Corrosion around thru hulls

[DLETF]

The bottom of my trawler was painted with Interlux Micron CSC antifouling paint in June 2019. The prior bottom paint was also Interlux Micron CSC paint. The cuprous oxide content of the paint is 37%.
The attached photos taken on January 7, 2020 and show some corrosion around 2 of the thru hull fittings. No corrosion was noted when the bottom was inspected in September 2019.
A galvanic isolator is installed in the boat and the boat is slipped in a marina. The thru hull fittings are all bonded together with a green wire.
I have the following questions/comments:
1. Am I correct to assume that the corrosion shown in the photos is related to electrolytic corrosion and not galvanic corrosion?
2. If the corrosion is the result of electrolytic corrosion, am I correct to assume that removing the entire bonding green wires should solve this issue?
Thanks

 

[diver dave]

Ouch; they look pink, with loss of zinc. Replace and isolate, I tend to think. How are props?

 

[Xsbank]

Those through-hulls are toast, methinks. What does the “green wire” attach to? Do you have a transom zinc? It sounds like you need to go all through that system and check connections. Remember that stainless steel is the WORST metal for conductivity and you will need to check all connections very carefully. The galvanic isolator is an excellent device but it needs careful installation. Like was said earlier, what do your wheel(s) look like?

 

[Ken E.]

Assuming you have a transom zinc, with a meter check to see if you have continuity between the zinc and your pink through-hulls. I suspect you don't, meaning you're not bonded and your zinc isn't protecting your underwater metal.

 

[C lectric]

I am going to suggest that you seriously consider getting an experienced in corrosion problems marine electrician to check your boat.

You need to find out why those through hull have done that. It may be the wiring is disconnected from the zincs, It could be your boat has electrical leakage into the water and is using the through hulls to get back to ground, it could be there is a problem in the marina wiring.

I have seen the bloom of the bottom paint before but usually it is the result of overzincing which raises the voltage too high and burns off the paint immediate to the through hull. But that usually will not destroy the through hulls.

However, it looks to me, and others, like the through hulls have actually suffered and good marine bronze [I am assuming] should not do that unless there is a serious problem.

If that same thing is going on with the other water immersed metals you could have a serious problem.

 

[SoWhat]

I think photos of your shaft and transom zincs might help Dx galvanic issues. I would start with basic continuity checks. Then proceed to check for galvanic activity with silver reference cell.(http://assets.fluke.com/appnotes/electricalpower/B0269b_u.pdf

Stray current is a bit harder to measure. (https://www.powerandmotoryacht.com/uncategorized/stray-current-test)
Note that stray currents can come from neighboring boats.

I use Micron CF. Seems to last the season about the same as CFC. No copper so a lot less toxic.

 

[Comodave]

I wish everyone would stop using zinc as a reference to anodes. All sacrificial anodes are not zinc. It adds confusion to the conversation. When someone says zinc I always wonder if they are in fresh, brackish or salt water. A member posted here the other day that his zincs had not wasted at all over the years. His boat is in fresh water so I wonder if he is really using zincs or just using zincs as a catch all and he is really using the proper aluminum or magnesium anode. If people would use the proper term, anode, it or even better aluminum anode, magnesium anode or zinc anode it would remove any or all confusion.

 

[Nickair]

Good job taking this seriously. Many boats have gone down due to through hull fittings giving way. Heard reports of through hull fittings looking ok visually, although actually compromised internal. What material are your through hull fittings made of. Marine bronze is the good stuff. Keep us posted on this one. May be as simple as the boats next to you running chargers and other.

 

[Cigatoo]

The bottom of my trawler was painted with Interlux Micron CSC antifouling paint in June 2019. The prior bottom paint was also Interlux Micron CSC paint. The cuprous oxide content of the paint is 37%.
The attached photos taken on January 7, 2020 and show some corrosion around 2 of the thru hull fittings. No corrosion was noted when the bottom was inspected in September 2019.
A galvanic isolator is installed in the boat and the boat is slipped in a marina. The thru hull fittings are all bonded together with a green wire.
I have the following questions/comments:
1. Am I correct to assume that the corrosion shown in the photos is related to electrolytic corrosion and not galvanic corrosion? What does your anode look like?
2. If the corrosion is the result of electrolytic corrosion, am I correct to assume that removing the entire bonding green wires should solve this issue?
Thanks

Same marina as before the corrosion? Same boats in the neighborhood? Could be an external source. Just a thought.
What’s the anode look like?

 

[OldDan1943]

Get a qualified marine electrician to your boat to check your bonding system.
IF he finds a fault, correct it and take your boat to a yard to replace ALL your through hulls. IF he doesn't find a fault, get another more qualified marine electrician.
Something is going on and you need to identify it and correct it. The other alternative is to ignore it and get a BIG supply of wooden damage control plugs.

