Stray Current Corrosion?

[ERTF]

I have discovered what seems to be a bad case of stray current corrosion. I'm hoping you guys can help me narrow down the haystack a bit.

My fiberglass trawler with twin inboards is moored over 1/4 mile from shore or other boats. It has zinc annodes on the shafts. My fiberglass skiff dangles 15ft back on a painter line and the outboard is always kept up. The outboard has aluminum annodes.

I discovered a series of metal decay all around the same time. An air con thru hull became extremely brittle (note: the bonding had been accidently disconnected). I was anchored for many months and then discovered 20 feet of chain closest to the boat had dissolved dangerously thin. When cleaning my bottom i noticed 1 half dime sized knick missing from a bronze prop. And also when changing the impelled on my skiff outboard i found bad corrosion to the aluminum of the lower unit near the stainless shaft. I put new zincs on my trawler shafts and in couple months 1 is 1/3rd dissolved (my zincs used to last a really long time), and its active enough that it is the one place underwater that has no growth. I even saw very faint bubbling from that zinc. Other zinc is fine.

I have purchased a silver reference annode and am trying to learn about all this. Obviously this is a very complicated needle in a hay stack situation. What i'm hoping is you guys can help me eliminate a few possiblities.

1. This is surely stray current corrosion, correct?
2. Would installing a galvanic isolator be useless since I am not plugged into shore power with other boats?
3. I read that A/C current doesn't cause this type of damage because it constantly alternates? I would like confirmation on this because otherwise I could install a standalone inverter just for my chest freezer (and turn off all the rest of A/C power).
4. What can be inferred that both my skiff and trawler are having corrosion issues? The skiff has zero thru hulls. The only metal connections to water are the outboard bracket, a transducer, and possibly 2 bilge pumps that are mounted on a flush self bailing deck (so pumps are mounted on a damp surface but rarely submerged).

Sorry thats alot, but also about as concise as I could get to include important details.

 

[Mac2]

Couple questions. Are you in fresh, or salt water? How often are you running the generator? Definitely is stray current. Very strange it would just start happening. The a/c bonding seems high on the suspect list. Were there other systems hooked to the ac thru hull bonding?

 

[ERTF]

Couple questions. Are you in fresh, or salt water? How often are you running the generator? Definitely is stray current. Very strange it would just start happening. The a/c bonding seems high on the suspect list. Were there other systems hooked to the ac thru hull bonding?

Saltwater.

When you say a/c, you are speaking of air con (not alternating current), correct? One thing I can say is i obviously still have the stray current (no growth on port zinc) yet my air con hasnt been run in months. The air con bonding i'm reffering to was just the bonding wire screwed to the strainer housing and got disconnected when clearing it. It was actually just the hose fittings on the strainer that failed (not thru hull).

I wouldnt say It couldnt be air con related, but i have corrosion in many different places, so im not sure what would make that the top prospect. For whatever its worth, the air con pump is 12v.

 

[ERTF]

How often are you running the generator?

I have a portable gas generator, that was getting maybe 6 hrs a week in the late summer. But now basically 0 hrs a week. I think the port zinc (but not starboard zinc) still rapidly deteriorating is an important detail.

 

[ERTF]

A couple more questions:
5. If i install on/off switches on my skiff house & start batteries, and turn off when not in use, will that protect my outboard?
6. I was already planning to install a triple danforth mooring. But i don't want to put new chain in the water til i know it won't get eaten up. Since the mooring will be attached to the boat by a nylon pendant (unlike my all chain anchor rode), will the mooring chain be immune from the stray current eating away its zinc?

 

[MVCalypso]

Since you've already ordered a 1/2 cell, I highly recommend this workbook to learn how to figure out what's going on: Boat & Yacht Corrosion Control Manual
It gives good practical info that leads you step by step diagnosing your boat's grounding & bonding systems.
You don't need the book's "special meter" just a good DMV (preferably one that has enough digits to show millivolts) and you'll want to make up some really long (like 15-20 ft each) test leads to reach fittings all around the boat.

 

[Mac2]

Air con is what I was referring to. On my last boat the green ground cable went to a thru hull and a second green wire went from that thru hull to the next thru hull. I would install on/off switches regardless. There are more knowledgeable people on this forum, but the stray current is eating your chain-what is the distance from the skiff to the anchor chain? As a precaution, Ive seen zincs with a clamp/wire you could attach to the anchor chain and skiff until you figure things out. Good luck.

 

[ERTF]

Skiff outboard to trawler anchor chain = 75ft

 

[Cigatoo]

Did you check the bilge for DC wires hanging in the bilge water? Bilge pump wires are a common source of stray current.

 

[ERTF]

Did you check the bilge for DC wires hanging in the bilge water? Bilge pump wires are a common source of stray current.

