Thread: What's the best corrosion resistant bolt?

I would think 316 stainless steel would be the way to go.
316 is marine grade, and commonly available in the size you described

Originally Posted by Ironwood

I would think 316 stainless steel would be the way to go.
316 is marine grade, and commonly available in the size you described

Thanks for your reply- I am a bit concerned about stainless steel having seen it rust.
I realise that there are different grades of ss- how does 316 ss stand up in the long term?
These bolts wont be able to be accessed once they are in- so they have got to last a few decades without corroding- would 316 be able to do that or would I be better to go for brass do you think?

Bronze is the way to go. Not brass.

It also depends on what the material is you are bolting?

Originally Posted by Papa

Bronze is the way to go. Not brass.

It also depends on what the material is you are bolting?

Yes I heard that boatbuilders use bronze bolts- I guess it is the most expensive option by far.
Material is fibreglass resin panels (two flanges of around 6mm each)- swimming pool retaining wall for liner (vinyl not ocean going)..

Are the best two options bronze or 316L/A4 Stainless steel and if so, does anyone know which of the two will last better ?

The issue is really about galvanic action, not rust. If you have dissimilar metals together in a saline environment, the less noble metal will erode.

316 stainless steel isn't any good in an oxygen deprived environment.

Here's some examples of galvanic erosion in bolts taken off my boat during its rebuild - the big ones due to dissimilar metals, the smaller bole a 1/4" SS bolt - due to corrosion caused by an anerobic environment.

Originally Posted by igatenby

The issue is really about galvanic action, not rust. If you have dissimilar metals together in a saline environment, the less noble metal will erode.

316 stainless steel isn't any good in an oxygen deprived environment.

Here's some examples of galvanic erosion in bolts taken off my boat during its rebuild - the big ones due to dissimilar metals, the smaller bole a 1/4" SS bolt - due to corrosion caused by an anerobic environment.

Thanks- can you suggest anything which will work for this application?

Originally Posted by stan250

Thanks- can you suggest anything which will work for this application?

I guess the issue is what other materials it will be in contact with and how much of an electrolyte there is. Why not ask the local pool guys - not so much what they use (they may not be interested in longevity) - maybe ask them what they have pulled out that had the least corrosion.

Maybe the answer is encapsulation - we used to use a tube of some sort of yellow goop back in the sailing days whenever we put dissimilar metals together. Maybe paint it with some Ormonoid tar or something?

Originally Posted by igatenby

I guess the issue is what other materials it will be in contact with and how much of an electrolyte there is. Why not ask the local pool guys - not so much what they use (they may not be interested in longevity) - maybe ask them what they have pulled out that had the least corrosion.

Maybe the answer is encapsulation - we used to use a tube of some sort of yellow goop back in the sailing days whenever we put dissimilar metals together. Maybe paint it with some Ormonoid tar or something?

In contact with sand (surrounded by it) moisture and the fibreglass.
Would silicon bronze bolts be ok in an anaerobic environment.
There would be no other metal in direct contact with these bolts although there will be some rebar embedded in concrete within a few inches.

Originally Posted by stan250

In contact with sand (surrounded by it) moisture and the fibreglass.
Would silicon bronze bolts be ok in an anaerobic environment.
There would be no other metal in direct contact with these bolts although there will be some rebar embedded in concrete within a few inches.

I replaced that stainless bolt in the photo with silicon bronze. I can't see the rebar being an issue. Its a bit of an unusual situation to what I normally deal with - but yeah if there's $$ or failure consequences, I'd go with silicon bronze over SS. Incidentally - I've used thousands of SS screws on my boat too - but only well above the waterline.

Stan

what are you building?

why must you use metal bolts?

Isn't there an all fibreglass or glass reinforced plastic alternative?



ian

Originally Posted by ian

Stan

what are you building?

why must you use metal bolts?

Isn't there an all fibreglass or glass reinforced plastic alternative?



ian

They dont have to be metal bolts but I thought that there was no other option.
Its an in ground GRP swimming pool made of bolt together wall sections.
The bolts must be strong and long lasting- thats the only criteria,
I have seen nylon bolts but I thought that they would perhaps not be strong enough??
I never heard of fibreglass bolts and cant find any mention of them on the net apart from very large ones used in the mining industry.
do you mean nylon?

Monel bolts are pretty much indestructible but the costs would be high.

Rhys

Hobbyhorse stole my answer. Monel would be the best, most corrosion resistant metal, but availability and cost could be prohibitive.

Next to this would be one of the more noble bronzes, like silicone. Bronzes come in many varieties, you'll want one with a fairly high copper content.

316 Stainless could work, it would be better if it was 316L (low carbon) in this environment. When you work with steels, you need to gauge the amount of expected corrosion, during an expected life span and add this amount to the dimensions of the fastener. An example would be the hulls on an oil tanker are about twice as thick as strength requirements dictate, but over it's life span (the ship's) it will lose some of this thickness to corrosion, so to not have a structural failure near the end of it's service life, it's twice as thick.

Galvanized steel could also be employed as could plan old mild steel. Both will need a coating (polyurethane) to offer additional protect against the underground environment. Again, size the fasteners to accept some corrosion during it's life.

Plastics could be put to reasonable effectiveness, but now you surely have to engineer the fasteners to insure strength requirements. There are several plastics, that would last hundreds of years in this environment, but not many manufactures of common fasteners, so you'll likely have to machine them up yourself (not as hard as you might think).

For example (in regard to plastic fasteners) you could simply make or buy some round bar stock and cut them to be used as pins. Heating and mushrooming each end in place on the job sight wouldn't be very difficult and the "sheer pin" would be basically "inert". Again, these pins would have to be sized and the proper material choices made to accommodate strength requirements (read call an engineer or have a fancy lawyer on stand by). I'd think one of the HDPU family would serve quite well with the option to use the ultra high modulus stuff.

Lastly, you could just go old school on the situation and butter everything up with plan old tar. Yep, that nasty, asphalt based roofing goo. It'll protect just about anything, stick to it too. Mild steel or galvanized fasteners (the cheapest and most readily available also) could be well lathered up with tar, then buried. The pool may well not out live bolts treated this way. Of course the kicker is if you miss a spot with the tar and a fastener lets go at a bad time (like when the pool is full of lawyers and politicians, during an election year). You may end up with more then a soggy back yarrd.

How's that Brad?