over & under zinced

[mlydon]

Just came across a thread discussing zinc fish, with the cables etc, and someone mentioned they are likely used when you are 'under zinc'd'.

I've also seen mention of being 'over zinc'd' as being bad....forget the details.

Can someone explain how we get to 'just right' on the zincing?

Matt

[smac999]

Use a silver silver cell and volt meter to measure your current protection levels. Add or remove as needed.

You also need a galvanic isolator or iso transformer if at a dock.

[mlydon]

What is a silver silver cell, and how would i use a vm to measure... Resistance? Voltage? From where to where?

Matt

[S/V Illusion]

There is no “just right” - what we strive for is minimal electrolysis to preclude or minimize galvanic corrosion. Simply adding/removing an anode such as zinc serves only to alter the battery created by introducing any anode.

Anodes are bandaids which minimize the damage. Unfortunately, many people mistake more for better. The size and location vary depending on what you want to protect but realize too much can cause as much if not more damage as too little.

[mlydon]

"Anodes are bandaids which minimize the damage. Unfortunately, many people mistake more for better. The size and location vary depending on what you want to protect but realize too much can cause as much if not more damage as too little."

There you go - that's what I mean by 'just right' - not too much, not too little.

now - HOW does one determine that?

Matt

[S/V Illusion]

Originally Posted by mlydon

"Anodes are bandaids which minimize the damage. Unfortunately, many people mistake more for better. The size and location vary depending on what you want to protect but realize too much can cause as much if not more damage as too little."

There you go - that's what I mean by 'just right' - not too much, not too little.

now - HOW does one determine that?

Matt

Let me try explaining it differently - there is no way to answer your question definitively without in-situ measurement.

If you have a prop zinc of any size on the shaft and it ‘wears’ quickly, you have a problem which should be identified and cured, not masked by adding more anode material. Consequently, you are asking an unanswerable question.

[Sailmonkey]

Originally Posted by mlydon

What is a silver silver cell, and how would i use a vm to measure... Resistance? Voltage? From where to where?

Matt

Start here, then head down the rabbit hole.

https://www.boatzincs.com/corrosion-...ode-specs.html

[fstbttms]

What kind of prop do you have?

[mlydon]

Maxprop

[RaymondR]

The amount of current produced by the zinc in the sea water electrolyte is dependent upon the exposed surface area of the zinc and the proximity of the other metal forming the galvanic cell and it's exposed area.

The current generated by the zinc and it's complementary protected metal flows in a direction which reverses the tendency of metal loss so that it is the zinc which is eroded away rather than the metal being protected, hence the term "sacrificial anode".

If the metal being protected is also coated to ensure exclusion of the corrosive sea water, reactions at the metal/water interface caused by the excessive electrical current will lift the coating away from the metal substrate causing blistering and consequent failure of the coating.

Whilst the underlying metal will be protected by the current flow the reduction of the zinc area and consequent reduction of current will eventually result in insufficient protection of the underlying metal and removal of metal will occur.

The silver/silver chloride cell is used to measure the galvanic potential of a metal in an electrolyte. In the short term you can get a similar result using a piece of 316 SS on one leg of a digital multimetre.

Putting a piece of zinc on one terminal and a piece of 316 SS on the other with both in a glass of water will show about 0.6 Volt difference of potential whether it is fresh or sea water. However the amount of current through salty or sea water will be far greater at the 0.6 Volt potential and hence greater metal removal in the sea water.

[fstbttms]

Originally Posted by mlydon

Maxprop

You’re overthinking this. You should have an anode on the prop and one on your very limited length of exposed prop shaft. If you need more than that, you have an electrical problem aboard or nearby and adding additional anodes becomes a bandaid, not a solution.

[rgleason]

I noticed that after spray painting prop with barnacle anti fouling paint, that my zincs easily lasted 2 years. Perhaps I should reduce the number of zincs to one on the tail of the prop, removing the zinc on the shaft, which is extra anyway?

Shaft is short and for an appeture.

[Capt.Don]

Originally Posted by fstbttms

You’re overthinking this. You should have an anode on the prop and one on your very limited length of exposed prop shaft. If you need more than that, you have an electrical problem aboard or nearby and adding additional anodes becomes a bandaid, not a solution.

I'm glad you contributed to this post!

In addition to the prop-nut and shaft zincs, I also have one 3" rudder zinc on my strut. I was thinking about adding another one to the other side. Now reading this, I probably won't add the additional zinc.

Thanks
Don

[Phillip]

Originally Posted by mlydon

What is a silver silver cell, and how would i use a vm to measure... Resistance? Voltage? From where to where?

Matt

A Silver/Silver Chloride half cell is a reference electrode.

Pro Mariner makes one. Link: Corrosion Test Meter Half Cell | ProMariner
You would measure voltage from the reference electrode to various metal parts on the vessel that are in contact with water.
The voltage (in Millivolts) is an indication of whether the metal is protected, freely eroding or over protected.
More information at this link:

assets.fluke.com/appnotes/electricalpower/B0269b_u.pdf
Good luck!

[FabioC]

In my experience, the "zincing" in practice is much less scientific than what suggested by most of the comments here.

I am an electrical engineer and I have hired marine experts twice to understand the galvanic corrosion on my boat. The issue I was facing was unusual corrosion on the strut holding the prop shaft on my boat. Incidentally, there were two boats identical to mine within 20 yards of mine in the marina, and they did not show such issue. The boat is a Jeanneau, which complicated the investigation somewhat, since Jeanneau's are not bonded.

Anyway, the "scientific approach" was inconclusive. All measurements we performed did not point out measurable currents nor measurable differences between my boat and the boats that did not have galvanic corrosion. In fact, after performing many measurements, my distinct impression is that measurements would only identify very obvious galvanic effects (for example, if your neighbor's boat is not isolated), but are ineffective in catching and allowing to model what is really going on in most situations. In addition, there is no good model to anticipate the effects of adding a zinc or bonding a tru-hull.

I think zincing in practice is very much a trial and error procedure, as engineering often is. You can do some measurements if you really like it, but then put the instruments aside and proceed prudently in steps, introducing a single modification at each step (e.g., adding or removing one zinc or making a single modification on the bonding), get plenty of pictures so you can easily compare, and see what happens at every step. You should be able to converge to an empirical but effective solution very fast.