Anodes -- Zinc vs. Aluminium

B

baz

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Fluid Motion Model
C-24 C
About 18 months ago my Diver service for maintaining our R-25's under water anodes started to switch them from zinc to aluminum. Over this time period I've noted that the aluminum anodes will become encrusted with a white cauliflower-like material. The previous zinc anodes did not become encrusted to such an extent nor as quickly.

I believe the cauliflower-like material inhibits the proper affect the anodes are supposed to have and because of this the aluminum anodes are having to be replaced more often.

This is counter intuitive to me because if the cauliflower-like material is inhibiting the proper action of the anode then surely they would last longer, but at the expense of the boat's metal being eaten away.

My R25 is kept in salt water all year long.

I brought this subject up some time ago (Sep 2013)and did get some replies. However, I wonder if there's any new info on this subject. Re: http://www.tugnuts.com/viewtopic.php?f=2&t=4177

Anyone care to comment on what I've stated ? Thanks. 🙂
 
My understanding is that the cauliflower build up that you see does not inhibit the purpose of the zinc/aluminum anode.
 
I am very versed and familiar with the properties of both as we use them to specifically "dial up" the electrolysis in the water as an aid to catching fish... along with the use of a Scotty "black box". Alum alloy zincs "cook" hot and start working immediately. The "porrage" or "califlour" look is exactly what they will look like if they are working. If you get this look on normal zincs they are not working. The negative of Alluminum zincs is ... though they work immediate and are measurably hotter... they "stop on a dime". They do not have the same lifespan as a regular and do not handle coming in and out of the water well and will stop working without warning even though they will look like there is lots of zinc left. To measure their affect you can take a volt meter, connecting the negative (ground) to your motor and hang a bare wire into the water and connect the positive to it. Monitoring this is a good habit. I would not suggest switching out all zincs to aluminum unless you have a specific reason, like fishing, to do it. However, having a combination.... like 50/50 on the trim tabs is not a bad idea.
 
The biggest advantage to aluminum is that it is effective in both salt and fresh water. Traditional zincs become useless once they are introduced into a fresh water environment and remain ineffective upon returning to salt water from fresh water. For boats traveling back and forth through the locks in Seattle, this would be an issue.
 
kim&tim":19nad2qg said:
The biggest advantage to aluminum is that it is effective in both salt and fresh water. Traditional zincs become useless once they are introduced into a fresh water environment and remain ineffective upon returning to salt water from fresh water. For boats traveling back and forth through the locks in Seattle, this would be an issue.
Click to expand...

Can you explain why once a traditional zinc is in fresh water and then returns to salt water it becomes useless.
 
We learned this while engaging marine consultant Steve D'Antonio ( also technical editor for Passagemaker ) to assess a boat we were looking to purchase. The boat was kept in Lake Union, thus the reason the subject came up. We had no idea that exposure to fresh water comprimised the effectiveness of zinc. The marine surveyor on the project also confirmed what Steve was telling us. Upon being told this we "googled around" and found articles to support this theory.
 
So, I suppose if someone were to do the loop, they should change there zincs to aluminum?
 
Thanks for the replies and extra information on the affect of the cauliflower deposits. I had a long chat with my Diver service and they indicated this issues is strictly related to the aluminum anodes.... they kind of boil off the substance. The Diver will brush of the cauliflower to expose the anode surface and typically its quite pitted -- meaning the anode is doing its job.

For me, the switch to aluminum has meant my anodes now last 1 to 2 months longer, and my Diver service has now extended the period between underwater examinations.

The anode that gets replaced more frequently than any other is the prop shaft anode.
 
Just to clarify, since I will be going to the North Channel (fresh water) in a couple of weeks. I should change my zinc anodes to aluminum and I will be good to go back in salt water? And I guess, if later on the aluminum anodes need replacement and I plan on staying in salt water just reinstall the old zincs?
 
it is my understanding that aluminium anodes perform equally well in both salt and fresh....and back and forth in between the 2 as well. There would be no need to switch back to zinc.
 
My zincs have been in fresh water in the summer and salt in the winter and they eat away quite nicely in both liquids without changing. There is no chemical reason for fresh water to "poison" a zinc that I can fish out of my aging brain's recollection of chemistry 101 some 50 years ago. If you are concerned then simply give the surface of the zinc a quick polish with some 120 grit.
As far as Aluminum I have always wondered why it is considered better than Zinc in fresh water. When part of a cathodic battery it is very close to zinc with the nominal difference being only 0.03 volts and in the wrong direction to make it better for freshwater..

Voltage of Al is a nominal -1.00 volts (range is -.76 to -1.00)
Voltage of Zn is a nominal -1.03 volts (range is -.98 to -1.03)
Voltage of Mg is a nominal -1.60 volts (range is -1.60 to -1.63)

So we can see that Mg has far more electrons it is willing to donate compared to aluminum. With fresh water being a more dilute electrolyte than sea water, Mg is the metal of choice for fresh water protection.

Here is what Defender has to say on the subject.
http://www.defender.com/html/zincs_info.html
 
I shot an e-mail off to boatzincs.com asking about switching back and forth from fresh to salt and here was the reply:

Zinc anodes will passivate (become inoperable) when in fresh water for an
extended period of time. They develop and oxide coating that interferes
with the corrosion process. The oxide coating will eventually wear off when
the anode is returned to salt water, or you can simply wire brush them to
quickly re-activate.

Best regards,

Bob Olsen

BoatZincs.com
The Online Superstore for Zinc Anodes

Tel: 978-841-9978 Fax: 866-763-2628
bob@BoatZincs.com www.BoatZincs.com
 
There is an article in Cruising World, March 27,2014...."Zinc and Aliminum Scarificial Anodes"..." So you think you know the difference between zinc and aluminum scarificial anodes? " ....does a good job of explaining the differences. Google the article and check it out.
 
kim&tim":2g5lcizh said:
There is an article in Cruising World, March 27,2014...."Zinc and Aliminum Scarificial Anodes"..." So you think you know the difference between zinc and aluminum scarificial anodes? " ....does a good job of explaining the differences. Google the article and check it out.
Click to expand...

It's an interesting article. I like reading Steve D'Antonio's stuff.

Click here http://www.cruisingworld.com/how-to/mai ... ial-anodes

/david
 
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