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 Post subject: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Tue May 22, 2012 4:04 pm 
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Posts: 321
Location: Pocasset, MA
I've been doing a lot of work on anodic protection systems for my job, and I figured that I'd share the info with my fellow CP'ers.

There is a lot of misinformation going around about protecting your engine/hull/bracket/tabs, etc from galvanic corrosion. Most people understand that you have to protect underwater metals from stray current, but don't understand how to do so properly.

Basically, every metal has an electric potential. Some are higher (gold, bronze, brass, SS, etc), and some are lower (aluminum, zinc, magnesium). When you connect these differing metals in a conductive environment (seawater, fresh water, or brackish water), the more noble metal becomes the cathode, and the less noble metal becomes the anode in a very weak, primitive battery. Any of you ever make a clock than ran on a lemon with a zinc strip and a copper strip embedded in it? Same thing.
Here's a good chart of metal potentials:
Image
To understand what's going on with your situation:
1. Find the different metals on your boat. Prop, shafting, bracket, outdrive/outboard engine, trim tabs, exposed fasteners, and transducer.
2. Locate each metal on the chart. In our case (2520 w/ a F225), we've got Stainless (Prop, tabs, and exposed fasteners), bronze (transducer), and aluminum (bracket and outboard).
3. See which metal is the least noble (left-most) on the above chart. That's the metal that's going to turn into the anode without any protection. The other metals will steal electrons from that item, and it will corrode.

In our case, that'd be the motor and the bracket. We'd prefer that those don't corrode. The answer is to install something else to be the anode, something made of a metal that's less noble (lower electrical potential) than anything else on the boat. Many people think the answer is simple:
Zinc.

Well, zinc is less noble than mostly everything, but it has some major problems. First of all, if you take a zinc into fresh water, it'll generate an oxide layer, which effectively insulates it from the hull, and prevents it from doing anything (except looking nice). Secondly, if you haul your boat, and let the zinc dry out, it'll insulate itself in much the same fashion. Again, useless at protection.

Ok, how about magnesium. Magnesium is the least noble of the elements listed in the chart, so why not just use that? Well, it'll protect your metals for sure, but it can overprotect aluminum. When people talk about overprotection, they are referring to the generation of hydrogen bubbles on the cathode, similar to what happens in Hydrolysis. Basically, the electrical potential is strong enough that the water molecules split, generating hydrogen at the cathode and oxygen at the anode. This hydrogen is generated on a molecular scale, and will lift the paint on your aluminum outdrive or bracket. Plus, magnesium will very rapidly dissolve in brackish or sea water.

This leaves aluminum, but not just any aluminum. There are special alloys which include zinc and trace amounts of a rare metal called indium. The indium keeps the aluminum from making an oxide layer when wet or dry, and the zinc helps lower its potential to below that of structural (aircraft) or marine aluminum alloys.

Here's a chart of acceptable metals/hulls/motors/water types:
Image

In any case, pretty much everyone here should be running aluminum anodes on their engines, hulls, brackets, and trim tabs. If you mix and match anode materials, you'll run into problems. Mercury is aware of this issue, and won't cover corrosion damage on outdrives or outboards if you use zinc anodes. Yamaha factory anodes are aluminum, and should always be replaced with aluminum.

Feel free to ask more questions regarding corrosion.


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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Wed May 23, 2012 6:26 pm 
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Joined: Fri Jun 12, 2009 6:58 am
Posts: 433
Location: Stafford, Virginia
Thank you for the very informative post.

Steve

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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Thu May 24, 2012 7:31 am 
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Location: Lewes, DE
Wow. Good information.
Too valuable not to be made a sticky.
Done!

Thanks!

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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Wed Jun 27, 2012 10:52 am 
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Joined: Tue Nov 20, 2007 7:13 pm
Posts: 173
My tiny brain is a little sore from all of that information! Definately some very useful stuff for me as I am at the point of replacing anodes and never really gave it much thought as to what material to use. Thanks for the lesson/ advice!


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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Wed Jun 27, 2012 11:47 am 
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Location: Newbury, MA
FYI, http://www.boatzincs.com is the largest seller of anodes, and I just checked and they have aluminum anodes available for Merc, Bomb/OMC, and Yamaha OBs.

Attached is their chart for aluminum vs. zinc anodes.


Attachments:
Anodes.JPG
Anodes.JPG [ 101.28 KiB | Viewed 1109 times ]

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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Fri Jun 29, 2012 8:20 am 
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Location: Newbury, MA
FYI: More info from www.boatzincs.com about aluminum anodes:

Without question, aluminum anodes are technically superior to zinc anodes in every important performance metric, including:

-Output galvanic voltage
-Total energy
-Self-cleaning behavior
-Weight

The only ‘problem’ with them is that they have a much higher melting temperature than zinc, which unfortunately makes them considerably more difficult for the manufactures to cast into anode products. Economically, aluminum anodes today have a material cost of about ½ that of zinc, but market supply/demand/competition can sometimes negate that advantage as seen by the consumer.