 

[kchace]

Same as everyone else has noted, it looks to me like excessive galvanic current has been going through those two fittings. Even good marine bronze can’t put up with that forever. You mention that all thru hulls are connected with a green wire, but it’s a horrible environment for wiring - the green anode wire must be checked by verifying LESS THAN 1 ohm of resistance between every bonded fitting AND all anodes and ground to the boat. I would also verify that your galvanic isolator is properly connected and functional.

Ken

 

[DLETF]

reply

Ouch; they look pink, with loss of zinc. Replace and isolate, I tend to think. How are props?

diver dave: the props do not show any evidence of corrosion

 

[DLETF]

reply

Those through-hulls are toast, methinks. What does the “green wire” attach to? Do you have a transom zinc? It sounds like you need to go all through that system and check connections. Remember that stainless steel is the WORST metal for conductivity and you will need to check all connections very carefully. The galvanic isolator is an excellent device but it needs careful installation. Like was said earlier, what do your wheel(s) look like?

Xsbank: the green bonding wire is attached to all the thru hull fittings. The green bonding wire also connects to the following:
1. An aluminum anode mounted on the outside of the hull near the transom
2. A Dynaplate bronze ground plate mounted on the outside of the starboard hull
3. One engine motor mount
I do not understand the question about:"... what do your wheel(s) look like?"
Thanks

 

[DLETF]

reply

Good job taking this seriously. Many boats have gone down due to through hull fittings giving way. Heard reports of through hull fittings looking ok visually, although actually compromised internal. What material are your through hull fittings made of. Marine bronze is the good stuff. Keep us posted on this one. May be as simple as the boats next to you running chargers and other.

Nickair: I assume the thru hull fittings are made of silicon bronze but do not have an invoice to confirm that opinion.
Thanks

 

[DLETF]

reply

Same marina as before the corrosion? Same boats in the neighborhood? Could be an external source. Just a thought.
What’s the anode look like?

Cigatoo: yes; same marina with same boats in the neighborhood. The aluminum prop shaft anode lasted 9 months and the aluminum transom anode was installed 12 months ago and is not ready to be changed.
Thanks

 

[DLETF]

reply

Get a qualified marine electrician to your boat to check your bonding system.
IF he finds a fault, correct it and take your boat to a yard to replace ALL your through hulls. IF he doesn't find a fault, get another more qualified marine electrician.
Something is going on and you need to identify it and correct it. The other alternative is to ignore it and get a BIG supply of wooden damage control plugs.

OldDan1943: I do have a supply of wooden damage control plugs but do not plan to ignore the issue. I have contacted a local marine electrician.
Thanks

 

[diver dave]

Nickair: I assume the thru hull fittings are made of silicon bronze but do not have an invoice to confirm that opinion.

Thanks

Bronze fasteners are usually 95% Cu balance silicon. Typical thru hulls closer to half Cu and around 30% Zn. (From memory) when the Zn leaches out, u will see the remaining pink Cu, ending up grainy and weak.

 

[Duvie]

My feeling is that if a piece of under water metal will NEVER connect to another piece of underwater metal then do not bond. By bonding an electrically isolated piece of under water metal you have introduced the possibility of both galvanic or electrolytic corrosion by intentionally bonding it to other underwater metals or power sources that may have leakage issues.

In a case such as a rudder I would protect with an anode directly or by bonding. The rudder could be connected to other metals through hydraulic lines, cables and such. In the case of a through hull fitting I would not bond. The likelihood of it connecting to other under water metals is minimal.

So I would isolate your through hulls from the bonding system but you should still investigate the cause of the corrosion. This is because everything connected to that bonding system may be subject to the same corrosion issue.

I know that many say bond, bond, bond but I disagree.

 

[kchace]

the aluminum transom anode was installed 12 months ago and is not ready to be changed.
Thanks

This is not a good sign all by itself.

And just because bonding wires go to certain components, their connections can go bad over time. As I said before, all connections must be verified to be under 1 ohm.

 

[Simi 60]

Get a qualified marine electrician to your boat to check your bonding system.
IF he finds a fault, correct it and take your boat to a yard to replace ALL your through hulls. IF he doesn't find a fault, get another more qualified marine electrician.

There's qualified marine electricians then there is those who actually know what they are doing.

Electrickery is a very black art.

 

[Cigatoo]

Cigatoo: yes; same marina with same boats in the neighborhood. The aluminum prop shaft anode lasted 9 months and the aluminum transom anode was installed 12 months ago and is not ready to be changed.
Thanks

As others have recommended check the connections / resistance on the transom anode. If that checks out as being within spec (1 ohm or less) get an electrician involved. If not find the open connection. Your prop anode is working as it should. The transom anode is not. My 2 cents.

