Crevice Corrosion

triton318 · Post by **triton318** » Sat Oct 04, 2008 7:38 pm

Does anyone know if silicon bronze is as susceptible to crevice corrosion as stainless steel?

Rachel · Post by **Rachel** » Sun Oct 05, 2008 1:20 pm

Not nearly. Actually, I'm not sure it's subject to crevice corrosion (specifically) at all. It can de-zincify (right term?), but that's different. That's not a "lack of oxygen down inside something" issue.

It's just a lot less "sneaky" than stainless ;)

Rachel
<admittedly biased bronze fan>

LazyGuy · Post by **LazyGuy** » Sun Oct 05, 2008 4:11 pm

I had some training in metallurgy a "few" years age so don't take this as gospel, but I believe it is only steel that is susceptible to crevice corrosion and stainless steel is more susceptible than mild steel. It has something to do with certain elements within the steel such as Chromium, Magnesium and Molybdenum which are all parts of the various alloys of stainless steel.

Short answer.... Bronze is not susceptible to crevice corrosion. That is why they use it for fasteners in quality wooden boats. They never use stainless below the waterline. Another option that is not susceptible is Monel.

bcooke · Post by **bcooke** » Sun Oct 05, 2008 6:28 pm

The bigger question is, why is this important?

Inquiring minds want to know.

triton318 · Post by **triton318** » Sun Oct 05, 2008 9:31 pm

bcooke wrote:The bigger question is, why is this important?

Well...when I was redoing my Triton seven or so years ago, I removed the A-4. I also built a new rudder because the original one was in really bad shape. Because I was going to use an outboard, I built the rudder without the propeller aperture. I had a new rudder post fabricated out of 316 stainless, with two sets of heavy stainless straps welded onto the post and a 1/2" pin machined on the lower end (with threads on the very bottom). The bottom pin fits into a stainless gudgeon. The rudder slides between the straps and is bolted on. Then there is a stainless steel pintle and gudgeon at the bottom of the rudder. Obviously, I hadn't done enough research. (I am pleased that everything has worked fine these past seven years.)

The boat is out of the water and I removed the rudder, rudder post, gudgeons, and pintle. I have yet to clean up the rudder post, but a cursory inspection showed it to be in good shape. (I'm going to clean it up with a wire wheel and inspect it thoroughly with a magnifying glass.) There was, however, a lot of crevice corrosion on the bolt threads where the nuts were. This makes sense, based on my newfound understanding of crevice corrosion.

So now I'm wondering how to proceed when I go to put everything back together. If I find that the rudder post and straps are okay, I'd rather not have a new rudder post fabricated in bronze. (Which is what I should have done originally, I guess.) But, I know I'm not going to want to reuse stainless steel bolts. (Unless I find there's some way to prevent the crevice corrosion.)

Oh hindsight...

David · Post by **David** » Sun Oct 05, 2008 9:56 pm

If you used 316L stainless I don't think you should have a problem. 316 is much less prone to crevice corrosion than lesser stainless alloys; plus the rudder assembly is in free flowing water. Crevice corrosion is formed in oxygen deprived stagnant seawater.

David

triton318 · Post by **triton318** » Mon Oct 06, 2008 8:40 am

David wrote:Crevice corrosion is formed in oxygen deprived stagnant seawater.

Right--as in the area of the threads of a bolt where the nut is. That's where the corrosion was pretty bad. I'm wondering if I could use bronze bolts/washers/nuts on the stainless straps. Bronze and stainless are very close together in the galvanic series, so maybe there wouldn't be a problem. Plus, I could install a sacrificial zinc anode on the rudder near the straps. I don't know...I guess I'll keep doing research. And, of course, any advice from this forum would be appreciated.

David · Post by **David** » Mon Oct 06, 2008 10:07 am

I think you are better off staying away from dissimmilar metals on your rudder.

Post by **Tim** » Mon Oct 06, 2008 10:16 am

triton318 wrote:I could install a sacrificial zinc anode on the rudder near the straps.

If the zinc isn't on the metal itself, or electrically connected thereunto, it won't provide any protection.

Rachel · Post by **Rachel** » Mon Oct 06, 2008 6:32 pm

I think it's uncommon, but on the W-32 there was a zinc for each gudgeon, and one for the bobstay fitting (stainless). They were mounted to 'glass a short distance from the fittings and connected via wire.

Not saying it was the most desirable set-up; but it is done. The zincs were shaped like half of a hard-boiled egg and had molded in grooves on the back (flat) side for wire.

Rachel

Post by **Tim** » Tue Oct 07, 2008 6:08 am

Rachel wrote:...connected via wire...

This being the key to a zinc located away from the metal part, and the same as

I wrote:...electrically connected thereunto...

triton318 · Post by **triton318** » Tue Oct 07, 2008 9:53 am

Tim wrote:If the zinc isn't on the metal itself, or electrically connected thereunto, it won't provide any protection.

I guess I've misunderstood this all these years. I always thought that by being immersed in saltwater (the electrolyte), the two dissimilar metals were electrically connected. But you're saying the two metals have to actually be touching or connected by some conductive material (excluding seawater), i.e. a wire. Saltwater does conduct electricity, doesn't it?

So I could have a stainless steel rudder post and a stainless steel upper gudgeon (contacting the SS rudder post) and a bronze gudgeon and pintle on the lower end of the rudder, and I wouldn't have to worry about the bronze corroding preferentially to the stainless?

Rachel · Post by **Rachel** » Tue Oct 07, 2008 11:46 am

Tim,

Sorry if it seemed like I was implying that you hadn't addressed the connectivity issue. I was just giving an example (vs. saying something different or contradictory) of how they were set up on the W-32 because I haven't seen that many boats with "remote" zincs like it had.

Could have been more clear on my part.

Rachel

Sailordave · Post by **Sailordave** » Wed Oct 08, 2008 6:33 pm

Wellllll, several years ago the Triton in my (then)sailing club dropped the rudder under the Chesapeake Bay Bridge. I took on the job of building a new one after the guy who was supposed to didn't. (long story there!)
Anyway, what we did was take a length of BRONZE rod, had the machine shop bend it to the correct angle and cut a groove about 12 in. long and then brazed a piece of 1/4" bronze plate into the groove. This gave us a huge lever that was inbedded in the made up rudder and spread the load out over a larger area.

On the original rudder the bronze stock just had SS FLHD screws going into the rudder.

I sandwiched two pieces of 3/4" marine ply cut out to the shape of the rudder around the bronze plated and recessed the rod from the rudder stock into the leading edge. Worked like a charm, AFAIK it's still sailing like new! Oh, and both pintles were hung on the gudgeons on the original; I installed one upside down so it COULD NOT come off unless it was intentionally taken off. I wasn't worried about excess wear on the one pintle/gudgeon as the rudder was almost neutral buoyancy, plus I believe when they installed it (I was out of the country sailing!) they put a washer on the pintle.

Oh, and to comment on the original ?... Don't use SS, use bronze.

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Crevice Corrosion

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