Unzincs

[Doug Hoople]

Has anyone ever experimented with sacrificial anodes made
of materials more noble than zinc?

We've got a wood boat that gave us an extravagantly expensive
lession in electrolysis when we first got it over a year ago, and
now I know that zinc drives voltage potentials too high for the
good of the wood, >1000 millivolts, where it should be
>500 and <700. I've got two kinds of metal that see salt water
on my boat, silicon bronze struts, shaft logs, rudders,
through-hulls, fasteners, and monel the prop and rudder shafts.
Actually, to be totally honest, I don't know what bronze alloy the
prop is made from. And I'm sure there's some stainless in the
odd fastener here and there.

I've just noticed, for example, that there's a class of aluminum
anode. Aluminum would drive the voltage potential to around
600-700mv, which would be perfect for the bronze. Bringing
the numbers down would make the anode less hot, draw less
current and make the anodes last longer, it would seem to me.
And aluminum is ignoble enough that it probably wouldn't lead
to terrible side-effects in leaching midnobles out of the prop
alloy, but here's were I'm edging onto thin ice.

Apparently, there's no mil-spec designation, but there is
something called the B605 alloy specification, the origins of which
are a mystery to me, having only heard of it since yesterday.

The other alternative is an electrical controller. But knowing
that the wrong move could fry everything in a matter of days, I'd
be happy to find a more natural solution, so I'd like to keep that
topic out of this thread, at least for the time being.

My zincs have been drawing 1000mv through my bronze for a
year since the boat's been in the water. I'm a year more educated,
and the sealant around the struts and shaft logs in the new planks is
a year closer to getting wet. I'm supposed to be getting new zincs
this week, and so now's the time to start getting this right.

Am I playing with fire or putting out the fire?

Doug Hoople
M/V Bomar
1963 Stephens Bros. 50 FDMY
Sausalito, CA

[nedL]

Doug, Sad to say I can't really comment here, though I do understand exactly what you are saying. I will be quite interested in hearing what some of the "more noble" forumites have to say :D Interesting question you raise.

[Doug Hoople]

Just a quick addendum. There are 'aluminum' anodes on the market, but these aren't what I'm talking about. They're actually alloy anodes heavily mixed with very ignoble metals that make them 'hotter' than zinc, if you can believe that. Calling them aluminum is a bit misleading. It's a little like calling manganese bronze bronze, when it's 30-40% zinc.

I'm looking at the possibility of finding (or even making) an anode that would drive the galvanic potential to the 600-700mv range, safe for most bronze used in wooden boats, and healthy for the wood, instead of the industry-standard +1000mv that you get with zincs. +1000mv is over-protected and enough to trigger electrolysis. Marine-grade aluminum looks to fit the bill pretty nicely, at least in terms of its galvanic rating 650-700mv.

Wooden boats are a special case, and I'm sure the market is too small to warrant the research by the makers. It's not too small for us, though, is it? It would take a bit of finesse and reasonable intellect to get this right, as the topic is rife with subtleties. But wooden boat people are used to subtleties, no?

[Frank Wentzel]

Doug

The idea sounds reasonable but I am afraid that, as most things in life, it is more complicated than it appears. Aluminum may give the right voltage but I'm afraid you may have a problem with passivation. Aluminum should corrode like mad except for the impervious oxide film that forms on the surface. That makes it great for applications that require corrosion resistance but bad for this purpose.

Check out the link below. These aluminum anodes(AS 2239 A2) anodes are 5% zinc, which may eliminate the passivation problem. I believe these are intended for underground cable and pipe protection but you should be able to evaluate them based on voltage readings taken after a few days equilibration time

http://www.allmar.com.au/anodes.html

/// Frank ///

[Doug Hoople]

Hi Frank,

Yeah, it's a fresh idea, so I hadn't thought through all the possibilities. Looks like aluminum might not be it.

As for the alu/zinc alloy, the worry then is that the zinc would leach out in a hurry, like the zinc in manganese bronze, and you'd be left with the original problem you've described.

