Current in bonding wire

[patja]

At my last haulout the thru hulls showed the high copper content antifouling paint had been blasted off and there was some minor corrosion on the lead keel which required some grinding and fresh barrier coating. So I am surveying my electrical system to try to track down the source.

For resources I have a Fluke 117 DMM, a silver/silver chloride reference anode, and an Extech 380942 clamp-on ammeter. I also have several recommended books on marine electrics and corrosion: Collier, Wing, Sherman, and of course Calder.

Everything is bonded. There is a good-sized zinc on the hull connected to the bonding system, and a zinc fish hung over the side, also connected to the bonding system. Then there are two shaft zincs and the cone-shaped zinc on the end of the MaxProp. I get a consistent -.960 volt reading from the silver-silver chloride reference anode to the bonding system and every metal object in the boat. It goes up to about -.990 when I connect the zinc fish. As the recommended range maxes at -.900 maybe I am over-protected? I've read that can result in paint blasting.

My plan is to exhaustively measure and document my findings and engage a local marine electrician to corroborate my findings and help solve the problem. I'm trying to do as much legwork as I can to make things simpler for the electrician.

One item which I've found that is perplexing me right now is that there is about 30 mA of DC current flowing through some of my bonding wires, even when all batteries and shore power are completely disconnected.

What are some plausible explanations for this current in the bonding wire when I've disconnected the batteries and shore power? I left them off overnight to be sure there wasn't a capacitor draining somewhere.

[Paul Elliott]

Current is supposed to flow in a bonding wire, that's how the zinc anode can protect multiple bonded through-hulls (etc) that don't have their own zincs. Consider the zinc as the anode, and bronze the cathode of a battery. Salt water is the electrolyte. When you connect the anode to the cathode via the bonding wire (or by bolting the zinc to the propshaft, etc.) then current will flow. You have effectively short-circuited the zinc-bronze battery.

960mV does sound a bit high though, and you may indeed be over-zinc'd.

Here are some possibly useful links on the subject:
Boat & Yacht Corrosion Control Manual
http://assets.fluke.com/appnotes/ele...r/B0269b_u.pdf
Marine corrosion survey
Cathodic Protection 101

[smac999]

what is your hull? you can't over zinc a fiberglass boat.

on wood it is very important.

[Paul Elliott]

Originally Posted by smac999

what is your hull? you can't over zinc a fiberglass boat.

on wood it is very important.

I understand that over zincing on a fiberglass hull can result in "burning" of the copper-loaded bottom paint around the bronze through hulls.

This is counter-intuitive to me though. I would have thought that the excess zinc would be plating the copper, not causing erosion. Perhaps it's the plating that causes burning?

[senormechanico]

I disagree. Too much zinc will cause paint burn around thruhulls.
I had that happen when I thought I was doing the right thing by installing a larger zinc. It wound up being too big, the voltage stayed too high (no resistive drop due to surface area).

A more reasonable voltage is (and this is only my opinion thru experience and measuring) around 0.550 to 0.700.


My boat has ONE zinc, (the same one as before) mounted near the centerboard,
Everything is bonded, and run through an automatic circuit of which I have previously posted here. No corrosion, or loss of metal in ten years, and the bottom paint around the thru hulls etc doesn't show any voltage stress.

I replaced the zinc one time about 4 years ago, not because it needed it, but to keep the Admiral happy. Imho, it probably would be due for replacement this year at haul out.








Flame away !

[sailorboy1]

Originally Posted by patja

Everything is bonded.

One item which I've found that is perplexing me right now is that there is about 30 mA of DC current flowing through some of my bonding wires, even when all batteries and shore power are completely disconnected.

What are some plausible explanations for this current in the bonding wire when I've disconnected the batteries and shore power? I left them off overnight to be sure there wasn't a capacitor draining somewhere.

Bet there is a corroded ground wire to something that is also bonded. My boat almost sank because all that was holding in a 1.5" hull fitting was caulking after the the ground wire for the fuel tank corroded so the tank gage decided it would be acceptable to ground via the bonding line that was connected to the aluminum tank, which was connected to the hull fitting and for which the next bonding connection was corroded. So it just went to the water at hull fitting.

I'm now aganist bonding anything to something that might have a current.

[Paul Elliott]

I had been wondering about the practice of bonding otherwise isolated bronze through-hulls. Perhaps dezincification? This happens to brass, and some bronzes, so perhaps even with a good bronze alloy this could be an issue, and a sacrificial zinc anode will provide protection. Also, there can be an electrical connection to the through-hull via the saltwater in the hoses. Cooling water intakes will have this type of connection to the engine or heat-exchanger. It gets pretty complicated!

[chris95040]

The voltage across a battery is based on the galvanic series of the two metals forming the battery.

The *quantities* of those metals should *not* impact the voltage (and therefore, should *not* impact the amount of current that flows in a circuit attached to that battery)

Put a *small* pile of anode material and a *small* pile of cathode material in an electrolyte and wire them together through a resistor. Note the voltage and the amount of current flowing.

Now do it with huge piles of the same materials. The result should be the same.