I was actually wo dering if there is a good way to check the bikge water for voltage? The reference annode says its important not to get oil on it. Can i just dangle a regular multimeter probe in the bilge water?

 

[ERTF]

If i dangle a positive multimeter probe in trawler bilge water with negative clamped to 12v ground, i get about .25v dc. However when i turn off all battery switches the .25v reading does not change, so i'm not sure what to make of that.

These are the type of further confusions i expected as I get granular. Which is why i was hoping for answers to some of my original questions first.

 

[SteveK]

1. This is surely stray current corrosion, correct? sounds like
2. Would installing a galvanic isolator be useless since I am not plugged into shore power with other boats? NO
3. I read that A/C current doesn't cause this type of damage because it constantly alternates? I would like confirmation on this because otherwise I could install a standalone inverter just for my chest freezer (and turn off all the rest of A/C power). You may convince me once we find your cause
4. What can be inferred that both my skiff and trawler are having corrosion issues? The skiff has zero thru hulls. The only metal connections to water are the outboard bracket, a transducer, and possibly 2 bilge pumps that are mounted on a flush self bailing deck (so pumps are mounted on a damp surface but rarely submerged).

#4 Sounds like the skiff has a battery, how is it kept charged?
Inverters can be a cause.
@Steve DAntonio needs to offer an opinion

 

[ERTF]

#4 Sounds like the skiff has a battery, how is it kept charged?
Inverters can be a cause.
@Steve DAntonio needs to offer an opinion

It has an on board charger that i plug in to ext cord from trawler inverted 120v only occasionaly until skiff house is topped. But for example yesterday when i saw my trawler shaft zinc faintly fizzing, the skiff was not hooked up.

 

[tiltrider1]

To have this much stray current corrosion something must be operating pretty much continuously. With the boat on the hook what is running? How are you recharging?

 

[ERTF]

To have this much stray current corrosion something must be operating pretty much continuously. With the boat on the hook what is running? How are you recharging?

I have 1600w of solar.

i have an old xantrex inverter that is on 24/7. 120v chest freezer. 12v fridge.

This is why I was inquiring specifically about 120v because that is something I could work around. Especially because 120v confuses me, so it would be nice to just eliminate that possibikity. But most what I have read says its usually DC current.

Again I am hesitant to get too granular pondering the numerous little gremlins on a boat, but in the last year I did have problems with several electric tooth brushes charged on both 12v & 120v. Never had a problem with them for several years previously. Finally gave up buying new ones. No obvious problems with other devices plugged in to outlets.

 

[ERTF]

Basically i'm assuming im gonna have to devote the next few months of my life to finding a needle in a haystack -- something i miswired or a wire/connection that has gone bad.

But if there are any semi-simple things I can try right now to maybe not find the exact source, but at least stop expensive parts on my boat from being eaten, please let me know.

 

[SteveK]

It has an on board charger that i plug in to ext cord from trawler inverted 120v only occasionaly until skiff house is topped. But for example yesterday when i saw my trawler shaft zinc faintly fizzing, the skiff was not hooked up.

OK, if the extension cord was not hooked up at the main boat then rule the skiff out. So no wires from main boat to skiff.
The inverter then is my next thought and you just said it is on 24/7.
Is the inverter chassis ground connected to the bonding system? Is the ground wire of the A/C 120V connected to the bonding system?

 

[mvweebles]

Steve D has a couple decent articles on stray current that are definitely worth a read. Apologies for not including rheir links as I usually do but they are all in PDF and he's protective of them so difficult to link. Google "steve dantonio stray current" and you'll find several good articles. Tougher than I would have thought. Solar with DC certainly would be suspect.

Gone with the Wynn's YouTubers took possession of a brand new HH Sailing Cat almost a year ago. They've been struggling with severe stray current issues - degraded their anchor chain too which is amazing. Worth a view. HH published a rebuttal a few days later that tracked the problem to an electric cooling pump off one of the engines (if I remember correctly)

Peter

 

[ERTF]

OK, if the extension cord was not hooked up at the main boat then rule the skiff out. So no wires from main boat to skiff.
The inverter then is my next thought and you just said it is on 24/7.
Is the inverter chassis ground connected to the bonding system? Is the ground wire of the A/C 120V connected to the bonding system?

The inverter chasis ground routes to a bus bar with about 15 green 120v wires in the electrical closet. It's impossible to visually trace if any of those wires route directly to bonding wires. I tried testing resistance from that bus bar to nearby thru hull that has bonding wires on it and got about 94 ohms. I'm not confident in the bonding connections on this 35 year old boat as the wires run all over in damp, difficult to access locations and I've just left them as they were when i got the boat 6 years ago.

If there is a better way to test, please advise. My grasp of AC current is minimal.

