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Old 28-03-2017, 10:33   #1
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Sacrificial anode for aluminum hull?

Hi,

I have a197? 43' aluminum hull houseboat on a freshwater lake near Atlanta, GA. AFAIK it has never been out of the water. It's located in a fairly large marina. I've been told that steel hull boats should have a sacrificial anode applied to help keep the hull from breaking down, but recently someone said that aluminum boats should have one also. Is that true, and if so can anyone tell me just how to do it or suggest a page or video that explains to how to apply one to aluminum hull boats in fresh water?

Thank you for any help!
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Old 28-03-2017, 11:31   #2
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Re: Sacrificial anode for aluminum hull?

In freshwater the galvanic effect is reduced, but it doesn't hurt to have one....since the hull is aluminum, the anode would really need to be zinc.
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Old 28-03-2017, 11:48   #3
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Re: Sacrificial anode for aluminum hull?

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In freshwater the galvanic effect is reduced, but it doesn't hurt to have one....since the hull is aluminum, the anode would really need to be zinc.
Zinc for salt water, Aluminum for brackish water, Magnesium for fresh water
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Old 28-03-2017, 21:17   #4
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Re: Sacrificial anode for aluminum hull?

A friend with a 44' Kings Craft aluminum houseboat had aluminum brackets welded on to the hull so it could have more anodes than the ones on the shafts. He is also freshwater on the upper Mississippi River and uses magnesium plates bolted to the brackets.
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Old 29-03-2017, 21:44   #5
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Re: Sacrificial anode for aluminum hull?

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A friend with a 44' Kings Craft aluminum houseboat had aluminum brackets welded on to the hull so it could have more anodes than the ones on the shafts. He is also freshwater on the upper Mississippi River and uses magnesium plates bolted to the brackets.
Not only a good idea, a very good one. Sea water conducts better, but galvanic corrosion can happen anywhere where electric currents exist, and stray currents in marinas are a definite possibility. 'Tis better to sacrifice you anode (magnesium probably, but I have not looked) than one or both of your aluminum hulls. At least that is the idea.
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Old 29-03-2017, 22:15   #6
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Re: Sacrificial anode for aluminum hull?

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Old 01-04-2017, 12:59   #7
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Re: Sacrificial anode for aluminum hull?

What possible good could be provided by using aluminum as a sacrificial metal with an aluminum hull? Maybe the proviso recorded here (Zinc/Aluminum/Magnesium) is for a steel hull? Warning: I am newbie to sailing and boating.
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Old 01-04-2017, 13:51   #8
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Re: Sacrificial anode for aluminum hull?

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What possible good could be provided by using aluminum as a sacrificial metal with an aluminum hull? Maybe the proviso recorded here (Zinc/Aluminum/Magnesium) is for a steel hull? Warning: I am newbie to sailing and boating.
The referenced table is for generically fiberglass vessels. It would do you very little good to protect an aluminium hull with aluminium, however seeing as magnesium is less noble than aluminium, it will provide corrosion protection to that vessel in fresh water.
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Old 02-04-2017, 06:06   #9
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Re: Sacrificial anode for aluminum hull?

Corrosion Protection Guidelines for Aluminum Hulls
Pages 6 & 7 ➥ http://www.sname.org/HigherLogic/Sys...c-42415ecab0a3
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Old 02-04-2017, 07:48   #10
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Re: Sacrificial anode for aluminum hull?

This is what you want but magnesium costs extra I'm afraid. Clip one each side to some part of the aluminium hull structure. The toe rail will do if it is welded to the hull.

Martyr Grouper Style Universal Sacrificial Anode - Magnesium
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Old 02-04-2017, 08:06   #11
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Re: Sacrificial anode for aluminum hull?

If you feel like making up your own, these are bigger and will last longer. Drill out one of the tabs and attach some copper wire and a clip. I don't know what they cost in magnesium but it will probably be over $50.

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Old 04-04-2017, 13:42   #12
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Re: Sacrificial anode for aluminum hull?

