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Old 13-08-2014, 14:15   #31
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Re: Sense or nonsense hanging zinc anode overboard?

I'm siding with SC on this one, and this is why. As long as the "Guppie" zinc is electrically bonded to the metals your trying to protect as a less noble metal it will be sacrificial and that means it is protecting the other metals it's bonded to.
If it's not electrically bonded to the metals your trying to protect, then I can see your just wasting money with it.
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Old 13-08-2014, 14:24   #32
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Re: Sense or nonsense hanging zinc anode overboard?

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Originally Posted by a64pilot View Post
I'm siding with SC on this one, and this is why. As long as the "Guppie" zinc is electrically bonded to the metals your trying to protect as a less noble metal it will be sacrificial and that means it is protecting the other metals it's bonded to.
If it's not electrically bonded to the metals your trying to protect, then I can see your just wasting money with it.
The anode's ability to protect a part is reduced by a long mechanical, resistive connection to the part and also by physical distance from that part. Both of these conditions exist when using a guppy zinc. It is entirely possible that while you have a guppy zinc in some sort of electrical contact with your running gear, that those parts are undergoing more corrosion than the anode is capable of protecting them from. This is why the mere presence of the guppy zinc is no assurance that everything is OK.
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Old 13-08-2014, 14:35   #33
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Re: Sense or nonsense hanging zinc anode overboard?

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Originally Posted by Scot McPherson View Post
If there are two dissimilar metals in the salt water, then they already are electrically connected, whether they are wired together on the boat or not. Now connectictinf them with a wire will speed up the corrosion, but disconnecting them will NOT stop it.
I don't think is correct. The requirements for galvanic corrosion are an electrical connection and immersion in an electrolyte.

This is also true of a hanging zinc it must be connected to the metal it is trying to protect. Just hanging a zinc next to prop is of no help. There must be an electrical connection.
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Old 13-08-2014, 14:53   #34
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Re: Sense or nonsense hanging zinc anode overboard?

I don't think anyone is saying a "guppie" zinc is a guarantee of anything, there are very few absolutes.
I do believe it may help, it won't hurt and if I see the Guppie being eaten up , I'll investigate as that may be an indicator of increased electrolysis.
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Old 13-08-2014, 15:02   #35
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Re: Sense or nonsense hanging zinc anode overboard?

It is correct...the electrolytic solution in this case is also a good conductor, just not as good as a bright to bright connection. Disconnection may slow corrosion because the electrical connection is more resistive than a solid bright to bright connection, but it's still there. Therefore disconnecting will slow corrosion, but it won't eliminate it...if it did, we wouldn't need sacrificial anodes, we'd just need to isolate any exposed metals.

And to be quite frank...dissimilar metals just means the metal surfaces have different levels of resistivity and ion charge. Even a single piece of metal free floating (as if that were possible) in the middle of the sea will suffer galvanic corrosion, the pockets of surface will have shifting resistivity and ion exchange...what we see this as is pretty much rust (at least for iron and steel)

The iron ion is pulled from one place, oxidizes and get deposited back in a slightly different position as rust at the reduction point.

Understand an ion and electron are not the same...in a galvanic circuit, ions flow one way while electrons flow the other if the medium is the same....only by giving a less resistive path by means of a short wire for instance, does it appear to flow in a circle, but you need to realize even then it's just the electrons getting back to the anode, so the anode can realize ions that flow to the cathode.
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Old 13-08-2014, 15:37   #36
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Sense or nonsense hanging zinc anode overboard?

Quote:
Originally Posted by Scot McPherson View Post
It is correct...the electrolytic solution in this case is also a good conductor, just not as good as a bright to bright connection. Disconnection may slow corrosion because the electrical connection is more resistive than a solid bright to bright connection, but it's still there. Therefore disconnecting will slow corrosion, but it won't eliminate it...if it did, we wouldn't need sacrificial anodes, we'd just need to isolate any exposed metals.

And to be quite frank...dissimilar metals just means the metal surfaces have different levels of resistivity and ion charge. Even a single piece of metal free floating (as if that were possible) in the middle of the sea will suffer galvanic corrosion, the pockets of surface will have shifting resistivity and ion exchange...what we see this as is pretty much rust (at least for iron and steel)

The iron ion is pulled from one place, oxidizes and get deposited back in a slightly different position as rust at the reduction point.

Understand an ion and electron are not the same...in a galvanic circuit, ions flow one way while electrons flow the other if the medium is the same....only by giving a less resistive path by means of a short wire for instance, does it appear to flow in a circle, but you need to realize even then it's just the electrons getting back to the anode, so the anode can realize ions that flow to the cathode.

I'm sorry there are four conditions for a galvanic cell to operate , an anode, cathode , an electrolyte and a current path. Otherwise every battery we know would just disintegrate before our eyes.

Merely hanging an isolated zinc over the side , in itself does nothing. That's why all these zincs have a lead that in theory is clipped into the bonding circuit of the vessel.

