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How do you know if you gave "over zinced"


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I've heard b4 today that its possible to over zinc (have to much cathodic protection) but never had it explained what it means in a practical sense.

Got a mate whose Jeanneau has 4 anodes, rudder/shaft/ and one each side of keel.while tightening the nut that holds one of the keel anodes the threaded stud broke off -i know him well enough to believe he wasn't going gangbusters on the lever - and now he's talking exposing the top of and pulling encapsulated keel bolts just to see if he has a can of worms or not.

He made observation that the keel anodes haven't degraded at all where as rudder and shaft seem to do what they should - annual replacement being the norm for last few years.

So - is it a bad idea to have too much zinc and why?

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Yes. You (I can do this if required) can measure the protection level with a silver/Silver chloride anode. That will give you a millivolt reading, which needs to be in a certain range, depending on Hull material. Too high and the boat will corrode, to low means to many zincs.

To much protection will cause additional marine fouling and other issues.

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thanks IT, boat down country so probably practical for you.

 

Its a production boat late 70s early 80s I guess,  fibreglass french built 

 

can you expand? I'm hearing "too high" = high millivolt reading which means under protected? What do you mean by "boat will corrode"?

Ditto too much protection means the millivolt reading is low?

What are "other issues" you allude to?

 

I can tell you that fouling doesn't seem to be an issue whether its because of boat location or superior anti foul etc I wouldn't know

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If the boat is fibreglass, reading is - 550 to - 1100 mv, that's OK. Outside that range is not. Higher (lower numbers, remember it's a negative value, unless a really serious issue!) means insufficient anode, lower is too much.

Those numbers are ONLY CORRECT FOR A FIBREGLASS HULL. Other materials are different.

If I'm home at a decent time tonight I'll link in some tables and more info.

 

If only some of the anodes are eroding, so consumption at different rates, then I'd suggest the metal objects they are attached to are not bonded together.

What do the anodes look like after being immersed for a while?

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They wouldn’t. Common anode materials are:

 

Magnesium(Mg)

Aluminium(Al)

Zinc (Zn)

Chromium(Cr)

Iron(Fe)

Nickel(Ni)

 

But most of us use zinc. All our anodes are in fact alloys, with other metals included.

Sacrificial anodes which are protecting the parent material must lie higher in the electromotive series or galvanic series of metals, otherwise they won’t work!

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Here is the galvanic series, in seawater at 25deg C.

Platinum

Gold

Graphite

Titanium

Silver

Chlorimet 3 (62 Ni + 18 Cr + 18 Mo)

HASTELLOY®; alloy C (62Ni + 17 Cr + 15 Mo)

INCONEL® alloy 625

INCONEL® alloy 825

Stainless steel, type 316 (passive)

Stainless steel, type 304 (passive)

Stainless steel, type 410 (passive)

Monel alloy 400

INCONEL® alloy 600 (passive)

Nickel 200 (passive)

Copper alloy C71500 (Cu 30% Ni)

Copper alloy C23000 (red brass 85% Cu)

Copper alloy C27000 (yellow brass 65% Cu)

HASTELLOY® alloy B

INCONEL® alloy 600 (active)

Nickel 200 (active)

Copper alloy C46400

Copper alloy C46500

Copper alloy C46600

Copper alloy C46700 (naval brass)

Tin

Lead

Lead-Tin solders

Hastelloy A

Stainless steel, type 316 (active)

Stainless steel, type 304 (active)

50 - 50 lead tin solder

Stainless steel, type 410 (active)

Cast iron

Wrought iron

Low carbon steel

Aluminum alloy 2117

Aluminum alloy 2017

Aluminum alloy 2024

Cadmium

Aluminum alloy 5052

Aluminum alloy 3004

Aluminum alloy 3003

Aluminum alloy 1100

Aluminum alloy 6053

Galvanized steel

Zinc

Magnesium alloys

Magnesium

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Article in latest BoatingNZ says that zinc anodes have been superceded. 

Aluminium anodes are superior and now the way to go!

What they told me in Europe (for a steel boat)

Zinc anodes if the boat is in seawater.

Aluminium anodes for dirty old canal fresh water.

Magnesium anodes for pristine fresh water.

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Article in latest BoatingNZ says that zinc anodes have been superceded. 

Aluminium anodes are superior and now the way to go!

 

How do they work on a alloy yacht or what should be used on alloy?

 

Indeed that sounds like dangerous half information if that's actually in the article. As Harrytom says, how on earth is that gonna work on an alloy yacht? And given that probably half the boats on the water are unpainted aluminium Fishos, I can't see how that's gonna protect them if they sit around in the water for long... 

 

 

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What they told me in Europe (for a steel boat)

Zinc anodes if the boat is in seawater.

Aluminium anodes for dirty old canal fresh water.

Magnesium anodes for pristine fresh water.

We use Alluminium Indium anodes on our Alloy yacht, they were recommended by a corrosion expert. The previous owners used Zinc. In either case there was and is no signs of corrosion on the hull. One thing I did learn though is that Rubber hoses can corrode alluminium, they have a carbon content.

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Here is the article:

https://boatingnz.co.nz/articles/alternative-sacrificial-anodes/

 

"“All of the engine manufacturers now put aluminum anodes on their product instead of zinc,” says Michael Szwez, director of marketing for anode manufacturer CMP Global. He should know, since CMP makes the anodes used by most of the major outboard engine manufacturers."

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Interesting. There MUST be a difference in the nobility of the two metals you're trying to protect. The least noble will be the anode, the most the cathode. If you were to use an aluminium anode on a boat with an aluminium saildrive, that would not work. However, if they can use aluminium anodes on an aluminium outboard, then they must be very different grades of aluminium for it to work. If they are too closely matched, then both metals will erode.

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