Galvanic Corrosion

kenny · Post by **kenny** » Fri Feb 10, 2006 11:22 pm

aya16 wrote:I saw what a leaky electric motor did to a lower unit of the outboard on the same boat. It burned through the outboard metal in a couple hours of use.

aya16, can you clarify what "I saw what a leaky electric motor did to a lower unit of the outboard on the same boat. It burned through the outboard metal in a couple hours of use" means.
Do you mean it was leaking current or ?
I'd like to understand this possibility better; having never considered it.

Edit by Mod: aya16's story about the electricity leak and the related discussion have been re-filed here, creating this topic on Galvanic Corrosion. (from Elec. Kicker/Generator topic)

aya16 · Post by **aya16** » Sat Feb 11, 2006 5:53 am

Current yes. The electric motor was a couple years old and on a small 16 foot fiberglass boat. The e motor was a foot controlled bow mount. it was used in salt water to fish the sea break wall in Long Beach mostly. When the current started to leak it took out the zinks on the main outboard first then attacked the lower unit and created pot holes in the skeg on up. It also took out the shaft on the electric outboard. and pot marked the metal on the transom. I have also seen tin boats attacked in the same way.

kenny · Post by **kenny** » Sat Feb 11, 2006 10:19 am

aya16, thanks for the clarification, I thought that was what you meant BUT couldn't really believe it. Did this happen quickly or over a long time period.

Tom Root · Post by **Tom Root** » Sun Feb 12, 2006 12:51 pm

aya16 wrote:Current yes. The electric motor was a couple years old and on a small 16 foot fiberglass boat. The e motor was a foot controlled bow mount. it was used in salt water to fish the sea break wall in Long Beach mostly. When the current started to leak it took out the zinks on the main outboard first then attacked the lower unit and created pot holes in the skeg on up. It also took out the shaft on the electric outboard. and pot marked the metal on the transom. I have also seen tin boats attacked in the same way.

I wondered the same thing on what you meant by "leaking"?

I just want to mention that I find it highly impropable that just the Electric motor did the dastardly deeds you speak of.

Of course, I may not know all the variables/conditions/circumstances..... whatever, but the zinc and lower end problems you speak of may be normal corrosive effects, or stray voltage "leaking", if you will from a dock electrical source possibly.

The whole reason I hypothesised this is simply because the threat is removed once the batteries are depleted, well there just isn't a source for EMF period, therefore the electrolosis cannot occur!

Maybe I am missing something, but just a comment nonetheless, as I cannot invision an electric motor as the cause in any way.

I do know every one I have ever used is good for about a season or two at most in a salt water environment, unless specifically designed for salt water. They corrode in short order.

A Salt Away, bath helps, otherwise it is usually toast!

As it is, all our baots are sitting in a solution that is similar chemically in SOME respects to a big battery whereas metals want to resort back to their natural state (corrode) One of the big reasons I'd never own an Aluminum or Steel Hulled boat! And of couse worms, mold and general rotting effects loves the medium of wood! They don't know when Fiberglass dies yet, so I am pretty confident in it's makeup, to hold together till I draw my last breath anyway!

One of the FEW exceptions is the 'Star of India' a Steel sailing vessel from the 1800's in San Diego Harbor. It amazes me that it is not scrap yet!

aya16 · Post by **aya16** » Sun Feb 12, 2006 4:33 pm

well tom either I made it up or it happened, sooooo a trailer boat used in salt water will not have much effect from the salt in the amount of use it will get. When a fairly corrosion free pot free motor in a day turns to crater world and all the zincs are gone and the electric motor is toast
I would guess it was caused by the electric motor as that fried first but was leaking elec. in the water before it did.

Tom Root · Post by **Tom Root** » Sun Feb 12, 2006 8:02 pm

aya16 wrote:well tom either I made it up or it happened, sooooo a trailer boat used in salt water will not have much effect from the salt in the amount of use it will get. When a fairly corrosion free pot free motor in a day turns to crater world and all the zincs are gone and the electric motor is toast
I would guess it was caused by the electric motor as that fried first but was leaking elec. in the water before it did.

Wow, in a day's time! That really is incredible!

OK, I'll take your word for it! Again, if you had her tied to a dock, I would find that suspect.

To do that much damage is something that would concern me for sure!

What brand/model and prior condition was the trolling motor?

Back on the topic, I think that using the method that MadMike used on his X is a good method, and stick with a gas motor vice electric for practical purposes.

