Hydrogen Embrittlement ASTM A574

[TugboatEng]

Today I have inherited several vessels with obsolete drive units and no factory support. The drives have submerged.structural flanges and are notorious for breaking bolts. 30 years later, I think it's time to look into a cause. The fastener are socket head caps screws of ASTM A574 standard and some shoulder bolts of no specific standard. My understanding is that the shoulder bolts have never been found broken. It appears the shoulder bolts are 7/8" and 32 HRC. The bolts that are breaking are 1-1/8" and 37 HRC. Another manufacturer of similar equipment uses exclusively class 8.8 fasteners, I assume to reduce the risk of hydrogen embrittlement.

Here is a picture of one failed fastener. Are their any visual clues of HE? Can HE be determined if the fracture faces are corroded?

I'm thinking of specifying ASTM A193 B7. The problem is that lower strength socket head cap screws are not commercially available in imperial dimensions.

 

[mfgenggear]

Yes, salt water. There are zinc anodes but I don't believe they offer much protection to the bolt threads in the hole. This is also adjacent to the stainless steel nozzle liner. With that said, I am surprised how little corrosion there is on the threads.

I disagree. belive anodes are important and need replace when needed. as an ex small outboard 70 hp 2 stroke. in salt water, anodes prevented
corrosion. as long as I flushed the engine and and outside with clean fresh water.
in your case if that is not possible zinc chromate coating is a must. and a maintenance of so many cycles replaced after the coating has been dissipated.
a prevention maintenance sop.

 

[mfgenggear]

there is a big deal white papers on fasteners
obtaining hydrogen embrittlement , is and has been a problem on only high strength
fasteners. it is a good read. when plating is involved, it is important to sop to post bake
to remove hydrogen. I did it, for high strength
aircraft components for decades. only had one issue but it was rare.

 

[TugboatEng]

The boat has many anodes. I don't believe they offer much protection in the bolt holes themselves because they are mostly sealed (the holes are wet due to damaged faying surfaces) and don't have a clear line of sight to the anode.

 

[mfgenggear]

In cases where there was issues with contamination or ingress of moisture or other contamination. I would apply grease.
While this may or may not solve your problem. The other issue would be to obtain a small test stand. To correctly apply the correct torque and or elongation.
Seen lectures from fastener manufactures on this. It is very helpful. But you have to research with Lubricant would be correct.
Maybe an oring design may be possible.
Anodes do help , but as stated anodes need replace ment. Standard propane water heaters have an anode installed inside of the tank. And does work. But it does sacrificial use and eventually needs replaced.
On engines with a dual issue of corrosion
And corrosion I would use anti seize.
And it would immensely help.
And othe trick is to hit the head with a soft
Hammer or brass . And heat it with propane or acetylene torch. I would chase the threads with a tap. To remove corrosion or drill out
Tap for an insert.

 

[mfgenggear]

Also let's talk about capillary issues.
Faying surfaces with suck moisture or liquids . So there needs some type of mitigation to prevent it. I would reach out to sales engineers . I sure they are approached by this common problem. There should be no sense of reinventing the wheel. I am sure they can solve your issue. And can recommend which type of fastener , thread lube or thread locker. Or ?

 

[Orange_kun]

To me, they all look like they failed at random thread root locations. Considering that the shoulder bolts have the same threadform, but haven't ever (?) failed, and also seem to look consistently less corroded... I would want to know 3 things:
1. Are the fully threaded bolts confirmed to be roll-formed threads? Or is this unknown?
2. What does the joint look like (cross section)? It seems like the shoulder might be acting as a pin and preventing some bending loads from being applied to the threadform.
3. Are the shoulder bolts the same material grade as the fully threaded bolts?

You've got a harder material showing consistent failure at the points with the highest stress concentrations. It also looks like the threads deepest in the hole are faring the best. In the shoulder bolt's case, it has a lovely huge fillet to lead from the tensioning threads to the rest of the bolt. It's also a more ductile fastener based on the hardness tests.

It seems like addressing either the stress concentration issue or the corrosion issue would help improve longevity. What kind of load do the bolts see? Is there a reason they aren't stainless?

Edit: I saw the comment about the Krytox 227 proposal. Probably a great idea. If you can get your hands on the XP stuff, that might work even better (liquid anti-corrosion additives, vs soluble ones). The krytox isn't the best at resisting washout, but hidden in the bolt holes I think it'll be fine. I'd still worry about the bolt head fillet though. Shoulder bolts and krytox would be the simplest it seems.

 

[TugboatEng]

I wasn't able to source the Krytox in time so we had to use Tef-Gel. The entire length of the bolt below the head is coated. The outer circumference of the flange is getting welded. The heads of the bolts are going to be encapsulated with fairing compound which will most likely be West System epoxy with the 410 filler. As long as the circumference weld doesn't crack in feel this should provide longevity. I am thinking of purchasing an ultrasonic thickness tester to simplify finding broken bolts.