Anodes

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Anodes

#1

Post by Challenge »

Hello All,

Stanley's' post has me thinking about Anodes. I did a search and it seems that folks here are mostly using zinc or aluminum anodes for salt water. Also I looked at the BoatZincs and their chart:

http://www.boatzincs.com/pdfs/Chart_B-W ... _I_Use.pdf

I think both of my boats have Zinc (pacific OEM & Boat Zinc) & I have never had either of my boat zincs start sacrificing. Both the motor anodes show some corrosion. I have an aluminum prop on one boat and stainless on the other. Both boats are on salt water moorings in a small boat mooring field so few or most likely no "Hot" boats around but I do travel to a few commercial harbors.

According to the the table I should use one Zinc (aluminum prop) and one Aluminum (stainless prop). I'm thinking about switching both to Aluminum Anodes.

What Do you guys think and what are you using????

Thanks in advance

Rick
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Re: Anodes

#2

Post by 3f8 »

Navalloy
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Re: Anodes

#3

Post by Chaps »

Your experience is something I'm seeing everyday. Zinc anodes on aluminum boats just are not reactive enough so the aluminum anodes on the engines end up doing all the work and I guarantee the O/B engineers did not have that in mind. Its funny (and sad at the same time) when I have alloy boat clients bring their boats in and tell me that they are happy with their zinc anodes because "they last a long time and in fact I've never had to replace 'em but those ones on the motor go bad real fast!" Like 3F8 said, Navalloy, or any quality aluminum anode should be used on our hulls and engines. Even the boat builders make the same mistake of putting on zinc and usually itty bitty ones. Only recently has North River started installing aluminum hull anodes on their new builds.

Incidentally I absolutely disagree with the boatzincs chart, those guys know better than that (or they should)
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Re: Anodes

#4

Post by 3f8 »

Here is an old post on this topic.search.php?keywords=navalloy
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Beyond that, we all enter the food chain, and not always right at the top."
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Re: Anodes

#5

Post by MacGyver »

Selecting the best anodes for your boat is not just picking the type of anode you should use.
Location of anodes, how long you want them to last, water temperature, is your boat trailer or a slip queen, do you a have any internal or external stray current problems just to name a few possible area that will need to be fix. If you want the your boat to have the best corrosion protection.

There a couple of things you can do, one is select the anodes you think will work the best. Inspect, document (picture) the hull inside and outside for any signs of corrosion and see if every thing is still ok in the fall. Just remember all the anodes on the hull and engine must be the same type.

Or you can have a corrosion survey done by a professional to find and correct any problems and install the proper size and location to provide the best possible protection.
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Re: Anodes

#6

Post by Tookalook »

If I slip the boat in fresh water and have the anodes from the MerCruser dealer on the outdrive I should install the same material anode on the Hull? On the transam I take it but where? What do you mean by stray current. and what kind of survey professional covers that kind of thing Sorry for my ignorance just a newbie Thanks
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Re: Anodes

#7

Post by MacGyver »

* If I slip the boat in fresh water and have the anodes from the MerCruser dealer on the outdrive I should install the same material anode on the Hull?

Yes they must be the same type anodes. As long as the MerCruser dealer is using the proper anodes for fresh water and the hull and prop material.

*On the transam I take it but where?

Both sides of the engine, below the water line.

*What do you mean by stray current.

Stray current, in this post is DC current that is cause by the boats battery system leaking current into the hull, bypassing the ground wiring. Or stray current from other boats that are near your slip.
It not as simple of a subject is I'm making it sound like. You might Google it for a better answer.

*and what kind of survey professional covers that kind of thing Sorry for my ignorance just a newbie Thanks

A Marine electrician with the experience and test equipment to find and fix corrosion problems.
Again, You might Google it for a better answer. I love Google.

Don't feed bad as a new boater. When I decided to learn about boat corrosion I had know idea how confusing and complicated subject it is.
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Re: Anodes

#8

Post by flyingcow »

So I've been lurking around these parts for some time, but now I have a reason to post! Ic this should be it's own topic, feel free to yell at me.