This is why typically on US yachts all underwater fittings are bonded together and connected to the zinc. The express design is to actually create a galvanic cell, where the zinc cathode is the less noble metal over other inadvertent galvanic cells that might form.

This is why for example propellor shafts have zincs clamped to them and similarly with rudders and trim tabs. The sea water provides the electrolyte and the direct metal to metal provides the current path.

The US all bonded approach is of course a very debatable process. In my view it potentially increases the risk of galvanic corrosion , doubly so when the risk of impressed current corrosion is added by connecting the shore power into the DC negative / bonding system.

Hence the practice of clipping the lead to the guardrail and lowering a zinc may in fact do little especially if the railing is electrically insulated.

The process originated in aluminium boats, for very good reasons. I don't think I've ever ever seen it in Europe on a GRP boat. I would seriously question it's usefulness

Your free floating metal example is correctly called a concentration cell. It's responsible for things like crevice corrosion etc and occurs because the concentration of the electrolyte is not identical along the surface of the material causing an effective and cathode to form on the same metal, ( or oxygen depletion etc) again all four conditions must be and are present in concentration cell.



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Old 13-08-2014, 15:44   #37
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Re: Sense or nonsense hanging zinc anode overboard?

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Originally Posted by Scot McPherson View Post
It is correct...the electrolytic solution in this case is also a good conductor, just not as good as a bright to bright connection.
I still don't think this is right.
Electrolytes are conductors, but you require a completed circuit. It is difficult to explain in simple terms but you are relying on the electrolyte to conduct both sides of the circuit. With two dissimilar metals and an electrolyte you have an open circuit like a battery with a single wire leading to a lightbulb nothing happens until you complete the circuit.

Seawater is conductive and can work as the wire for one side of the circuit, but not both at the same time.

Quote:
Originally Posted by Scot McPherson View Post
..if it did, we wouldn't need sacrificial anodes, we'd just need to isolate any exposed metals.
Yes and this is a perfectly acceptable and common way of eliminating galvanic corrosion.

However, it is very hard to completely isolate dissimilar metals especially with moving parts. Hence it often more practical to allow the dissimilar metals to be in electrical contact but to add a zinc anode that is also in electrical contact with the aim of allowing the zinc to corrode.

Quote:
Originally Posted by Scot McPherson View Post
Even a single piece of metal free floating (as if that were possible) in the middle of the sea will suffer galvanic corrosion, the pockets of surface will have shifting resistivity and ion exchange..
Yes it is not usually called galvanic corrosion, but the principles are the same.

The dissimilar parts of the metal are electrically connected and immersed in an electrolyte hence it fulfils all the requirements for corrosion.


The important principal is the metals have to be electrically connected and in contact with an electrolyte for galvanic corrosion.
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Old 13-08-2014, 15:57   #38
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Re: Sense or nonsense hanging zinc anode overboard?

Quote:
Originally Posted by noelex 77 View Post
I still don't think this is right.
Electolytes are conductors, but you require a completed circuit. It is difficult to explain in simple terms but you are relying on the electrolyte to conduct both sides of the circuit. With two dissimilar metals and an electrolyte you have an open circuit like a battery with a single wire leading to a lightbulb nothing happens until you complete the circuit.

Seawater is conductive and can work as the wire for one side of the circuit, but not both at the same time.


Yes and this is a perfectly acceptable and common way of eliminating galvanic corrosion. However, it is very hard to completely isolate dissimilar metals especially with moving parts.
I believe the answer is that sea water isn't a good enough conductor of electrons? However, this doesn't mean it can't happen at all.

When a NiCad battery self discharges, isn't that because it is completing the circuit internally through the electrolyte? Whereas Li Ion batteries have much less of this effect? So perhaps it is dependent on the geometry (distance to travel, surface area), and the electrolyte.
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Old 13-08-2014, 16:00   #39
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Re: Sense or nonsense hanging zinc anode overboard?

Quote:
Originally Posted by noelex 77 View Post
I still don't think this is right.
Electolytes are conductors, but you require a completed circuit. It is difficult to explain in simple terms but you are relying on the electrolyte to conduct both sides of the circuit. With two dissimilar metals and an electrolyte you have an open circuit like a battery with a single wire leading to a lightbulb nothing happens until you complete the circuit.

Seawater is conductive and can work as the wire for one side of the circuit, but not both at the same time.


Yes and this is a perfectly acceptable and common way of eliminating galvanic corrosion. However, it is very hard to completely isolate dissimilar metals especially with moving parts.

Full marks. For example the Statue of Liberty now has a PTFE insulating layer between the copper sheet and the wrought iron support structure for just the purpose you outline


The principle of European corrosion protection is to rely more on isolated metals then bonding


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Old 13-08-2014, 16:04   #40
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Re: Sense or nonsense hanging zinc anode overboard?

