THICK white deposit in zincs

[NotCook]

I’ve made 3 dives so far this week to scrape barnacles, and even growths of coral like marine life, off our bottom. I was alarmed to find the zincs encrusted with half inch thick deposits almost like the sheetrock inside a drywall panel. It comes off easily enough, in big chunks, but what is it, why is it there, and how alarmed should I be? My first reaction is how can the zincs function with that deposit on them, and my second reaction was do I have a serious electrical issue creating this situation and eating away at our steel hull?

Anyone KNOW for sure what’s going on and what to do about it?

 

[jssailem]

Brian, what did the zincs look like after you scrapped the "thick Deposits" off?

How long have the zincs been on the boat? Last change date?

Are these hull zincs?

Are you seeing any hull metal, prop metal, or shaft metal being affected?

 

[Gunni]

It is zinc oxide and indicates that your anode is too passive for your waters. The oxide acts as an insulator and makes your zinc anode much less effective. Are you in brackish waters? I found that in brackish water an aluminum anode is much more effective and doesn't oxidize like zinc - it may last a bit longer too, but the important thing is that you get the galvanic protection you need.

 

[dlochner]

The white stuff is probably zinc oxide.

Is this a new problem?

 

[JamesG161]

BTW all Zincs do that. The Zinc Oxide is normally "cleaned off" buy a spinning shaft. It is a classic problem with "pencil zincs".

I suspect he has a Hull Zinc.
Jim...

 

[JamesG161]

The oxide acts as an insulator

Not so much as an insulator, but a surface area of Zinc reducer. Electrons will still pass through the porous Zinc Oxide. It still reduces the Zinc's effectiveness.
Jim...

 

[NotCook]

Thanks for the comments so far. To answer some of the questions, we are in salt water, in the Caribbean, the zincs are on the hull, not pencils, and are 9” long and weigh 5# each, on an all steel vessel. This is the first I have noticed this buildup. These are actual zinc, not other material. They’ve been on the boat for several years in fresh wayer, where they were probably not the best composition, but have been in saltwater the past 3 years. I have not been able to detect any metal loss in the hull or prop. We do have a galvanic isolator onboard, but it indicates a lower level of protection than is appropriate for steel, and always has. I am not certain if it or needs to be calibrated.

 

[dlochner]

I'm still going with zinc oxide on the anode. Clean it all off and see how long it takes to form again.

Do you regularly measure the anodes effectiveness with a corrosion reference electrode? If something is amiss there could be increased galvanic activity that accelerates the zinc oxide formation. Here's a link to one: http://www.boatzincs.com/corrosion-reference-electrode.html

I have heard from reliable sources that there are permanently installed reference electrodes that monitor the galvanic potential in a steel boat, but I can't tell you where to buy them.

This book may help:
The Boatowner's Guide to Corrosion: A Complete Reference for Boatowners and Marine Professionals

 

[NotCook]

Do you regularly measure the anodes effectiveness with a corrosion reference electrode? If something is amiss there could be increased galvanic activity that accelerates the zinc oxide formation. Here's a link to one: http://www.boatzincs.com/corrosion-reference-electrode.html

Thanks dlochner,

I am not sure if this is what we have or not. I’ll take a look.

 

[JamesG161]

We do have a galvanic isolator onboard

A Galvanic Isolator is normally used to stop a DC current from entering your boat through the Ground Fault line from connected shore power.

I'm still going with zinc oxide on the anode.

Me too!

How long has it been since you last checked your Zinc and a Full Sail run?
That may the time you last "Cleaned" the Zinc, by sailing.
_____
We have "Drop Zinc" from the same link by dlochner at BoatZinc.com. You can get one and connect to your hull and drop over the side near your Hull Zinc. Then you can monitor it's weight and build up easier.
Jim...