 

[ERTF]

The corrosion reference annode instructs to connect the other probe to the "vessels main dc ground". If I connect to the shunt for the house loads, i get some milivolt changes when flipping breakers. But if I connect to the start battery negative (which is also connected to the house bank negative, as well as engine grounds), I get basically no fluctuations when flipping breakers. I also get basically no fluctuation when connecting directly to the engine.

Which is the preferred "main dc ground" point? It would seem to me that they all should lead to the same place?

 

[Pascall]

You say that the stray current was not present before, has anything changed in the installation of the ship?
Has something been added, pump or something like that?
If the problem never occurred, something must have changed that is causing it.

 

[SteveK]

An air con thru hull became extremely brittle (note: the bonding had been accidently disconnected)

Was this thru hull wet with bilge water?
If yes then replace all float switches as they have DC+ in same water.

 

[ERTF]

Was this thru hull wet with bilge water?
If yes then replace all float switches as they have DC+ in same water.

I actually misspoke. It was the hose barbs on both sides of the a/c strainer. So no they weren't in bilge water

 

[cafesport]

Sounds like this is more of a recent condition and nothing long term .

Quite a few things could be going on here, so it might be helpful to also list any recent electrical projects that might have been started.

Also I think 95 ohms is too much resistance for a bonding wire and the .25v in bilge water has to be coming from a dc source. You may have to start disconnecting battery cables to find it. Sounds like your shaft zincs are doing most of the work and your bonding connections aren't doing a great job anymore. Zinc eaten off the chain may need to be replaced with a zinc fish until you get things sorted out.

The outboard corrosion could be a separate and unrelated galvanic problem stemming from a lack of service. A neighbor recently encounterd a similar problem with a dinghy he stored on his boat deck under a cover. He only serviced it this week to sell it since it had not been used for in 4 years.

 

[Taras]

what I would do is disconnect all batteries in the boat by pulling the battery cables off the batteries and then re-check for that voltage in the bilge. I'm thinking your bilge pumps maybe connected directly to the battery and not on a switched circuit.

 

[Steve DAntonio]

While tracking down the source of stray current may seem daunting, in fact it is not, it can be done methodically and should not take very long.

Keep the following in mind...

1. Stray current corrosion is nearly always DC in nature.
2. Stray current is nearly always derived from the vessel's battery. If you shut the battery disconnect off and you still are measuring stray current, make sure nothing is bypassing the switch. Bilge pumps and float switches are often the source of stray current, and many are wired to the line side of the switch, i.e., between the switch and the battery.
3. In addition to the reference cell test, a multimeter can also be used to measure current flow from the battery. For very low current, the clamp meter may not be sensitive enough, you may have to do it the old fashioned way, placing the meter's leads in series with the load.

Based on what you have shared, I cannot see how the tender would be affected by stray current originating on the mouther ship, there is no way for the current path to be completed, the water is only the ion half of the circuit, you need an electron path as well, which is wire or metal.

The reference cell test should help you narrow this down, and it's easier when you are on a mooring. During the test you switch on-off every breaker/load and look for changes in the reading. That's where I would start.

These articles may be helpful.

Feature Article: Unraveling the Corrosion Mystery – Editorial: Technical Competence and Job Skills | Steve D'Antonio Marine Consulting

Getting to Know Your Vessel – Bonding Systems and Corrosion Prevention | Steve D'Antonio Marine Consulting

Reference Cell Testing; Know Thy Corrosion Protection Level – Editorial: Old vs. New | Steve D'Antonio Marine Consulting

Feature: Busting the “Hot Marina” Myth – Editorial: A Blue-Collar Resurgence | Steve D'Antonio Marine Consulting

 

[CharlieJ]

OP
The four references provided by SDA #26 will provide a graduate level understanding of the corrosion processes at play. To maximize your enjoyable time on your boat and to minimize your frustration as you learn the black art, I suggest that you read SDA’s contribution and then work side by side with an ABYC Certified Corrosion Tech as you track down and correct the problem. It will be time well spent and you will be the smartest guy in the harbor on the subject!

 

[Bmarler]

I used a reference cell to find a very similar issue. My shaft zincs were going at an alarming rate.
My culprit was a battery maintainer I was trying out for the start bank.
Point is, the disconnecting of anything a.c. powered and watching for change in the reference cell is a great place to start.

 

[CharlieJ]

@Bmarier #28

Point is, the disconnecting of anything a.c. powered and watching for change in the reference cell is a great place to start.

You do mean DC, don’t you. AC rarely causes stray current corrosion.

 

[Steve DAntonio]

@Bmarier #28

You do mean DC, don’t you. AC rarely causes stray current corrosion.

Right but I think he means in this case it was an AC powered battery charger.

In this case it sounds like the vessel is on a mooring, so nothing AC unless it's running on the inverter, but certainly not a battery charger.