Hmmm... wonder if there's another alloy/metal we could look at.

Doug

[Frank Wentzel]

Doug

I would guess that the zinc in the aluminum "zinc" is in there to prevent passivation. I’m assuming that since the alloy is used for anodes and wouldn’t be of much use if it went passive after a few weeks. Voltage readings taken over the first few days and maybe even weeks will tell the tale. Of course the fact that the company is in Oz is a bit of a problem

By the way, your prop is probably manganese bronze. You can test it with a strong magnet. Manganese bronze is very slightly magnetic. You will need a strong magnet - not a refrigerator magnet. A rare earth magnet or a strong ferrite magnet taken from the magnetron tube in a dead microwave oven will do the trick. (There are two very strong ferrite ring magnets in virtually every magnetron tube - and I LOVE magnets.) The prop will just barely attract the magnet if it is manganese bronze.

Another suggestion for your anode would be metallic lead. Note that lead would not protect ferrous metals or "active" stainless steels, but it will protect manganese bronze. Due to the much lower potential you might want as much as a couple of square feet (lead flashing) depending on the surface area of the metal you want to protect. Once again your voltage readings will indicate whether you will be successful.

If you do try these alternative anodes out, please report the results back to the Forum. I am sure there are many others who will be interested.

/// Frank ///

[Matt Middleton]

How does one take these voltage readings? If it's the voltage potential between anode and component of interest, we'll say prop, for example, do you just put a lead from a multimeter to each of the two? Is the electrical system required to be off or on to do the measurement, or is it irrelevant?

[Alan D. Hyde]

Wasn't there a "capac" system years ago that used perhaps a platinum node?

That's all I remember about it right now.

I think Ross Norgrove talked about it in one of his books.

Alan

[Doug Hoople]

The test setup is based on a multimeter measuring millivolts. One of the leads is long, say 25 ft, and terminated with a silver/silver-chloride half cell. You drop the half cell in the water, through any convenient port. You touch the other lead to your underwater metals, all throughout the system. If you've got a decent electrical connection to your underwater zincs, the reading should reflect that, meaning it should be somewhere near 1000mv. If you measure less, then check your connections for the failure point. If you touch an isolated silicon bronze through-hull, on the other hand, you should get the normal reading for silicon bronze, somewhere in the 250mv vicinity.

Don't worry about positive or negative. Switching the leads just reverses the sign. Actually, I take that back, if you have an impressed current protection system, then DO pay attention to the signs, but not before reading a lot more technical material than this.

On a boat with a properly installed bonding system, touching any bonded underwater metal should show the same reading as all the others, because they're all attached to the same circuit. The reading should be that of the zincs.

The half cell, by the way, is sold as part of an overpriced testing kit that's made by Guest, among others. Electro-Guard makes one, too, but at least they include a respectable multimeter, the Fluke 70, but then they use zinc in their reference half-cell so you have to redo your figures. Either way, the kit includes the multimeter, a standard probe, and a long-lead probe with the half cell. I think that you can get just the long lead probe by itself these days, and use the multimeter you've already got, which is what I do. The meter in the Guest kit is really just a simple voltmeter with color coding on the dial so you don't have to think things through.

[Doug Hoople]

Frank,

I've noticed that the standard 'aluminum' anodes are supposed to be even more active than zinc anodes, measuring +1100mv. I find that strange, as the percentages are as you've mentioned, with zinc at 5%, presumably to prevent passivation, and aluminum for the remainder. The intended market for these seems to be for applications where the metals are very base, and where zinc may be nobler than what's being protected.

Do you know, are the struts and rudders on older wooden boats also made of manganese bronze? I'll run the magnet check you suggested on mine this weekend. I'll bet it's all manganese bronze, come to think of it. The diver I'm working with is a wooden boat owner, too, and he's pretty interested in helping out and finding out more himself.

[Ian G Wright]

Zinc in seawater, magnesium in fresh and forget "bonding".
That's all,,,,,,,

IanW.

