Current in bonding wire

[flyingfin]

Originally Posted by boatpoker

No. It is just not worth the arguments and ridicule from the dock experts. I've learned my lesson.

AMEN!

[chris95040]

Originally Posted by patja

Thanks Gord. More data supporting the hypothesis that what I am seeing is consistent with what one would expect in an overprotection situation.

I pulled my zinc fish anode out of the water, which still puts me at .960 volts on the silver/silver- chloride anode connected to the bonding, which seems to still be in the overprotection range. Granted the zincs are all brand spanking new as the boat is fresh from the yard with new zincs so the protection level will only go down. But I don't want it blasting my paint off again. The extra prep on my bottom paint job was almost $1k. On the plus side, the metals were protected -- no pitting or sign of any loss of metal.

I am considering having a diver out to swap my large bonding zinc that is on the hull for a smaller one to drop my protection level hopefully down into the .800 range. The current zinc is a Martyr MZ404

In the pdf from ProMariner which Paul linked, they recommend their own product (go figure) called the Deluxe Corrosion Controller. I am having trouble finding much in the way of documentation on it, but it looks like it comes with its own silver/silver chloride anode which is to be mounted in a thru hull for constant monitoring, and then it has a potentiometer to dial up or down your protection level (voltage). On its own it is not too expensive, but I guess I would have to swap out a transducer for its anode, and even then that might not work...the thru hull fitting pictured on the product looks to be smaller than the ones used by my transducers, so it might need yet another hole in the boat to accommodate it, which I am loathe to do (I just spent a month on the hard in the yard!). Does anyone have experience with this thing and know how it adjusts the protection level?

I think I'm over trivializing the antifouling paint problem. Interesting.

So the understanding I'm reaching is as follows, and I think this jives with your observations:

From the perspective of your metal things you are trying to protect, you can't over-zinc.

But from the perspective of your bottom paint job, you might not want "full" protection current - you want your anodes just strong enough to keep the current flowing in the right direction, but not so strong that the resultant current blows your paint away.

Now, if it were me, I'd definitely be more concerned about my through hulls than my paint job, so downsizing the anode sounds scary. You'll be replacing them more often, and more likely to accidentally spend some time without protection.

Why not keep the anodes the same size, but switch to a more noble material like Aluminum?

[Paul Elliott]

Originally Posted by chris95040

But from the perspective of your bottom paint job, you might not want "full" protection current - you want your anodes just strong enough to keep the current flowing in the right direction, but not so strong that the resultant current blows your paint away.

Chris, I believe that there's a fairly broad current range that will give protection to your underwater bronze before causing burning of the bottom paint. As long as you stay below that level, then bigger zincs are better because they will last longer.

But here's a question that I've not been able to answer, and I am hoping that one of the corrosion experts can explain:

It has to do with the need for anodic protection when you have a fiberglass hull with no dissimilar metals in the water and no shore power connection.

For starters, assume that you have a single bronze through-hull. No prop, propshaft, etc, just the one piece of metal. Say it's not connected to your battery ground. Where's the circuit? Now connect it to your battery ground. Again, where's the circuit? I see no way for damaging current to flow.

Now add more underwater bronze. Bonded or not, where is the circuit that allows current to flow? All underwater metal is at the same place in the Galvanic series and is at equal potential.

So why do we need zincs? Here's my conjecture:

Perhaps my prop, propshaft, and through-hull aren't exactly the same metal. There are different grades of bronze. Some propshafts, rudder posts, etc, are made from a stainless alloy. My keel is lead. My keel bolts are stainless or monel. So a zinc (bonded to everything else) will sacrifice itself to save the other metals.

But what if these metals weren't bonded together? Submerge a piece of lead, and a piece of stainless and a piece of bronze in close proximity, but don't connect them together. Where's the current flow?

More conjecture: Most modern metals are alloys. Bronze and stainless are alloys, and alloys can be their own bimetallic galvanic cell. Attaching (or bonding) a zinc to the alloy will protect the alloy.

Still more conjecture: Even without bonding we still have the salt water in the cooling water intake hose providing an electrical connection to the engine or heat exchanger, which themselves are usually connected to the boat's battery ground via the starter. The propshaft may also be electrically connected to the engine. So there are some connections, whether we bond things or not.

