Material Science Primer for Cat Sailors

I was recently hanging out with a friend of mine who makes high-tech kayaks. In our discussion of dream boat designs, my friend mentioned that carbon fiber and aluminum are +/- electrical potential and therefore create a nice battery that dissolves the material around both sides of the connection. He explained how someone brought him a 5K carbon bicycle where the paint on the aluminum came off and the frame dissolved along with the aluminum. This last weekend, we went to fix a carbon rudder housing + carbon tiller arm on one of our A-cats. The old aluminum rivets showed excessive corrosion, but after drilling them out, it was clear the carbon holes were enlarged with lots of corrosion signs. We had to repair it with a stainless sleeve and stainless rivets. In talking with sailors, I am suprised how few people know about this. To recap, this seems to be the basics. Perhaps some real engineers can correct any errors:

Fiberglass + Alum/Stainless/Carbon> ok, watch for tearing
CarbonFiber + Alum> NO-- significant electrical reaction
CarbonFiber + Stainess/Fiberglass> ok
Aluminum + Stainless> materials will electrically bond (so you cannot remove screws), but no structural risk

Yup - it's called galvanic corrosion or a galvanic reaction. You know how most outboard motors have zinc anodes on them? That's so the zinc, much lower on the galvanic scale, will take the corrosion hit and sacrifice itself saving the metals that are higher on the galvanic scale...hence the term "sacrificial anode". Eventually the zinc will corrode away and need to be replaced. Most steel (high pressure) natural gas pipelines take this reaction pretty seriously and are protected with similar system (I used to be a cathodic protection technician for a gas company).

Carbon falls under "graphite" in this list...though the carbon shouldn't be corroding when in contact with aluminum (I suspect that the aluminum corroded and became loose in the hole - the wear caused the opening of the carbon)...the aluminum will corrode quickly when in contact with carbon graphite in the same way the zinc anode does.

Wikipedia has a list of common materials on the galvanic scale...the further apart these materials are from each other on this list, the more severe the corrosive reaction will be. All dissimilar metals will have some sort of galvanic reaction.

http://en.wikipedia.org/wiki/Galvanic_series

That's right


There is absolutly a structural risk, in general the alu part will suffer more and fails first.

If you attache aluminium parts to carbon fibre, you should use a layer of glass fibres as isolation.
Also not all aluminium alloys are corrosion proof by itself. Some are, some not, as steel. In general the stronger ones (e.g. 2000 and 7000 families) are sensitive to corrosion.

Cheers,

Klaus

Jake, do you use anodes on your beach cat parts (mast, boom, beam)?

PS, how did you give the monkey and eye test to know he needs glasses? and doesnt he know smoking will stunt his growth?

What monkey? That's Jake's live webcam...

Nah, no anodes. I don't sail in salt water that much.

The monkey? He says he has trouble reading tax law so I got him the glasses...as far as smoking, he's pretty stunted already...what's the difference? He's the first one to get thrown to the wolves if I get audited.

I agree - aluminum and stainless + salt water = corrosion of the aluminum (not bonding). Definitely structural. I'm also fairly certain that carbon and stainless steel can eventually corrode.

Another important thing to keep in mind with this is that unlike fiberglass, carbon fiber is highly conductive, so you should be just a careful with a carbon mast as an aluminum mast when it comes to power lines.

sm

That is not a monkey but a primate, there is a difference.

Primates are all related to human beings in the way species are qualified and grouped together as "families".

Wouter

You should know. :-P

There's a special insulating paste you can (probably should) use whenever you need to assemble dissimilar metals together. It's quite expensive stuff but unfortunately I forgot the brand name.

Duralac

However, I've also used sticking tape, sticking sailcloth (were sail numbers are made from) and plain epoxy-glue or metal glue to seperate the two metals. All methods work pretty well. Only the plain sticking tape wears away after a while. All other stuff, including sticking sailcloth holds up for several in not many years.

Wouter

Is the paste / sticking tape / sailcloth / etc intended to go between the rivet and the walls of the hole (that the rivet goes into), or is this intended to go between the surfaces of the items being joined?

The paste (duralac) can also go between the rivet and the walls of the hole, the other isolators like tape can of course not. Although, I do use sticking sail cloth to isolate my aluminium axles from the stainless steel ballbearing surfaces of the wheel on my landyachts. Basically, the sticking sailcloth is 0.125 mm thick (1/200th inch) and can be wrapped really tight around any bolts or other elements that pass through other materials.

I got sticking sailcloth again under things like my diamond wires chainplates and the mast, so here it is between both surfaces. The reasoning is pretty much that the less large the contact surface is to less severe the corrosion. This is not entirely a proportional relationship but it does make a difference.

Most rivets on beach cats are Monel rivets which are a mixture of Nickel and Copper which are very attractive in a glavanic sense. As far as I can tell on my own sail craft thus far is that these rivets hardly corrode at all. You can practically use them without isolators like duralac.

I use some stainless steel (parker) screws on my boat (to secure the spinnaker cleat to the mainbeam) and the corrosion around those holes is much more significant. But I'm actually thankful for that as it locks up the screws. It takes a good number of years for the hole to corrode to a level were the screw may break out. Then it is just a matter of sliding the fitting some 10 mm to one side or rotate its base plate and screwing it back to the beam again. This has not been necessary so far and my F16 boat is now 4 years old and stored on the beach itself and sails 99.5% of the time in salt water. I replaced the spi cleat on other Taipan a while ago and that boat had it on there for some 6 years already. I think I redrilled the holes then; but more because the owner wanted to be sure about them.

I'm surprised how long this corrosion can go on and still hold fittings to their place. Again this was between typical aluminium (beam 6068-T5) and pure Stainless steel. Other fasteners like monel blind rivets see much less corrosion and I expect them to hold for many many years (15+years).

At least this is may experience so far. I have been building and modifying sailboats and landyachts (also use on the beach) for 10 years now.

The picture is of an aluminium (6060-T6) axle and a wheel of one of my sailkart landyachts. The black stuff is the self sticking sailcloth. The black rings are O-rings used to isolate the bearings from the alu axle and alu frame to the sides.





This setup works really well, I have never had so little play in the wheel as with this setup. Even is the fabrication proces is not entirely dependable you just fill the additional void with a single layer of self sticking sailcloth. That stuff is also very resistant to wear. I have never tried heat shrink but maybe that works too. Best advantage of sticking cloth it that you can dependent on it being constant is thickness. Just as with paper sheets the control of the thickness is tightly controlled during fabrication. And of course you can just trim a long piece of cloth to just the right length to remove all play. On my landyacht axles I use just shy of two wraps around the axle. Besides the stuff is both cheap (cut-offs of my sail number and class logo) and can be bought and shipped from anywhere to anywhere.

Wouter

Thanks Wouter - that's just the information I have been wanting to know, but never got round to asking!

Lanocote is another "assembly" paste. Another method (usually suggested for use on the Corsairs) is to coat the matching surfaces with a thin coat of silicon, dip the rivets in it, assemble it, but wait until the silicon has cured to pop the rivets.