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Discussion Starter #1
On a recent prop barnacle thread, the question of zincs and underwater metals was brought up. So when are zincs needed?
1. Why do we not need a zinc on an un-bonded bronze through hull fitting?

2. Why do we not need a zinc on the rudder hinges that on my boat look to be bronze?

3. If you have a silicon bronze prop, silicon bronze shaft and silicon bronze strut, do you need a zinc?

4. Why do suppliers use zinc mixed in with bronze if the bronze can de-zincify? Seems a silicon bronze would be best.
 

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On a recent prop barnacle thread, the question of zincs and underwater metals was brought up. So when are zincs needed?
1. Why do we not need a zinc on an un-bonded bronze through hull fitting?

2. Why do we not need a zinc on the rudder hinges that on my boat look to be bronze?

3. If you have a silicon bronze prop, silicon bronze shaft and silicon bronze strut, do you need a zinc?

4. Why do suppliers use zinc mixed in with bronze if the bronze can de-zincify? Seems a silicon bronze would be best.
Zincs are needed whenever the percent of zinc in the alloy exceeds 5%. (Refer to the prior thread for more detail.) The primary determinant of the type bronze used for a given application is cost.
 

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On a recent prop barnacle thread, the question of zincs and underwater metals was brought up. So when are zincs needed?
Whenever you have dissimilar metals below water.. Your engines RW circuit is also filled with electrolyte and has numerous dissimilar metals...

1. Why do we not need a zinc on an un-bonded bronze through hull fitting?
You usually don't unless you are mixing brass and bronze, which MANY boaters and builders do.. Dangerous, but it happens all the time... This was taken in 2011 of a 2011 production boat:



This is what can happen when you mix 85-5-5-5 bronze with el-cheapo yellow brass Home Depot fittings. The yellow brass male adapter literally crumbled.


So long as your boat is electrically safe, as in stray DC current, and your seacocks are all of a similar metal, then bonding can certainly be optional. My own boat is not bonded but It has no DC or wiring issues and 85-5-5-5 bronze seacocks...

2. Why do we not need a zinc on the rudder hinges that on my boat look to be bronze?
Sometimes those metals are isolated and not in contact with other metals other than through the electrolyte. They may also be a high quality 85-5-5-5 bronze... That said there are plenty of "isolated" prop struts that snap due to dezincification, usually because they were cheaply cast manganese bronze.

3. If you have a silicon bronze prop, silicon bronze shaft and silicon bronze strut, do you need a zinc?
That's a big "if"... I have never seen a silicon bronze recreational prop or shaft most likely due to how machinable the material is or is not... Shafts are either SS, Aqualoy, Nitronic, manganese bronze or Tobin bronze and props are usually manganese bronze, SS or NiBrAl...

Most props, the majority, are Manganese Bronze which is upwards of 35-40% zinc. Older Tobin Bronze shafts were also upwards of 40% zinc. Slap a manganese prop on an AQ-22 shaft and you're going to need a zinc.. Now add one of the graphite packings and you will even more DEFINITELY need a zinc.... Today no one sells bronze shafts and they are SS or one of the Nitronic or Aqualoy variants. The most common saltwater shafting today is Aqualoy 22..

4. Why do suppliers use zinc mixed in with bronze if the bronze can de-zincify? Seems a silicon bronze would be best.
Often machining ability & casting ability and how it deals with pitch adjustments etc.. Manganese bronze is highly formable but they add some tin to it to help minimize dezincification...

Most good quality bronze seacocks, Apollo/Conbraco, Buck Algonquin, Spartan, Groco etc. etc. are cast from 85-5-5-5 bronze which is very, very, very resistant to dezincification and far better than a manganese prop or shaft.. There is a big misconception that these are silicon bronze when in-fact they are 85-5-5-5 bronze (copper, lead, tin, zinc).

Some rudder shoes etc. are made of this too and some are actually made of silicon bronze.. Cheaper builders through still use manganese bronze for p-struts and rudder stuff and they can and do fail due to dezincification...

This is what graphite packing can do locally to a bronze shaft even with an intact zinc, though poorly installed, at the other end of the shaft..... It took less than a season to destroy this bronze shaft when graphite was added to the mix. The damage was localized to the area where the packing made contact with the shaft...


Keep in mind that the electrical potential of graphite is +0.20V to +0.30V and manganese bronze is -0.27V to -0.34V. This creates a HUGE corrosion spread!!!! The further away your metals are in the galvanic scale the more need for zincs....
 

