Electrolosis ??

[RodSBT]

Did I spell it right? (..and no, I'm not talking about removing unwanted hair !:D)

Anyway, the subject came on another thread and has me wanting to learn more.

So what causes it, what happens when it occurs and how do you prevent it?

[willmarsh3]

Electrolysis is a process that occurs when two dissimilar metals in contact sit in seawater. They set up an electrical current like a battery. The metal lower on the galvanic series gets dissolved - and rather fast in some cases. The good thing is that the metal higher on the scale gets protected from corrosion.

The galvanic series is described here: http://en.wikipedia.org/wiki/Galvanic_series

This occurs also in fresh water but at a lower rate because it is less conductive than seawater.

A common example is a zinc attached to the rudder. The zinc undergoes electrolysis and corrodes away while protecting the rudder.

[Peerie Maa]

When you have two dissimilar metals connected by conducting liquid (salt water) they form an electric battery. This can occur where the connection is made by wet wood into which the dissimilar metals are fixed. Several things then happen.

the less "noble" metal corrodes away. For example the zink in brass corrodes out, leaving weak spongy copper behind.
The ions carrying the current through wet wood create alkaline conditions which destroys the lignin in the timber, causing it to crumble away.

A similar effect can be caused by stray currents from your electrics finding their way through the wet wood of your hull.
Here's a link:
http://marinesurvey.com/surveyguide/wood1.htm

[James McMullen]

It's bad news in any boat, but wooden boats can have even more problems as the changed PH issues at either end of the anode/cathode pair can chemically weaken the wood. Typically, the worst problems I've seen are from de-lignification at the higher PH (base) end.

[rbgarr]

Electrolysis is a method to permanently remove hair.

Galvanic corrosion is the issue regarding dissimilar metals and boats.

[Cooper]

rbgarr - thanks for the enlightening correction:
"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte."
"electrolysis is a method of using an electric current to drive an otherwise non-spontaneous chemical reaction. " (gotta love wikipedia). So I guess all galvanic corrosion is electrolytic, but not all electrolysis is galvanic corrosion (?)

[Cooper]

how do you know how advanced Galvanic Corrosion is by looking at, say in a bronze fitting in the bilge? Is the color and type of surface corrosion any indicator? I know 'electrolysed' bronze goes pink but one has to cut or file it to check that way.
thanks
C

[rbgarr]

how do you know how advanced Galvanic Corrosion is by looking at, say in a bronze fitting in the bilge? Is the color and type of surface corrosion any indicator? I know 'electrolysed' bronze goes pink but one has to cut or file it to check that way.
thanks
C

Check the link mentioned above for signs of corrosion types: http://marinesurvey.com/surveyguide/wood1.htm#Electrolysis

[georgel]

a subsidiary question is How do you prevent it. Or how many zincs are enough. # 1 do not use automotive battery chargers on boats. Besides the danger of electrocuting nearby divers, as they are based on autotransformers and dump current into the "neutral" or ground side of the circuit. Thus accelerating problems. One approach to protect your boat is to use lots of zincs. It is wrong. You want to use just the right # in the right locations. too many will accelerate the galvanic current. causing deterioration of wood and cook off the paint on metals and probably damage metals as well. I have tested moorages where the AC outlets on the docks were hooked up backwards. A lot of AC equipment doesnt care. Light bulbs, motors. But it causes the water to become an electyical conducter carrying a large amont of current. Any way Its a complex subject with no simple answers.

[RodSBT]

Thanks, interesting problems to solve.

[Excalibur]

rbgarr - thanks for the enlightening correction:
"Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte."
"electrolysis is a method of using an electric current to drive an otherwise non-spontaneous chemical reaction. " (gotta love wikipedia). So I guess all galvanic corrosion is electrolytic, but not all electrolysis is galvanic corrosion (?)

Correct. Galvanic corrosion is a "passive" process where dissimilar metals that are connected together electrically are immersed in an electrolyte (seawater in many cases), and current flow simply occurs through the connection. You can also get "active" corrosion where an electric current is forced though the water from metal to metal. In this case, the seawater is acting as a conductor, not as an electrolyte. This usually occurs at docks where a boat (or the dock itself) has some metalic part of it's systems in the water with a voltage applied to it. It often happens when the dock has poor grounding. If any boat on that dock's AC system doesn't have an isolation transformer or galvanic isolator, the voltage that appears at that boats grounding plate will induce current flow with other boats. This kind of active corrosion can be very damaging very fast.

