Isolating Keel Bolts at Flange

[SeaJay]

Guys,

I have a keel attachment similar to one that can be seen at:
http://www.apm-keels.com/ (go to "keels" / "high strength fixed keels" and its the photo of the two guys loading a keel on a trailer). My keel is a steel weldment with cast lead inside. The keel bolts are not embedded in the lead as is often done, but rather, bolted through the steel flange. I plan to use monel bolts and am wondering how to galvantically isolate the bolts from the steel flange? Or is this even necessary or desirable?

The heads of the bolts will be epoxy coated and the shank protected by sealant at the flange / hull interface. However, I cannot help but think that some water will work its way between the two dissimilar metals. Does anyone have a recommendation they'd like to share?

Regards,

SeaJay

[tazmann]

SeaJay
Wondering if maybe hot dipped galvanizes bolts would be a better option?
Tom

[Eric Sponberg]

Monel is way at the top of the galvanic scale, far away from steel, and you are right to be concerned about corrosion--in the steel, not the monel. A better choice would probably be stainless steel bolts so that galvanic corrosion is much less of a concern--the bolts and the keel are much closer together on the galvanic scale and do of similar electolytic potential.

Another alternative if using Monel bolts is to install a suitable number of zinc anodes around the keel to protect the steel--the anodes will corrode before the steel will, if done properly.

Eric

[SeaJay]

Hot dipped galvanized? That's an idea. I'd have to increase the diameter of the bolts but that may not be a problem. However, I'd think I'd still have the same issue and maybe even a bit worse as I'd think the galvinization would tend to act like a sacraficial anode.

[SeaJay]

Eric,

Thanks for the reply. The design is a modification of a Dudley Dix design and he uses stainless steel bolts. Given the issues surrounding SS keel bolts that has been discussed often in these forums I was trying to think of an improvement. However, there is one great advantage in this arrangement in that the bolts are relatively east to inspect and infinately easier to replace than embedded bolts. I will, as you have suggested, be sure to use proper anode protection as my primary line of defense.

In this thread it was suggested to use duplex 2205 grade ss bolts.

http://www.boatdesign.net/forums/materials/monel-keel-bolts-24072.html

I will do some research to see about the availablity of bolts in this material but is there another similar material I should be looking at as well?

Thanks for your input.

[gonzo]

If you use 316S type stainless, crevice corrosion shouldn't be a problem

[SeaJay]

Well, I made another pass at the literature on this subject and the opinions appear to be all over the map. Clearly the subject is very dependent upon the specific situation; i.e. ss bolts through wood, ss embeded in lead, galvanized in iron, etc.

There is a chart in the bottom portion of this webpage:

http://www.estainlesssteel.com/corrosion.shtml

that shows the relationships of various fastener materials with various base materials. I think this is an excellent reference piece for anyone interested in the subject. This information confirms Eric's thoughts on monel in my particular application; gives some credence to Tom's galvanized bolt suggestion (although confirms my concerns about the corrosion of the galvinization); and lines up with Eric's and Gonzo's ss recommendation.

Of course this only addresses galvanic corrosion (which was the topic of my thread), and still leaves open the crevice corrosion and galling issue with respect to ss. Nonetheless, at this point the Duplex 2205 ss appears to be the front runner. It's yield strength, 65 ksi, is impressive. I see that it is readily available but haven't checked price against 316.

Regards to all

Sea Jay

[TeddyDiver]

If you use 316S type stainless, crevice corrosion shouldn't be a problem
Please explain how's that possible? Anyway I've allways thought that it's a problem common to all stainless steels..

[apex1]

It IS Teddy...........

[Ad Hoc]

If you use 316S type stainless, crevice corrosion shouldn't be a problem

How??....crevice corrosion is just shielding of the oxide layer, thus starving the steel of oxygen. How is the grade of SS going to stop this occurring under a bolt head or washer?

Confused? :confused:

[Eric Sponberg]

Eric,

Thanks for the reply. The design is a modification of a Dudley Dix design and he uses stainless steel bolts. Given the issues surrounding SS keel bolts that has been discussed often in these forums I was trying to think of an improvement. However, there is one great advantage in this arrangement in that the bolts are relatively east to inspect and infinately easier to replace than embedded bolts. I will, as you have suggested, be sure to use proper anode protection as my primary line of defense.