[Doug Hoople]

Wish it was that simple. When I bought my boat, I had to replace two long planks that carried the running gear, and the whole transom log and a few lower transom planks. Not to mention all the backing blocks. All at great expense. The guy who had it before me said "If a little zinc is good, then a lot must be better".

Wood boats that don't get tuned right see the yard for this kind of work every 10 years.

[wolfietuk]

If someone is near a college with a decent electrical engineering dept. go ask a professor. They love to give their students thinks like this for projects. Who knows, they might come up with something profitable for the college and boat owners.

Rick

[Matt Middleton]

I'm not meeaning to muddy up this thread with my questions, but let me see if I get this or not. The silver/silver chloride or zinc half cell is literally half of the "battery" being created by dissimilar metals, the other half being the metal component in question- is that right? If so, it would seem that you'd need to use your anode material (and size- surface area corresponds with resistance) as your half cell, and that you could do your experiments with this setup. Just find some different alloys, use them as your half cell, take your readings, and monitor them over time to look for possible issues with passivation and the like.
I do realize that there's prob'ly a lot more going on under my boat than I realize, but this is just the way things make sense to me at this stage in my learning. But could it be this simple?
BTW, thanks for the response to my earlier question.
Matt

[Ian G Wright]

I suspect that fitting anodes "'cos you should" and bonding all you can reach is a mistake and can/does cause more problems than it cures.
If a boat has no problem then don't fix it.
Mine, copper fastened with an iron keel needs only a small (2inch dia) zinc anode to protect the bronze heel fitting and bolts. The sea cocks are Blakes bronze and untouched by any corrosion after 12 years.

IanW.

[Doug Hoople]

Hi Matt,

Actually, you're right, it is that simple. Any of the metals in the galvanic series could be used as a test cell (Electro-Guard, for example, uses zinc). The advantage to the silver/silver-chloride test cell is that the galvanic series itself uses it as its reference base, as do the numbers published in most of the literature on corrosion, so there's no mental math to do every time you're looking something up.

On the other hand, the math involved is simple addition and subtraction, so no big deal. Making up your own test probe from a zinc and a piece of wire is something you could probably do right now from stuff you've got on board.

[Doug Hoople]

So, um, Ian, not to be flip, but...

What if it IS broke, and you're broke because it's broke, and you don't want it to break no mo' 'cause you don't want to be broke no mo'?

[Doug Hoople]

Ian, on a more serious note, do you have an engine and a prop shaft and a shaft log and a propeller? Unbeknownst to many, that combination is a completely unavoidable mini-bonding system unto itself, and the shaft log has to be taken into account when zincing the prop, which most people zinc the crap out of, oblivious to the connection.

Understanding how to size your zincs is pretty important, unless you have no interactions at all to worry about, and that's a relatively rare boat.

[Matt Middleton]

Thanks for being patient with me Doug. I have another question- I don't understand why you would need to reference your numbers obtained using zinc to silver/silver chloride. It seems to me that the voltage reading you get while using a zinc half cell would be the same voltage that exists when you use zinc as an anode. And so if you used another alloy, your voltage reading would be the same as if you used the above alloy as you anode. Why would a difference exist between the test setup and the actual corrosion prevention system?
Actually, I may be one with a simple enough system not to have to worry, but I still want to understand these things. I have 2 thru hulls- salt water pump pickup and sink drain- that stay in the water. The outboard usually gets tilted up out of the way. The construction is cold molded, so unless the wood is wet I don't have a complete circuit anywhere (thru hulls not bonded.)

[Doug Hoople]

Matt,

Check any book on metal corrosion. They'll have a table called the galvanic series, that lists common metals ordered from noble to base. That's where you'll find typical numbers like -1000mv for zinc and -250mv for silicon bronze. They came up with -1000mv for zinc by putting zinc and silver/silver-chloride into an electrolyte solution and measuring the voltage difference. So silver/silver-chloride is just the reference benchmark.

If you wanted instead to use silicon bronze as your reference test cell, then measuring zinc would give you around -750mv in voltage difference. If you wanted to used zinc as your reference test cell, then measuring zinc should yield 0mv difference.