Feel free to post my comments. Best regards,
Bob Olsen
ABYC Certified Corrosion Technician

www.boatzincs.com

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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Thu Dec 20, 2012 12:23 pm 
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Joined: Mon Nov 03, 2008 9:58 pm
Posts: 321
Location: Pocasset, MA
Just a note on this topic:

We recently got home with our 26' Aluminum research boat after a bunch of work in NYC (a notoriously bad galvanic environment), marine work in CT and MA, as well as work in freshwater lakes in NY, CT, and MA. I've been on most of the trips, starting in January. We put the new anodes on the boat in May (around the time of my post). Our previous anodes were zinc and had been installed for three years. They had lost ~10% of their original mass.

Our new anodes have lost at least 30% of their mass in ~7 months.


Needless to say, the aluminum anodes are doing a much better job protecting our boat.

The anodes on our pontoon boat, which has only seen 14 days of use this year, have lost ~10% of their mass as well. Its worth noting that the pontoon is not painted, and our 26' is painted with a 3-stage Pettit coating system (Aluma-protect primer, Pettit-protect primer, and Ultima Eco bottom paint).


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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Wed Sep 04, 2013 12:22 pm 
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Joined: Sat Oct 23, 2010 5:37 pm
Posts: 450
Location: Santa Barbara, CA
Wait... Are you saying that we should all replace our zinc anodes with aluminum ones?? I just replaced the anodes on my Honda OB with the zinc ones from boatzincs.com

Should I go back and replace with aluminum? The boat is slipped in the briny Pacific...

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Last edited by SBH2OMan on Wed Sep 04, 2013 2:06 pm, edited 1 time in total.

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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Wed Sep 04, 2013 2:02 pm 
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Joined: Wed Mar 19, 2008 6:45 pm
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Location: Greenwood, Indiana & Charlotte Harbor, Florida
I read the recommendation is aluminum for sea water (salt water) and magnesium in fresh water, and if magnesium used in sea water will have shorter life span than zinc, oth you will replacing them often

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1994 Parker 2320 Extended Cabin
2002 Yamaha F225 TXRA 4 stroke


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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Thu Sep 26, 2013 7:44 pm 
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Joined: Mon Nov 03, 2008 9:58 pm
Posts: 321
Location: Pocasset, MA
SBH2OMan wrote:
Wait... Are you saying that we should all replace our zinc anodes with aluminum ones?? I just replaced the anodes on my Honda OB with the zinc ones from boatzincs.com

Should I go back and replace with aluminum? The boat is slipped in the briny Pacific...

Are you sure that what you installed were zinc? If so, I'd consider swapping them if you venture into fresher water, or ever haul the boat. If it stays wet, you'll be OK with Zinc. If it dries out, it'll slime over and no longer react.


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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Sat Sep 03, 2016 6:38 am 
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Joined: Wed Jun 15, 2016 7:17 am
Posts: 8
Location: Brightwaters Canal
As I wait for my 2320 to be trailered up to LI, I'm checking out the various forums. This information is absolutely great. I had no idea of the various anodes and how they differ depending on where you use your boat and now I'm informed.

Things like this and the members of CP are what makes this site a valuable source for any Parker owner, and just about any boat owner as well.

Thanks!

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2015 2320 SC
2015 Yamaha 250 XCA
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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Sat Sep 03, 2016 7:24 am 
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Joined: Thu Jun 30, 2016 10:32 pm
Posts: 176
Location: St. George, Maine
Hmm. I get that engine anodes need to be aluminum. What about my trim tabs? They are currently zinc.
BTW, all the anodes on my FL-based diesel trawler are zinc, including the ones in my Volvo engine.

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Parker 2520 XL


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 Post subject: Re: Catch-all Motor/Hull/Tab Anode Information
PostPosted: Sat Sep 03, 2016 10:44 am 
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Joined: Sat Mar 07, 2015 9:48 pm
Posts: 159
Location: James River, Richmond VA
Great post! I'd like to add a little vocabulary. Sparky was discussing galvanic corrosion, It is NOT electrolysis! If you use the word electrolysis you mark yourself a rookie. He referred to stray current corrosion, which of course is a different animal.

Galvanic corrosion is a process that happens when parts of the engine/drive/boat interact with each other. It all happens on your boat and your boat alone. Stray current corrosion is the addition of current from an outside source. May be from your boat, may be from shore power. Anodes won't protect from stray current corrosion!

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