Revelations about zinc electrodes.

Agreed. We have had some of these "failures" on our boats..especially around waterjets. It is not a plug and play solution.

 

Not trivial. I first became aware of these systems talking to the crew that preserves the Star of India at the San Diego Maritime Museum (an iron hull tall ship). That was the spark that led me to follow the entire thread.

 

You made some lucid remarks about zinc electrodes, but now you've taken a wrong turn!

First of all, the conductivity of seawater is assumed to be 5000 millimhos/meter or 5 Siemens/m (ITT Reference Data for Radio Engineers). That gives a theoretical resistance of 0.2 ohms per meter or 0.6 ohms for 3 meters. Seawater doesn't conduct as good as metal, but it comes pretty close. The value is used among other things for determining the efficiency as a ground plane in telecommunications.

Measurements with a multimeter are meaningless because of electrochemical activity at the probes and their small size. The calculation of resistivity between 2 points is a complex integral, analog to an infinite number of wires between these points with increasing length. I don't want to bother anybody with it here.

A zinc electrode in seawater generates an electric potential of 0.5 to 0.7V towards ferrous metals, a lot less towards aluminum. If this is the voltage range we are happy with, I do not see the need of increasing the voltage in an impressed current system.
Active systems with platinum or palladium electrodes and a silver-chloride reference need a higher voltage because they are very small due to the materials price. With a lump of platinum the size of a zinc electrode 0.5 volts would be enough to perform like zinc does.

The Mercathode system demonstrates the above. It has, to say it politely, a rather limited lifetime. If you dissect a unit after just a few years of (assumed) service, you'll find a hole in the reference electrode where is once was soldered to its wire. Especially if it has been staring at two stainless props its life is sheer agony.
The electronics involved are stone age technology where the electrode voltage is increased until the reference value is reached. With a gaping hole in the silver reference electrode the noble one is near the + Battery voltage, which is lethal even for one micron of platinum.

But there is a better way to do things.
The electronic circuit must be a switch mode converter delivering a minimal voltage over a current range from 0 to the required current based on the surface to be protected, whether this is 10 milliamps /sq. meter or 50 is a matter of personal preference.
I've measured 30 mA for an 8 sq.cm. bare surface, so 10% paint damage is an impossible goal unless we are discussing toy boats. A large zinc electrode of 200x200 mm gives 1.5 Amp and dies in the process (it looses 0.82 grams for each Amp/hour.

There is absolutely no need for a reference electrode, that is just complicating things. The AgCl electrode is nice for a measurement but must stay in the water as short as possible, subsequently rinsed, dried and stored.
The converter I described above should have a simple current measuring circuit. Again it is a matter of personal taste to choose an old fashioned analog instrument, a numeric display or a flashing "ATTENTION, CURRENT LIMIT EXCEEDED".
A drop in current means bad wiring or a lost electrode, an increase means the integrity of the paint layer has become questionable.
Boat used in both fresh water and at sea need two current markers because the conductivities are different. Or a simple switch if two markers are confusing.

And then there is the electrode.
No metals destined for women's jewelry, but something cheap and simple, so making a 3 inch electrode surface doesn't flatten your wallet.
A material that conducts well and (almost) doesn't take part in electrochemical processes. Used to deposit zinc on steel, gold on copper, to split water into H2 and O.
Yes: CARBON!
Sintered on a copper substrate (or copper deposited) and potted in a resin and a glass filled cup. The bottom has a threaded hole and an O-ring groove so you can simply screw it on an insulated through hull stainless bolt.

Does this exist? I don't know.
If I could figure this out on a Saturday morning while painting a ceramic floor there must be dozens of guys who figured it out in the past.
If not, someone should start doing so on Monday morning.

 

Thanks all, i have not come back on thread since i queried the merits of the active system back on post #150 but i have been following along and you have more than answered my question thanks. I can see there are merits to update this system using more modern control circuits and materials if somewhere lurking out there it is not already on the market. It sounds like it could be more effective with less guesswork if it could be made reliable. I do know there is a similar system being marketed to protect automobiles but have no info wheather it is reliable or not. Actually i think i'll search that down and look up the consumer report on it and get back but meanwhile if someone has that info already by all means save me the time. Tnx. Geo.

A yacht is not defined by the vessel but by the care and love of her owner.

 

Yes sorry not pure seawater, I pulled the figure from a local active protection system blurb and that's the worst case quoted Which is brackish estuarine. Looking again at that article and they work on up to 50 volts with an easily replacable graphite electrode ! So I think you are re-inventing the wheel.

