Zinc passivation

New to me boat purchased last summer in Ontario Canada. Seller and surveyor were unable to identify the anodes. As soon as I got into salt water I checked the hull potential and is was -200mv.

At least everywhere in the bonding system measured the same within 5mv or so.

I tried wire brushing the anodes without success so used a hanging anode which dropped the potential to around -600mv until I could haul the boat.

It took a hammer to expose base material.

It's zinc not aluminum.


This is what the transom anode looked like:

I’ve had the same thing happen. I switched to Aluminum... my bottom cleaning diver kept telling me everything was fine.... politely refused to change... Well the small “zink’s” in the cooling system were doing the job. After firmly asking/demanding the diver to change zinks. He said, told you, they looked great almost new.... now am normal “wear”....

Lots of knowledge on this site.
https://boatzincs.com/corrosion-reference-electrode.html

Zinc in fresh water develops a coating of zinc hydroxide and calcareous material. These materials effectively insulate the zinc and render them useless. Some people wire brush the anode til' shiny and figure that's fine. They forget (or don't know) that the coating creeps over the entire surface including the underside and under fasteners. The anodes must be removed and the contact points cleaned.

PS. Gotta question a surveyor who could not identify anode materials.

^^
Now to figure how much protection the boat will need to get to -1100mv. I fear some trial and error next spring.





Re - surveyor - when I got the report he had the engines identified as raw water cooled. Useless.

Zink

I was out walking the dock in the fall and noticed a fiberglass trawler at the dock with some zinks laid out about every foot on the deck ( small screw in zinks ), Scratching my head I just walked by thinking huh must have forgotten to toss them out and just left them on the deck of the boat . Now we are into winter and they are still sitting there. I noticed that the deck of the boat had NO growth (green, black or ect) on it. Is it possible that there's something to this ?

What is recommended for brackish water?

If an anode is not being used up, the first thing to check (apart from suitability of material) is whether it is in 100% no resistance contact with whatever it is there to protect. A multimeter is your friend.

Nocanvas said:

I was out walking the dock in the fall and noticed a fiberglass trawler at the dock with some zinks laid out about every foot on the deck ( small screw in zinks ), Scratching my head I just walked by thinking huh must have forgotten to toss them out and just left them on the deck of the boat . Now we are into winter and they are still sitting there. I noticed that the deck of the boat had NO growth (green, black or ect) on it. Is it possible that there's something to this ?

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A zinc strip is often used to deter moss on the north side of an asphalt roof.

Jim

Copper too

JDCAVE said:

A zinc strip is often used to deter moss on the north side of an asphalt roof.

Jim

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Ok makes sense now because those Zinks I saw were laying on the north side of the boat.

Sorry responding so late. You will be overshooting trying for -1100mv. Realistically you need no more than -700mv or so depending on the quality of the materials. I suspect your propellers (probably Nibral) are what you are aiming to protect from the stainless-steel under your boat and they range between -310 to -420. ABYC indicates protecting above -1100mv may be detrimental to antifouling paint coatings.

2.5.1 A cathodic protection system shall be capable of inducing and maintaining a minimum negative shift of 200 mV relative to the corrosion potential of the least noble metal being protected.

Wood said:

^^
Now to figure how much protection the boat will need to get to -1100mv. I fear some trial and error next spring.

Re - surveyor - when I got the report he had the engines identified as raw water cooled. Useless.

Click to expand...