I made the switch to aluminum last year from an itchy hull with an outboard. I now have a 22' Starcraft with a Mercruiser I/O. After a 10 day stint in fresh water, I noticed some substantial pitting on the stern.
Image
Image

Now a few key things. I later discovered that I am missing 3 anodes off my outdrive, and the one on the hull doesn't seem to be doing much of anything. So my plan is to replace all the outdrive anodes and the hull anode with new aluminum anodes. The boat will be mostly in fresh water, but it will have an extended stay in brackish water as well. Is there anything else I should be looking out for? I did have a bilge blower failure (which might be causing voltage in the hull) but I'll be fixing that before it goes back in the water.

Thanks!
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Re: Anodes

#9

Post by kmorin »

flyingcow, there are couple of items in the photos that could be improved IMO.

All the screws/bolt heads are going to be acting as galvanic cell sites- inducing corrosion between the SS (304? 316L- passivated?) transom because there is a significant potential between the two metals- see the galvanic series chart to understand that any time these two metals are adjacent to one another and wet- there will be some current flow.

Goops, gels, tape, and sealants can reduce the area in contact but.. the threads will touch somewhere and there will be current flow- hence galvanic corrosion. Hot dipped zinc screws (if you can source them?) would be better choices for all that hardware- less galvanic potential- less corrosion or more slowly corroded.

Next is the scuffs and bangs in the paint- those points of opening in the paint film will allow water in, and that water will stay in a ring around the point of entry and begin a crevice (poultice) corrosion site so if possible sanding them open and painting them is better than leaving them to do their work. Also using a base solution like baking soda in water or ammonia in water to wet those cell sites will help to delay the acid acting on the aluminum but its only a temporary fix as that will not be a cure all.

I'm not sure on this last point- does the boat have a hollow transom? Meaning is the transom itself built with a core? or all welded aluminum framing? If there is a wooden or foam core with aluminum on both sides ? (I'm not sure but have seen a few Starcrafts years ago with this design/build feature?) The large number of screws/bolts/holes in the transom work to provide water paths into the core.

Between the core material and the outer transom 'covering' sheet of aluminum, in the boat I looked at; there was significant crevice corrosion where water seeped in around screws (similar to what is shown) and was unable to dry out. So the water lost its oxygen to the aluminum initially, but then became acidic so the wood surface holding the (now) acid solution to the aluminum became the agent to corrode the entire transom surface in large palm of the hand sized patches- that showed through in small pin holes - not unlike the ones you show in the photo's.

I'm not saying this is what's happened in your boat- but.... there are pits showing on the outside that appear to be deep into the metal of the hull- that is what I'm remarking about.

The boat I was asked to look at was eventually stripped inside- the wood removed and all, the result was that hull needed an entire new transom. So, given this possibility I'd suggest you consider exploring

#1 the construction in detail to understand how the transom is fabricated- in cross section.

#2 I'd also suggest you consider getting the boat where you can pull some of the screws or bolts and see what's inside that they hold too- all metal or composite of some type?

#3. I'd also suggest considering a look around for hot dipped zinc fasteners- if you're able to replace the screws in place?

# 4 last, if the screws have to remain SS then I'd suggest getting some passivating gel, and some heavy viscosity thread sealant and pull- clean and passivate all fasteners and replace them once they hvae less reactive surfaces and fresh, water-excluding sealant.

Just my take based on the images you posted.

Cheers,
Kevin Morin
Kenai, AK
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Re: Anodes

#10

Post by flyingcow »

kmorin wrote:
The boat I was asked to look at was eventually stripped inside- the wood removed and all, the result was that hull needed an entire new transom. So, given this possibility I'd suggest you consider exploring

#1 the construction in detail to understand how the transom is fabricated- in cross section.

#2 I'd also suggest you consider getting the boat where you can pull some of the screws or bolts and see what's inside that they hold too- all metal or composite of some type?

#3. I'd also suggest considering a look around for hot dipped zinc fasteners- if you're able to replace the screws in place?

# 4 last, if the screws have to remain SS then I'd suggest getting some passivating gel, and some heavy viscosity thread sealant and pull- clean and passivate all fasteners and replace them once they hvae less reactive surfaces and fresh, water-excluding sealant.

Just my take based on the images you posted.

Cheers,
Kevin Morin
Kenai, AK

Thanks much!

