Rot due to electrolysis???

[Trojan Dog]

Have a look at the attached pic: http://www.robertedney.com/scary_holes.jpg
You are looking aft at the base of the transom from the interior (28' 1963 Trojan sedan cruiser). Those two holes are below the waterline (the boat is on a sling in its berth). There was a large zinc plate attached to the exterior with two bronze bolts. The holes are about the size of the washers, and the zinc had just about fallen off. There is noticeable deterioration of the wood for a few inches extending out from the attachment points. This, along with slight weeping of water, were the only clues of a problem (the zinc was not visible from the exterior because of the swim platform). Moreover, the transom is clearly fir marine ply, while the rest of the boat is mahogany lapstrake -- strongly suggesting that it's not original. Is this electrolysis rot -- and how do you prevent it???

Thanks!

[Bob Cleek]

Yes it is, and about as clear an illustration of it as one might imagine. I'll wager the zinc wasn't connected to any other metal, either. In any event, what you have there is "over-zincing." The balance between the big zinc and the bronze bolts just made a great little battery there. Cooked the wood around the bolts. Happens all the time. LESS zinc. DO NOT "bond" the zinc to anything other than the shaft and prop via the stern bearing hagger bolts. (Wrap the wire and tighten down with a second nut on one of the hanger bolts.) Take a look around your harbor and see what size zincs others are using. LESS is better.

[Elco's]

This situation is unsettling...BUT, my opinion differs from Mr. Cleek's. Because there weren't any wires attached to the bolts holding the zinc into place, the damage was not cuases by galvanic action (electrolic corrosion). Thus, the zinc was not part of the bonding....and It would not be.
Bonding is used to protect underwater metals such as the struts and rudder.
As for "over zincing", the zincs attached to a stainless steel propshaft protects the bronze propeller(a LESS noble metal) on a stainless steel shaft( a MORE noble metal), too many zincs on that shaft negate the protection and the bronze propeller would corrode as if there were no zincs. To say that the zinc protected the bronze bolts that attached the zinc to the hull and thus "burned" holes into the plywood, ignores what the zincs actually do...they are "sacrificial" to the MORE noble metals. The zinc corrodes instead of the bronze.

It looks to me, that the rot in the plywood is a seperate situation from the attached zinc. I would suspect the plywood has rotted along an interior lamanite course.

[ 11-16-2004, 09:44 AM: Message edited by: Elco's ]

[Ian McColgin]

Interesting.

I'm not familiar with rot, per se, caused by electrolysis. I've certainly seen more than my share of rot in the way of fastenings. Iron in oak produces interesting chemistry. If electrolysis diminishes a fastening, then you may have conditions that encourage rot around the fastening. Etc.

It makes sense to figure out if anyone had anything coherant in mind with that zinc. Was it (ever?) bonded to anything? Maybe someone retro'd the platform on and thought a zinc would help the fastenings. Who knows. maybe it was attached to the top of what looks like the rudder shaft for reasons now unknown.

In general, everyone's right that it's way too easy to over-zinc. It's just the before I dismiss an idea, I like to figure out what the genius before me might have had in mind. Sometimes even if their exact solution is half-baked or over-engineered it's still worth recognising the underlying plan.

Anyway, prope about the holes and along the transom to see if the rot has spread. I do this with a bit of unpremeditated violence with a morticing chisel as anything that's black and comes out easily is a problem anywho. As you gouge and destroy, you'll quickly see whether this is patchable or whether the damage has spread so far that you're looking at transom replacement. If there's trouble there, look at other through-transom spots like the exhaust and up where the sole, gunnels and such attach.

If you must replace, consider solid wood.

G'luck

[Bob Smalser]

Those washer-size rot holes seem much too tidy for the the infiltration rot I'm familiar with in improperly-sealed plywood holes, even if the appliance was pulled off by force.

My money would be on electrolysis, which certainly will turn wood punky.

[TimothyB]

Not to be picky, but I can't help myself :)

What we are talking about is Galvanic Corrosion, which is -not- the process of electrolysis. Here is a reference for Galvanic Corrosion:

http://www.corrosion-doctors.org/Aircraft/galvdefi.htm

Galvanic corrosion, Electrolysis and Electrolytic corrosion are all seperate processes.

Electrolytic corrosion only takes place when an -outside- current affects a closed system, like stray current or an improperly bonded or grounded battery/generator system.

Electrolysis only refers to the electrolyte itself, not the other components causing the current to flow.

And we all know what Galvanic Corrosion is, since it is what we are talking about.

Let me also quote a boat surveyor on the subject:


It has been said by many electrical (gurus) that the term electrolysis is the most confusing and misused term around the boatyard. Electrolysis is a term that is loosely applied to the corrosion processes. The term refers to "solution phenomena" and not to corrosion.

There is a type of corrosion called electrolytic corrosion but it is not electrolysis. Electrolysis refers to the degradation of an electrolyte that occurs as a result of passing electrical current through it. Electrolytic corrosion is caused by a current from an external source, often the boat's battery or shore supply. The current that causes electrolytic action is called "stray current" and usually emanates from a poorly installed electrical circuit or a bad earthing (ground) arrangement, like a poorly installed electrical circuit or bad ground arrangement on a radio or power tool or a current leak due to damp connections.

The confusion is usually between the terms galvanic corrosion and electrolytic corrosion. Galvanic corrosion is caused by an electric current generated by two different metals in a conducting medium such as seawater. When we talk about galvanic corrosion we're talking about an electric exchange. The results of each type of corrosion can be similar and can occur at an alarming rate. The bottom line however is that you can have a properly wired boat moored at a properly wired dock and still be at risk of corrosion. Addendum: In the case of the bronze burning the wood, the zinc should have sacrificed itself to protect the bronze. The washers may have been steel, which could have caused the problem galvanically. The zinc would have protected things, but the current flowing between the washers and the zinc would have been significant. There are a lot of possibilities.