My bronze prop is attached to my stainless (I believe) propshaft. This right there is a galvanic cell, and without a zinc the bronze will sacrifice itself to the stainless. Adding a zinc (which is more active, or anodic than the bronze) will protect the propeller.

Now if we add a shorepower connection, or let your bilge pump's cracked wiring leak +12V into the bilgewater, have a steel or aluminum hull, and it gets even more complicated. But for now I would just like to understand the corrosion current paths on an isolated fiberglass hull.

Anyone care to critique my conjectures? Please?

[glcalahan]

Boatpoker,

Coincidentally, I am now making decisions on this topic and are very interested in your opinion.

Would you feel comfortable with sharing an opinion??

Thank you in advance,

GLC

[CarinaPDX]

I would like to add my voice to the request for @BoatPoker to share his wisdom. Many of us here are just seeking to understand. Some will offer their understanding, which may or may not be correct, but most (definitely not all) will accept being corrected by someone who really knows, and can explain. I don't think there is a need to get into a pissing match with the kibitzers, just lay it out for us and add clarification if asked.

As for those whose "contributions" are mostly zingers, please try to control yourselves (and get a life).

Greg

[qwert]

"Why not keep the anodes the same size, but switch to a more noble material like Aluminum?"

I used to have a wooden boat. The propshaft went through a thickwalled bronze tube which was bedded in tar where it went through the timber. The bronze tube was connected to the external zinc. I was getting alkaline rot in the timber. I changed my anode to a piece of mild steel to reduce the potential difference..........problem solved.

[Capt Gill]

I was going to chime in, but realised my limitations which is best expressed in this quote:
"I suspect that whatever cannot be said clearly is probably not being thought clearly either"
--- Peter Singer
This subject can be confusing when the one who is attempting to explain it does not fully understand it or take the time to clearly state their answer.
Not pointing any fingers, just saying ...

[Blue Stocking]

This thread often reminds me of a story related by a high school teacher of mine, 60 years back.
Story goes : teacher asked a nerdy kid to simply explain, " birds of a feather flock together "
Nerd's response, " ornithological specimens of identical plumage invariably congregate in adjacent proximity ".

BTW, boatpoker has been beat up on, sorry 'bout that Mate ! :cheers.

[chris95040]

[emoji849]

[CharlieJ]

OK. Here goes. I will stick with facts and clearly id opinions.

1. For galvanic corrosion to occur there must be two dissimilar metals, electrically connected immersed in the same electrolyte.
2. To eliminate corrosion of a metal immersed in an electrolyte, its potential must be driven > -200mVDC from its freely corroding potential lists on the galvanic series. Hence a lead keel has a freely corroding potential range of -190mVDC to -250mVDC. To protect the keel from corroding, its potential must be shifted more negative at least to -450mVDC.
3. (Opinion) The vast majority of paint system failures is the result of all or some of the following:
>> Poor substrate preparation.
>> Insufficient film thickness
>> Not following manufacturer's instructions (RTFM)
4. Anti-fouling paints do not work well to control or insulate the underwater metal surfaces.
5. Neither galvanic current nor cathodic protection current will travel down a hose further than a couple of diameters.

The hydrogen disbandment observed ("haloing", "burning", etc.) when using high copper anti-fouling paint is caused by the copper in the paint system matrix being anodic to most if not all of the underwater metal components. To prevent this condition, the underwater metal components, typically the thru hulls, should be properly prepared and then barrier coated with an epoxy polyamide coating that will electrically isolate the copper in the paint from the bronze thru hull.

Now to Paul Elliott's Post #33.

For starters, assume that you have a single bronze through-hull. No prop, propshaft, etc, just the one piece of metal. Say it's not connected to your battery ground. Where's the circuit? Now connect it to your battery ground. Again, where's the circuit? I see no way for damaging current to flow.

Correct, there is no circuit and the only current that will flow is that created by the bronze alloy. The corrosion rate will be very, very low and the fitting will last for decades.

Perhaps my prop, propshaft, and through-hull aren't exactly the same metal. There are different grades of bronze. Some propshafts, rudder posts, etc, are made from a stainless alloy. My keel is lead. My keel bolts are stainless or monel. So a zinc (bonded to everything else) will sacrifice itself to save the other metals.