The transom is a wood core that's fastened to a thin aluminum skin just like you had mentioned. The the wood core is very new (<3 yrs) and is marine grade plywood. The previous owner had the work done by a reputable local shop. The challenge is, around here, even the reputable local shops don't know aluminum. I'm pretty sure all those screws are 303 stainless and are what fastens the wood core to the aluminum skin. If I can't find new fasteners, I can get some nitric gel and passivate them. I'm new to aluminum, but my background in stainless goes for years. :thumbsup:

Thanks again!
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Re: Anodes

#11

Post by kmorin »

flyingcow, I hope the marine grade ply is not the copper impregnated type! This treated wood, and some water, will dissolve aluminum in weeks! Had a home repair transom mount - just a piece of ply. He used the green (water proof) plywood and in less than a summer melted' the entire transom (3/16") where the ply was bolted on!

We had someone post here a few years back showing trailer bunks made of the 'water proof' lumber! And there were holes in the bottom of the boat too! All lined up on the bunks made of bare, copper impregnated 2x6's.

Clearly there is already wood/SS/water corrosion going on at the screw penetrations in the transom. Might be worth trying to find out how the 'reputable local shop' did the work? Uniform layer of sealant? Troweled on 5200? or a bead around the edges??? I think the idea of a tile grout job application may have some merit- but the idea that a bead or two will seal- well I think the evidence may be in the photos?

just a word to the wise?

Cheers,
Kevin Morin
Kenai, AK
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Re: Anodes

#12

Post by flyingcow »

kmorin wrote:flyingcow, I hope the marine grade ply is not the copper impregnated type! This treated wood, and some water, will dissolve aluminum in weeks! Had a home repair transom mount - just a piece of ply. He used the green (water proof) plywood and in less than a summer melted' the entire transom (3/16") where the ply was bolted on!

We had someone post here a few years back showing trailer bunks made of the 'water proof' lumber! And there were holes in the bottom of the boat too! All lined up on the bunks made of bare, copper impregnated 2x6's.

Clearly there is already wood/SS/water corrosion going on at the screw penetrations in the transom. Might be worth trying to find out how the 'reputable local shop' did the work? Uniform layer of sealant? Troweled on 5200? or a bead around the edges??? I think the idea of a tile grout job application may have some merit- but the idea that a bead or two will seal- well I think the evidence may be in the photos?

just a word to the wise?

Cheers,
Kevin Morin
Kenai, AK
So it's covered in West system, but there's no obvious green tint. I'm going to pull some of those screws and see what comes out. If I have wood rotting already, then the problem is obvious. I really hope they weren't dumb enough to put pressure treated in there.

This might get fun!
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Re: Anodes

#13

Post by kmorin »

flyingcow, WEST coverage is good news- should encapsulate the ply's glues- likely not done to green ply so another good note. The surface of WEST will however work just like any other flat surface to cause a thin film/capillary filled void that will promote acid corrosion (crevice/poultice) so worth a look to see how much water is invaded between ply and transom?

Good step to pull some screws, also might consider 'cooking the boat' in a garage with the heat turned up- no water, dried out boat= much less catalytic fluids to promote galvanic potential electron flow. So if possible to clean up fasteners and passivate? the while that's happening a nice 'bake-off' would help the openings for screws to be replaced into a dry wood and metal interface? Drop of 5200 on each fastener w/a donut of tape, clean during install, easy wiped clean up- pull the tape and the sealant is left and the transom is clean around screw/bolt heads.

I use a roll of blue tape (masking) take a tube 3/8" - 1/2" sharpen end, press into tape so when the tape unravels off roll; there are perforations the size of the screw heads. These are torn off and placed over the openings so the 5200 comes out on the tape, a quick wipe and pull the tape, clean job, nice seal- pretty fast to do.

cheers,
Kevin Morin
Kenai AK
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Re: Anodes

#14

Post by flyingcow »

kmorin wrote:flyingcow, WEST coverage is good news- should encapsulate the ply's glues- likely not done to green ply so another good note. The surface of WEST will however work just like any other flat surface to cause a thin film/capillary filled void that will promote acid corrosion (crevice/poultice) so worth a look to see how much water is invaded between ply and transom?

