Stainless cracking. Metalurgists, puzzel this one

[thinwater]

Most of the bolts I see with damamge are pitted somewhere inside the hull, a distance from both the head and the nut:



[Vyv Cox]



But on Farrier trimarans, a not uncommon failure is for the heads of the bolts holding the lower folding struts to fail right under the head. There are 4-6 bolts in shear, through an anodized aluminum plate about 3/8-inch thick (varies with the model). There is only a split lock nut between the bolt head and the flange. These bolts support the full heeling load of the boat. The plates and struts are conservatively engineered and never fail and the bolts never bend or get loose. The heads fall off. And typically only one will just fail. The others hold without distortion. But typically when they are pulled for replacement, most show serious damamge. The anodized aluminum flange is typically undamaged.





Why would the damamge be right under the bolt heads instead of farther in? Does the anodized aluminum have something to do with this (it seem it must)?



[This strut is in tension. I'd have to run some numbers, but the max working load is probably 3-4 tons spread over 16 bolts (2 on each side of 4 plates).]

[Jammer]

Split lock provides a crack to admit water. 5200 or whatever sealant they use is good enough to preclude migration of the water further down the shoulder of the bolt. Simple answer

I am not a metallurgist however and maybe that's not it.

I no longer use split locks for much of anything. They're awful in so many ways. In that application I doubt if they had any effect at all since the sealant would have a much higher breakaway torque than the lockwasher.

[PippaB]

I'm with Jammer, probably the split washer. Do the hinge bolts ever fail in the same manner? Looks like the installation is slightly different.

Do you have enough data to correlate the direction of the split in the washer on the bolts that failed? For instance, if the split is on the high side, does it introduce a place for salt water to get to the head, and get trapped, and deplete oxygen, and start crevice corrosion? While if the split is on the low side it allows water to drain, and those bolts are the ones that do not fail?

[thinwater]

Originally Posted by Jammer

Split lock provides a crack to admit water. 5200 or whatever sealant they use is good enough to preclude migration of the water further down the shoulder of the bolt. Simple answer

I am not a metallurgist however and maybe that's not it.

I no longer use split locks for much of anything. They're awful in so many ways. In that application I doubt if they had any effect at all since the sealant would have a much higher breakaway torque than the lockwasher.

Correction. I looked at my boat and they are SAE bolting washers. You would never put a lockwasher under a bolt head. No purpose.

[thinwater]

Originally Posted by PippaB

I'm with Jammer, probably the split washer. Do the hinge bolts ever fail in the same manner? Looks like the installation is slightly different.

Do you have enough data to correlate the direction of the split in the washer on the bolts that failed? For instance, if the split is on the high side, does it introduce a place for salt water to get to the head, and get trapped, and deplete oxygen, and start crevice corrosion? While if the split is on the low side it allows water to drain, and those bolts are the ones that do not fail?

The hinge bolts never fail and are never corroded or damaged. Never heard of a case.

No split washer. My bad, please read correction. But more likely if there was free flow of water and oxygen that would actually be better. Pretty sure. Consider the hinge pins. Heck, the tiny circlips on the pins hold up as well as the bolts. Fortunately, they are loads easier to replace; 15 seconds and you carry spares.

All of the failures are as shown, right up under the bolt heads.

I'm puzzled by the location more than the failure due to cracking.


---


Could a more noble 316 washer next to a less noble (stronger alloy) bolt be a problem?

[dlj]

Thinwater,

How far south of Annapolis are you? I'd love to come look at your broken bolts. I'll be back down to my boat in a week or two.

If you'd like, send me a PM.

dj

[dlj]

Originally Posted by thinwater

Could a more noble 316 washer next to a less noble (stronger alloy) bolt be a problem?

What alloy are the bolts made from? Do you still have the washers that were next to each broken head? didn't see them in the photos.

dj

[thinwater]

Originally Posted by dlj

What alloy are the bolts made from? Do you still have the washers that were next to each broken head? didn't see them in the photos.

dj

Not my boat. But I just asked those questions.

[dlj]

Originally Posted by thinwater

Not my boat. But I just asked those questions.

