Bronze Propeller anode depletion to bonding bronze skin fittings

haydude said:
In the meantime I found out that Volvo Penta decalares that their folding propellers are a nichel+aluminium+copper alloy.
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Volvo say here that these props are a nickel-aluminium bronze. That is just about the best material it is possible to use for a propeller, strong and highly resistant to corrosion and cavitation. I am quite surprised that they consider an anode necessary at all, if my understanding is correct that the prop is isolated electrically from the shaft. The material is widely used in ships, where AFAIK no anodes are attached to them. There is an obscure corrosion mechanism whereby phases of the metal form a galvanic couple but this should easily be controlled during manufacture.
 
Generally once the anode goes i would expect you aluminium saildrive leg to be the next attacked component.

The anode is doing its job.

it could be another boat moored nearby or a 240v problem with the pontoon? You could get the boat checked in the water by a good marine electrician.

Do you have a copper coated hull, copper bot, coppercoat etc?
 
Bandit said:
Generally once the anode goes i would expect you aluminium saildrive leg to be the next attacked component.

The anode is doing its job.
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You dont indicate which post you are replying to. :confused:

The prop anode is doing nothing to protect the saildrive leg.
The anode on the leg is doing that. Once that has gone then corrosion of the aluminium alloy leg may well follow.

total loss of the prop anode may lead to corrosion of the prop although as Vyv points out NiAl Bronze is just about the most corrosion resistant alloy available for props.
 
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vyv_cox said:
Volvo say here that these props are a nickel-aluminium bronze. That is just about the best material it is possible to use for a propeller, strong and highly resistant to corrosion and cavitation. I am quite surprised that they consider an anode necessary at all, if my understanding is correct that the prop is isolated electrically from the shaft. The material is widely used in ships, where AFAIK no anodes are attached to them. There is an obscure corrosion mechanism whereby phases of the metal form a galvanic couple but this should easily be controlled during manufacture.
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Sadly my experiance is that the anodes on volvo folding propellers last about six months at best and once their gone the props soon show the classic signs of the loss of zinc. :(

This based on two boats with two or three bladed folding props moored a variety of locations.
 
simon14b said:
Sadly my experiance is that the anodes on volvo folding propellers last about six months at best and once their gone the props soon show the classic signs of the loss of zinc. :(

This based on two boats with two or three bladed folding props moored a variety of locations.
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They should not show any signs of loss of zinc if they are nickel aluminium bronze . no zinc in them in the first place!

Only brass will show signs of loss of zinc.
 
The manual for my boat says that anode depletion rates are initially higher due to the copper in the antifouling.
By chance, I swopped to a copper free antifoul last year and am due to lift out in three weeks. I wonder if there will be any difference.
 
simon14b said:
The last two have shown gone pink in places when the anode expired before I was able to replace it - I thought was loss of zinc?
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Presumably you are describing a different type of propeller. Perhaps the majority in general service are manganese bronze, not a bronze at all but a brass with a few element additions. The Volvo folding prop, as described on their website, is a very different matter.
 
Vyv

Thanks for your very helpful comments. I shall keep on replacing the prop anode, of course, but you have convinced me that our Volvo prop will not de-zincify, go pink, and fall to bits immediately if I am a bit slow in replacing it - very reassuring!
 
vyv_cox said:
Presumably you are describing a different type of propeller. Perhaps the majority in general service are manganese bronze, not a bronze at all but a brass with a few element additions. The Volvo folding prop, as described on their website, is a very different matter.
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The prop was is genuine Volvo part as was the one before that on my previous boat. As the volvo saildrive/folding propeller combo is infamous for eating anodes and then moving on to the propeller I am rather surprised how this thread has developed.

The only thing I can think of is that they have changed composition since 2005?

Edit - I notice this post http://www.ybw.com/forums/showpost.php?p=3409413&postcount=17 says Volvo changed the composition but that made the problem worse!
 
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simon14b said:
Sadly my experience is that the anodes on volvo folding propellers last about six months at best and once their gone the props soon show the classic signs of the loss of zinc. :(

This based on two boats with two or three bladed folding props moored a variety of locations.
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This is exactly the kind of experience giving me concern.

