Stray Current Corrosion Problem

[BigEasy]

Apparently, I am having a new corrosion problem. Diver told me this week that my shaft zinc was completely deteriorated and prop showing evidence of pinking & deterioration. I had the bottom cleaned and shaft zinc replaced six months ago with approximately 20% of old zinc remaining and no evidence of corrosion. There were no modifications to the electrical system and all onboard systems are working normally. Battery charger and refrigerator are usually on. There is NO galvanic isolator nor isolation transformer installed at present. Same marina and slip; same dock neighbors. He advised that I get the boat hauled asap.
So, I spoke to a marine electrician and waiting for him to check it out and correct the problem before I haul out to repair damages. DC system is now completely disconnected and AC power cord unplugged. I purchased a reference Ag/AgCl reference probe and did some investigation this morning.
The hull potential measured 689mV with the AC power cord unplugged and the DC system completely shut down with negative disconnect. Salinity level is 1.9ppt and water temp is 87 degrees. All onboard AC & DC systems checked out OK; however, after plugging the AC power cord in, with the dock AC breaker off, the hull potential dropped from 689mv to 556. With the dock AC breaker on, it dropped further to 520mV. I had both adjacent dock neighbors unplug their AC cords and did not detect any change in the mV values.
What are the possible problems?
I have a ProSafe / Fail Safe 60 amp galvanic isolator, on hand, that I can install; however, I am thinking that an isolation transformer might be a better option.
Any and all opinions appreciated. Once I get this issue under control, I plan to haul out to change folding prop blades, and inspect the shaft and thru hull fittings. First time I have experienced this kind of problem in 10 years of ownership at same dock/slip.

 

[jaysweet]

BigEasy: sorry that I have no expertise to share on this, just a personal anecdote. I've always put 2 zincs on our 1" prop shaft, and for many years, they were fine. Then I had a few years where they were either really eaten up at the end of the season, and two years where one of them was totally missing. Last year, I moved slips from E6 to E14 (so 4 slips further down on the same side), and when my boat was hauled this year, both zincs were almost pristine: better than I'd ever seen them after a season in the water. I actually considered re-using them. My theory is that the powerboat next to my old slip was causing problems. No particular evidence for this, other than a general policy of: whenever possible, blame power boaters... Also, that particular guy being next to me very roughly corresponded to my years of zinc troubles.

Anyway: good luck fixing your problem. (We have an isolation transformer on our Hunter 306, BTW.)

 

[Will Gilmore]

Aged wiring and insulation chafe? Either your boat or the slip's supply? The difficulties in trouble shooting these types of problems are, age and wear are always approaching a critical point, but rarely show up gradually, and few boat owners have a historic baseline for their measurements.

Sounds like you're doing everything you can do. Is it possible the zinc was poorly installed or something got painted that wasn't painted before, or vice versa?

Good luck, I have faith you'll figure it out.

-Will

 

[dlochner]

The connection between your boat and others can be beyond the immediate neighbors, depending on how the marina is wired. All boats that are connected to the same panel will share a ground, if that ground is shared with other panels they too would be connected.

An isolation transformer is the gold standard in protection and like all gold standards you pay for it. Isolation transformers are large, heavy and expensive.

On your boat look at the DC wiring, especially wires that may be in the bilge, like bilge pump wires or shower sump pump wires. Stray AC causes problems, but not with the kind of anode wasting you are having. Bob Olson from Boatzincs.com is a good resource. I've talked to him a few times about various issues.

If it is any consolation, for years I never had any issue with anodes until last summer. Took the boat from fresh water to salt water and anodes have been disappearing like crazy. I'm on my 4th set since last July. Can't wait to get back to fresh water!

 

[BigEasy]

Perhaps I am in denial; however, I really believe the problem is external to the AC / DC system on my boat. My bilge stays bone dry. Checked all the circuits with the reference anode and every thing checks out OK with exception of the AC ground on the power cord. Going to swap out the cord just to rule out a power cord leak. Good idea Dave, I may contact Bob, as I purchased the reference probe and diagnostic manual from his firm.

 

[fstbttms]

If it is any consolation, for years I never had any issue with anodes until last summer. Took the boat from fresh water to salt water and anodes have been disappearing like crazy. I'm on my 4th set since last July. Can't wait to get back to fresh water!

What material are your anodes made from?

 

[JamesG161]

What are the possible problems?

Hook a Drop Zinc to your shaft ASAP. [ I Have a Spare, if you can not get one locally]

Since I have the same AgCl probe as you do, perhaps a PM and phone call Sunday after 5pm might be helpful.
Jim...

 

[BigEasy]

What material are your anodes made from?

Zinc, collar on shaft.

 

[BigEasy]

Since I have the same AgCl probe as you do, perhaps a PM and phone call Sunday after 5pm might be helpful.
Jim...

Thanks Jim, will give you a call.

 

[BigEasy]

Anyway: good luck fixing your problem. (We have an isolation transformer on our Hunter 306, BTW.)

Jay,
What manufacturer and size (watts) isolation transformer do you have on board?

 

[jaysweet]

Jay,
What manufacturer and size (watts) isolation transformer do you have on board?