 

[Cigatoo]

Also check on any dc leakage into your bilge water. Any wire that is in the water should not be. Check to see if any neighbors are having same problem as you OR are chewing up anodes at a rapid pace.

 

[diver dave]

Note that using the typical DMM on a bonding system with the boat in the water is problematic. At least it is if you use Ohm function. As you measure across a problem connection point, the galvanic derived V will cause measurement issues. One sign of this is seeing negative resistance values.
So, use the low volts function.
On a boat on the hard, use low Ohms function.

 

[rgano]

Get a qualified marine electrician to your boat to check your bonding system.
IF he finds a fault, correct it and take your boat to a yard to replace ALL your through hulls. IF he doesn't find a fault, get another more qualified marine electrician.
Something is going on and you need to identify it and correct it. The other alternative is to ignore it and get a BIG supply of wooden damage control plugs.

Through hulls can be tested for general integrity with a simple hammer test. If tapping the thing with a hammer results in a thud rather than a tink, well you probably have and issue.

 

[timb7734]

Replace the through hulls and hire a marine architect not just a surveyor, also check with the boat manufacturer if still around.

 

[catalinajack]

Had that happen during a survey on a boat on which we made an offer. The props went thunk instead of ting. After having had to shut down an engine for overheating on the three-mile trip to the haul-out, we stopped any further survey work and travelled back to the boat's marina. On the way back, the other engine overheated. Walked away, no, ran away.

Through hulls can be tested for general integrity with a simple hammer test. If tapping the thing with a hammer results in a thud rather than a tink, well you probably have and issue.

 

[Steve DAntonio]

Are the thru-hulls themselves actually corroded? It's difficult to tell if this is corrosion or an accumulation. If the latter, and the paint is lifting/failing around bonded underwater hardware, this is often indicative of over-protection, rather than under-protection. When over-protected, an alkaline solution is produced around cathodes, the protected metals, which is not harmful to the metal, but it does attack the paint. Therefore, confirm this first.

No continuity testing between underwater metals and the bonding system or anodes can be carried out while the vessel is afloat. However, a reference electrode test can be performed and it will tell you if underwater, bonded metals are properly protected. More on that testing here https://stevedmarineconsulting.com/wp-content/uploads/2019/10/Anodes.pdf

I would also recommend confirming your galvanic isolator is wired and working properly, that can be done using a multimeter, which you'll need for the ref electrode test as well.

If the thru-hulls are corroding, and they are actually pink, that is indicative of dezincification, a phenomenon that can occur when brass is used in a raw water application, a practice that should be strictly avoided. For more on this see https://stevedmarineconsulting.com/know-your-underwater-alloys/

This may be a compound case of brass dezincification as well as inadequate cathodic protection, i.e. the bonding system has high resistance and thus the anodes are no longer protecting the brass, something they should not be expected to do in any event. The maximum allowable resistance between any protected metal and its anode is 1 ohm, a low threshold indeed, and one that's easily exceeded with just a little corrosion or a loose bonding wire terminal.

Electrolytic corrosion, commonly called stray current corrosion, is typically a rapid process, it can cause significant damage in days, even hours. It nearly always originates aboard the vessel suffering the corrosion, from the vessel's own DC system/batteries, and not shore power. There are exceptions, but they are rare.

In all corrosion analysis cases, there should be absolutely no guess-work what so ever, don't take any action until all the facts are know. I see this approach all too often, even by otherwise well-meaning pro's who don't understand the corrosion process.

Ultimately, unless you are experienced in corrosion analysis, your best bet is to hire an ABYC Certified Corrosion Technician (a certified electrician is good, but that certification does not include advanced corrosion analysis). You can find these on ABYC's website, www.abycinc.org

Once again, don't let anyone guess, ask for definitive results.

 

[SteveZ]

thru hull corrosion

The attached article provides a good discussion on this topic.

If the fitting itself is pitted or pink, the cause is most likely a DC leak into the bonding system.

If the fitting is good, but the bottom paint around it is distorted (halo) the problem is most likely an AC stray current leak OR too much sacrificial anode.

The presence of stray current can be found by dropping a silver chloride half cell into the water and connecting it to a DVOM set to DC voltage. By energizing each individual circuit you can identify which source causes a change in the voltage. The baseline should be around 700 mV, assuming you have some sacrificial anodes tied into the bonding system.

Steve Zimmerman

 

[Boatcrazy]

following

 

[rgano]

I would check the integrity of the bonding system by first disconnecting the entire harness from all items and then running continuity tests from end to end and then from every branch to one end or the other. Then ensure solid reconnection. NOW you know it is good. Then on to the silver-silver chloride cell testing, etc, etc.