Note: this assumes we are talking about small currents, non limiting scenarios. With large currents that are on the verge of browning out the battery, the amount of material matters. (A bigger bank with more anode + cathode material can provide more max current than a smaller bank before having it's voltage drop out.)

This is why people say "you can't over-zinc". I'm not saying "you can't over-zinc", I'm just saying that's why people say it. I personally don't understand enough about what goes wrong with bottom paints or wooden boats or whatever to say one way or the other. But if this guy's seeing too much current in his bonding wire, it's because of a big difference in galvanic series of the materials, not because there's too much zinc.

[Paul Elliott]

Originally Posted by chris95040

The voltage across a battery is based on the galvanic series of the two metals forming the battery.

The *quantities* of those metals should *not* impact the voltage (and therefore, should *not* impact the amount of current that flows in a circuit attached to that battery)

Ah, but the quantities of the metals affect the surface area, and that area as well as the distance between the two metals affects the resistance (because seawater has a non-zero bulk resistance). This is essentially the internal resistance of the battery, and this, as well as the voltage differential, controls the current.

When you connect a zinc anode to a bronze (cathode) via a bonding wire you are putting a short-circuit across the zinc-bronze battery.

So size matters.

[chris95040]

Originally Posted by Paul Elliott

Ah, but the quantities of the metals affect the surface area, and that area as well as the distance between the two metals affects the resistance (because seawater has a non-zero bulk resistance). This is essentially the internal resistance of the battery, and this, as well as the voltage differential, controls the current.

When you connect a zinc anode to a bronze (cathode) via a bonding wire you are putting a short-circuit across the zinc-bronze battery.

So size matters.

No, I don't think it does.

Say you've got a bronze through hull and a zinc anode and they are bonded in the boat.

Now say I come along and replace your "small" anode with a "really really big" anode.

If the zinc anode is "really really big", we might see the actual theoretical voltage difference between the two materials (say, 0.5V or so.) Big deal? So we see hundreds of millivolts flowing in your bonding wire, and then the zinc anode corrodes kinda quickly at first, until its "small" again. So what? The bronze through hull has been protected the whole time.

The "really really big" anode might be a little less cost effective because of the time it spent corroding a little faster, but I don't see any problem other than that. More anode material really just means more time between replacement.

[patja]

Originally Posted by Paul Elliott

Current is supposed to flow in a bonding wire, that's how the zinc anode can protect multiple bonded through-hulls (etc) that don't have their own zincs. Consider the zinc as the anode, and bronze the cathode of a battery. Salt water is the electrolyte. When you connect the anode to the cathode via the bonding wire (or by bolting the zinc to the propshaft, etc.) then current will flow. You have effectively short-circuited the zinc-bronze battery.

If I understand this, even with my boat's batteries out of the picture there is a battery of sorts formed between the zinc, the saltwater, and the dissimilar metal of the bronze through hulls through the bonding wire. So it is expected that there will be current running through it all the time?

My concern is that my research leads me to think that 30 mA is way too much. And that a reliable method of finding a DC current leak is to measure the current in the bonding system: disconnect circuits until it goes away and you've found the problem. But if current is running through the bonding circuit even when the batteries are completely disconnected, that poses a challenge.

[patja]

Originally Posted by Paul Elliott

I had been wondering about the practice of bonding otherwise isolated bronze through-hulls. Perhaps dezincification? This happens to brass, and some bronzes, so perhaps even with a good bronze alloy this could be an issue, and a sacrificial zinc anode will provide protection. Also, there can be an electrical connection to the through-hull via the saltwater in the hoses. Cooling water intakes will have this type of connection to the engine or heat-exchanger. It gets pretty complicated!

The way I've come to think of it (maybe right, maybe wrong!) is that in a perfect world your DC current only returns via the DC - wire on each circuit and nobody needs bonding.

But as soon as you have that stray DC+ wire in a wet spot or frayed insulation or a poor connection combined with an alternate ground path, such as the saltwater (including as you mention water in hoses), you can get current flowing through your fittings and corrosion. In a perfect world where there were no alternate paths for ground there is no need for bonding. But as soon as there is some stray current, better to bond and get that current connected to a sacrificial zinc anode via a low resistance path through the bonding wire.

[boatpoker]

I am a retired Certified Marine Corrosion Analyst and no longer offer opinions on threads like this. Let me just say to the OP that there is a tremendous amount of misinformation in this thread. Do not pay attention to any of it. Hire an ABYC Certified Marine Corrosion Analyst and get it right the first time.

[Paul Elliott]

Originally Posted by boatpoker

I am a retired Certified Marine Corrosion Analyst and no longer offer opinions on threads like this. Let me just say to the OP that there is a tremendous amount of misinformation in this thread. Do not pay attention to any of it. Hire an ABYC Certified Marine Corrosion Analyst and get it right the first time.

Perhaps you might break your rule just this once, and give us a hint as to what is wrong here (and what is right, if anything)? Be as general or specific as you want, but just saying it's all wrong is a bit, shall we say, unsatisfying.

[boatpoker]

Originally Posted by Paul Elliott

Perhaps you might break your rule just this once, and give us a hint as to what is wrong here (and what is right, if anything)? Be as general or specific as you want, but just saying it's all wrong is a bit, shall we say, unsatisfying.

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