Sense or Nonsense Hanging Zinc Anode Overboard?

[sailorchic34]

Originally Posted by Scot McPherson

In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism.[1] During corrosion there are two reactions, oxidation (equation 1), where electrons leave the metal (and results in the actual loss of metal) and reduction, where the electrons are used to convert water or oxygen to hydroxides (equations 2 and 3).[2]

Fe → Fe2+ + 2e−




(1)
O2 + 2H2O + 4e− → 4OH−




(2)
2H2O + 2e− → H2 + 2OH−




(3)
In most environments, the hydroxide ions and ferrous ions combine to form ferrous hydroxide, which eventually becomes the familiar brown rust:[3]

Fe2+ + 2OH− → Fe(OH)2




(4)
As corrosion takes place, oxidation and reduction reactions occur and electrochemical cells are formed on the surface of the metal so that some areas will become anodic (oxidation) and some cathodic (reduction). Electric current will flow from the anodic areas into the electrolyte as the metal corrodes. Conversely, as the electric current flows from the electrolyte to the cathodic areas the rate of corrosion is reduced.[4] (In this example, 'electric current' is referring to conventional current flow, rather than the flow of ions).

As the metal continues to corrode, the local potentials on the surface of the metal will change and the anodic and cathodic areas will change and move. As a result, in ferrous metals, a general covering of rust is formed over the whole surface, which will eventually consume all the metal. This is rather a simplified view of the corrosion process, because it can occur in several different forms.[5]

CP works by introducing another metal (the galvanic anode) with a much more anodic surface, so that all the current will flow from the introduced anode and the metal to be protected becomes cathodic in comparison to the anode. This effectively stops the oxidation reactions on the metal surface by transferring them to the galvanic anode, which will be sacrificed in favour of the structure under protection.[6]

Exactly right.

Well except for " more anodic surface" which should read "lower anodic voltage potential", though maybe I'm splitting hairs.

However current flow without a conductor between the anode and cathodic metals will be far too low to protect the metal with the higher voltage potential.

This part "metal will change and the anodic and cathodic areas will change and move" is a bit dodge too. A pure metal will not have a anode or cathode as the metal is at the same voltage potential. Take iron, Yes you'll get bonding of o2 to Fe for ferrousoxide, but that's not a galvanic reaction, but a chemical bond/reaction.

But take say a 85,5,5,5 bronze prop, with 85% copper, 5% tin, 5% zinc and 5% lead or more likely aluminum now a days. In sea water that 5% zinc will leach out due to micro galvanic action. Here we do get true galvanic corrosion.

This is also why brass with higher 20-30% zinc should not be used in contact with seawater. It will become brittle as the zinc in the mix is sacrificed via galvanic action.

There are also bacterial caused corrosion, Galvanized iron water pipe can suffer from this. Quite interesting.

It is for that zinc in the bronze props, that we place separate large zincs in circuit to protect the prop from micro galvanic corrosion. That Zinc anode, being at a lower voltage potential then everything else but magnesium, makes it a really nice anode.

[boatpoker]

Originally Posted by a64pilot

It's does indeed take DC to have electrolysis, doesn't it? I had forgotten that.

Electrolysis has nothing to do with it. Electrolysis refers to the chemical change in the electrolyte caused by the passage of electrical current. You are dealing with galvanic or stray current, not electrolysis.

[boatpoker]

Originally Posted by transmitterdan

That does not cause stray DC current. The main fear is that the insulation could be compromised and allow the hot wire to leak current. Nearby swimmers, especially in fresh water marinas, can drown due to paralysis caused by the AC current in the water. This is why all marinas should have residual current devices at the power pedestal. But most in the US do not have them.

"leak current" is stray current.

[boatpoker]

Originally Posted by a64pilot

It's does indeed take DC to have electrolysis, doesn't it? I had forgotten that.

No, electrolytic corrosion (not electrolysis) can be caused by AC but at a much slower rate.

[fstbttms]

Originally Posted by Cheechako

But the odds are good that if a zinc is disappearing... something else isn't!

Pretty sketchy logic. If there was a bad problem, you would expect the anodes to deplete, right along with your prop. You want to trust your boat to nothing more than pulling up a guppy zinc and having a look at that- hey, it's your dime. Bottom line- Nothing can replace a set of eyes on the gear to acertain it's status. I guarantee none of the owners of the props below had a clue there was anything wrong until someone got in the water and inspected them.

[noelex 77]

Originally Posted by fstbttms

Bottom line- Nothing can replace a set of eyes on the gear to acertain it's status.

