Now if that shopping cart was plugged into Shorepower, that would be a problem!!
Stray Current Corrosion Problem
- Thread starter BigEasy
- Start date
Corrosion issues can also be caused by a stray DC current in the water as well. DC will actually cause a much faster deterioration than AC will. I know it sounds strange as most of us are worried about the effects of the stray AC as that is far more common. Since this is a new thing to your boat and all things have been roughly the same for the last 10 years, do you have any new dock neighbors within a 100 feet or so? here is why I ask. DC is nothing more than a difference in potential. Most of us (if not all by this point) will ground our engines and thus external metals to a negative potential. But that is ONLY a negative potential to our own boats. If there is someone nearby that has a more negative potential than you do, your boat becomes the positive or anode in the circuit, and thus the corrosion effects are much more pronounced. The ability for electricity to find its way back to the source is amazing and confusing at the same time.
I know this isn’t a boat thing but this is the EXACT same thing you are potentially dealing with (pun is intended). I have a utility trailer that gets a single taillight on when 2 things happen. One the harness is plugged into my truck and 2 the ball is on the receiver of the trailer. This somehow makes a circuit through the trailers right tail light. I check the trailer and everything seems right. Same with the truck. I detect no obvious stray voltages on the truck, and there is no battery in the trailer so the power is coming from the chassis of the truck and feeding back through one of the lighting circuits.
I know this isn’t a boat thing but this is the EXACT same thing you are potentially dealing with (pun is intended). I have a utility trailer that gets a single taillight on when 2 things happen. One the harness is plugged into my truck and 2 the ball is on the receiver of the trailer. This somehow makes a circuit through the trailers right tail light. I check the trailer and everything seems right. Same with the truck. I detect no obvious stray voltages on the truck, and there is no battery in the trailer so the power is coming from the chassis of the truck and feeding back through one of the lighting circuits.
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I was reading too fast and missed the guy about 40’ away with an issue. Serves me right. Lol. But I would venture a guess he is your issue. I would bet if he dropped the AC and disconnected his batteries he would stop your, and everyone else’s issue. Or you could run a cable from his negative to yours so you are at the same potential!
He moved to another dock about 3 weeks ago.I was reading too fast and missed the guy about 40’ away with an issue. Serves me right. Lol. But I would venture a guess he is your issue. I would bet if he dropped the AC and disconnected his batteries he would stop your, and everyone else’s issue. Or you could run a cable from his negative to yours so you are at the same potential!
But the thing that concerns me is that I am getting a baseline hull potential of 690 mV with AC cord unplugged and DC disconnected and it drops to about 530mV when I plug in the AC cord. Also going to use @JamesG161 detector to check around dock and adjacent boats.
My thinking is, the shopping cart, partially buried in the muddy bottom, is also grounded and may represent a potential because of the electrolyte. If another boat forty feet away can steal electrons from your brass prop, why wouldn't a steel mesh embedded in the ground beneath you that's only a foot or so away? Is there metal on metal contact between the boat three slips over? It all depends on the size of the electrical pressure difference.
-Will
No, it doesn’t. What you just described may (or may not) be a source of some form of corrosion but whatever it is, it is not the source of galvanic corrosion.
This was also part of David and my discussion on Stray Currents in his Marina.
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Well almost true.
A Classic example of AC Stray Currents was a Metal Hull Boat with a on board generator that was running at 50 Hz vs 60 Hz.
Grounding that boat to yours.... Ouch!
The Manual on the AgCl probe mentions that, and I had to buy a Hertz frequency detector, when I did my first boat AgCl survey.
When I first started up my whole house generator, I did use that Hertz meter to adjust the generator to 60 Hz exactly or my old Fluorescent Bulbs would show a "Wave" till then.
Jim...
PS: Do not forget to do your baseline Survey @BigEasy when all is cleared up.
Yes, I plan to obtain another baseline measurement when everything is squared away and recheck Hull potential periodically. The Ag/AgCl probe can be used on a regular basis to check on the adequacy of your anodes to ensure that your underwater gear is sufficiently protected. If the hull potential measurement drops to an “unsafe” level, the anodes have depleted to the extent that it is time to replace them.
Made significant progress today addressing the corrosion problem. Completed installation of a Pro Mariner galvanic isolator and added a 6 lb zinc drop anode attached to the SS prop shaft. Base line (corrected) hull potential reading was initially 640 mV with one donut zinc anode attached between the cutlass bearing & prop hub and ACpower cord disconnected. Hull potential dropped to 520 mV with power cord connected and shore power on. After installing the GI and drop anode today, the Hull potential increased to 820mV with insignificant difference with power cord unplugged vs plugged in and power on. The galvanic isolator and additional anode has made a huge difference. I feel confident that my underwater gear is now adequately protected. Finally was able to talk to boat owner across and 2 boats up from my dock who had severe corrosion problems ( shaft, thru hulls, & prop). Apparently the electrician discovered significant problems such as old battery charger with common & ground wired together as well as other problems. That stray current could have been source of my problem. Still researching methods for testing stray current & galvanic corrosion from within my boat; however, haven’t found anything yet. Unable to get a marine electrician to inspect thus far.
Two big things that I have learned so far, unless you test your boat’s Hull potential using an Ag/AgCl probe and digital multi meter you don’t have a clue as to whether your underwater gear is adequately protected and don’t know when it is time to replace the anode(s). Also, after reading many articles and watching videos about corrosion, a galvanic isolator is an absolute minimum necessity, better yet, an isolation transformer, if you are in a marina environment connected to shore power.
This is going to be an expensive ordeal including haulout, and replacement of folding prop blades. Hopefully there was no damage to the prop hub, shaft, and thru Hull fittings.
Two big things that I have learned so far, unless you test your boat’s Hull potential using an Ag/AgCl probe and digital multi meter you don’t have a clue as to whether your underwater gear is adequately protected and don’t know when it is time to replace the anode(s). Also, after reading many articles and watching videos about corrosion, a galvanic isolator is an absolute minimum necessity, better yet, an isolation transformer, if you are in a marina environment connected to shore power.
This is going to be an expensive ordeal including haulout, and replacement of folding prop blades. Hopefully there was no damage to the prop hub, shaft, and thru Hull fittings.
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This is exactly what happened in our Marina Stray Current attack.
For a while after, I weighed the Drop Zinc for loss of Zinc versus Visual surface Corrosion. The PO had left one that had the shape of a Fish.
Sounds like you have corrosion under control.
Jim...
Thanks to all of you who contributed helpful info for resolving this corrosion problem. Special mention to @jssailem and @JamesG161 who spent over an hour providing insight about galvanic corrosion / stray current and methods to fix.