The answer would depend upon alloys used. Do you have the bolts? The photos are such it's not possible to give any kind of answer other than a WAG (Wild A$$ Guess). Something the internet is exceptionally prolific at...

dj

[Searles]

Probable all 304 s/s,plus how old are they,a bit late to be checking them now, try for 2205 s/s at least ,have rarlely see nyloc nuts in 316 ,best to remove the lot and inspect and clean every couple of years .⚓️⛵️

[thinwater]

Originally Posted by dlj

The answer would depend upon alloys used. Do you have the bolts? The photos are such it's not possible to give any kind of answer other than a WAG (Wild A$$ Guess). Something the internet is exceptionally prolific at...

dj

From the owner:


"All the ones (bolts, washers and nuts) I removed were 304 series (A2), it is isolated from the aluminium bracket with a glass reinforced acetal washer. This sort of crevice corrosion is quite common in saline environments where a water film can get trapped and not flushed. It is a significant area of interest in offshore oil and gas where the use of duplex steels (which have austenitic and ferritic phases) is common which are designed to be more resistant to this type of corrosion. One of this issues is there is no great way to tell if a fastener has crevice corrosion. PERN number, which is a ratio of chromium, molybdenum, nickel, and nitrogen content is often used to compare the resistance of steel to this type of corrosion. PERN is approx18 for A2 (304), 24 for A4 (316) and mid 30s for duplex. I notice the farrier sailing manual suggests no maintenance is required except flushing the bolts with fresh water if feasible."
_._,_._,_

[s/v Jedi]

Originally Posted by thinwater

From the owner:


"All the ones (bolts, washers and nuts) I removed were 304 series (A2), it is isolated from the aluminium bracket with a glass reinforced acetal washer. This sort of crevice corrosion is quite common in saline environments where a water film can get trapped and not flushed. It is a significant area of interest in offshore oil and gas where the use of duplex steels (which have austenitic and ferritic phases) is common which are designed to be more resistant to this type of corrosion. One of this issues is there is no great way to tell if a fastener has crevice corrosion. PERN number, which is a ratio of chromium, molybdenum, nickel, and nitrogen content is often used to compare the resistance of steel to this type of corrosion. PERN is approx18 for A2 (304), 24 for A4 (316) and mid 30s for duplex. I notice the farrier sailing manual suggests no maintenance is required except flushing the bolts with fresh water if feasible."
_._,_._,_

This explanation of it being crevice corrosion and hiw it happens is correct.

[thinwater]

Originally Posted by Searles

Probable all 304 s/s,plus how old are they,a bit late to be checking them now, try for 2205 s/s at least ,have rarlely see nyloc nuts in 316 ,best to remove the lot and inspect and clean every couple of years .⚓️⛵️

The problem with the "inspect every few years" approach is that they are epoxied in place.


The interesting thing is that only certain models within the manufacturer seem to have this problem. My boat, for example, is virtually identical and the 4 bolts I removed last year were fine (I had a small leak to fix). But there are differences: mine is in brackish water, perhaps the rain washes over the flange more efficiently, the washers are stainless not acetal, the installation torque (preload) may have been less, etc. My boat is over 20 years old, so the difference is not age.

[sailorboy1]

Originally Posted by thinwater

Most of the bolts I see with damamge are pitted somewhere inside the hull, a distance from both the head and the nut:



[Vyv Cox]



But on Farrier trimarans, a not uncommon failure is for the heads of the bolts holding the lower folding struts to fail right under the head. There are 4-6 bolts in shear, through an anodized aluminum plate about 3/8-inch thick (varies with the model). There is only a split lock nut between the bolt head and the flange. These bolts support the full heeling load of the boat. The plates and struts are conservatively engineered and never fail and the bolts never bend or get loose. The heads fall off. And typically only one will just fail. The others hold without distortion. But typically when they are pulled for replacement, most show serious damamge. The anodized aluminum flange is typically undamaged.





Why would the damamge be right under the bolt heads instead of farther in? Does the anodized aluminum have something to do with this (it seem it must)?



[This strut is in tension. I'd have to run some numbers, but the max working load is probably 3-4 tons spread over 16 bolts (2 on each side of 4 plates).]

Well to me that seems a shear or bearing failure instead of a pitting corrosion issue. That suggests that bolt strength is too low, either from the material of size.

[Madehn]

I think the crevice corrosion is the answer. I have seen bolt heads shear like that. Never under load but when u put a wrench on them to remove them.