Over Zinc'd??

[Taras]

Hey team,
My shaft pick ups need to be replaced in my bonding system. Until I get these replaced, I put shaft zincs on. My corrosion control system now reads "over protected".
What is the issue with being over protected?
I need to replace my shaft pick ups, but in the mean time I wanted to be sure my props were protected.
So what's the problem with being "over Zinc'd"?

Thanks for your input.

Ex Taras

 

[Marin]

I think your problem is with your corrosion control system, not the way you have the boat zinced. Over-zincing can be a problem on a wood boat. But so far as I know, it's not a problem with a fiberglass boat, but I could be wrong about that.

On our boat we have two transom zincs ("license plate" zincs), two shaft zincs on each shaft, and shaft wipers.

Because our harbor is in a bay with a fair size stream entering next to the harbor and a large-ish river entering a couple of miles away, the upper surface of the water in the harbor is often fresh or brackish. So we, like a lot of people in our marina, hang a zinc on a cable off the side and down about six or eight feet. The other end of the cable is connected to the boat's bonding system. This ensures that there is good connectivity between the bonding system and the surrounding water even when the upper layer--- the layer the transom zincs are in--- is fresh or nearly fresh water.

We don't have any sort of electronic corrosion control system other than a galvanic isolator if that can be considered "electronic." So we don't have any system that gives us warnings. But our boat is set up the way almost all the similar boas are in our harbor and the setup and our hanging zinc were recommended by the marine electric shop we use. We've not had any problems in the 17 years we've owned this particular boat.

So in your case I would ignore the warning from whatever your corrosion control system is. The whole subject of corrosion and electrolysis control on a boat has an element of black magic to it anyway. So I would be inclined to not put much stock in a "control" system since I have no idea if they are programmed to account for the twenty million variables in corrosion control.

 

[Larry M]

If in doubt get a corrosion analysis done with someone who knows how to use silver/silver-chloride electrode such as a ABYC Certified Technician. We had a corrosion analysis done a few years ago to check our DC, AC, shore power and bonding system. It took about an hour. Here's some information:


Corrosion Reference Electrode

 

[Pgitug]

For some expert advice talk to the fellows at boatzincs.com. They are very helpful.

 

[Taras]

I've emailed boat zincs.com
I'll post what they say.
Thank you for the tip!

Any other advise on this question is welcomed

 

[what_barnacles]

If in doubt get a corrosion analysis done with someone who knows how to use silver/silver-chloride electrode such as a ABYC Certified Technician. We had a corrosion analysis done a few years ago to check our DC, AC, shore power and bonding system. It took about an hour. Here's some information:


Corrosion Reference Electrode

the quiz's first question in your link says that too negative a voltage (for the given hull material) indicates underwater metals connected to the bonding system are over protected and causing vessel damage.

 

[bglad]

That would be the case if the hull were wood. Overprotecting a fiberglass hull doesn't damage the hull but can be tough on coatings. It causes reactions to the metals in antifouling paint (haloing) and can blister paint off metals.

the quiz's first question in your link says that too negative a voltage (for the given hull material) indicates underwater metals connected to the bonding system are over protected and causing vessel damage.

 

[what_barnacles]

right, the way it reads thats the point they were making. Kind of assumes that boats needing bonding, probably need bottom paint too.

 

[Capt Kangeroo]

Marin said it well "The whole subject has an element of black magic".

Like many others who are having corrosion problems, I have been studying this subject for quite some time looking for clear, concise answers based on scientific facts. I wasn't long coming to the conclusion that a great deal of the information to be found, besides being contradictory, is either vague unscientific misinformation by pseudo experts or pure rubbish by those trying to sell you something. "Not unlike like many of the anchor studies that abound on the internet"

Taras question regarding being over Zinc'd clearly falls within Marin's "black magic" reference. At the risk of contributing to the myth and hearsay out there, the only scientific information I have been able to find on the subject is that too much zinc increases the potential between the anode and cathode, this increased potential "can" under certain conditions create hydrogen which is absorbed by the cathode and in turn causes hydrogen embrittlement. This embrittlement can cause sudden failure under load or stress cracks which in turn provide the potential entry points for crevice corrosion. In other words excessive negative potential can cause the very thing cathodic protection is intended to prevent. All that said I have yet to find anything that quantifies "how much zinc is too much zinc or how real "to the boating community" the threat of hydrogen embrittlement really is. I suspect it is not.

http://www.aimnet.it/allpdf/pdf_pubbli/lug14/Bellezze.pdf
http://www.npl.co.uk/upload/pdf/cathodic_protection.pdf
http://www.braemaradjusting.com/ass...brittlement of Subsea Structures(website).pdf
https://en.wikipedia.org/wiki/Hydrogen_embrittlement
Corrosion electrochemistry