Surprisingly, it's right here in the Hunter manual. It says I have a "3.6 KVA ISOGUARD (120 v.a.c.)". Googling this gives a hit on Amazon to an unavailable product:

Amazon.com: New Iso-Guard Isolation Transformers Charles Marine Products 93isog26a ISOGUARD 3.6 KVA 30 amp 12V 60Hz : Automotive

Buy New Iso-Guard Isolation Transformers Charles Marine Products 93isog26a ISOGUARD 3.6 KVA 30 amp 12V 60Hz: Engine Parts - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

The picture does look like what I recall seeing in our aft cockpit locker. I'll just say that it didn't keep me from losing zincs when I was next to that (presumably) leaky boat. I do see a couple of Victron isolation transformers at Hodges Marine. One is $530 (2000W) and the other is $842 (3600W). I guess 3.6 KVA = 3600W? Pretty pricey, anyway.

 

[fstbttms]

Zinc, collar on shaft.

You aren't using zinc anodes in freshwater when you're there, are you?

 

[JamesG161]

Thanks Jim, will give you a call.

A few thoughts and questions...

1) Did you do a base line AgCl probe survey on you boat before this Discovery?
2) If you find a Drop Zinc, Drop it at ≈ Depth and adjacent to the old Zinc. Then use AgCl to verify you are Protected again
3) You can use your AgCl probe to detect a stray current in the water. You will need a small piece of copper tubing for this.
4) We have had many lightning storms lately, did you find your shore power breaker tripped recently?
5) Did you follow the AgCl manual for a full boat survey?

Jim...

 

[JamesG161]

@BigEasy and I did share info yesterday.

Sound like someone is causing "Stray Current Corrosion"

He will be checking with the Marina Electrician on a few follow up items, such as...
1) Did Hurricane IDA flood his shore power box?
2) Where is his Finger Pier Grounding rod?
3) Any new boats with metal hulls transit the Marina?

David, may build a "Stray Current Detector"
If he does, hopefully he will send pictures of it.

The Diver did replace his shaft Zinc and his new AgCl probe shows his boat is again protected.

Jim...

 

[jssailem]

I wonder about the water and the choice of "zinc's" vs a more fresh water biased material.

Lake Pontchartrain

Salinity varies from negligible at the northern cusp west of Mandeville up to nearly half the salinity of seawater at its eastern bulge near Interstate 10. Lake Maurepas, a true freshwater lake, connects with Lake Pontchartrain on the west via Pass Manchac. The Industrial Canal connects the Mississippi River with the lake at New Orleans. Bonnet Carré Spillway diverts water from the Mississippi into the lake during times of river flooding.​

When I looked up the location of Slidell which is near the eastern bulge of the lake, is became evident the boat is in that area with "half the salinity of seawater". A true zinc makes sense.

Hope the @BigEasy can identify the source and put the trouble to rest. Will be interested to see the capabilities of a floating divining rod.

 

[fstbttms]

When I looked up the location of Slidell which is near the eastern bulge of the lake, is became evident the boat is in that area with "half the salinity of seawater". A true zinc makes sense.

This is why I asked the OP a few posts ago if he was using zinc anodes in freshwater. Zinc anodes should not be used in fresh or slightly brackish water. Magnesium and aluminum anodes should be used in these conditions.

BTW- I wish people would stop saying "zincs" as a generic term when referring to anodes. It frequently confuses the issue now that magnesium and especially aluminum anodes are in common use.

 

[JamesG161]

Salinity level is 1.9ppt and water temp is 87 degrees.

Although the marina can go almost fresh water, it seldom does that.

I have the same situation @fstbttms , Zinc does still work fine.

It only takes a small amount of Salts to conduct the Galvanic current.

The main reason for Zinc use here is the Gulf of Mexico is short ride away.

Never mix sacrificial anodes types.
Jim...

 

[Will Gilmore]

While visiting a marina in Scotland, I saw something like this just under the water at the back of a boat slip.

It looked like it had been there for a while and I wondered what effect it might have on the galvanic action of a boat that was backed into that slip. "I've been in this slip for ten years and never had a problem, but for the last two years, I just can't figure out what's changed!?"

-Will

 

[fstbttms]

While visiting a marina in Scotland, I saw something like this just under the water at the back of a boat slip.
View attachment 206219
It looked like it had been there for a while and I wondered what effect it might have on the galvanic action of a boat that was backed into that slip. "I've been in this slip for ten years and never had a problem, but for the last two years, I just can't figure out what's changed!?"

-Will

Galvanic corrosion takes place when two dissimilar metals are in contact with each other while immersed in an electrolyte. Since the shopping cart is not in contact with any metal on any nearby boats, galvanic corrosion due to its presence is not going to be possible.

 

[BigEasy]

Zinc anodes should not be used in fresh or slightly brackish water. Magnesium and aluminum anodes should be used in these conditions.

Have been using zinc anodes for past 10 years w/o a problem. Now in past 6 months, zinc is totally depleted with corrosion on the prop. I am not opposed to trying a different anode; however, that is not the problem. Aggressive galvanic or stray current is the issue. In process of finding the source. I suspect it is from another boat. Guy across from my dock and 3 boats down (appx 40 ft) had electrical problem recently, that originated in his boat & caused severe corrosion to his shaft & prop. I suspect that was the source of my problem. Will keep investigating & have electrician check it out. Meanwhile I will install a galvanic isolator that I already have on hand. Going to check with marine electrician; however, I am sure that everyone around here is using zinc anodes.