In this thread it was suggested to use duplex 2205 grade ss bolts.

http://www.boatdesign.net/forums/materials/monel-keel-bolts-24072.html

I will do some research to see about the availablity of bolts in this material but is there another similar material I should be looking at as well?

Thanks for your input.

Sea Jay,

Back in the old days, bronze alloys were the preferred keel bolt material, but in modern times, bronze became more expensive than stainless steels. Boatbuilders, always trying to save a buck, adopted stainless steel for keel bolts, much to the chagrin of old-time builders and marine surveyors. I like to specify silicon bronze for keel bolts when I can, which is a widely accepted alloy. Copper-nickel alloys such as 90-10 CuNi, are also very good. However, if your boat is built out of aluminum, then bronze and any copper-based alloy is a no-no--your aluminum hull will fizzle faster than an Alka-Seltzer.

Stainless alloys of 304, 304L, 316, and 316L stainless are very common in keel bolts. They will last years, and eventually they will corrode, the non-L versions faster than the L versions. But they will last a long time, and longer if you protect them well and maintain/replace them when necessary.

Another alternative is Aquamet 22 and its sisters.

http://marinemachiningandmanufa2.liveonatt.com/aquamet22.nxg

I am not sure of their more generic alloy designation, but Aquamet 22 (and Aqualoy 22) are the best, most highly corrosion-resistant alloys that are used for propeller shafting--same environment as your keel bolts, under the water and near the surface, also in contact with a wider variety of materials when you consider the whole drive train. You could have bolts machined out of Aquamet these materials. You would have to check the price.

Eric

[TeddyDiver]

It IS Teddy...........
Soy yo :D

[SeaJay]

Eric,

According to the reference I posted above, and along the same lines as your comments on Monel bolts, bronze bolts would likely accelerate the corrosion of the steel flange (I realize your comment regarding silicon bronze was a general statement.) I've also considered Aquamet 22 and here is a reference that states that it is similar to 17-4 and 15-5 stainless.

http://www.eng-tips.com/viewthread.cfm?qid=3731&page=8

(It is also a pretty good general discussion on ss corrosion in saltwater. I found it interesting that one poster said that all things considered, mild steel fasteners can be a good choice as their rate of corrosion is so predictable)

From my earlier rePossible alternative grades to grade 2205 stainless steels are given in table 5.

Table 5. Grade specification comparisons for 2205 grade stainless steels

Grade
Why it might be chosen instead of 2205

904L
Better formability is needed, with similar corrosion resistance and lower strength.

UR52N+
High resistance to corrosion is required, eg resistance to higher temperature seawater.

6%Mo
Higher corrosion resistance is required, but with lower strength and better formability.

316L
The high corrosion resistance and strength of 2205 are not needed… 316L is lower cost.

Corrosion Resistance
Excellent general corrosion resistance; superior to Grade 316 in most environments. Excellent resistance to localised corrosion including intergranular, pitting and crevice corrosion; the CPT of 2205 is generally at least 35°C. The grade is also resistant to chloride stress corrosion cracking (SCC) at temperatures of up to about 150°C. Grade 2205 will often perform well in environments which cause premature failure of austenitic grades. It has better resistance to sea water than grade 316.

I think my take-away from this discussion is that for my particular situation, and despite the noted issues with stainless, 2205 or an Aquament type product is probably the best choice. This choice would also be predicated upon following these precautions: a) Protecting the bolts from water intrusion as much as possible b) proper cathodic protection and c) an appropriate inspection schedule.

[Eric Sponberg]

I agree, all smart choices. Yes, Aquamet 22, as I recall now, is similar to 17-4 stainless (I don't necessarily check my library every time as I write stuff like this). We are using 17-4 stainless for shafting material is a wind turbines that I have been working on this year, and we chose it because it is relatively easy to get, has very good corrosion resistance (high chromium content), and it is going to be used in a Gulf Coast environment (very corrosive atmosphere, as anyone living in Florida can attest). I am sure we readers will all be curious to know what you find out about costs.