There are probably other factors in the choice of silver/silver-chloride having to do with its chemical purity or stability, but I'm out of my territory on this.

So again, keeping it simple, start by measuring an isolated silicon bronze through-hull. Drop the silver/silver-chloride test cell in the drink, touch the multimeter probe to the through-hull. You should read a voltage similar to what's listed in the galvanic series table, somewhere between 250-300mv negative. Next, connect the through-hull to a wire that terminates at an underwater zinc. Touching the multimeter not to the zinc, but to the through-hull, you should now read the voltage for the zinc, around 1000mv negative. The bronze through-hull will assume the voltage of the zinc. Don't forget that it may take a couple of hours for the metals to polarize before the numbers stabilize.

Repeating the above with a zinc test cell, the isolated through-hull should measure 700-750mv positive and, after attaching the zinc, should measure zero. You may have noticed that the voltage numbers are negative with the silver/silver-chloride half cell, and positive for the zinc half cell. Read the galvanic series table, and you'll see why.

Hope this hasn't gone on too long, and hope it helps.

[rodcross]

Doug, I think you are obsessing on an issue that has been debated for many years and will continue to be, although much of that argueing will be among powerboaters with aluminum outdrives. You've already pointed out something this forum has spent a lot of time on: Too much zinc can be a killer.

I'm with Ian, here. I've started with a little button of zinc. At every haulout, I'll examine the condition of the underwater metal and decide what needs to be done.

If you're going to focus on something that really tears the metal apart, focus on stray current caused by faulty wiring; improper grounding, shore-power leaks, etc.

With too little zinc, my prop may dissolve in 15 years. Improper wiring can cause it to fall off in a day and kill people swimming near the boat, to boot.

[Doug Hoople]

Rod,

Ian's boat is dead simple. Yours might be, too. You don't have problems, then fine, don't fix them.

I was either unlucky or dumb enough to have bought a boat in advanced stages of alkali rot that the surveyor didn't pick up. As mentioned above, instead of just backing plates, the transom log, transom planks and both running gear planks had to be replaced at a cost of 40k.

If you think it's obssessive to want to try to understand the problem and avoid a future visit to the yard, then you've got a lot more money than I.

My boat was built with the best of practices in 1963, and the original bonding system was still working beautifully when I got the boat. To bond or not to bond? That wasn't the question. It already was. I've unbonded since, but I don't think I've figured everything out yet, take the prop shaft-shaft log-prop-prop zinc problem. I know of someone who suffered alkali attack on his shaft log just from his prop zinc.

I don't have stray current problems, I don't have disappearing zincs, I do have an isolation transformer. The electrical practices on the boat are pretty good, if a bit old.

What I also may have is a diver who will zinc the crap out of the propeller and start the shaft log down the road to ruin.

For all I know, I might be fine now. The original problem happened on this boat because the owner overzinced. I don't think I'm doing that. But I'm not going to wait until the boat's being hauled out again to find out.

So forgive me my little obssession, ok? Lots of threads here. Find one you like.

Can you tell me how?

[Doug Hoople]

Sorry for the tirade. I can get pretty excited about this subject.

To sum up, I'm still looking for a way to achieve the Holy Grail, a galvanically balanced wooden boat. This boat has two engines and a fair amount of underwater metals including the usual stainless steel/bronze combos on prop and rudder shafts.

In my definition, a galvanically balanced boat of this type would be one where the millivolt levels of the underwater metals were high enough to prevent the noble metals from corroding and low enough that the sacrificial metals aren't creating undesirable side effects.

In addition, I'd like to not have to wait until the boat's out of the water to observe damage, because that's too late.

If you balance your boat, you can keep your metals AND your wood, and that's what I'd like to do. This thread has helped me a lot to collect my thoughts on the topic.

Thanks to you all.