I think in this case there may have been some other subtle clues that there was a problem

[fstbttms]

Originally Posted by noelex 77

I think in this case there may have been some other subtle clues that there was a problem

You would be wrong. I was cleaning the hull and grabbed the prop to pull myself to the other side of the boat. That's when the first blade snapped off in my hand. The owner was standing right there. He was shocked. Admittedly, nobody had been under that particular boat in a long time.

[Goudurix]

Now this is where I like to see the discussion go! Interesting all your replies and debates.
I like the empirical way but backed by some sound electrochemical processes.

Sailorchic interesting to read about the 0.64 voltage delta you have measured. I remeber Nigel Calder mentions measuring the protection level but by means of an reference electrode (Ag/AgCl?). have to check, haven't got the book here at hand.

The thing I'm most sceptical about is the "low resistivity" connection between a ("guppy" is what you guys cll it?) overboard zinc anode and the engine block or DC-ground and /or bonding system.
I guess that with a rather bad connection or eg the S/S wires often used for this one, or just hanging it on a (bonded) stanchion or toerail, often gets a "free floating system" in this case a zinc, connected in a better or worse mechanical way, to a copper wire or S/S cable, and hanging in the seawter (electrolyte). If it is ± free floating, you should see the zinc reacting, as it will "protect" the copper or S/S wire that is at higher potential.
Hence one would see the zinc working and thinking it protects the propeller and the propeller shaft but it would rather protect the copper or S/ wire.
Am I wrong or am I wrong?

Could be measured out I guess.

What I learn from this: galvanic (and stray current) corrosion are complex phenomenons.
There is also a lot of nonsense circulating about it.
I will plunge in my Nigel Calder's and (try to) revive my long gone eclectrochemical reaction knowledges....

Jan

[goboatingnow]

The key to establishing whether any CP is working , is to invest in a half cell reference and use a good DVM with a 10 MOhm input resistance. Then you can establish the potentials of all the supposedly protected metals and determine if in fact they are being protected at all.


dave

[noelex 77]

Originally Posted by Goudurix

Am I wrong or am I wrong?

A bit of both . Yes there is an effect, but the wire is plastic coated so the amount in contact with seawater is only small in relation to the size of zinc (at least for the better models)
If the zinc is not doing anything there is very minimal "self wastage".

[Goudurix]

Originally Posted by goboatingnow

The key to establishing whether any CP is working , is to invest in a half cell reference and use a good DVM with a 10 MOhm input resistance. Then you can establish the potentials of all the supposedly protected metals and determine if in fact they are being protected at all.


dave

That's my boy!

If I remeber well about -0.5V below the metal's own potential is a good level of protection, no?

Jan

[sailorboy1]

While hanging a zinc into the water and connecting it to the boat ground system meets all the requirements I just don't see it doing much except under a severe problem. Current is lazy and I doubt that the connections are so good that it is going to find its' way to protecting a prop when it is so much easier to travel the short distance to the zinc on a shaft or prop instead.

Did I mess a post on this thread when someone was having a severe shaft zinc wasting problem and solved it by hanging a zinc overboard?

[fstbttms]

Originally Posted by sailorboy1

Did I mess a post on this thread when someone was having a severe shaft zinc wasting problem and solved it by hanging a zinc overboard?

Of course not. Again, a guppy is just a bandaid. If you have a severe corrosion issue, adding anodes is not solving the problem.

It occurs to me that if hanging an anode overboard with a wire were a truly effective form of protection (as some here would have you believe), nobody would bother mounting them on the shaft, prop, strut, rudder etc., etc.

[Scot McPherson]

@sailorboy...you are absolutely correct. The shaft and propeller are in too close proximity, so you do need a shaft zinc. However electric fields aren't intuitively understood as has been demonstrated here.

You can have galvanic corrosion with a single piece of isolated metal, and even when the engineering example is copy and pasted from an engineering site, and everyone agrees with it, they seem to miss or gloss over that part.

A bright to bright connection is better, but it's by no means the cause of the savrificial anode corroding, it simply makes it happen faster. If the electric fields of two pieces of metal are in contact, then you have both ion and electron flow following the megnetix lines of the electrical field. It is higher resistivity, but it's still there.

Someone use the example of the statue if liberty having it's copper sheathing isolated from the steel structure, but that's in air, not seawater.

[noelex 77]

Originally Posted by Scot McPherson

Someone use the example of the statue if liberty having it's copper sheathing isolated from the steel structure, but that's in air, not seawater.

Rainwater provides the electrolyte.
It is a good example of why you need electrical contact. If I understand correctly it was a breakdown (and lack of maintenance) of the insulation between the two dissimilar metals that caused the problem.