is that the test does not have any predictive value with respect to hydrogen embrittlement. There is a similarly undocumented test that consists of immersing the article in hot silicone. I don't think it works, either.

And why not? Because what you are seeing as outgassing does not and cannot embrittle the part. It's gone. Porous deposits such as mechanical plating and phosphating will outgas; that's why they are hydrogen-embrittlement-free.

Many experts in hydrogen embrittlement feel that it is monomolecular hydrogen and not hydrogen gas (as seen in the paraffin test) that causes hydrogen embrittlement by reacting with the alloy at the grain boundaries.

If you want a test that really works to see if the parts are embrittled, put a reasonable sized sample of parts under load (75% - 85%). If they break within 48 hours it is probably hydrogen embrittlement. To be sure, look at the fracture face under 500 - 800 x magnification; if the fracture face has a "rock candy" aspect it is certainly hydrogen embrittlement.

Keep in mind that ASTM B850 [link by ed. to spec at TechStreet] sets forth the position that baking reduces the risk of hydrogen embrittlement. Baking does not eliminate the risk. But this specification does have recommended baking schedules for those who are comfortable with the inherent risk.

If you want a process that will not embrittle the articles, you should be using phos and oil, or, if you need sacrificial protection, mechanical plating (as Simon Cook says), or (at a higher cost) dip-spin zinc-rich coatings, which these days are often applied over a base of dry zinc phosphate.

In my humble opinion, all fasteners with a Rockwell 'C' hardness above 32 should be finished with a hydrogen-embrittlement-free process. If you do plate and bake, for every point you go up in Rockwell, the risk goes up.

And if your articles are under 32 Rockwell 'C', you can stop worrying about hydrogen embrittlement.

tom_rochester Tom Rochester
Plating Systems & Technologies, Inc.  

Jackson, Michigan, USA

September 2014

thumbsup2Very informative Tom. Thanks.


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

October 5, 2014

A. Hi Tom,

I disagree with your statement "if the fracture face has a 'rock candy' aspect it is certainly hydrogen embrittlement".

Tempered martensite embrittlement (TME) and stress corrosion cracking (SCC) will also show a brittle fracture surface.

A brittle fracture in combination with intergranular decohesion ("cavities") is generally believed to be caused by hydrogen. However, we have evidence of fracture surfaces which had never been exposed to any form of hydrogen and showed this same picture...

Best regards,


Jeroen Visser
- Geldrop, The Netherlands

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