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Old 11-13-2017, 07:30 AM
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Aluminum Anodes

We just pulled the boat and got a chance to look at the results of our one-season experiment with aluminum anodes on the Upper Chesapeake.

The anodes on the prop and the shaft (all aluminum) are substantially more degraded than we have traditionally seen with zinc. Of course, something other than the selection of material could theoretically explain the difference, like new problems in a neighboring slip, but nevertheless, the difference is striking.

Now, is the increase in degradation a good thing or a bad thing? Should we stay with aluminum at least one more year?

I've never seen anodes for engines or heat exchangers offered with anything other than zinc. despite the fact that least one reference I've seen sternly cautions against mixing types on the same vessel.

Bill
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Old 11-13-2017, 08:45 AM
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My take is that the Al anodes are doing their job and as long as they are not too far degraded mid season, then I would stick with Al (less $ and weight). They are more noble (slightly) than Zn and yet they are degrading faster, which could say something about the area this year.
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Old 11-13-2017, 10:57 AM
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Quote:
...something about the area this year
The salinity in this portion of the Chesapeake is strongly influenced by the weather. Over the last 30 days alone, the salinity reported by the NOAA weather buoy near Annapolis has been as low as 10.84 PSU and as high as 15.65.

Looked it up...

Quote:
Ocean salinity is generally defined as the salt concentration (e.g., Sodium and Chlorure) in sea water. It is measured in unit of PSU (Practical Salinity Unit), which is a unit based on the properties of sea water conductivity.
So the buoy is measuring conductivity directly, and not salinity?

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Old 11-13-2017, 07:25 PM
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My son the NOAA oceanographer

My son is a NOAA oceanographer scientist. He is a sailor AND lives in MD. I will ask him.

Mary

Last edited by HOTFLASH; 11-13-2017 at 07:35 PM. Reason: additional infor
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Old 11-14-2017, 08:54 PM
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Old 11-14-2017, 09:09 PM
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Anodes and salinity

This is the reply from my son, Andrew Eichmann, NOAA ocean scientist, who whenever the time comes, will probably inherit my '74 Tartan 27 yawl #570 powered by a the original A4, and sailed only in fresh water: Lakes Ontario, Lake Champlain and Lake Michigan. I have an aluminum donut anode on my shaft where I have very little clearance so not much option, but would have a magnesium one if I could find one.

I sent Andy the thread, and this is his reply:

"Oh, sorry, meant to address this: so I think roughly speaking yes,
PSUs are calibrated to conductivity -- probably temperature is part of
the conversion. Conductivity sensors are how salinity is usually
measured in situ (buoys, vs satellites) so for most purposes you can
think of PSUs as being the same as salinity. (The reason this is
complicated is that getting a direct measure of salinity involves
getting the mass of the seawater, evaporating the water, isolating the
salts from the crud -- you get the idea.)

How salinity values affect corrosion rates I don't know; *it may not
be linear* (that is, it may not be the more salinity, the more
corrosion, over all possible salinity values)

Hope this helps."

Mary
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Old 11-14-2017, 09:25 PM
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Nice to have an "active duty" ocean scientist in the house.
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Old 11-15-2017, 02:05 AM
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I thought the value of aluminum was for those boats that live in freshwater or estuarine water. The closer to the sea, the more they'll erode. Way up-river, the zincs on my boat are probably inert and useless.Certainly, they never erode. And I never get around to replacing them with something more suitable.

BTW, I'm restoring a venerable Sailomat 3040 - all aluminum. It's got some corroded spots, near the mounting bolts, so could probably use some kind of anodic protection - not to mention nylon washers and teff-gel - but what material? The most corroded piece is the windvane locking bolt, which appears to be made of some lighter alloy. Could be a clue.
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Old 11-15-2017, 02:09 AM
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BTW, I googled "aluminum pencil anode" and got all kinds of hits, from all the usual sources. Most appear to be "Martyr" brand.
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Old 11-15-2017, 09:00 AM
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Quote:
Originally Posted by toddster View Post
I thought the value of aluminum was for those boats that live in freshwater or estuarine water. The closer to the sea, the more they'll erode. Way up-river, the zincs on my boat are probably inert and useless.Certainly, they never erode. And I never get around to replacing them with something more suitable.

BTW, I'm restoring a venerable Sailomat 3040 - all aluminum. It's got some corroded spots, near the mounting bolts, so could probably use some kind of anodic protection - not to mention nylon washers and teff-gel - but what material? The most corroded piece is the windvane locking bolt, which appears to be made of some lighter alloy. Could be a clue.
For your Al I would go with Zn or Mg. Canada Metal Martyr is starting to push Mg for saltwater more as it works almost as well as Zn but weighs less and is less expensive; but either will work well as they are much less noble than Al and will sacrifice well before the Al.

Mg is the way to go in fresh water lakes as it is less noble than Zn and in the less conductive fresh water will still sacrifice more readily than the Zn.
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Old 11-15-2017, 10:51 AM
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Quote:
Originally Posted by HOTFLASH View Post
...How salinity values affect corrosion rates I don't know; *it may not
be linear* (that is, it may not be the more salinity, the more
corrosion, over all possible salinity values)...
I don't have any quantative data on this, but from personal experience I do know that the higher salinity of saltwater in the Bahamas is vastly more corrosive than the brackish water we have here in the Chesapeake. We experienced significant rust staining and "bleeding" of all of the deck level stainless hardware and tubing, which would reappear within 2-4 weeks of a thorough cleaning and removal.
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