mixing bronze with S/S below waterline

[JLM]

getting ready to replace some bottom planks and i dont have enough bronze machine screws.but i have about 150 S/S screws. ?is do i risk galvanic erosion by mixing the 2 together.I know its not good to mix metals but i was not sure about S/S.

Thanks JLM

[Ed Harrow]

IMHO I would not mix the two, I've seen some pretty scary bits of SS that came from boats. Crevice corrosion problems when it can't create and keep its protective oxice layer.

Now, having said that, I took quite a number of SS fasteners out of Phoenix (guessing the fastners had been there >15 years) and most of them looked really OK. There were a few, however, that were, ummm, rather diminished, if you get my drift.

Now I am curious as to machine screws... (there were a few of them in Phoenix as well, into bronze floors that were added around her mast step)

[PugetSound]

Your concerns regarding galvanic corrosion are well founded. CRES (stainless steel) comes in an astounding number of alloys most of which do not do all that well in a marine environment. That said, I should mention that CRES alloy 316 (and even better: alloy 316L - the 'L' stands for low iron content) will do OK in a marine environment. Not great, mind you, just OK.

I think that it is a case of false economy to substitute CRES at this point. Go to the extra effort and get some additional bronze fasteners.

There probably isn't all that much to worry about as far as the bronze fasteners reacting with the CRES fasteners because they will be in separate locations in the plank but CRES (even 316 alloy) can adversely react to the local environment by going from 'passive' to 'active'. Turns out that CRES requires a regular flow of water over it in order to remain passive. Remove it from the flow and you're in trouble.

[paladin]

Just a really general overall thought...as a normal rule...Stainless under water ain't stainless no more.......

[Art Read]

I'll jump on the anti-stainless bandwagon too. If a supply source is your biggest problem, Jamestown will ship 'em at reasonable prices. (Order in 100 count lots though...)

http://www.jamestowndistributors.com/cgi-local/SoftCart.exe/index.htm?E+scstore

After all the work you'll put into re-planking her, the additional delay and expense will hardly signify. Why ASK for trouble if you don't have to?

(Why machine screws though? Is that what was used originally, are you just going for additional "holding" power, or some other reason? Flat head wood screws or rivets are much more common fastenings for hull planking. Lot cheaper too! Just a thought.)

[ 07-10-2002, 12:01 PM: Message edited by: Art Read ]

[Ian McColgin]

On the other hand, if you wire a few dozen fittings in series you can get to twelve volts and save on batteries . . .

[jimstitt]

I was at the beach two days ago with the dog, and found some interesting bits of boats to salvage parts from. I got a hatchcover with about 2 dozen bronze screws, which I've already put to use, but I also found the side of some poor persons boat. It had originally been fastened with iron boat nails, but many replacement stainless steel wood screws were adjacent to the iron nails. The ss screws were in super condition, but the boat WAS is scraps...
Jim

[JLM]

ART to answer your ? the reason im using machine screws is because its what was used to build her.
another ? for you all found some s/s in other parts of the boat (all above the waterline)from repairs from the old owner do i have to replace these.
thank you all JLM

[Ed Harrow]

Hi JLM, To what do those screws (machine screws) fasten? Are they thru the frames and nutted on the inside? Possibly replacements for rivets? What is the boat?

It might be worth your while to contact Paul Haley (check WoodenBoats links and look for Giffy Full and Associates) and get his opinion. My opinion (and it's only that) is that I'd be less concerned about fasteners above the water line. Note that I said "less" not "not".

[Rich VanValkenburg]

Is there a difference in performance of the SS in fresh water vs. salt? Uh....um...Sonja's keelbolts are 316 SS and I've never seen any sign of the red bleeding I was told to watch for when SS starts into hydrogen embrittlement.

Rich

[JLM]

ED the boat is a 34' pembroke lapstrake and every
pembroke i have seen had bronze machine screws through the lap and #10 2"bronze wood screw in the
rib. Thanks ED

[ford]

I have had the unfortunate experience of stainless below the waterline and the result of it was a boat full of copper nails corroded with 250 milliamps running thru' the hull. Massive repair job, the ss was put in in 1984 316 grade keelbolts and yup crevice corrosion to boot, to date I have only removed 2 of the bolts each have been eaten away by about 30%, neither showed any visible signs that there was anything wrong. Use ss below the waterline at your own peril. :mad:

[Stan Derelian]

All this stainless and bronze talk is worrisome. The original galvanized fastenings have been replaced with bronze, but I've had to replace a couple of 12 1/2" x 9/16 bolts that were originally galvanized iron. Can't get 9/16 bronze bolt--or rod--. Haven't wanted to use 1/2" because of the looseness around it. I did get 9/16 - 316 SS rod threaded on each end, and have painted them with red lead metal primer. On top of that I have coated the hole first with CPES, and then with melted Slick Seam to try to isolate them from water contact....But this thread has me worried, and I am listening to anyone more experienced and wiser who wants to comment......

[chesterm]

well now my brow is furrowed. Our mahogany triple project has silicon bronze fasteners except for stainless screws in the stem and the prop shaft is stainless. We have zincs to bolt to the bronze rudder and stainless shaft. should see water by the end of the summer hopefully. is there a worrry? thanks, mitchell in CT

[Martens]

For whatever it's worth, and if my memory serves me correctly, I'd just like to add a small bit of information regarding stainless steels. The 300-series stainless steels (also known as "18-8" stainless} are basically alloys of iron, nickel and chromium with nominally 18% nickel, 8% chromium and the balance iron. Type 316L is more corrosion resistant than plain 316 because the "L" means lower carbon in the alloy. Carbon can form carbides with the chromium which depletes the amount of chromium present in a solid solution and thereby reduces the corrosion resistance. The chromium carbides can appear as a precipitate within the grain boundaries and result in intergranular corrosion.

[videoguy]

You guys are scaring the hell out of me I ambuilding a redwing 21 and the plans call for
all stainless steel screws and bolts ,to late
to change now .

[ford]

The problem is not the corrosion of stainless steel, the problem is the battery you set up by using it below the waterline, the more noble less noble thingy, the end result being copper and bronze are eaten.
Geo, have you tried to obtain other bronze mixes for your bolts? ie aluminium bronze or what about good old copper bolts, they've been around for years and work well. smile.gif