Good step to pull some screws, also might consider 'cooking the boat' in a garage with the heat turned up- no water, dried out boat= much less catalytic fluids to promote galvanic potential electron flow. So if possible to clean up fasteners and passivate? the while that's happening a nice 'bake-off' would help the openings for screws to be replaced into a dry wood and metal interface? Drop of 5200 on each fastener w/a donut of tape, clean during install, easy wiped clean up- pull the tape and the sealant is left and the transom is clean around screw/bolt heads.

I use a roll of blue tape (masking) take a tube 3/8" - 1/2" sharpen end, press into tape so when the tape unravels off roll; there are perforations the size of the screw heads. These are torn off and placed over the openings so the 5200 comes out on the tape, a quick wipe and pull the tape, clean job, nice seal- pretty fast to do.

cheers,
Kevin Morin
Kenai AK

It's so nice having people who know what to do to ease a newbie's panic. I think (hope) if it's pressure treated, I'll recognize it when I pull the screws. Passivating and re-sealing the screws is no problem. Thanks!
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Re: Anodes

#15

Post by kmorin »

flyingcow, not too likely the shop would try to coat a green wood with epoxy- not all that compatible. So the odds you've got real live marine ply is good. The remaining issue is the mastic/goop/bedding used and technique to apply it.

On thing you may consider but its a bit 'out there' ?; after baking the skiff 'inject' 5200 into the screw holes before the screws are replaced. The idea being to 'frack-the-fill' or pressure inject something that seals ALL the holes so water is excluded.

opinions are cheap, boats are expensive, hope we're helping not hindering?

Cheers,
Kevin Morin
Kenai, Ak
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Re: Anodes

#16

Post by flyingcow »

Kevin,

Can I go full on theoretical for a minute with you? I'm a Chemical Engineer with the better part of a decade in the food and beverage industry, hence why I know stainless very well. Again, this is purely geeking out.

If I passivate a 300 series stainless fitting and get it near aluminum, doesn't the Chromium Oxide layer just push the fitting further down the galvanic series? On the flip side, if I wanted active 300 stainless, I'd need to hit it with nitric or citric then keep it wet for eternity to prevent the oxide layer...

Just curious. Does an oxide layer affect the position on the galvanic series?
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Re: Anodes

#17

Post by flyingcow »

kmorin wrote:flyingcow, not too likely the shop would try to coat a green wood with epoxy- not all that compatible. So the odds you've got real live marine ply is good. The remaining issue is the mastic/goop/bedding used and technique to apply it.

On thing you may consider but its a bit 'out there' ?; after baking the skiff 'inject' 5200 into the screw holes before the screws are replaced. The idea being to 'frack-the-fill' or pressure inject something that seals ALL the holes so water is excluded.

opinions are cheap, boats are expensive, hope we're helping not hindering?

Cheers,
Kevin Morin
Kenai, Ak

Well, this makes me wonder just how much of this is caused by the virtual absence of anodes. Checking for pulp in the AM.
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Re: Anodes

#18

Post by kmorin »

flyingcow, zincs (term used as a generic sacrificial metal anode)of any alloy only provide protection against stray current. They protect when the battery current is using the hull to 'return' and not for the screws of different metals (galvanic) and not for crevice (thin films of water that become acidic (when DE-aerated) stray current uses the boat's aluminum as a sacrificial anode- so.... cathodic protection involves mounted an electrically bonded (don't use the term ground or the discussion will deteriorate) to the aluminum alloy hull.

The anode will 'go first' it will be dissolved into the solution of salt water before the hull when there is a stray current 'bleeding' from the battery (or shore power) to the water- through the hull's metal.

(of the) Three types of corrosion you may have the first two, and if you had a shorted bilge blower motor you may have had a short term dose of the third type as well? Usually motors OPEN circuit fail so there is not stray current to hull- also lots of bilge blowers are plastic so there is no winding to hull failure- unless its submersed? but then it might have a hard time working!

The images clearly show the galvanic cells and (may) indicate the crevice corrosion inside the transom but..... since the zinc looks intact? (not sure of the alloy of that fitting) the idea that you had stray current is not as valid, IMO, as the idea your hull is experiencing some dissimilar metal (galvanic) and perhaps some crevice (pitting inside an inadequately coated filler/core) corrosion.