Eric

[SeaJay]

Apex,

Could you please explain why you feel Aquamet is a bad choice???

Regards,

Sea Jay

[apex1]

That was a very dumb mistake I made there, sorry for that!
I deleted the post.

Regards
Richard

[tazmann]

Sea Jay
I cant tell in the picture but are the bolt heads resest in the flange and faired over with filler or exposed ? moisture migrating around the the bolts shouldnt be an issue if you fill with marine caulking " no air void around stem of bolts"
Tom

[SeaJay]

Tom,

My keel is similar to the one in the photo, but not exactly the same. The bolts will be inside "pockets". My plan is to epoxy coat the flange and the inside of the bolt holes. Between the flange and hull, I'll use a sealant as typical, with special attention around the bolts themselves. After the keel is in place and the bolts tightened, I thought I'd put a "grease cap" over each bolt. After that is done, I'll foam the pockets and seal them with epoxy/glass.

I think water intrustion from the pockets should be limited, but my concern is more from the joint between the keel and hull. It would seem to me that is just the environment where stainless has a problem with crevice corrosion and pitting. This was why I was leaning toward monel, but then you get the galvanic corrosion between the bolts and the steel flange...damned if you do, damned if you don't.

Regards,

SeaJay

[tazmann]

SeaJay
I see what you mean damned if you do and damed if you dont. I wouldnt use stainless there eather with only part of the bolts being exposed to the inside of boat. If the keel fits nicely to the hull and the surfaces are painted with an underwater epoxy then when you put in on use a thin layer of marine caulking on both surfaces, bolting it up tight will squeeze the excess caulking out. Good tight seal with no air voids you would never have a problem unless the keel came loose some how? I think the only place you will have problem with corosion is the nuts washers and exposed threads on the inside of boat but thats easily taken care of paint ,grease ext.
Tom

[SeaJay]

Eric,

I have some preliminary pricing info. (I asked for a quote on Duplex 2205 as well as 17-4 or 15-5 (Aquamet type material) if they had it. Here is the response to my 3/4" x 12" inquiry:

Bolt in 2205 duplex $80.00E Nut $12.00E Ship in 2 weeks after receipt of
order. The other materials would be wildly more expensive

(At $92.00 per set, one can only imaging what "wildly more expensive" means!)

By comparison I found a 3/4" 6" 316 ss (not 316L) bolt for $12.00. Figure maybe $25 for a 12" bolt and $5 for the nut, and it's about 30% of the Duplex 2205. For my application that's about a $850 difference...hmmm, I'll have to think about this.

SeaJay

[SeaJay]

(6 months later) I've been out bolt shopping and the difference between what engineering calcs call for and what is available in the real world has reared its ugly head. As noted earlier the costs of the duplex and monel (if you can find them) bolts are enough to give one pause. And even after all of that, I still have the crevice corrosion and galvanic corrosion issues that I've noted before.

So tell me if I'm nuts...what about just using Grade 8 steel bolts and coating/embedding them in epoxy? I can get 12" x 1" Grade 8 bolts from McMaster-Carr for $18.66 each (interestingly, the same bolt zinc coated is only $13.08...go figure!). The Grade 8 steel is as strong or stronger than the other options and crevice / galvanic corrosion are not an issue. Of course I still have steel in salt water, but it isn't like that is an unknown combination. And remember, my bolts are easily accessable for inspection and/or replacement.

What say you gentlemen?

[Brent Swain]

The easily accessible is the key. You can check them early after lauching then less often if problems don't appear. If there are major problems, they will show themsleves quickly.
Grade 8 bolts seem your best choice at the moment. Zincs will protect them outside , inside should be checked regularly, where zincs wont protect them. The only safe way to guarantee contact with a zinc is to weld them on.

[TeddyDiver]

Agree with Brent in this one :)

[Brent Swain]

For bolt on zincs, where I don't have the opportunity to weld them on right away, I like to use welded on stainless bolts, and put a bit of ss weld around the bolt hole on the zinc strap. That way, you have stainless in contact with stainless, with far less chance of a bit of corrosion insulating the zinc from the hull. Then I get a welded tack on it at the first opportunity.

[SeaJay]

Thanks for the input Brent.