Zinc Wasting

[senormechanico]

All I know is that my settings got us through four years of cruising in Mexico with no underwater metal damage, and none in the PNW during the last twenty.

Who sets the (imho) high 910mv as the optimum number?

I will bet it’s the ZINC manufacturers so they can sell more product...

[rbk]

Originally Posted by a64pilot

I actually hung my guppie zinc today, I usually do in a marina but haven’t trying to resolve this issue. I did connect a 10ga wire directly to the engine block that has a ring terminal on it with a silicone bronze bolt that is now in the cockpit ice box so that I have a direct connection for the guppie zinc to the engine block.
Previously I connected it to a chain plate and as SS is such a poor electrical conductor I’m not so sure how well that worked, now I have Ti chainplates, and Ti is an even worse conductor than SS, so I ran the wire.

Bronze isn’t a great or even a good conductor. Coppers best or if you can find a plated bolt would be far better.

[CharlieJ]

Post #31

Who sets the (imho) high 910mv as the optimum number?

Actually, it is the electrochemical nature of the beast and -910mVDC is not the "optimum number".

Experimentation shows that if an underwater metal's potential is driven >200mVDC more negative (in reference to a silver-silver chloride electrode) than its freely corroding potential, then cathodic protection is achieved and galvanic corrosion halts.

Through experience, the ABYC recommends that a hull potential of -550mVDC to -1100mvDC is the range of protection for a fiberglass vessel. A review of the galvanic series will give an indication of why this range is recommended as all commonly used underwater metal will be protected.

Not a conspiracy...just science.

[Paul Elliott]

Originally Posted by a64pilot

I believe to be any good a zinc has to be in direct contact with the water on the outside of the Boat, plus my shaft is about four feet above the bilge.
I have another month before I can inspect and see the zinc.
I disconnected and removed ALL power on the boat, and still had the 950mv, which is I believe what you want, you don’t want zero voltage.
Apparently the silver half Cell is one side of a battery Cell and the other side is the zinc, put them into electrolyte (salt water) and they generate current. If you have stray current, then of course you should measure way more than the static voltage of the Boat with all power removed
Or at least that is they way I think the half Cell works anyway, I may be wrong

The AgCl half cell is *not* measuring current, just voltage difference between the AgCl and the rest of your boat's immersed metal, with the seawater acting as the "battery" electrolyte. Since you are using a high-impedance voltmeter to measure this voltage, the current through the half cell will typically be less than 0.1 microamp (i.e. essentially zero).

It's just like measuring the voltage across the terminals of a battery using a high-impedance voltmeter: no appreciable current will flow.

What the half-cell measurement tells us is if the zincs are working to protect any other submerged metal. This measurement only makes sense if all underwater metal is bonded together, or if you measure between the cell and the individual unbonded through-hulls (etc).

Honestly, I don't know why the AgCl cell is the "standard" reference cell, since it seems to me that any other conductive electrode will suffice as long as we know the electrode's place in the galvanic table. Perhaps the AgCl is less prone to voltage shift caused by corrosion?

I recently added a galvanic isolator (diode) in the shore power ground connection on a new boat. It didn't make a big difference in the half-cell measurements, but it did stop about 0.1A of DC current (*). I am hoping this will help my Zincs last a bit longer.

(*) Current measurement from memory, I could be mistaken. I got the half-cell only recently and I took a lot of measurements on both my boats.

[senormechanico]

"Experimentation shows that if an underwater metal's potential is driven >200mVDC more negative (in reference to a silver-silver chloride electrode) than its freely corroding potential, then cathodic protection is achieved and galvanic corrosion halts."
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"
What the half-cell measurement tells us is if the zincs are working to protect any other submerged metal. This measurement only makes sense if all underwater metal is bonded together, or if you measure between the cell and the individual unbonded through-hulls (etc).

Honestly, I don't know why the AgCl cell is the "standard" reference cell, since it seems to me that any other conductive electrode will suffice as long as we know the electrode's place in the galvanic table. Perhaps the AgCl is less prone to voltage shift caused by corrosion?"

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Which is why I didn't spend $150 USD for a AgCl cell and instead used a simple silver electrode.
Before using a 90% pre 1965 US coin as the electrode, I experimented with a .995 silver earring. The difference in voltage was about 0.03V between the two.
As a AgCl cell is 0.0v and a .995 silver electrode is -0.1v and the 90% silver coin is -0.13v, I figured that into my calibrations when adjusting the bonding system voltage setting.
Why burn more zinc than you need to if no corrosion is taking place? This has been working for me for decades!

[a64pilot]

Update.
It appears that disconnecting my inverters neutral to ground tie has greatly diminished the zinc wasting. Zinc used to only last maybe two months, it’s been about two and a half months since last zinc change and inverter being disconnected.
I didn’t Dive the Boat myself, as water temp is around 50F and I only have a 3mil wet suit, so I paid a diver. I gave him a new zinc and told him it’s likely the old one may be completely gone. After diving the Boat and wiping the hull and cleaning the prop, he gave me my new zinc back, telling me the one on the prop wasn’t 50% gone yet.
So it seems my excessive zinc wasting has hopefully been solved