To summarize: the propeller:

1) is electrically isolated from the shaft
2) is nichel+aluminium+copper bronze alloy, the best you can have
3) in an Hallberg Rassy thread Volvo Penta replied that they have added the anode because of "market requirement" and an anode would otherwise be unncessary

However:

a) the anode depletes in three to six months depending on the case
b) I read reports like the one above that are in total contraddiction to (2) and (3)

(a) would not be a concern if (3) were true.

What do I believe?

Simon14b, what signs of "loss of zinc" did you see if Volvo Penta propellers are not supposed to be the bronze alloy type based on zinc?
 
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haydude said:
Simon14b, what signs of "loss of zinc" did you see if Volvo Penta propellers are not supposed to be the bronze alloy type based on zinc?
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What I took to be loss of zinc is pink patches on the propeller blades and the hub.

The boat gets new anodes when it launches in March and they are replaced in late September when the are usually gone or just about to go!
 
If you look at the galvanic series in seawater here you see that the voltage of nickel aluminium bronze is -0.2 whereas that of zinc is about -1.0. By connecting the two together you are creating a pretty active cell, so naturally the zinc depletes rapidly. The combination would appear to do nothing to protect the propeller but lots to swell Volvo's profits. This is assuming the prop construction is all NAB, or at least that what appears to be stainless steel pins and a collar(?) is duplex.
 
vyv_cox said:
If you look at the galvanic series in seawater here you see that the voltage of nickel aluminium bronze is -0.2 whereas that of zinc is about -1.0. By connecting the two together you are creating a pretty active cell, so naturally the zinc depletes rapidly. The combination would appear to do nothing to protect the propeller but lots to swell Volvo's profits. This is assuming the prop construction is all NAB, or at least that what appears to be stainless steel pins and a collar(?) is duplex.
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Thank you Viv, from the first part I understand that the great difference in voltage is what consumes the zinc prematurely and NAB would make the anodes superfluous, is this correct?

I am afraid you lost me with the last sentence "This is assuming ..." please could you explain? Pins and holding screws _are_ stainless steel. Are you suggesting that their presence could cause a premature corrosion of NAB once the zinc anodes have depleted? Or would their influence be negligible?
 
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There is a small voltage difference between NAB and 300 series stainless steel, so there is a possibility of galvanic reaction between them, although it would be very minor. If the stainless is duplex, not shown in the table, then there could very possibly be no reaction at all.
 
I just had a conversation with the dealer who fitted the prop and he said they have an example of a boat with an older VP folding prop, exactly as mine, but without anodes. He says the prop is fine.

I suppose that fitting an Ambassador rope cutter (which was on my list for next year's haul out) at this point would create a continuity between prop and shaft/saildrive, thus putting at serious risk the saildrive due to the prop electrical properties.
 
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simon14b said:
What I took to be loss of zinc is pink patches on the propeller blades and the hub.

The boat gets new anodes when it launches in March and they are replaced in late September when the are usually gone or just about to go!
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Given that the prop is not supposed to be based on zinc+copper brass, are you sure that your pink patches aren't something else? Did you actually find any pitting or weakness?
Do you have a rope cutter?
 
haydude said:
I just had a conversation with the dealer who fitted the prop and he said they have an example of a boat with an older VP folding prop, exactly as mine, but without anodes. He says the prop is fine.

I suppose that fitting an Ambassador rope cutter (which was on my list for next year's haul out) at this point would create a continuity between prop and shaft/saildrive, thus putting at serious risk the saildrive due to the prop electrical properties.
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No. The Stripper has no effect. It is all 316. You will see literally hundreds of saildrives fitted with them on both fixed and folding props.
 
haydude said:
Given that the prop is not supposed to be based on zinc+copper brass, are you sure that your pink patches aren't something else? Did you actually find any pitting or weakness?
Do you have a rope cutter?
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I assume that Volvo changed the material at some time between 2005 and now. Here is a picture I took when it had just come out of the water in the middle of January, make of it what you will.

As an aside on another thread it was reccomended that the prop anodes are painted around the 'screw' hole as it was considered that they fell off when the anode eroded around them. But as might be visible on the photo, this area is the last to corrode.

Photo0068.jpg
 
simon14b said:
I assume that Volvo changed the material at some time between 2005 and now. Here is a picture I took when it had just come out of the water in the middle of January, make of it what you will.
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Right ... I think that your pink patches are just fouling. You can spot pink dezincfication only by scratching the surface to reveal bare metal.

Here is the link to an example I found googling:

http://www.pbase.com/image/108122498
 
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