Aluminum Alloy "Zincs" Better than Zinc "Zincs"?

[rardiH36]

Yesterday at the Oakland Strictly Sail show, I spent some time talking to a vendor of anti- corrosion products. And even bought a shaft brush to ensure the best electrical link between my prop shaft from inside the boat and the zinc fish I have been hanging over the side to augment the shaft collar zinc on the shaft in the water. (http://www.saltydoggs.com/product_info.php?products_id=22&osCsid=d768c74d12601789f084aae165683eee). My photo of the device is attached below.

The gentleman also described the benefits of aluminum anodes over zinc. Later, back home, an internet search revealed more info -- mostly in favor of aluminum - even in salt water. (Some links are below.) Seems it's necessary that "aluminum" for anodes be an alloy that contains a small amount of Indium and also some zinc and is manufactured adhering to a military spec. The aluminum anode material is only slightly more active than zinc (-1.10v for Al vs. -1.05v for zn in saltwater against a Silver-Silver Chloride reference electrode), is lighter, lasts longer, and is more environmentally friendly.

My thought is to use the aluminum alloy anode material for my fish anode while at the dock. I would think that being slightly more active than zinc, it might also reduce the depletion rate of my zinc shaft collar anode = fewer times to pay a diver to replace?

Anybody have experience with this aluminum anode verses zinc topic?

http://www.martyranodes.com/content/martyr-resources/alloy-specifications.php
http://www.performancemetals.com/images/pdfs/Aluminum Anodes.pdf

I found that shaft collar anodes are also available in aluminum/indium formulation. http://www.defender.com/product.jsp?path=-1|299255|319700&id=1052567

(Sorry SailboatOwners, but I didn't find aluminum shaft anodes in your store.)

Thanks,

 

[Don S/V ILLusion]

I read a recent article in Professional Boatbuilder on the same subject advocating alum alloy anodes but I didn't see what the alloy components are so assume it is at least in part zinc or some equivalent noble metal. Don't ignore the galvanic scale of metals in making up your own mind.

 

[srhummer]

Might want to consider that if you use an Al anode and it is the most active, will it attract any elec in the water and consume your Al anode instead of all the zinc around and you will use up the Al anode really fast. Just a thought here and I will count on others on the site who know more thank I do to chime in.

 

[Gunni]

Has anyone ever seen a zinc fish actually sacrifice?

 

[Don S/V ILLusion]

Has anyone ever seen a zinc fish actually sacrifice?

Yes. But only when it was properly connected to something other than a chainplate or shroud which serves little purpose because of resistance.

 

[rardiH36]

Yes. But only when it was properly connected to something other than a chainplate or shroud which serves little purpose because of resistance.

For the past nine months or so my zinc "fish" definitely have sacrificed. Until my purchase of the propeller shaft brush a few days ago, I've had my grounding point for the fish zinc at the engine. The 1.25 fish zinc's I've gone through are off-the-shelf collar zincs onto which I've clamped a few feet of recycled 3/16" standing rigging wire which then is clamped to tinned copper wire for the run into the engine compartment. Even though some of the run is SS instead of copper, I figure that the short run and the thick 3/16" should offer negligible resistance considering such low voltage/current. I feel SS for the part of the run that is in the water will hold up better over time than copper.

 

[fstbttms]

Yes. But only when it was properly connected to something other than a chainplate or shroud which serves little purpose because of resistance.

Terminating a "fish" or "guppy" zinc to a chainplate or shroud isn't ineffective because of resistance. It's ineffective because in order for an anode to protect any item, it must be in direct electrical contact with that item and both the zinc and item being protected must be immersed in an electrolyte (like seawater.) Clipping a zinc to a chainplate or shroud serves no purpose, unless the chainplate or shroud is electrically bonded to something underwater requiring protection.

 

[Don S/V ILLusion]

For the past nine months or so my zinc "fish" definitely have sacrificed. Until my purchase of the propeller shaft brush a few days ago, I've had my grounding point for the fish zinc at the engine. The 1.25 fish zinc's I've gone through are off-the-shelf collar zincs onto which I've clamped a few feet of recycled 3/16" standing rigging wire which then is clamped to tinned copper wire for the run into the engine compartment. Even though some of the run is SS instead of copper, I figure that the short run and the thick 3/16" should offer negligible resistance considering such low voltage/current. I feel SS for the part of the run that is in the water will hold up better over time than copper.

That's what I meant by "properly". Lots of folks connect a guppy to a chainplate presuming it is grounded by a low resistance path if at all. What you describe in connecting it directly to the engine is the best approach as it precludes multiple ground points and ensure efficacy allowing the anode to do it's job. Connecting it to some different underwater metal thinking it is "grounded" sets up a worse scenario which actually can promote electrolysis.

 

[rardiH36]

Terminating a "fish" or "guppy" zinc to a chainplate or shroud isn't ineffective because of resistance. It's ineffective because in order for an anode to protect any item, it must be in direct electrical contact with that item and both the zinc and item being protected must be immersed in an electrolyte (like seawater.) Clipping a zinc to a chainplate or shroud serves no purpose, unless the chainplate or shroud is electrically bonded to something underwater requiring protection.