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Bronze is a combination of copper and tin, not zinc. Brass is a combination of copper and zinc. The concentration of tin in bronze varies depending upon the alloy, but is in the neighborhood of 12%. Silicon and phosphor bronze have around 1 -2% of these metals in the alloy. Unbonded through hulls will eventually leach out their tin and that is why bonding is a good idea and one should pay attention to the color of them during haul-outs. One of my old boats had a zinc on the rudder which I’m assuming was there to protect the pintles and gudgeons. Both Phosphor bronze and “regular” bronze have an electrical potential of -2.5 so not much is gained by going to phosphor (stainless steel is -1.5). The other considerations is cost and the hardness of the phosphor alloy would make it impossible for a prop shop to mechanically balance and adjust prop pitch. Even if all our running gear is a version of bronze, it is still attached to a ferrous engine and to make matters worse, that engine uses the block as a ground!
 

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Discussion Starter #6
Whenever you have dissimilar metals below water.. Your engines RW circuit is also filled with electrolyte and has numerous dissimilar metals...



You usually don't unless you are mixing brass and bronze, which MANY boaters and builders do.. Dangerous, but it happens all the time... This was taken in 2011 of a 2011 production boat:



This is what can happen when you mix 85-5-5-5 bronze with el-cheapo yellow brass Home Depot fittings. The yellow brass male adapter literally crumbled.


So long as your boat is electrically safe, as in stray DC current, and your seacocks are all of a similar metal, then bonding can certainly be optional. My own boat is not bonded but It has no DC or wiring issues and 85-5-5-5 bronze seacocks...



Sometimes those metals are isolated and not in contact with other metals other than through the electrolyte. They may also be a high quality 85-5-5-5 bronze... That said there are plenty of "isolated" prop struts that snap due to dezincification, usually because they were cheaply cast manganese bronze.



That's a big "if"... I have never seen a silicon bronze recreational prop or shaft most likely due to how machinable the material is or is not... Shafts are either SS, Aqualoy, Nitronic, manganese bronze or Tobin bronze and props are usually manganese bronze, SS or NiBrAl...

Most props, the majority, are Manganese Bronze which is upwards of 35-40% zinc. Older Tobin Bronze shafts were also upwards of 40% zinc. Slap a manganese prop on an AQ-22 shaft and you're going to need a zinc.. Now add one of the graphite packings and you will even more DEFINITELY need a zinc.... Today no one sells bronze shafts and they are SS or one of the Nitronic or Aqualoy variants. The most common saltwater shafting today is Aqualoy 22..



Often machining ability & casting ability and how it deals with pitch adjustments etc.. Manganese bronze is highly formable but they add some tin to it to help minimize dezincification...

Most good quality bronze seacocks, Apollo/Conbraco, Buck Algonquin, Spartan, Groco etc. etc. are cast from 85-5-5-5 bronze which is very, very, very resistant to dezincification and far better than a manganese prop or shaft.. There is a big misconception that these are silicon bronze when in-fact they are 85-5-5-5 bronze (copper, lead, tin, zinc).

Some rudder shoes etc. are made of this too and some are actually made of silicon bronze.. Cheaper builders through still use manganese bronze for p-struts and rudder stuff and they can and do fail due to dezincification...

This is what graphite packing can do locally to a bronze shaft even with an intact zinc, though poorly installed, at the other end of the shaft..... It took less than a season to destroy this bronze shaft when graphite was added to the mix. The damage was localized to the area where the packing made contact with the shaft...


Keep in mind that the electrical potential of graphite is +0.20V to +0.30V and manganese bronze is -0.27V to -0.34V. This creates a HUGE corrosion spread!!!! The further away your metals are in the galvanic scale the more need for zincs....
Thanks Maine Sail, Ill print this and keep as reference.
 

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Thanks for the link. Clicked through the various alloys of bronze and brass and I can’t see “rhyme or reason” or any consistency in the naming process. Did you know that “Naval Brass” is copper and tin while manganese bronze has zinc? Phosphor Bronze has almost the same composition as naval brass with the exception of a little phosphor added. The next time I meet up with my metallurgist, I have a few questions for her! Needless to say, manganese bronze, “regular” bronze, and phosphor bronze all have the same galvanic potential. All need a much stronger cathode like zinc for corrosion protection.

Now I have a question. Magnesium Bronze being copper (potential approx. -.33 ) and zinc (~ -1.2) (major) components – is this difference in electric potential the reason that an isolated through hull can corrode? And if so, why not use silicon bronze as the copper and tin component both have a very similar potential?
 