I am sorry, but adding a wire as a very low risistivity conductor only speeds up the process, it does not create it.

Read the read of my post, do the research....it's all there.

Batteries discharge slowly when not being used, some designs hold charges for longer than others but they all discharge slowly.

Anyway, I don't want to fight with anyone so last I am saying about it, but anyone who wants to learn about it can do plenty of googling, and don't just fixate on the first thing on the list. You want science and engineering articles not a comment by a boat supply shop.
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Old 13-08-2014, 16:19   #41
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Re: Sense or nonsense hanging zinc anode overboard?

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The anode's ability to protect a part is reduced by a long mechanical, resistive connection to the part and also by physical distance from that part. Both of these conditions exist when using a guppy zinc. It is entirely possible that while you have a guppy zinc in some sort of electrical contact with your running gear, that those parts are undergoing more corrosion than the anode is capable of protecting them from. This is why the mere presence of the guppy zinc is no assurance that everything is OK.
Obviously Fastbottoms loves to get paid to change zincs. But to continue on his vein NOTHING of any kind "assures that everything is OK". But the odds are good that if a zinc is disappearing... something else isn't!
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Old 13-08-2014, 16:40   #42
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Re: Sense or nonsense hanging zinc anode overboard?

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Originally Posted by fstbttms View Post
The anode's ability to protect a part is reduced by a long mechanical, resistive connection to the part and also by physical distance from that part. Both of these conditions exist when using a guppy zinc. It is entirely possible that while you have a guppy zinc in some sort of electrical contact with your running gear, that those parts are undergoing more corrosion than the anode is capable of protecting them from. This is why the mere presence of the guppy zinc is no assurance that everything is OK.
Yes there is a maximum distance from a cathode to a galvanic anode. In dirt that is about 50 feet, depending on conductor size, soil type, etc. But that is from the anode to any part of the rest of the metal. Which is why hundreds of feet of pipe line can be protected by a single 10-20 pound anode.

In salt water that distance would be even longer due to the higher conductance of salt water.

There is a relationship to amount of metal being protected and size of the anode. Though mainly to keep the change out times to a longer duration. However a one ounce zinc will provide all the protected needed, but it will however corrode rather quickly. The important part is that there is a strong voltage potential between the metals.

In my case where I have the zinc fish connected to the engine, there is a voltage potential of 0.64V, which happens to be the difference between bronze and zinc on the galvanic voltage table. Were it stainless steel only, the voltage difference would be something less the 0.64V (-1.05V for zinc and -0.4V ish for bronze).

So in my case, I'm fairly confident that my prop is protected. The voltage differential tells me so.

Both goboatingnow and Noelex 77 are of course correct.

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Old 13-08-2014, 16:51   #43
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Sense or nonsense hanging zinc anode overboard?

Quote:
Originally Posted by Scot McPherson View Post
I am sorry, but adding a wire as a very low risistivity conductor only speeds up the process, it does not create it.

Read the read of my post, do the research....it's all there.

Batteries discharge slowly when not being used, some designs hold charges for longer than others but they all discharge slowly.

Anyway, I don't want to fight with anyone so last I am saying about it, but anyone who wants to learn about it can do plenty of googling, and don't just fixate on the first thing on the list. You want science and engineering articles not a comment by a boat supply shop.

Battery self discharge is a complex process, and is not solely due to migration of electrons in the electrolyte.

The purpose of a zinc on a vessel is to specifically create a galvanic cell. In particular to raise the galvanic voltage 250 mV over the " free voltage " potential of the possible cathode immersed in the electrolyte.

Merely placing an unconnected zinc in the electrolyte provides virtually nothing as it cannot raise the free potential of the other underwater material by suffice mV to effect any real protection

This can be easily verified by use of a good quality digital multimeter and a sliver chloride half cell reference. Place the cell in the seawater and measure potential of the say stainless shaft. You will see that it is not protected , place a zinc anode in the water nearby ( not bonded in, just say on a string ) you will not see anything like sufficient rise in the cathode voltage to perform any sort of protection


Again placing an unconnected zinc into seawater does basically nothing
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Old 13-08-2014, 16:51   #44
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Re: Sense or nonsense hanging zinc anode overboard?

[QUOTE=Scot McPherson;1605320 You want science and engineering articles not a comment by a boat supply shop.[/QUOTE]

Honny, First I am an engineer and have read up on corrosion control from real honest to god engineering books no less.

With only a single path, IE prop to sea water to zinc fish there is no current flow, or rather there is no protection given to the cathode metals. The sea water is the electrolyte but only one side of the circuit.

You need an electrical conductor for the flow of electrons to flow from a anode to a cathode. Without an electrical connection there isn't galvanic protection to the more noble metal. Why its called cathodic protection as the Anode protects the Cathode.
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Old 13-08-2014, 20:36   #45
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Re: Sense or nonsense hanging zinc anode overboard?

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]
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