Good points. For most boats the shrouds/chainplates are bonded to the ground system via the mast and eventually to the engine block? But it is a circuitous route.

The main items on my fiberglass hull sailboat that I want protected are the prop and the shaft (the prop would get eaten up before the SS shaft is attacked). Everything else such as the cutlass bearing and my bronze thru-hulls aren't bonded. So not subject to galvanic corrosion very much.

The shaft and the prop are physically connected to the engine through my standard shaft coupling and through the gearbox. But I have read that rust build-up at the coupling, and also gear lube in the gearbox can potentially degrade the electrical conductivity. So that's why a shaft brush inside the boat can be recommended.

Also I have read that its best that the hanging zinc "fish" anode be as close as possible to prop. I hang mine over the rear pulpit = the zinc is about 3-4 feet from the prop.

 

[jibes138]

Since the engine is connected to the negative terminal of the battery you should be able to just connect the fish to the negative battery terminal. Electrically it is the same point as connecting it to the engine ground or the shaft. Any electricians out there see a problem with this?

 

[Rick486]

Interesting discussion. In using a fish while tied up at the marina, I found a closed circuit from my throttle control on the pedestal to the engine. So I clamp the fish to the throttle handle, hoping that I'm doing some good for the propeller and shaft. And there is good continuity between the engine and shaft. So hopefully it's like that old song, "....the hip bone is connected to the leg bone.....".
Any thoughts?

 

[fstbttms]

Also I have read that its best that the hanging zinc "fish" anode be as close as possible to prop. I hang mine over the rear pulpit = the zinc is about 3-4 feet from the prop.

It is the length of the electrical path from anode to protected item that is the critical consideration, not the zinc's physical proximity to the prop. For the sake of argument, let's say you have a guppy zinc, 20 feet of wire and a battery clip. Clip the wire to the prop shaft and it doesn't matter if you hang the zinc 10 inches from the prop or 10 feet. It will provide equal protection either way.

 

[rardiH36]

I suspect that by now, 10 or so posts into the thread, new participants in the topic are unlikely.

Going back to the question on my original start of the topic, seems that nobody has info/opinions about zinc anode performance compared to the Al-Zn-In alloy anode that a vendor was advocating at the West Coast Strictly Sail show last week.

So I have attempted to research/answer my own question. I have touched base with a few purveyors of both zinc and aluminum anodes. Citing liability concerns, one would say nothing of pros and cons, instead recommending that I contact a professional corrosion consultant. A couple of others ventured that hanging an Aluminum anode over the side would likely extend the life of my shaft zinc anodes as compared to a zinc "fish" and otherwise would have no negative effects. The -1.10V of the Al vs the -1.05V of the zinc (as measured against the Ag-Ag-Cl reference anode standard) would cause the Al fish anode to sacrifice faster than the zinc anode on the shaft and also protect my prop.

I'm inclined to do Al to replace Zn for the hanging anode but continue with Zn for the fixed anode on the shaft and/or prop nut.

Can't see much downside. I'll observe and report back in six or so months.

 

[mitiempo]

I would stick with a zinc if you keep the boat in salt water. Aluminum is better in brackish water, and magnesium in fresh water. In any case why introduce another metal?

Good information here http://www.mgduff.co.uk/

 

[fstbttms]

I would stick with a zinc if you keep the boat in salt water. Aluminum is better in brackish water, and magnesium in fresh water.

Aluminum anodes are excellent in saltwater and last longer than zinc.

 

[mitiempo]

But not a combination of zinc and aluminum. One or the other.

If aluminum lasts longer, as its place on the galvanic series suggests, the zinc will deplete just as fast if it remains because it is more active or anodic.

And of course this all goes out the window if you have a saildrive as aluminum will not protect aluminum.

 

[fstbttms]

And of course this all goes out the window if you have a saildrive as aluminum will not protect aluminum.

So I'm assuming that none of these saildrive legs are aluminum?

Yanmar SD20, SD30, SD31, SD40, SD50 and Volvo Penta 110, 120, 130 and 150

Because aluminum anodes are available for all of them.

 

[mitiempo]

As far as I know all saildrive legs are aluminum. It makes sense to me that you would want the anode to be more active than the saildrive leg - not equally active or closer to it.

But getting back to the op's original thought of having both zinc and aluminum anodes at the same time wouldn't the zinc go as fast as before as it is protecting the boats underwater metal as well as the aluminum anode?

 

[fstbttms]

As far as I know all saildrive legs are aluminum. It makes sense to me that you would want the anode to be more active than the saildrive leg - not equally active or closer to it.

Then I guess I misunderstood your previous post. Sounded as if you were saying that an aluminum anode would not protect an aluminum saildrive leg.

 

[mitiempo]

That is what I meant. Wouldn't you want a more active anode than the material you were trying to protect?