[mobjack68]

just don't put copper bottom paint on an aluminum boat......
I posed a question to my brother who at the time was studying electronics and seawater.....I asked how much seawater would be required to create enough potential to power an electric vehicle...You know, a seawater battery in the trunk !! when you start going slow, you drive to the beach, pump out the old seawater, pump in fresh...obviously under the cover of darkness, drive on!!

[Excalibur]

just don't put copper bottom paint on an aluminum boat......
I posed a question to my brother who at the time was studying electronics and seawater.....I asked how much seawater would be required to create enough potential to power an electric vehicle...You know, a seawater battery in the trunk !! when you start going slow, you drive to the beach, pump out the old seawater, pump in fresh...obviously under the cover of darkness, drive on!!

And your brother went "huh?". In the galvanic process, it's not the electolyte that's being used up, it's the anode. When you get to going slow, you don't need fresh electolyte, you need another anode!

[JPhoenix]

After my survey last week, my surveyor recommended in his report that I consider an "active impressed current system" in lieu of all the zincs I have. Anybody ever heard of one of these systems and where I can find a dscription of one?

Thanks,

Jm

[AstoriaDave]

Excalibur wrote: Correct. Galvanic corrosion is a "passive" process where dissimilar metals that are connected together electrically are immersed in an electrolyte (seawater in many cases), and current flow simply occurs through the connection. You can also get "active" corrosion where an electric current is forced though the water from metal to metal. In this case, the seawater is acting as a conductor, not as an electrolyte.

Dave the chemist responding here. Might expand on what Excalibur has written a bit.

Galvanic corrosion demands that some chemical process (typically breakdown of the sea water) occur on the cathode surface to absorb the free electrons generated within the metal that receives those electrons from the anode metal. In the case of zinc on a steel hull, zinc is the anode, releasing electrons into the zinc, which then pass into the steel hull (the cathode). The steel can not continue to build up electrons, so it releases them to water in contact with it, and the water breaks down, generating hydroxide ion (where the electrons reside as they move into the sea water), and hydrogen gas, typically.

So, the water is functioning as a conductor (and electrolyte) in galvanic corrosion, also.

In electrolysis, some other source of EMF drives the process (a battery or another DC source with its positive lead in the water, for example). In the case of a positive ion current passing into a hull (steel, for example) from sea water, the current carriers in sea water are ions, hydrogen ions most commonly (sodium ion is not easily reduced, so it is present but not chemically active). These are chemically transformed at the metal surface into other species. If the current flowing into the hull consists of positive ions, they take electrons from the hull, making the hull an anode, and oxidizing the hull -- e.g., the iron metal is transformed into soluble ferrous or ferric ions, and the steel corrodes. This is what people usually mean when they say "electrolytic corrosion occured."

In contrast, if the negative wire from the DC source is in the water, negative hydroxide ions (or chloride ions) travel from the wire to a steel hull, and at the hull, oxygen gas or chlorine gas are produced, and free electrons move into the hull. The steel remains intact, because it is the water or chloride ions which are changing chemically, not the steel. This would also be an example of electrolysis, and the water is a conductor and an electrolyte. But, there is no corrosion, because the steel remains intact. The electrons released into the steel maintain it as a cathode, protecting it from corrosion.

[KAIROS]

Here's what I assumed was due to electrolosis on my boat. We had used the boat for a season, then got into a restoration, which led to a repower. When we had the old motor hoisted out, there it was. An awl is shown here, but you might have stuck your finger into it. The center of the problem is a motor mount hole, visible here. This is afromosia wood.

The mystery to me is that this 'delignification' was completely isolated. It stopped just where it appears to stop. The wood a half inch from the apparent problem, in every direction, was rock hard (as this wood is.....much like teak).

Where was the current going to? Current can't just radiate outward.....doesn't it have to be going somewhere else?

We carefully checked wood it might also have been traveling through nearby....such as the through-hull bronze bolts at right and left. Those bolt heads on the outside of the hull were countersunk and bunged, and there was no bad wood around them.

Electro-doctors: what gives?

http://www.yachtflyers.com/forum_images/electrolosis.jpg

[KAIROS]

I should add that the old motor wiring was in terrible shape. The ground wire to the engine block was loose too...

[AstoriaDave]

Kairos,

We do not have enough information, really, to sort this out. Likely, an active source of current was pumping electricity through that area, and dissipating itself through the wood pores themselves. That area looks as though it was wet a good bit of the time. A loose or disconnected ground wire may be responsible, but a better analysis would have to be done to answer your question. Probably not an example of over-zincing, given the degree of damage.