[Ian G Wright]

Originally posted by Doug Hoople:
Ian, on a more serious note, do you have an engine and a prop shaft and a shaft log and a propeller.I do. Perhaps I cheated a little. From new the engine and gear box are electrically isolated from the prop shaft, which is monel metal not stainless, with the help of a plastic (nylon?) connector/isolator.
There were no anodes at first but when I noticed that the six bolts holding on the rudder/keel heel fitting were a, brass not bronze, and b, had de-zinced to the point that a 3/8 bolt could be bent and broken with no more than finger pressure I relaced the bolts and added a small 2inch zinc. That was 11 years ago,since when no problem.

IanW.

[Doug Hoople]

Ian,

You've got it all right, it seems. We should all have your native talent in corrosion control. Would save us a ton of obsession time and yard time and money :).

Quick question, though. The prop, prop shaft, and shaft log form an isolated galvanic cell, if you're using a standard dripping stuffing box. The ground connection to the engine is irrelevant, unless stray current is involved or your tracking bonding system interactions, which you're not.

Presumably your prop is made of something besides monel, so you'll almost certainly have a prop zinc that hasn't entered the discussion. What size is that, and how's your prop? I'm assuming the shaft log's fine, since you haven't mentioned it.

[Ken Hall]

Galvanically balanced wooden boat=trunnel fastened, no motor? :D

I guess I shouldn't be flip on a serious topic. So, in all seriousness, would the above solution work? Obviously, the application of such a boat mught be limited, but is the principle sound? Or am I missing something relevant?

[ 05-03-2002, 05:18 PM: Message edited by: Ken Hall ]

[Ian G Wright]

Originally posted by Doug Hoople:


Presumably your prop is made of something besides monel, so you'll almost certainly have a prop zinc that hasn't entered the discussion. What size is that, and how's your prop? I'm assuming the shaft log's fine, since you haven't mentioned it.The prop is bronze of some sort, (admiralty brass perhaps?), there is a bronze/rubber cutlass bearing, a bronze stern tube and a proprietory bronze adjustable stuffing box with remote greaser. There is no prop or shaft zinc, but, astonishingly, no sign of corrosion. I put it down to clean living, good luck and no bonding,,,,,

IanW.

[rodcross]

Oh yeah, there's something else, Doug. I have an isolation transformer, too. The problem was, there were nagging electrical gremlins that drained the batteries in the boat over the years and a succession of electricians were called in to diagnose the problem. (The problem was 250Ah batteries with a 35A automotive alternator)

I found a cute 10g (red, if you can believe it) wire leading from boat DC Neg. to the ground pin on the shore power plug. Some bright guy thought he'd solved the problem...'Oh my God!, the boats not connected to ground.'

Check to be sure you didn't have a genius like that working on your boat.

I'm sticking with my guns, here. Eliminate the bonding and start with a minimum of zinc. Check the tips of the props and the flanges of the through-hulls every year. You've already cured the source of wood damage and you'll have a measure of your success when the surveyor pulls some fasteners every three years, or so.

Don't pitch the multimeter

[Doug Hoople]

Ian,

It's a miracle.

[Doug Hoople]

Rod,

Mine was a simple case of overzincing by the ex-owner. Huge mutha zincs attached to the transom log, that were no longer there by the day of the survey, but the damage was done, the framing piece at the bottom of the transom turned to mush, and no way to see it because it was below and behind everything. It was so bad that we worried about the running gear ripping free during the first two weeks we had the boat.

The ex-owner allowed as how the absence of zincs during the survey might have been because he suspected an overzincing problem. He didn't spend a lot of time elaborating.

So no mysteries, really. I get clean readings on all my electrical connections.

Wish I could try Ian's approach, but I think his karma's better than mine in this category. Monel shaft and bronze prop and no prop zinc and no corrosion? Jeez, some guys have all the luck.

Ian, do you actually launch your boat, or does it sit in the yard year-round? :) I'm still astonished.

BTW, Out here on the west coast, we keep 'em in the water for two years at a time, so we don't get the inspection intervals that you do.