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Discussion Starter #9 (Edited)
Now I have a question. Magnesium Bronze being copper (potential approx. -.33 ) and zinc (~ -1.2) (major) components – is this difference in electric potential the reason that an isolated through hull can corrode? And if so, why not use silicon bronze as the copper and tin component both have a very similar potential?
Agree. I have herd the argument copper is expensive and that is why zinc is put into bronze, to reduce cost. But that does not really make sense- how much cost difference is there really between zinc and copper- not much. And look how expensive bronze fittings are, I would think most sailors would pay a few dollars more to get the zinc out of there underwater fittings.

Maine Sail hit on a few points, like being able to work a prop to get the pitch right- might not be able to do that with silicon bronze.

OK, I did check $ for copper ($3/lb) and zinc ($1/lb), so for that 2 lb fitting, no zinc would cost $4.00 more, peanuts compared to the total cost.
 

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Casy, I did notice that the silicon bronze has a hardness factor of 2.5 times the magnesium bronze. It might mean that it is too hard to bend which would be important if you are trying to true up a propeller. But that wouldn't explain why they couldn't use the silicon bronze in cast fittings? Maine Sail did have a graphic example of a yellow brass (45% zinc) nipple corroding on a magnesium bronze (8-12% zinc) thruhull. The big moral to the story is check your zincs often!

Now here's another interesting question. Given that there is a higher percentage of salt in the water in the tropics, making for a more potent electolyte, Do zincs wear out faster in tropical waters than far northern waters?
 

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Discussion Starter #11
Casy, I did notice that the silicon bronze has a hardness factor of 2.5 times the magnesium bronze. It might mean that it is too hard to bend which would be important if you are trying to true up a propeller. But that wouldn't explain why they couldn't use the silicon bronze in cast fittings? Maine Sail did have a graphic example of a yellow brass (45% zinc) nipple corroding on a magnesium bronze (8-12% zinc) thruhull. The big moral to the story is check your zincs often!

Now here's another interesting question. Given that there is a higher percentage of salt in the water in the tropics, making for a more potent electolyte, Do zincs wear out faster in tropical waters than far northern waters?
I get about a year out of my zincs. However, a year ago my new prop zinc fell off and I have not replaced it. I do not notice any corrosion of de-zincification of my bronze prop, bronze strut, or bronze prop shaft. What kind of bronze I have I am not sure, but I believe the prop, shaft and strut, as well as my bronze through hulls (actually bronze pipe imbedded in the glass hull) are all original- that would make them 35 years old. I do have severe barnacle problems- need to dive boat once a month to keep the metals clean. My strut did not have a zinc, but when boat was surveyed 4 years ago, surveyor said to install one, which I did. I also have one zinc on the prop shaft. I have made sure I have no brass fittings any place on the boat, and my through hulls are not bonded. FWIW, I do not plug the boat into dock power.
 

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Casey, that's pretty good. On my Catalina in San Francisco Bay I have a button zinc on the strut, a cone at the end of my flex-o-fold and one to two "egg" zincs on the shaft. Fastbottoms does the changing and it seems that he is replacing a zinc about every three to six months. We don't always plug in (I do have a galvanic isolator), but I am berthed next to a dock transformer, have several live-aboards as neighbors and usually run the refrigerator during the week in the summertime. So my experience may not be typlical. My folding prop (Flex-o-Fold tells me that it is “bronze”) is a thing of beauty and expensive so I don’t mind spending a little money to keep it in good condition.
 

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Casey, that's pretty good. On my Catalina in San Francisco Bay I have a button zinc on the strut, a cone at the end of my flex-o-fold and one to two "egg" zincs on the shaft. Fastbottoms does the changing and it seems that he is replacing a zinc about every three to six months. We don't always plug in (I do have a galvanic isolator), but I am berthed next to a dock transformer, have several live-aboards as neighbors and usually run the refrigerator during the week in the summertime. So my experience may not be typlical. My folding prop (Flex-o-Fold tells me that it is “bronze”) is a thing of beauty and expensive so I don’t mind spending a little money to keep it in good condition.
First, thanks to Mainesail for his invaluable post.
Second ..... These dialogues make a lot more sense if we stop using the generic "zincs", when in fact they may be aluminum indium of magnesium and these have different properties. We have forum members in salt, brackish and fresh water and not all understand that there are differences, so I don't think I am being picky about semantics. If we call them "anodes" these issues become clearer in discussion.
 
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