Correct. Remember the conditions for galvanic current. (#1 above.)

But what if these metals weren't bonded together? Submerge a piece of lead, and a piece of stainless and a piece of bronze in close proximity, but don't connect them together. Where's the current flow?

Correct, there will be no current flow between the components. But each of the alloys will corrode...slowly.

More conjecture: Most modern metals are alloys. Bronze and stainless are alloys, and alloys can be their own bimetallic galvanic cell. Attaching (or bonding) a zinc to the alloy will protect the alloy.

Absolutely!!

Still more conjecture: Even without bonding we still have the salt water in the cooling water intake hose providing an electrical connection to the engine or heat exchanger, which themselves are usually connected to the boat's battery ground via the starter. The propshaft may also be electrically connected to the engine. So there are some connections, whether we bond things or not.

Galvanic current will not travel more than a couple of hose/pipe diameters so no real connection there. The prop shaft connection to the engine is tenuous at best due to the hydraulic transmission and less than perfect electrical contact. BTW, the spec for a cathodic protection system electrical connection is < 1 ohm of resistance. Remember, we are only dealing with a maximum of about 1VDC with normal anodes so it does not take much resistance to stop the current.

Hope this helps.

[patja]

On Tuesday the electrician and I spent three hours comparing measurements and investigating what's going on. His Fluke DMM and clamp ammeter both went to an additional digit (tenth of mA current, tenth of millivolt voltage) beyond mine, but did not come up with any different measurements from what I had found.

There was no definitive conclusion as to what is causing the paint corrosion. The silver lining is that at least I know I am not missing anything obvious with my own DIY survey.

He was generally skeptical of the theory that it is a "problem" of over-protection (specifically for a fiberglass hull...it is a real problem for wood boats), reasoning that if it happened you would see and hear a lot more about it happening as there are many fiberglass boats that have way more zinc on them than are needed. He does a ton of corrosion surveys and says with the exception of a few problematic antifouling paint products, he hasn't seen it.

The bonding system looked great to him, much better than most he has seen (which in a way is a clue...at some point a prior owner invested heavily in a really sound bonding system...was there a prior problem the PO was trying to solve with bonding?) and the reference cell voltage never budged much no matter what we did with connecting/disconnecting shore power and DC circuits. He said when you have a DC leak, it is pretty obvious and moves the voltage quite a bit. The only time we saw the voltage move (which was mostly measured in the tenth of mV digit), it moved in a direction that increased the protection (more negative). He said this is perhaps indicative of a DC ground loop, but it was barely measurable. The DC system seems good.

On the AC side, he saw the same stray current in the shore power cord that I did, even when the boat AC main breaker was off, which indicates it is coming from outside the boat. My AC system is so simple, and was all completely re-done in the US at some point (not factory-original from France), such that it seems hard to imagine that the problem is coming from the boat-side of my AC panel. With that said, he had stories about AC devices that seemed innocuous causing problems -- he had a recent example of a Keurig coffee maker that had a neutral to ground leak that only happened when the coffee maker was running. He clamped my neighbor's shore power cord and it had even more stray AC current, 60 mA as compared to 25-ish on mine (other nearby boats had less than 5 mA)...so that might be a clue. My neighbor is a Taiwan-made CHB trawler, and the electrician's experience was that substandard AC systems and wiring are common in boats built in Taiwan. He said my galvanic isolator is an early model and probably has a crap capacitor compared to today's models, and anyways it will only block stray DC current when working correctly and allow stray AC current to flow. Given the stray current measured on the shore power cable, the slightly suspect age of the GI, and the seemingly clean DC system, we agreed that an isolation transformer certainly wouldn't be a bad idea and might make a difference, so that's my next step. There was also some speculation about the lead keel, 70% copper content bottom paint, all tied into a very good bonding system may make my boat an attractive route for stray current from outside the boat.

I am reassured that my tools and measurement methods were corroborated.
I'm going to install the isolation transformer and keep a close eye on things:
regular reference cell / ammeter measurements, maybe rig up an underwater camera on a stick to inspect the through hulls and keel regularly, and I have a diver wipe the bottom every 3 - 4 months and will ask him for more detailed observations.