[rodcross]

one more thing, Doug, then I'm outta here. Let the surveyor, and especially the insurance company, know that the surveyor is not competant to survey boats. If you're worried about commiting libel, ask around. You'll find the bad wood could have been found with a few taps of a hammer and the removal of a few fasteners. You did insist that the survey included the removal of fasteners, didn't you?

[patriot]

http://nace.org/nace/index.asp
If In Doubt Ask The Experts!

[Ian G Wright]

Originally posted by Doug Hoople:

Ian, do you actually launch your boat, or does it sit in the yard year-round? :) I'm still astonished.,,,,,,,,,,,,,,,,,,,
we don't get the inspection intervals that you do.I do actually sail from time to time, just "British home trade waters, Brest to the Elbe," and haul once a year for maintenance and paint. Around 2000 miles of gentle cruising.

I sugest that you try only to use those zincs that actualy CURE a problem rather than use to many in the hope that this might prevent problems.
It might, if you are lucky, but more often will cause them.
,,,,,,,,,and haul once a year, it gives you a chance to have a good look at the wet part of the boat, plus re-antifoul.

IanW.

[ 05-04-2002, 10:02 AM: Message edited by: Ian G Wright ]

[Doug Hoople]

Ian,

So it's those magic, non-corroding waters on the eastern Atlantic then, is it?

Zincing only when there's a problem is a good rule of thumb in the absence of any more precise a guildeline. Around here, it's the divers who put on the zincs, and they seem to operate on the more-is-better approach.

I've got a diver now who's got his own wood boat. He's willing to work with me to calibrate and track things, and he's interested in finding out himself.

[Concordia..41]

The following is from the book Concordia Yawls The First Fifty Years By Elizabeth Meyer. The section is Surveying The Yawls by G. W. "Giff" Full.

"I would like to speak to the issue of "bonding" at this point. I recently surveyed two Concordia 39s and found numerous rotted areas around through-hull fittings. Immediately I knew that some Expert had bonded the boat. Taking a quick check inside proved this to be the case. Basically no wooden boat needs to be bonded. It is quite detrimental all around. All that should be necessary is one small "tear-drop" anode of zinc affexed to the stern bearing."

This may not be the case with a twin engine power boat loaded with electrical equipment, but one of the old time surveyors should be able to determine the amount of zinc needed. After all this log we are sliding down - they already have the bark off of!

Dave
Sun over the foreyard

[Doug Hoople]

Dave,

Thanks. Your quote seems to confirm the standing wisdom. To bond is surely worse than not to bond.

But that isn't the question.

This thread started with the question, is there another type of anode that can be used instead of zinc to reduce the potential for overprotection?

The goal is balance. One wishes to save one's wood AND one's metal, and one would also wish for a way of determining by the day or the week or the month, rather than the 'haulout-and-look-for-catastrophes' approach.

The 'haulout-and-look-for-catastrophes' seems to be the best we've got at the moment, as does the conventional wisdom not to bond.

I'm still casting about for something a little more precise. May be a fool's errand, but then I'm not claiming to be anything but a fool

I own a wood boat, don't I?

BTW, Nigel Warren, of 'Metal Corrosion in Boats' fame also concludes that something more noble than zinc might be useful in lowering the potentials below zinc's -950mv levels which, in fact, does lead to overprotection. He cites steel as a potential sacrificial anode (p. 138, 2nd edition), and the chart shows that it's good for most typical underwater metals, including manganese bronze(!).

[Ian G Wright]

Originally posted by Doug Hoople:
Dave,

He cites steel as a potential sacrificial anode (p. 138, 2nd edition), and the chart shows that it's good for most typical underwater metals, including manganese bronze(!).There you go then,,,,,,,,, Patience has 2.5 tons of the stuff hanging underneath her,I wondered what it was for and now I know,,,,,,, ;)

IanW.

[Doug Hoople]

Ian,

You are, of course, having me on, no?

[Ian G Wright]

Originally posted by Doug Hoople:
Ian,

You are, of course, having me on, no?What? Me? Not hardly,,,,,,,,
though Patience does have a cast iron/steel ballast keel of 2.2(ish)tons.

IanW.