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A Tough Lesson: Hydrogen Embrittlement Please Read If You Plate Hardware

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Fanclutchnut
Posts: 178
Joined: Jan 05, 2021 7:32 PM
Location: Westchester Co NY

A Tough Lesson: Hydrogen Embrittlement Please Read If You Plate Hardware

Post by Fanclutchnut »

You guys may know us from our other build thread viewtopic.php?t=156139
We’ve moved into the next phase of the project. The rear axle rebuild. About a month ago, we fully disassembled the rear subframe assembly. Our goal was to refresh anything that was obviously worn and refinish everything. Powder coating the subframe, trailing arms, brake disc dust covers and subframe support brackets. The hardware was to be plated. I didn’t have the easiest time finding a plater but eventually did. They offered trivalent yellow zinc with seal coating. After spending about 15 hours degreasing, sand blasting, wire wheeling all of the parts, I dropped them off. I picked them up 2 days later and they looked amazing. I checked in with a fellow forumite Jens (jhh925) who has been super helpful and supportive during our build. He informed me of something called hydrogen embrittlement. This occurs when the steel gets weakened by hydrogen molecules during the plating process.  It can lead to stress fractures in critical hardware. It can be avoided by baking the parts immediately after plating. 

After learning about this, I called my plater and he told me that they didn’t bake it because I didn’t ask. I obviously was really upset by this not only for the time and money, but the risk I might have been put in by using these fasteners. He was well aware that they were for an automobile restoration and didn’t recommend it.  

So, I’ll be spending the next few hours looking up all of the bolts on BelMetric.  Most of this beautiful hardware will be going in the garbage as a result.

I’m hoping that this story saves someone time, money and possibly something far more valuable.  

Super big shout out to Jens for the knowledge and help keeping us safe. 


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stuartinmn
Posts: 9584
Joined: Feb 12, 2006 12:00 PM
Location: Minneapolis

Re: A Tough Lesson: Hydrogen Embrittlement Please Read If You Plate Hardware

Post by stuartinmn »

I knew hydrogen embrittlement is an issue with chrome plating but wasn't aware it also occurred with zinc plating.  If the parts weren't baked immediately after the plating process, can it be done at a later date?  I'm wondering if you could maybe do it yourself now by putting them in the kitchen oven for the required time and temperature.  My brother is a metallurgical engineer who knows about these things, I just sent him an email and will let you know what I find out.

Edited to add:  this is my brother's response.

After zinc plating, there is an immersion treatment in a zinc chromate solution. This conversion coating renders the zinc basis coating more passive in corrosive conditions while maintaining protection of the steel. 

Chromate treatment formulations can be based on hexavalent or trivalent chrome. Since baking degrades the chromate film, it has to be done after plating but before chromate conversion coating. 

In principle, the fasteners could be baked now but he’d lose the benefit of the conversion coating. Or he could strip and replate. 

Also, hydrogen embrittlement is only a concern with high strength fasteners- SAE Grade 8 or equivalent. Depending on the fastener spec, the baking step may not be important.


In any case, I wouldn't worry about the various small clips, washers, and nuts/bolts used for low strength purposes.
///M
Posts: 103
Joined: Apr 17, 2009 11:21 PM

Re: A Tough Lesson: Hydrogen Embrittlement Please Read If You Plate Hardware

Post by ///M »

On looking a the nicely plated items my first thought was I will take them for free. The original bolts that were not plated originally were 10.9 and 12.9 grade bolts. As was stated by the metallurgist these are the highest tensile bolts which were most susceptible to the problem.
8.8 grade bolts were plated from new and that goes back to the 1970's when they were cadmium plated. Cadmium was banned in Europe  a long time ago and new processes were used. 
I have seen from another metallurgist a suggestion of torquing the bolts to a certain value below the yield strength and if they did not fail then they were ok. If you buy trailing arm bolts from BMW (grade 10.9 from memory) they are not plated so that is what the manufacturer went with but I am sure there  are a lot of cars with nice shiny bolts and even stainless steel which are not the correct grade along with issues of electrolytic corrosion where the plain steel is sacrificed to the more noble stainless.
I have plated items before and the yellow chromate can be easily removed after they come out of the solution and the idea was to leave them somewhere warm for 48 hours. I found that by heating them that the yellow did not come off.

The industrial process should be a certain temperature for a fixed period of time. I doubt that many places are doing it by the book as it adds to the overheads of the business.
The Don
Posts: 228
Joined: Sep 17, 2007 11:46 PM

Re: A Tough Lesson: Hydrogen Embrittlement Please Read If You Plate Hardware

Post by The Don »

I would agree with the above statement. Anything under 1000MPa (so anything P.C. 8.8, or 9.8, or without threads) is okay to use. Nuts that are P.C. 10 are likely safe to use as there is not a "neck" area like there is in a bolt for the part to catastrophically snap. 

A lot of the OEMs and the ISO standards ask for a certain amount of time/temperature depending on the hardness of the part plated. One OEM recommends the following:
HV320-HV380 (P.C. 10.9), 4 hours, T = 190-220C
HV380HV500, 8 hours, T = 190-220C

I don't remember any P.C. 12.9 hardware on the suspension, so you should not have any of that. 

Even with the bake, the plater would have to perform some testing to understand if the bake worked or not. That testing is usually destructive. So what I am getting at, is the bake can definitely be done for the piece of mind, but without testing, it is not 100% guaranteed. You can certainly have the high hardness fasteners stripped, re-plated, baked, and chromated for piece of mind. 
Fanclutchnut
Posts: 178
Joined: Jan 05, 2021 7:32 PM
Location: Westchester Co NY

Re: A Tough Lesson: Hydrogen Embrittlement Please Read If You Plate Hardware

Post by Fanclutchnut »

I appreciate all the comments and info. I am going to use all of the hardware that is for non critical attachments and fixtures. Anything that sees direct force will be replaced. That will include differential, trailing arm, axle flange, driveshaft, shock, sway bar, brake carrier mounting bolts and nuts. 
Shawn D.
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Beamter
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Re: A Tough Lesson: Hydrogen Embrittlement Please Read If You Plate Hardware

Post by Shawn D. »

I'm late to the party, but how would stripping and replating eliminate the embrittlement issue?  Wouldn't the fasteners already be affected?
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