Oh Dear! It is just enough to look at the relative galvanic potential and realise that Zn will do absolutely zero for an Al hull, in fact the Al hull might sacrificially corrode to protect the zinc! An Al anode is obviously no good to protect Al. It is a no-brainer: you need to use Mg anodes.
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Old 08-04-2017, 22:34   #13
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Re: Sacrificial anode for aluminum hull?

I ran a fleet of large tourist boats in the Great Barrier Reef. Yes salt water.


We had 2 aluminium cats, a 29Mtr 330 passenger & a 26Mtr 204 passenger boat. These were built by a good builder, but even they can miss things.


The 29 Meter boat was powered by a couple of 1200 HP 16V 92 GM diesels, which were raw water cooled, with a 120 KVA generator & extensive air conditioning, which was also raw water cooled. The water was drawn from a sea chest built inside each engine room hull of about 350mm cube of 12mm alloy, open to the water by a series of holes in the 6mm alloy hull.


The builders in their wisdom attached 3 by 2" bronze gate valves to each sea chest, but forgot to fit the galvanic protection inside them.


At 5 months old one of these sea chests pin holed from electrolysis. Both were extensively corroded, & almost through in many spots. The boat had to go back to the builders dry dock to have the existing chests cut off, & new ones fitted, with anodes this time, with a way of changing them from with in the boat.


Incidentally The GM diesels were raw water cooled, with zinc anodes in the various water jackets within the engine block & heads. They were about 4" long by 3/4" diameter. These would be almost totally consumed by electrolysis protecting the cast iron about every 4 months.
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Old 10-04-2017, 07:53   #14
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Re: Sacrificial anode for aluminum hull?

Assuming your hull is made from marine grade aluminum (5000 series), then you basically need a paint job and that's it. If you haul out every few years for a bottom job, then you can literally pressure wash and repaint - a lot cheaper than on fiberglass boats. After numerous studies, it has been shown that a 5000 series alloy boat with a paint job needs no galvanic protection, even in salt water.

You're bigger problem for corrosion that we see in our design practice is when people (either as original design or as an add-on) put dissimilar metals in contact with the aluminum. This can be sneaky - galvanized bolts used to attach the topsides, stainless steel screws used to attach cleats.

Galvanized steel bolts have the least potential compared to marine grade aluminum, but the galvanization will wear off very quickly. The best solution is to use aluminum bolts ($$$$) or to weld everything together. Place an isolating layer between the aluminum and any other metal.

Two other quick notes: I have seen several houseboats built by alloy fabrication shops that usually fab building components. Most buildings are constructed with 6000 series. If they used something other than 5000 alloys, then it's anyone's bet as to how long the hull will last. Second quick note: The Navy used a dielectric material on several of their cruisers/destroyers designed in the 80's. They had a steel hull and aluminum topsides. These did not work very well in combat as Al burns at much lower temperatures than steel, and they did not age gracefully as the dielectric material was in need of constant repair.
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Old 10-04-2017, 08:35   #15
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Re: Sacrificial anode for aluminum hull?

6000 series alloys (with silicon as the main alloying element) are commonly used for extrusions, including a few types which are considered marine grade. 5000 series (magnesium alloys) are extremely corrosion resistant and strong, but they are not suitable for extrusion. By itself 5083 H321 is probably the most inert material which is commonly used in boatbuilding, but the trouble is that it's never by itself.

The ideal anode for aluminium hulls is a function of water temperature in addition to salinity. In places, freshwater ponds and lakes can reach a much higher temperature than seawater, and with zinc anodes that can be hazardous because of a type of reversal which leads to the (aluminium) hull protecting the (zinc) anode! Magnesium is not capable of that kind of reversal, and thus magnesium anodes are the safest for freshwater.

Aluminium anodes are made from alloys specially designed for the purpose - certainly not the same alloy as the hull material - and they are probably the best compromise for boats which spend time in both freshwater and marine environments.

Zinc is for seawater. Its relative proximity to aluminium on the corrosion scale means a correspondingly lower driving voltage, and thus a longer lifetime. Magnesium anodes in seawater will get eaten alive in a very short period (weeks in some cases), but zinc should last much longer (months or years).
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