I doubt they make an Electric powerful enough to provide any reasonable thrust, even for a dink...I've been there done it, and canned that idea quickly!

Maybe for a bow thruster.....but that would be it!

Tom Root · Post by **Tom Root** » Sun Feb 12, 2006 8:24 pm

aya16 wrote:well tom either I made it up or it happened, sooooo a trailer boat used in salt water will not have much effect from the salt in the amount of use it will get. When a fairly corrosion free pot free motor in a day turns to crater world and all the zincs are gone and the electric motor is toast
I would guess it was caused by the electric motor as that fried first but was leaking elec. in the water before it did.

Ohhh, and please catch me on my typos too, 'cause I sure have a plethora of mistakes.

But I do need to know what a pot free motor is that you speak of?

And THEN, it took a leak in the water....geez! I gotta quit this, I know!

Maybe that's what 'Fried it', perhaps?

They Theirs · Post by **They Theirs** » Sun Feb 12, 2006 11:51 pm

Current leaks are the most common form of stray current problems

Electrical Leaks

Tom Root · Post by **Tom Root** » Mon Feb 13, 2006 7:44 am

Great links Dave, I should go out and take pics of my Engine's lower half.

It has areas on it that seem to be ready to loose the paint already. It is bubbling and white in color. I used the boat frequently in salt water, and watched the Zincs, and am replacing them currently even though my Suzuki manual stated that thet need replacing after 2/3's of them are depleted. I'd say less than 1/3 if that, is gone on them, and am getting leery!

I use Salt Away judiciuosly, flushing each and every time, and replaced the Impeller and Thrust Plate. It looked fairly good inside the lower half's housing, it's the outside that concern's me!

I have tried keeping the painted surface clean and waxed, and am hesitant to put Anti-fouling on them due to the fact that you cannot actually see the corrosion, before it is too late!

All bonding wires appear to be in order also, so this is a concern of mine for sure!

The damage that AYA16 relates to seems to be extreme and as stated in the story he relates to, and I want to mitigate any damage that he related to!

Catigale · Post by **Catigale** » Mon Feb 13, 2006 8:46 am

One of the classic mistakes that can literally corrode your engine overnight is to common ground your DC and your shore power to a single point.

What happens then is someone leaks current from the shore power, it goes into your DC ground and then exits your boat through grounded metal in the water (like your zincs) and completes the circuit through the water to the boat leaking current.

30 Amps of current can be supplied (this would be extreme of course), so it can do a FAiR Days work destroying your motor once it consumes your zincs.

Yeeks..

Scott · Post by **Scott** » Mon Feb 13, 2006 10:04 am

AAAAAAHHHHHH, Cat. On the money again. I posted the pic of my forestay bolt ate 1/2 through. When I was re-wiring the boat this year I noticed that my ground lead from my shore power which I lead to my centerboard pivot, was wasted. Bright green and starting to deteriorate. This justso happens to be where I commoned my house grounds. Also it is at the base of the compression post by virtue of the centerboard cable. Maybe that explains my problem.

To answer the as yet unasked question, I did in fact remove the offending wire and seperated the ground circuits. No, I was not smart enough to figure out the problem, I just took it on faith that if there was that much corrosion I must have done something wrong.

Strange that a current leak low in the boat would manifest itself in the way of corrosion at the top of the mast.
Any engineers care to chew on that??

Catigale · Post by **Catigale** » Mon Feb 13, 2006 11:22 am

Scott - are you in fresh water? The higher resistance of fresh helps you here as it drops the potential of the leaking current to do damage...

Glad you found it though...if you get someone running an AC with an unbalanced shore power load you can get a lot of current running and wreaking havoc...

IIRC you had the forestay pin half chewed through, but it didnt look like wear, so we thought it had to be electrolytic corrosion?

Catigale

Frank C · Post by **Frank C** » Mon Feb 20, 2006 2:20 am

Regarding that forestay bolt ... I thought this corrosion problem required dissimilar metals, but the forestay tang, turnbuckle and forestay bolt should all be stainless? ... that sounds more likely to be caused by constant shock-loading of the rig?

James V · Post by **James V** » Tue Feb 21, 2006 12:02 pm

It could be from rust. In order for Stainless Steel not to rust, it needs O2. If I remeber your photo correctly, there was some rust. Wear and tear and rust?
This is somethings that need to be checked every year. Does anybody have a winter/spring/before you put in water checklist?

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Galvanic Corrosion

Galvanic Corrosion