AACK! Galvanic Corrosion!

[flydog]

My boat is too new for this! I believe that I have a case of galvanic corrosion on my recently built boat. I attached a brass half-oval to the skeg of my wood boat using silicon bronze woodscrews. I figured that the bronze and brass were compatable galvanically. I only used the boat 5 times in fresh water and I noticed a white residue building up around the bronze screw/brass interface. Would replacing the bronze screws with brass ones solve the problem?
Here's a pic:

flydog

[longliner45]

do other boats near yours have this problem?

[Valk]

You could go ahead and get a galvonic isolator or if you could buy a galvonic tester. Fluke makes a cheap one I think. Might be worth asking the local marine tech if he has one. This looks to me like it may be "dissimiliar metal corrosion". What is the rail and the fastener made out of? Could be as easy as cleaning and installing new fasteners. This type of corrosion is amplified in a saltwater environment. If you have an aluminum rail and stainless or cheap steel fasteners this may be your problem. You could go to an aluminum fastener or at least use a nonconductive past to isolate the two materials.

[flydog]

I trailer the boat and it only goes to a lake. I did use SS screws to hold the keel to the bottom. the rails are wood held on by silicon bronze screws.

flydog

[Valk]

Wow, if you trailer the boat then that is almost definatly dissimiliar metal corrosion. If that metal strip is aluminum, try some aluminum screws. Maybe remove a few screws and remove the corrosion. Then replace with alum screws. Be shore to remove ALL traces of corrosion. This may entail removing the metal strip. In which case ya might as well just replace all the screws... Try a brass wire wheel on a drill for the hard to reach areas behind the strip. If by chance the metal strip is SS steel use the same for fasteners. The two different types of metal touching each other is whats causing the problem. I would be willing to bet those are silicone bronze screws touching that alum or steel strip? (I used the term "rail" before because I thought that was a bumber strip.)

[flydog]

The strip is actually a brass half-oval held on by silicon bronze screws.

flydoghttp://www.flickr.com/photos/9381061@N02/

[Ike]

If the half oval is brass, not naval bronze, then that is the problem. genuine brass and aluminum do not mix well. In fact brass does not do well in water. It is an alloy of Copper and Zinc, both on the other end of the galvanic series from aluminum. What is actually used in most marine applications is naval bronze which is copper zinc and tin with some other things thrown in to make it even more corrosion resistant such a silicon. If it were naval bronze, which many people call brass, you wouldn't be having this problem. As was said. On the aluminum, use only aluminum screws. Get rid of the brass.

Oddly enough if you add aluminum to the copper tin alloy it actually makes it stronger and more corrosion resistant. Who knew?

Wikipedia on Brass http://en.wikipedia.org/wiki/Brass

If you want to see what I am talking about go buy a couple of common brass screws, just screw them in anywhere on your boat and watch them begin to disappear. Compare them to you silicon bronze screws and see the difference.

[Pierre R]

If the boat will only be used in fresh water and is trailered you can temporarily use brass screws to replace the silicon bronze. The brass will hold up so-so in fresh water. When the brass goes to pot you can replace the whole system with something more compatible with a marine environment. In the mean time replace and go enjoy the boat.

[timgoz]

Hey Flydog,

I'd do what it takes to make it right now. Having that imperfection at the back of your brain has to be annoying. CharLou deserves the best!

Have a good one and enjoy your summer. Hope all is well with you.

Tim

[artemis]

Looking at the first photo, enlarged, clearly indicates that the problem is with the brass rail/silicon bronze screws! Use a silicon bronze strip.

[Frosty]

What ever you decide to do you should do it quickly, or those screws will twist off.