I'm not arguing that the high alloy aluminum 'zinc' wouldn't be better for you application, just noting that all these protect against is stray current (#3 type) corrosion of the hull with poorly wired DC/AC systems. 'zincs' of any alloy won't protect a site where two different metals galvanically interact- OR sites where there is thin film of water that becomes acidic which can't dry out- these are not the types of corrosion that anodes can stop.

Cheers,
Kevin Morin
Kenai, AK
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Re: Anodes

#19

Post by flyingcow »

kmorin wrote:flyingcow, zincs (term used as a generic sacrificial metal anode)of any alloy only provide protection against stray current. They protect when the battery current is using the hull to 'return' and not for the screws of different metals (galvanic) and not for crevice (thin films of water that become acidic (when DE-aerated) stray current uses the boat's aluminum as a sacrificial anode- so.... cathodic protection involves mounted an electrically bonded (don't use the term ground or the discussion will deteriorate) to the aluminum alloy hull.

The anode will 'go first' it will be dissolved into the solution of salt water before the hull when there is a stray current 'bleeding' from the battery (or shore power) to the water- through the hull's metal.

(of the) Three types of corrosion you may have the first two, and if you had a shorted bilge blower motor you may have had a short term dose of the third type as well? Usually motors OPEN circuit fail so there is not stray current to hull- also lots of bilge blowers are plastic so there is no winding to hull failure- unless its submersed? but then it might have a hard time working!

The images clearly show the galvanic cells and (may) indicate the crevice corrosion inside the transom but..... since the zinc looks intact? (not sure of the alloy of that fitting) the idea that you had stray current is not as valid, IMO, as the idea your hull is experiencing some dissimilar metal (galvanic) and perhaps some crevice (pitting inside an inadequately coated filler/core) corrosion.

I'm not arguing that the high alloy aluminum 'zinc' wouldn't be better for you application, just noting that all these protect against is stray current (#3 type) corrosion of the hull with poorly wired DC/AC systems. 'zincs' of any alloy won't protect a site where two different metals galvanically interact- OR sites where there is thin film of water that becomes acidic which can't dry out- these are not the types of corrosion that anodes can stop.

Cheers,
Kevin Morin
Kenai, AK
Ok. I'm on board. #1 is dissimilar metals with fittings, #2 is internal aluminum oxide formation at the transom, #3 is the lack of anodes. I'll attack them in that order.
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Re: Anodes

#20

Post by flyingcow »

and PS. Thanks for the Bonding vs Grounding comment... that makes this process engineer VERY happy :)
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Re: Anodes

#21

Post by kmorin »

flying cow, just a note about #2 it is not really aluminum oxide - a very high heat melting point, tremendously hard and nearly imprevious layer of aluminum and oxygen that is the problem in #2. That film is degraded by the 'back and forth' of the oxygen molecule(s), when the aluminum metal finally 'loses' the fight for the free oxygen- it's left as aluminum hydroxide- white chalky stuff- that is no protection to underlying metal. That cell can be renewed into crevice corrosion by a drop of H2O what #2 is about is the film of water that (potentially) lays between the two surfaces.

This thin film allows the water to give up its original oxygen to the aluminum - at first. But in so doing the water's ph shifts upward- that allows the now acidic solution to 'claim' the oxygen back and the protective film of oxide is now unavailable to protect the aluminum under that self healing film. IF there is a constant addition of water to these cells and no real net oxygen (fresh air) then the aluminum can't form aluminum oxide to 'heal itself'.

So, anytime there is a surface to surface void allowing some water to fill by capillary action or vacuum- the water stagnates- becomes acidic and THAT attacks (bond attraction) the oxygen from the aluminum oxide- a protectant - now the very tough self healing aluminum oxide is left as aluminum hydroxide and that won't protect the underlying metal as it is not inert.

So this is the nature of the crevice corrosion cell inside the aluminum panel fit to the plywood doubler in your transom. To arrest this cell- eliminate water- to do that heat up the entire boat to dry it out. Then if possible bed the entire surface of the ply in 5200 so there is virtually no place for water to 'get in'.

Just making sure that we're both seeing the same chemical reaction in regard #2 type- crevice or poultice corrosion in the 5000 and 6000 series alloys of aluminum?

Cheers,
Kevin Morin
Kenai, AK
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