[ 11-17-2004, 09:55 AM: Message edited by: TimothyB ]

[Trojan Dog]

Interesting responses -- and a reminder as to why I am confused every time I read about this subject. For the record: This "rot" is very clearly associated with the bronze bolts (actually threaded rod) and washers. Yes, the washers are bronze too. The rot is most concentrated directly under the washers, and radiates outward, ending with solid wood a few inches away. It does not look at all like delamination. Every other part of the transom is solid. What I've done (pending her next haul out) is to bed a patching block on the inside in urethane goop and through bolt it to the tansom. I then re-attached the zinc. My reasoning here is that the boat will be hauled before long, that she's in fresh water (less electrolysis?) and that the zinc was originally put there for some -- possibly -- good reason. Now, about bonding? Should that zinc be electrically connected to anything else?

[TimothyB]

Found a reference work:

http://www.kastenmarine.com/mbqCref.pdf

Short answer: You should use insulated washers and bearings to reconnect the zinc so current does not flow through the water soaked wood. You should not bond. Read the whole paper to get the details.

[ 11-17-2004, 02:11 PM: Message edited by: TimothyB ]

[RonW]

Neat info Timothy. And like Trojan, I admitt That I am not clear on this subject. But with that being said, how about another possible theory.

The holes are below the waterline, and instead of tight fitting bolts, as Trojan says they are actually threaded rod. What about plain old simple seapage around the threads of the rods. The wood surrounding the threads will wick up the moisture. And 40 years later the wood surrounding the threaded rod has simply rotted due to constant seapage. Plus the threads actually leaves small air pockets that are filled with moisture in a confined space that dries too slowly not having much air circulation.

[TimothyB]

Sure Ron. That sounds plausible as well.

I think bedding it well with compound and insulating the bolts from the wood will cure the problem, regardless of it's origin.

--T

[RonW]

http://marinesurvey.com/#Articles

This might be a good place to add this website, scroll down to his articles, then the ones on corrosion.

[Stargazer14]

That photo looked very familiar -less the holes - I have a 28' 1968 Trojan.
I dont see any reason why you should bother putting that large zinc back on your transom, if you are hauling soon anyway I would think the best thing to do would be check the condition of your 'small' rudder and shaft zinc if possible(how cold is the water?) and then have another look upon hauling to see what happens without this stray mystery zinc.
That is of course if you have zinc in the place it should be....

[paul oman]

I would think that epoxy coating any surface that comes in contact with the metal or bolts would serve to insulate the lot from both moisture/water and tiny electrical fields.

I am curious like all of you about the cause of the problem (current or moisture) but think the epoxy would fix both.

paul oman

[Bob Cleek]

Listen to Stargazer. He's got a boat like yours. If I were you, I'd find out what he uses and how it works for him, then try that. And thanks to those who clarified the nomenclature. Yes, there's different types of corrosion. Whatever you call it, though, a BIG zinc and bronze bolts are going to mean that the surrounding wood will get "cooked." I'll leave how that happens to the scientists.

Your later question was telling. Any zinc MUST be "connected" to the metal it is intended to protect. A specialized shaft zinc has a metal to metal connection with the shaft. Your zinc block should have either a bare copper wire coming out of it for the purpose of wiring it to whatever it is intended to protect, or tabs cast in it for welding to a metal hull, which provides the connection. A zinc hanging without anything to protect is useless. You will know if it is doing what it should if it is wasted (corroded to flakey white chalky stuff). The degree of wastage will tell you how well it is working. If a zinc that has been around a while looks pristine (short of marine growth), odds are good it isn't doing its job and may even be hurting.

Also, when you think about galvanic action, "noble metal" and "less noble metal" isn't the only factor. There has to be a proper comparison of masses, one metal to the other. A big huge zinc with bronze bolts not connected to anything else is going to do something galvanic. The wood in contact with that "galvanic hunk" is likely to go first. I don't know why, but it always does.

BTW, do you have any idea why somebody through-bolted a zinc with such heavy fastenings. Generally, zincs on wood hulls are simply screwed onto the wood with small wood screws that don't penetrate the hull. Their only purpose is to keep it from falling off. You aren't going to need that kind of holding power for that.

[TimothyB]

Just a quick note about fastening zincs for purposes of protection..

According to all the info I've read about this, it is desirable to attach zincs directly to the objects they are protecting, but for galvanic protection it is not necessary to do so. This is because the zinc's presence near the metal mass causes the electrical circuit to form between the zinc and the other metal -instead- of the protected source. Many casual references seem to cit ethat you -must- connect them, but that is not so.. not to protect against galvanic corrosion.

Attaching a zinc directly to the object obviously gives you the very best proximity to it, and in addition helps protect against electrolytic corrosion (stray current) as a bonus. However also according to what I've read, you shouldn't arbitrarily attach heavy zincs to things, since you could end up creating a problem rather than solving it.

I'd recommend reading thoroughly the references I cited. They have a great deal of very detailed and cogent information on it.

Also, listen to Bob. Bob good. Bob knows Boat. ;)

--T

PS: All of this talk about stray current and I might just hook up two grounding plates for a lightning protection system instead of one, put em on opposite sides of the boat and bond them together with heavy duty wire. I'd be killing two volts with one ohm. smile.gif

[ 11-18-2004, 08:36 AM: Message edited by: TimothyB ]

[Elco's]

I love it when I learn something from a thread!