Bronze vs SS Bolts for Chainplates

[Cdn_bacon]

My boat doesn’t use traditional chainplates but the fitting on the deck that the stay connects to, is bolted thru the deck to a block which connects to a rod that anchors down in the hull. I pulled the bolts out to rebed the stay brackets to the deck and noticed my bolts are bronze and they have some crevice corrosion. Question is what to go back to get with? The deck stay brackets are SS, the backing block under the deck appears to be aluminum and the rod that connects down to the hull is SS. All this joined by bronze bolts with SS nuts.

Go back with bronze, and if so, Aluminum-Bronze or Silicon-bronze? There seems to be a great amount of opinions on here which to use. Or just go back with 316SS.

The one last complication is with the config od the brackets I don’t think a hex head will fit. The existing have a round head with a flat spot so it doesn’t rotate in the bracket.

[Wotname]

A good quality marine bronze rarely suffers from crevice corrosion. The corrosion showing in the pictures look more like surface pitting rather than true crevice corrosion - at least to my untrained eyes.

In most circumstances, a quality silicon bronze will be better than any stainless steel.

A good article to read and to explain the various types of bronze and their characteristics https://www.gerrmarine.com/Articles/marinemetalspt2.pdf

316 SS is cheap and easy to obtain compared to a marine bronze though.

Me - I would reuse the old bronze hardware!

[Rucksta]

The bronze bolts look fine.

The rod in photo #2.
How is it attached to the aluminium?

If it's a formed ball end sitting in a cup inspect it for crevice corrosion.

Look around the circumference of the ball where is flares away from the parent rod.

A dye test on the polished end, viewed under 4-8x magnification will reveal cracks if present.

Tef-Gel or Duralac would be suitable bedding compounds.

[Bowdrie]

We just had a 4-page thread on almost the same thing that used the same bolts.
https://www.cruisersforum.com/forums...al-284414.html

[GordMay]

For Chainplates & Tie Rods, I’d use C655 High Silicon Bronze, or Monel [nickel 400], at about twice the price.

Bz ➥ https://www.mcmaster.com/products/si...-threaded-rods

Ni ➥ https://www.mcmaster.com/products/monel-threaded-rods/

A couple of interesting & informative Technical Articles, by Dave Gerr:

"High-Strength Marine Metals, Part 1 - Stainless Steel"
Why Stainless Steel is Not the Ideal Marine Metal
➥ https://www.gerrmarine.com/Articles/MarineMetalsPt1.pdf

High-Strength Marine Metals, Part 2 - Copper & Nickel Alloys
High Strength with High Corrosion Resistance
➥ https://www.gerrmarine.com/Articles/marinemetalspt2.pdf

[Cdn_bacon]

So I was curious and took a run to the hardware store and grabbed a 1/2” hex head bolt. It’s not going to seat flat without machining and given the load is in tension I’m reluctant to do that. I may have to reuse the old bolts. Question is how bad is the corrosion pitting? It’s not all the bolts but a few.

[rslifkin]

See if you can find a socket head cap screw instead of a hex head. That might fit better in the available space. You could also use a grinder to trim the sides of the hex to make it fit if needed, doesn't have to be machined precisely as you're not touching a load bearing surface.

[Cdn_bacon]

Originally Posted by rslifkin

doesn't have to be machined precisely as you're not touching a load bearing surface.

The underside of the head is load bearing by traditional fastener design.

[rslifkin]

Originally Posted by Cdn_bacon

The underside of the head is load bearing by traditional fastener design.

Yes, but that's not the surface you're grinding. So grinds that aren't perfectly flat and even won't cause uneven loading on the bolt head. Although a socket head bolt would be a better solution than grinding if you can find one.

[Bowdrie]

One of the problems with socket-head bolts is that their head diameter is not all that larger than their shank diameter in many cases.
Another might be obtaining a bronze bolt with that style head, and then you have the socket itself which makes a nice collection point for water and nasties.
If I absolutely MUST have a close duplicate of the original, I'd be tempted to make a two-piece steel die with the appropriate thru-hole and counterbore and then with a good vise/hammer/torch, the head could be hot-formed/forged from a length of CDA 655 Silicon Bronze and then the flats ground/filed.
After determining length required, cut the shank to length and thread it.
It's a lot of work, but sometimes the last resort should have been the starting point.

