Why not use Aluminum anodes?

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
Trying to figure out why everyone with a fiberglass hull doesn?t use aluminum anodes? According to my research:

- they have more negative potential then zinc
- can protect in salt, brackish, & fresh (important to me)
- cheaper.

So better, more flexible & cheaper.... what?s the catch? Why doesn?t everyone use them?

Thanks
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,296
the three common anode materials are zinc, aluminum and magnesium

and your manual will indicate which material gets used in which environment.....salt, brackish or fresh.

on the galvanic chart, zinc is more anodic than aluminum, and magnesium is more anodic than either.

I prefer to change the zinc more often and have the zinc remain the sacrificial material than have a piece of aluminum bolted to a piece of aluminum and hope that i dont have a chip in the paint.

galvanic_series.png
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Ayuh,.... I believe you'll find that zinc, 'n magnesium are less noble than aluminum,...

'n seein' our gear cases are also aluminum, what's to say the anode goes away, 'n not the gear cases,....
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,296
Ayuh,.... I believe you'll find that zinc, 'n magnesium are less noble than aluminum,...

'n seein' our gear cases are also aluminum, what's to say the anode goes away, 'n not the gear cases,....

exactly.... can you guarantee that the random electrons know which piece of aluminum to remove?
 

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
Manual just says sterndrives are fitted with zinc anodes which should be replaced.... nothing about water type.

Bondo, your statement has me confused. I thought aluminum (more specifically the aluminum alloy used in anodes) being more noble would protect BETTER than zinc. Are you saying that?s not the case?
 

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
Everything I read showed aluminum being slightly more negative than zinc but that differs from what Scott?s chart shows.
 

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
No Title

Is this just slick marketing?
 

Attachments

  • photo279677.png
    photo279677.png
    767.1 KB · Views: 1

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Without doubt...zinc is for SW....magnesium is for Fresh....aluminium is a compromise, and in my opinion not really effective at a chemical level.
 

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
Yup. Other articles make similar points about benefits of aluminum alloy anodes but it seemed to good to be true which is why I?m asking the experts here what the downside is.

Are the articles wrong? Am I missing something? Hope someone weighs in with an explanation.
 

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
15,414
I switched from zinc to an aluminum alloy three years ago with very good results.

I run back and forth between salt and brackish but primarily brashish.

Never had any damage but my zincs suffered from oxide problems (white dust disease). Changed over to aluminum (alloy) anodes and no more wore brushing zincs 2-3 times a year to keep them active
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,296
Is this just slick marketing?

yes, aluminum anodes last longer because they are less effective. look at the galvanic chart I posted. Magnesium is the best, then Zinc then aluminum. in terms of life, aluminum last the longest, magnesium last the shortest.

i personally want an anode that does its sacrificial job and gets dissolved before my outdrive, then replaced vs one that will remain longer than 6 months
 

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
No Title

Good to know dingbat. I?m going to be primarily brackish.

Scott, I have same goals as you. Protection not longevity. I know your chart shows aluminum as good as -1.0 however the slick marketing shows -1.1. Oddly there have been comments about aluminum protecting an aluminum drive. Attached pic shows that ONLY aluminum protects aluminum drives in salt. Not trying to argue but lots of info out there shows aluminum as better. Is it all BS?
 

Attachments

  • photo279714.png
    photo279714.png
    1.1 MB · Views: 0

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
15,414
look at the galvanic chart I posted. Magnesium is the best, then Zinc then aluminum. in terms of life, aluminum last the longest, magnesium last the shortest.

Your chart is flawed in as its too general for this discussion.

Aluminum anodes are alloyed with zinc and indium giving them a -1.09 potential. That is 4 points higher than zinc at -1.05. In contrast, magnesium is -1.50

https://www.cruisingworld.com/how/zinc-and-aluminum-sacrificial-anodes#page-2

Aluminum anodes, alternatively, offer several advantages. They?re immune to the calcareous coating menace and are therefore well suited for use in seawater, as well as brackish and fresh water. Aluminum anodes also pack more of a punch; they either last longer than zinc anodes of the same weight, or provide the same protection as zinc in a lighter package. Their relative energy capacity is 1,108 amp hours per pound (significantly more than zinc), with a voltage of negative 1,100 millivolts.
 

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
That matches what I?ve been reading. So why isn?t everyone using them at a lower cost? I?d this a case of people not embracing new tech or ..??
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,296
having worked in both the heat exchanger world and now in the yacht world. we only used zinc for a variety of reasons

you dont hear of an aluminum anode pencil in a heat exchanger. if you walked into any of the large yacht builders and said hey, lets hang aluminum on the keel vs zinc, they would all toss you in the drink.

the Aluminum anodes came out because people complained about having to replace the zinc anodes every 6 months or having to sand off the aluminum hydroxide coating that forms on the zinc when having in fresh water prior.

pure aluminum ends up with an oxide layer that protects the anode. So Aluminum must be blended with zinc, iron, and indium (navalloy) or even lithium blends to have enough precipitation to actually continue to work. additionally the higher output of aluminum anodes causes localized embrittlement in steel structures, so now the rage is low voltage output aluminum anodes where they mix in copper

the aluminum/zinc/copper blends and the aluminum/zinc/lithium are a joke as the aluminum and copper or aluminum and lithium become its own battery source in the presence of calcium and sodium and the protection goes out the window as you can call one energizer and one eveready.

and yes, aluminum anodes are lighter than zinc. material density would indicate that. why use it as a marketing ploy. we size zincs (anodes) by their square area, not by weight.

also be wary of low cost aluminum anodes as they are not proper anodes. they are generally low-cost off--shore knock-offs that do not have the proper blend of aluminum/zinc/indium or aluminum/zinc/lithium to actually work as an anode.
 

RaceCarRich

Petty Officer 1st Class
Joined
May 31, 2015
Messages
233
Was at my Mercruiser dealer couple days ago and was told Merc no longer makes zinc anodes (at least for my B3) and only makes aluminum (alloy I assume).

If true, what?s my takeaway? After reading Scott?s post, was thinking Zinc but now thinking the OEM may have it right.
 

scoflaw

Ensign
Joined
Jun 2, 2010
Messages
962
I have had very good performance with Martyr aluminum on B3's slipped 6 months a year in salt in Mass.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
I switched from zinc to an aluminum alloy three years ago with very good results.

I run back and forth between salt and brackish but primarily brashish.

Never had any damage but my zincs suffered from oxide problems (white dust disease). Changed over to aluminum (alloy) anodes and no more wore brushing zincs 2-3 times a year to keep them active

Yep, that because your gearcase is corroding away. Having to clean your Zinc anodes 2-3 times a year is the proof they are doing their job. That white powder is also proof the zincs are sacrificing to save the aluminium gearcase. The fact that the new anodes aren't 'corroding' would be a serious concern to me.

Chris...........
(works in a marine environment on underwater robots. Anodes are the things we are constantly aware of.)
 

dingbat

Supreme Mariner
Joined
Nov 20, 2001
Messages
15,414
Yep, that because your gearcase is corroding away. Having to clean your Zinc anodes 2-3 times a year is the proof they are doing their job. That white powder is also proof the zincs are sacrificing to save the aluminium gearcase. The fact that the new anodes aren't 'corroding' would be a serious concern to me.
Sorry...the white powder is zinc oxide. The only thing it?s proof of is a compromised anode. Zinc oxide is an insulator.

New aluminum alloys have superseded zinc in most applications.

Suggest you read the attached http://www.performancemetals.com/anodes/AnodeFAQs.shtml
 
Last edited:
Top