Need help: Bizarre Bravo III freshwater galvanic corrosion quandry

[Lou C]

Corrosion Reference Electrode with User's Guide | BoatZincs.com

Corrosion Reference Electrode with User's Guide | BoatZincs.com | America's Superstore for Marine Anodes

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[cyclops222]

Luck beats skill every time in a problem like this.
Big marinas with power outlets in use ??

 

[Cdn20valve]

It was my understanding that using mercathode and having magnesium anodes cancel each other out by both over protecting. While using mercathode, you need to use aluminum anodes. This might be the source of your problem.

 

[alldodge]

It was my understanding that using mercathode and having magnesium anodes cancel each other out by both over protecting. While using mercathode, you need to use aluminum anodes. This might be the source of your problem.

Not correct, different metals have different properties and do effect current

 

[paulswagelock]

Do you have a galvanic isolator installed? You should.
if you have a pro sport charger, you can ground the chassis of the charger. Talk to their tech support, there was a known issue where that charger could contribute to the problem.

 

[drewm3i]

I'm an ABYC-certified corrosion technician, and here are my two cents....

You need to get a silver/silver chloride reference cell and throw it overboard and see what you get for readings. Anything other than that is guessing at things. If your anodes are NOT corroding, that would point to electrolytic corrosion. With electrolytic corrosion, the cathode corrodes, not the anode.
With a reference cell connected to your multimeter's negative input, and the meter's positive lead connected to battery ground, you should get -800 to -1100 millivolts DC with all electrical systems off, including the mercathode.

• If you are a high, you have cathodic corrosion going on, which means that your boat may have too many anodes, the wrong type of anodes, or a boat next to yours has too many anodes or the wrong type.
• If you are low, that is galvanic corrosion, which means too few anodes, the wrong type of anodes, or a boat or structure near your boat has a lot of underwater stainless steel that your boat needs to account for.

Next, start turning on items 1 at a time and see if the reading changes. The readings should not change. If the readings change, especially a shift from - to +, there is DC stray current in the water coming from whatever is being turned on. The common offenders are trim senders, trim limits switches, faulty mercathode, and faulty bilge pump.

Good advice, but I'm like 99% sure I solved this. Keep in mind this was not "my slip," but transient ones that we were at for like four nights at one place and five at another. This boat is not, nor has ever been, slipped beyond a few days. The corrosion was never present in salt or brackish water because it is more conductive than fresh.

Even with stray AC current, the only logical cause was the loose 12VDC trim sender wires that were powered until I disconnected them from the engine harness recently.

Also, the anodes are indeed corroding/pitting, but just doing so very slowly; but that is to be expected in the cold, crystal clear Great Lakes water for a trailer queen. The anodes are also essentially brand new--as is the Mercathode and bonding system--so...

I think this whole thread, it turned out, was much ado about nothing since the obvious culprit were the cut/mangled trim sender wires which were essentially touching the gimbal ring with 12 volts DC anytime the key was in the "on" position...

 

[drewm3i]

Do you have a galvanic isolator installed? You should.
if you have a pro sport charger, you can ground the chassis of the charger. Talk to their tech support, there was a known issue where that charger could contribute to the problem.

I don't have one, nor do I have a shorepower system--just the charger. The chassis is indeed grounded per the manufacturer's specifications.

 

[Scott06]

Good advice, but I'm like 99% sure I solved this. Keep in mind this was not "my slip," but transient ones that we were at for like four nights at one place and five at another. This boat is not, nor has ever been, slipped beyond a few days. The corrosion was never present in salt or brackish water because it is more conductive than fresh.

Even with stray AC current, the only logical cause was the loose 12VDC trim sender wires that were powered until I disconnected them from the engine harness recently.

Also, the anodes are indeed corroding/pitting, but just doing so very slowly; but that is to be expected in the cold, crystal clear Great Lakes water for a trailer queen. The anodes are also essentially brand new--as is the Mercathode and bonding system--so...

I think this whole thread, it turned out, was much ado about nothing since the obvious culprit were the cut/mangled trim sender wires which were essentially touching the gimbal ring with 12 volts DC anytime the key was in the "on" position...

delete

 

[JustJason]

Good advice, but I'm like 99% sure I solved this. Keep in mind this was not "my slip," but transient ones that we were at for like four nights at one place and five at another. This boat is not, nor has ever been, slipped beyond a few days. The corrosion was never present in salt or brackish water because it is more conductive than fresh.

Even with stray AC current, the only logical cause was the loose 12VDC trim sender wires that were powered until I disconnected them from the engine harness recently.

Also, the anodes are indeed corroding/pitting, but just doing so very slowly; but that is to be expected in the cold, crystal clear Great Lakes water for a trailer queen. The anodes are also essentially brand new--as is the Mercathode and bonding system--so...

I think this whole thread, it turned out, was much ado about nothing since the obvious culprit were the cut/mangled trim sender wires which were essentially touching the gimbal ring with 12 volts DC anytime the key was in the "on" position...

Happy to hear you figured it out. BTW, for all intents and purposes, AC stray current is generally not an issue for electrolytic corrosion in the marine enviorment. While AC stray current can cause electrolytic corrosion, it takes AC stray current a year to cause the same corrosion damage that DC can do in a day.