[thomm225]

My 50 year old SS chainplate bolts had a little surface rust was all.

Could probably go another 50 years.

[dlj]

Originally Posted by GordMay

A couple of interesting & informative Technical Articles, by Dave Gerr:

"High-Strength Marine Metals, Part 1 - Stainless Steel"
Why Stainless Steel is Not the Ideal Marine Metal
➥ https://www.gerrmarine.com/Articles/MarineMetalsPt1.pdf

High-Strength Marine Metals, Part 2 - Copper & Nickel Alloys
High Strength with High Corrosion Resistance
➥ https://www.gerrmarine.com/Articles/marinemetalspt2.pdf

Reading through the above articles, there are a number of inaccuracies that seem to be rampant within the marine industry.

I've only reviewed the article on stainless indepth. In that article there are several misconceptions that I feel should be recognized.

One that has been deeply ingrained is that the austenitic alloys are non-magnetic. That is simply wrong. They are magnetic. When they are in the fully annealed state, if you put a magnet on them, they appear non-magnetic. However, as the amount of cold.work increases, magnets will stick to them. I've often seen these alloys that magnets stick to nearly the same as plain carbon steel. In the fabrication of these alloys, often the end product is highly attracted to magnets. I recently had a fabricator ask me if some plate 316L was actually 316L because of how strongly a magnet stuck to it. This is simply not a valid method of determining "good" or "bad" stainless.

18-8 is a term that does not denote an alloy system. It's a marketing term. It was started when the fastener industry wanted to use 303 stainless steel in the manufacture of fasteners instead of 304. I would suggest anyone that is looking at specifying or purchasing a stainless steel that they not purchase a material that has this designation. This term has become used a lot even through very reputable suppliers.

There are a number of other aspects in that article that are not correct.

Another area was talking about welding. Welding of austenitic stainless steel, specifically 304/304L and 316/316L had numerous errors. The description of potential corrosion is not due to galvanic corrosion as described. The concern about welding 304 and 316 occurs when you do not have the 304L and 316L alloy, but rather when the base chemistry is in the region of the higher carbon content permitted by 304 and 316. There is the potential to produce a microstructure that is called sensitization. The corrosion problem then can exist if sensitization occurs. This is pretty much a non-issue with today's 304/304L and 316/316L alloys as virtually all production of this alloys is currently, and has been for more than the past 20 years, been done producing what is called dual certifiable alloys. That means most production today of 304 and 316 is done such that the mill can certify those alloys as either 304 or 304L and 316 or 316L. It is actually difficult to buy a strictly 304 or 316 alloy.

For what it's worth...

sj

[mbowser]

Originally Posted by Cdn_bacon

So I was curious and took a run to the hardware store and grabbed a 1/2” hex head bolt. It’s not going to seat flat without machining and given the load is in tension I’m reluctant to do that. I may have to reuse the old bolts. Question is how bad is the corrosion pitting? It’s not all the bolts but a few.

This is exactly the issue I'm facing. The hex heads definitely will need to be modified so they sit flat and my biggest concern is that the modified head will weaken the bolt.

The foreman who ran the rigging operation at Hinterhoeller in the 1980s (when my boat was built) told me that they could be replaced with either bronze or stainless, but thought that reusing the originals is probably the best bet as long as they aren't deformed. I don't think bronze suffers from crevice corrosion the way SS does. I would clean up and re-use the bolts you have if possible. I can't because I installed inserts into the deck that increase the required length of the bolts by .25"

[Don C L]

I didn’t know bronze suffers from crevice corrosion. Is that possible?
What struck me was the mix of metals in the rigging! What boat is this? Is it all original?
My experience is with bronze chainplates and bolts. No problems since 1962.

[dlj]

Originally Posted by Don C L

I didn’t know bronze suffers from crevice corrosion. Is that possible?
What struck me was the mix of metals in the rigging! What boat is this? Is it all original?
My experience is with bronze chainplates and bolts. No problems since 1962.

Bronze does not suffer from crevice corrosion. It does have other corrosion mechanisms. But really, what's in a name? The OP should have the current bolts looked at by a professional - I know - finding a good one is difficult...

dj