That boat was poorly maintained.

[KAIROS]

....That boat was poorly maintained.

I'll say. It was on the cusp of ruin. Somehow nothing bad happened to the structure......beyond the cockpit which was modified crap. There were several cases like this photo which were not caught in days-long surveys.

In this and most cases we didn't care since we were replacing it anyway. These problems even made us feel better about hacking out wood, which we would have done even if it were in good shape (:eek:) We had a plan, and that bed log wasn't part of it.

I know electrolosis is complex. Some folks looked at it and shook heads. It made no sense to me. We moved on.

[donald branscom]

just don't put copper bottom paint on an aluminum boat......
I posed a question to my brother who at the time was studying electronics and seawater.....I asked how much seawater would be required to create enough potential to power an electric vehicle...You know, a seawater battery in the trunk !! when you start going slow, you drive to the beach, pump out the old seawater, pump in fresh...obviously under the cover of darkness, drive on!!

The Egytians had electric lights with a ceramic jar with two dissimilar metals in it. The jar contained acid. The part they did not perfect was a tungsten bulb.

[Cooper]

this has become an interesting thread, I particularly appreciated contributions by astoriadave and excalibur. "This kind of active corrosion can be very damaging very fast." Yuh, scary. I feel better informed but not that much more enabled to deal with it :( It seems clear that bonding is out of fashion, so I'll rip that out. But it seems that electrolysis can go on in silicon bronze alloy, within the metal, in contact with sea water, and there seems no way to halt delignification.
Georgel asked the hanging question - one I can't answer - how much zinc is enough, how much is too much, and how would you calculate it?

[AstoriaDave]

Georgel asked the hanging question - one I can't answer - how much zinc is enough, how much is too much, and how would you calculate it?You can't. Instead, set the boat up in the water and use a silver/silver chloride electrode and a digital voltmeter to measure the potential of each thruhull (etc.) which is immersed in sea water. There are tables in references on marine electricity which give the accepted potential differences. See, for example, Nigel Calder's Boatowner's Mechanical and Electrical Manual, Third Edition, p 216 ff and other info in the same chapter.

http://www.amazon.com/Boatowners-Mechanical-Electrical-Manual-Essential/dp/0071432388/ref=dp_ob_title_bk

[Excalibur]

Thurman the computer geek here;
Thanks to Dave the Chemist for the additional info. I'd go with his recommendations, he's the pro, and I'm the amateur.
Kairos, as Dave said there is not enough info. My guess is that poor grounding on the motor and salt water in the bilge over the motor mount (unless there was no rubber isolation, in which case the water only had to touch the mount) allowed current flow from the engine to the sea, which dissipated over the large (water) surface area once it got away from the mount. In other words, what Dave said.
jm, I've heard of active impressed current systems, but I have no personal experience for you. Dave? This wiki pretty much tells all I know;
http://en.wikipedia.org/wiki/Cathodic_protection
They are writing about land based systems, but the marine systems work the same way.

[AstoriaDave]

Excalibur wrote: jm, I've heard of active impressed current systems, but I have no personal experience for you. Dave? This wiki pretty much tells all I know;
http://en.wikipedia.org/wiki/Cathodic_protection
They are writing about land based systems, but the marine systems work the same way.

Pretty much all I know also. I've seen the systems used for mothballed ships (Bremerton, WA), which have long wires over the rail, suspended in the salt, running electrons into the ship's hull via DC sources. Don't know what they hang on the end of the wire; probably something that is more active than steel, and does not form an impervious coating.

I think Calder might have something on this, but I have not looked.

[JPhoenix]

Yeah, the google brings up lots of info on big tankers, offshore rigs and such having these impressed systems, guess I'll ask my surveyor for more info, just thought they would be more common, like maybe Fisheries had them on the shelf ;-)

Jim

[Excalibur]

I can't imagine why your surveyor thinks you need one for a wooden boat. Is your boat copper clad? Please share when you ask him...

[AstoriaDave]

Yeah, I want to know, also. I suspect the surveyor thinks you will be able to balance the effects of the impressed current system more easily than by adjusting the surface area of the zincs. Or, it might be that you need cathodic protection from the impressed current in areas remote enough to zinc sites the zincs can not do the job.

The good thing about zincs is that they are simple and rarely compromised. The added wiring (etc.) for an impressed current system will require regular inspection and may be more susceptible to damage than external zincs. But, I am speaking from a position of near-ignorance here.