Over zinc'd?

[firstbase]

Having electrical work done on my boat, GI installed, a couple of lesser things, and the electrician mentioned that when he is done I should look at getting smaller zincs as the ones I have will be too much. I have shaft zincs and two 6" X 3" or so, not sure never measured, bars on the transom. One on each side. What test or test result tells me how much zinc to put on the hull? Decreasing the size is problematic with the boat in the water as I would think that I would have to drill new bolt holes for a smaller zinc.

 

[DavidM]

What is a GI?

I really doubt that you will have too much zincs. But if your electrician is so concerned ask him to hook up a half cell corrosion reference electrode and test for the proper voltage.

The boat does need to be in the water to do the test.

David

 

[firstbase]

Galvanic isolator, GI. Sorry. He did test everything with silver chloride electrode and got -.860 or so unplugged, -.580 plugged in. Sorry if I am criss crossing posts here. Put up another on my electrical situation a few minutes ago.

 

[Larry M]

I posted this on your other thread. For a fiberglass boat with inboards you should be reading -750mv to -1000mv. Less than that you are under protected. Are you in brackish water? If you are, you can change from zincs to aluminum anodes and that should get you a little more negative.

Edit: Edit: I checked ABYC recommendations. They say you are ok at -550 my. There are a lot of references out there.

ABYC Recommended range of cathodic
protection for boats of different hull
materials in saltwater
Hull Material Millivolt Range
Fiberglass -550 to -1100
Wood -550 to -600
Aluminum -950 to -1100
Steel -850 to -1100

Some info from BoatZincs.com
http://www.boatzincs.com/corrosion-reference-electrode.html

 

[Insequent]

Good info above.

But back to the OP's question. How do you know if you have too much zinc? What are the symptoms? What are the downsides?

 

[firstbase]

I posted this on your other thread. For a fiberglass boat with inboards you should be reading -750mv to -1000mv. Less than that you are under protected. Are you in brackish water? If you are, you can change from zincs to aluminum anodes and that should get you a little more negative.

Edit: Edit: I checked ABYC recommendations. They say you are ok at -550 my. There are a lot of references out there.

ABYC Recommended range of cathodic
protection for boats of different hull
materials in saltwater
Hull Material Millivolt Range
Fiberglass -550 to -1100
Wood -550 to -600
Aluminum -950 to -1100
Steel -850 to -1100

Some info from BoatZincs.com
Corrosion Reference Electrode: Corrosion Quiz

Thanks Larry. I am in salt water, about a mile from the Atlantic and 200 yards from the ICW so nothing brackish about it. When all is said and done I am going to get him to show me the "after" tests as well as ask about the 30ma dock breaker, how do I know that I won't set it off. I assume that would be due to current leaving my boat and going to the dock pedestal and tied to the ground/neutral situation? Something allowing current to go out the back door.

 

[CharlieJ]

He did test everything with silver chloride electrode and got -.860 or so unplugged, -.580 plugged in.

Those readings tell me that the vessel is not isolated from the shore safety ground system. The galvanic isolator is either bypassed or it has failed.

To be galvanically isolated the hull potential should not change between the shore power plugged in and the shore power unplugged readings.

A fiberglass boat with no underwater aluminum components (sail drive, outboards, Zeus pods, etc.) can not be "over zinc'd". This is another dockside myth.

 

[Gordon J]

I had similar readings

Those readings tell me that the vessel is not isolated from the shore safety ground system. The galvanic isolator is either bypassed or it has failed.

To be galvanically isolated the hull potential should not change between the shore power plugged in and the shore power unplugged readings.

A fiberglass boat with no underwater aluminum components (sail drive, outboards, Zeus pods, etc.) can not be "over zinc'd". This is another dockside myth.

And then found that the galvanic isolated installed by the ptrevious owner was mistakenly installed on generator ground, not the shore ground.

 

[C lectric]

Good info above.

But back to the OP, 's question. How do you know if you have too much zinc? What are the symptoms? What are the downsides?

The first is the voltage read from the Silver/silver chloride cell.
Too high from too much zinc can bubble paint off of metal parts, props, shafts, struts, etc., any immersed painted metals.

May cause direct damage to metals like aluminum.

It may cause a ring of paint bloom/bubble or disappearance around immersed through hulls.

On wooden boats it can cause actual damage to the wood surrounding through hulls or any other immersed metal parts or bolts/screw to secure them.

There are some good books that cover the subject.
--Metal Corrosion in boats: Nigel Warren
--The Boat Owners Guide to Corrosion: Everett Collier
I think Boatowners Mechanical and ELectrical Manual also: Nigel Calder also has a bit.

http://www.kastenmarine.com/_pdf/mbqCref.pdf


There are many other sources.

 

[CharlieJ]

C electric Post #9:
On a fiberglass hull with no underwater aluminum components, the ABYC E-2 recommendation hull potential range for protection is: -550mVDC to -1100mVDC when measured against a silver/silver-chloride reference cell. The Galvanic Series of Metals, also from ABYC E-2, shows Zinc as having a Corrosion Potential range, again when measured against a silver/silver-chloride reference cell; in sea water flowing at 8 to 13 ft/sec and within a temperature range of 50F to 80F of -980mVDC to -1030mVDC with Mil-Spec Zn anodes at the upper end of that range.

Too high from too much zinc can bubble paint off of metal parts, props, shafts, struts, etc., any immersed painted metals.

If you review the recommend range for cathodic protection and the available protecting potential from a zinc sacrificial anode, "over zincing" is not really possible on a fiberglass boat with a properly installed underwater paint system. I have performed many, many corrosion surveys in salt and brackish water and rarely find a hull potential more negative than -1000mVDC. The norm is probably -950mVDC.

The process of paint coming off of underwater metal parts is primarily the result of poor paint system application which is generally a result of poor substrate preparation. In some cases, it can be caused by VOC entrainment or the presence of soluble salts on the substrate that have not been removed prior to painting.

It may cause a ring of paint bloom/bubble or disappearance around immersed through hulls.

Haloing is caused by high copper content paint and its interaction with good quality bronze through hulls. The copper in the paint matrix is anodic to the bronze which is cathodic. To prevent this condition, the bronze through hull needs to be properly primer coated with a barrier coat such as the Interlux products.

May cause direct damage to metals like aluminum.

Absolutely correct! Aluminum is amphoteric which means it can be damaged by either acids or bases. Driving the potential of an aluminum hull or underwater aluminum component to >-1200mVDC against the reference cell with an impressed current cathodic protection system or because of a fault in the on board DC system will create an alkaline environment that will damage the aluminum hull or component.

On wooden boats it can cause actual damage to the wood surrounding through hulls or any other immersed metal parts or bolts/screw to secure them.

Absolutely correct! The range of protection for wooden boats is -550mVDC to -600mVDC; a very difficult to achieve 50mVDC range between under protected and overprotected. If the protection potential is above this range, an alkaline environment develops and the alkalis will attach the lignin in the wood, a condition known as delignification.

 

[firstbase]

Those readings tell me that the vessel is not isolated from the shore safety ground system. The galvanic isolator is either bypassed or it has failed.

Thanks Charlie. I don't have an isolator at all, nothing to bypass or fail. Will on Tuesday though.