The reference electrode allows the system to adjust for variation in operational parameters (water resistivity, surface films, surface passivation ). A lot of research has been conducted on these systems and the accepted level of effective protection which is to polarize the hull at least 0.85 V to 0.95V above the surrounding seawater. ),95 V is now the suggested minimum for low oxygen seawater.

To force a current into a surface is the wrong approach. it's the polarisation that you want in this case not the current. It's not the same process as the Zinc galvanic cell which is an electrochemical cell working on ion exchange. The Impressed system polarises the surface by shifting the voltage relative to sea water.

So consider that for all the systems that are installed using Platinum, graphite, even low wastage alloyed iron active electrodes why do they run their voltages so high when they are simply polarizing the hull by .85 to .95 Volts ?

As for Zincs and the level of damage they can protect, you know when you've had a grounding and the zinc lasts longer than you would expect because of film and calcareous deposit formation on the protected surface. There's an initially high current which quickly drops over a few hours so the zincs last a lot longer than you would initially think.

 

How does the voltage prevent electrolysis between dissimilar metals? If they are all biased at the same voltage, the potential difference between them is the same.

 

By applying a negative voltage on the electrodes (either zinc or impressed current), the hull gets a positive potential against the surrounding water. If the potential is greater than that caused by dissimilar metals, there will be relative differences, but the absolute value remains positive, so galvanic corrosion is inhibited.

On a stern drive equipped with a working Mercathode system, the zinc electrodes present (trim tabs, zinc on anti-cavitation plate) do not wear because with the water slightly more negative than the zinc, there is no electrolysis.

It is like a lamp drawing current from a battery under charge: as long as the charger voltage is above the battery voltage, current flows directly from the charger to the lamp.

 

CDK, excellent explanation with the battery charger, battery and lamp combo. The batterys integrity(hull and associated parts) is maintained as long as the charger is the supplier of the wants of the lamp.(galvanic wants of the hulls dissimilar metals) It makes the operation of the system so understandable. Basically the charge set up on the hull nullafy the wants of dissimilar metals associated with the hull to eat each other. The meat on the cage floor satifies the lion from eating it's caretaker. As promised above I am doing some research on modern systems on the market and have located one that sounds like it is designed for smaller craft ( up to a 30ft alum. hull) but as yet have not done any research into consumer reports on it's effectiveness or reliability. Once again thanks all for your input into this alchemi subject. Geo

 

OK, but the potential difference between dissimilar metals remains the same. That is like biasing AC with DC.

 

Not quite the same Gonzo.

The differential does exist, but the galvanic action is ruled by the potential vs. the electrolyte (water). Example: If a piece of steel is kept at 1 V positive, a connected piece of zinc is still at 0.4 V positive, so no galvanic action takes place.

 

What happens to a stainless shaft in an aluminum housing? They are both electrically connected so the bias would be the same. However, the potential difference between them remains the same. I may be missing something. I thought that bias protection only worked if you biased the lower metal in the scale.

 

No metal to metal contact is allowed between SS and aluminium in a shaft run. It would disintergate .

"O" rings, Lip seals, cutlass bearings , oil baths ,are always between the alum and SS.

The reason why you use grounding brushes on a prop shaft is because the SS shaft has no, or very poor, electrical connection to the hull. It is floating.

 

Yes, both have the same bias, but the aluminum isn't happy with that because it wants to be half a volt lower, so it starts emitting ions into the electrolyte, thereby lifting the potential of the whole. The current flowing determines the rate at which the Al dissolves.
If another source emits ions already at a higher rate and the potential is higher than the target of the Al, the current flow stops.

Analogies: swimming with the tide, driving downhill, eating with a full stomach etc.

The opposite is also true. Stray currents from nearby objects with inverse polarity make your zinc disappear faster.

 

Michael: Shafts and housings make contact always. The gears, bearings, etc are all in contact. Many are press fit. I've yet to see an outboard or outdrive with a cutlass bearing or grounding brushes. Seems like you just make stuff up.
CDK: So the bias only works if the metals are isolated?

 

This is a lip seal !!

Ive yet to see a lower unit without a lip seal protected oil bath gear box. The lip seal insures that there is no electrolyte submerged SS to aluminium contact and keeps the gear box full of oil . Its possible that you operate your vessels without lip seals and sea water in the lower units, but this is not normal practice and will greatly reduce the service life.