Anyone able to saw a 6mm SS screw in 1/2 to see if its corroded???

[tudorsailor]

On our last trip, my wife found the head of a screw separate from the screw. It was the pan head of a 6mm through-deck bolt that holds the stanchion to the deck. Fortunately the other two bolts are intact.

The head had come off the shaft so that no shaft was left attached to the head. The yacht was built in 2001 by Winboats. I assume the screw was/is 316 stainless. Is there a way to test?

I have talked to some other owners and it seems that this is not a unique occurrence. So I now wonder if I need to replace all the other stanchion screws? If I removed some intact screws, it would be interesting to have some split length ways to see if there is corrosion in seemingly intact screws.

Does anyone have the saw that might do this and the time as well?

Thanks

TudorSailor

 

[macd]

If there is corrosion, it'll probably be crevice corrosion and should be evident once the fasteners are removed. Even without removal, some light rust staining of 304 and even 316 fasteners isn't unusual on deck fittings. In my experience it's rare for it to seriously weaken the piece, but possible.

Most stainless steels, 304/316 included, have very unimpressive tensile strength. It's perfectly possibly to tear the head off a 6mm bolt simply by overtightening. Maybe they were weakened by some gorilla in the past?

 

[Hoolie]

I'm not a metallurgist, but I would think it's more likely the stainless has work hardened with continual stressing and become brittle. If that is the case I think it would be real challenge to split the screws lengthwise

 

[Fr J Hackett]

If it's corrosion and the head has come off then you would see the evidence at the two faces. It may be that it's a machining fault and the thread has undercut at the bolt head leaving it weak, if that were the case then it would be an entire batch of bolts affected. It could also be a flaw in the rod from which it was made which again you could see with a decent magnifying glass.
As for cutting a good sharp hacksaw blade taking it slowly so as not to build up heat and harden the steel using a little cutting fluid and it should be the work of a few minutes.

 

[Tranona]

Was there any sign of corrosion on the head or the remains of the screw. Was it a machine screw (threaded up to the head) or a bolt. As macd says a pan head 6mm machine screw is not strong in tension because of the limited amount of metal in the head.

Doubt sawing one up will help in any way. If there is corrosion it will be crevice corrosion under the head and this starts on the surface so is easy to spot, hence my questions above. There will not be corrosion inside because it starts from the outside when the protective film breaks down because of damp and oxygen starvation.

If you are worried about the others, just remove them and replace with 316, making sure you bed them well in sealant to exclude moisture.

 

[tudorsailor]

Was there any sign of corrosion on the head or the remains of the screw. Was it a machine screw (threaded up to the head) or a bolt. As macd says a pan head 6mm machine screw is not strong in tension because of the limited amount of metal in the head.

Doubt sawing one up will help in any way. If there is corrosion it will be crevice corrosion under the head and this starts on the surface so is easy to spot, hence my questions above. There will not be corrosion inside because it starts from the outside when the protective film breaks down because of damp and oxygen starvation.

If you are worried about the others, just remove them and replace with 316, making sure you bed them well in sealant to exclude moisture.

Thanks everyone

Yes there was corrosion on the top of the screw shaft and the underside of the head. It was a pan head machine screw, so with threads right up to the head

I guess replacing them all is sensible. Not a small job on my 49 footer!

Thanks for all the thoughts

TS

 

[Tranona]

I expect when you get them out they will all be perfect and the one that failed was just a rogue - or rather was not properly sealed.

I would just inspect all the heads for signs of bleeding staining or failed sealant and just replace those, although appreciate that is not fool proof as the failure is likely hidden.

 

[tudorsailor]

I expect when you get them out they will all be perfect and the one that failed was just a rogue - or rather was not properly sealed.

I would just inspect all the heads for signs of bleeding staining or failed sealant and just replace those, although appreciate that is not fool proof as the failure is likely hidden.

I am concerned that this is not a one off (literally) as several other owners of similar yachts have had heads come off. One said most of his have now been replaced.
I will go around with a screw driver and give the others a little tweak. If another head comes off then I guess I will replace them all.
Thanks

TS

 

[bbg]

If you replace them all you will certainly have peace of mind. If you don't you will have doubts.

 

[John the kiwi]

I am concerned that this is not a one off (literally) as several other owners of similar yachts have had heads come off. One said most of his have now been replaced.
I will go around with a screw driver and give the others a little tweak. If another head comes off then I guess I will replace them all.
Thanks

TS

Actually this is a good idea. Better if you use a torque wrench with a screwdriver bit in it.
A quick google suggests that tightening to 70% of proof load is appropriate and this will be a torque of 11.1 NM for an SS316 M6 bolt.

Of course this is based on tightening the nut which i assume you cant access (or doesnt exist?) The term Machine screw implies a nut.

Even so, If you apply this torque and the head stays on it might be OK and if not, not.

Anyone care to advance the methodology? Can we come up with better torque test value that will give the the OP assurance he wants without needing to replace the whole lot?

Cheers
John

 

[vyv_cox]

There is a very good chance that the cause is stress-corrosion cracking. In the past month I have seen several examples, some of them on my boat. I have yet to complete adding them all to my website but you can read the ones I have done under ' rig and rigging' < Westerly Oceanmaster and Sadler 34.

As ever with corrosion it does not happen if you keep the water out. A dab of sealant under the head of the replacement screws will solve the problem.

 

[Zing]

I’ve had lots of broken screws too. Not there though I think. I’d better check. Thanks for the heads-up.

I’d love to know how you will get the little broken stud out?