Stray- Current Corrosion, or Galvanic Corrosion, or?????...Help

[sailcrazy]

We sail a f/g hull Taswell 43, built in Taiwan initially for a Hong Kong client. She was built and wired for 230v, 3-wire, 50Hz power, and has a factory-installed bonding system for all the underwater metal items. We’ve owned this boat since 1999, have sailed her through Asia, the Medd and the Adriactic, and now in the Caribbean, and never had a thru-hull issue....until this past season. When we arrived in the Caribb 3 seasons ago, we changed our cruising style to a 6-mo on the boat, and then 6 mo up on the hard as we return to our MN home. The only time we spend in a marina is typically a 2-3 week period after resplash (to reprovision, rerigg, and fix any issues), and then about 2 weeks just before haul-out, to prep the boat for haul out and dry storage.
Almost all our installed equipment is 50Hz/60Hz tolerant, so after our Caribb arrival we bought a (Chinese-made) 5000w step up transformer(30amp at 110v=3300watt), plugged the 110v shore power into it and then the boat into the 230v output, and it seemed to work OK-except the transformer only lasted about 5 weeks before it failed! We eventually gave up on cheap transformers, bought a Victron isolation auto-sensing transformer, and installed it just after resplash this last Nov. And I removed the older Galvanic Isolator I had installed 12 years ago.
All seemed good until later that season, while diving on the hull to clean it, I discovered multiple strange “patches” around and trailing behind all the mid and aft bronze thru-hulls,–see pics below. But all the thru-hulls on the fwd part of the hull were normal. On haul-out, after the sailing season, the “patches” turned out to be areas where the bottom paint appears to be “blown off”, and the metal on the driveshaft and “P” strut has shallow scallop-like areas on it. Scraping the bronze thru-hulls, they remain bronze-looking, however, not yellow. The 2 hull zincs, cleaned up and looking almost like new after the previous season, were reinstalled; now, after the second season, they are still near whole. A quick check of underwater metal continuity showed:
-continuity between the hull zincs and all the affected thru-hulls, and NO continuity with the unaffected fwd thru-hulls,
-continuity between the “P”strut and the hull zincs,
-continuity between the drive shaft and its zinc, and no continuity between the hull zincs and the drive shaft.
Any ideas? Where do I look, and for what? Should I remove the bonding system? Should I add more (new) hull zincs?
Is this stray current corrosion, or something else?

[bletso]

It may be stray current. This is usually self generated, usually from a positive conductor with frayed insulation in the bilge such as bilge pump or its float switch. The problem with float switched is that it could be internal in the float mechanism.

Personally I would un-bond my through hulls, but that is a up to you. There are pros and cons to bonding.

As far as lack of continuity from shaft zinc to main bonding zinc - not surprising as shaft electrical connection to engine ground is always problematic. But you main zinc should have very good continuity to your bonded thru hulls.

https://www.westmarine.com/WestAdvis...unding-Systems

Just one source of many.

[fstbttms]

I believe it is improperly primed metals interacting with the copper-based anti fouling paint. We call this "burn-back."

[sailcrazy]

fsstbtms...thanks for the idea, hadn't thought of that before. But we've been using nothing but International Micron EU and Micron 66 bottom paint for the past 6 years or more-through 3 seasons in the Medd and the Adriatic, and now 3 seasons in the Caribb-all without issue. We've just done "light sanding" and then a 1 or 2 coat reapplication every season after 5 months dry time on the hard stand. No adverse reaction, and no signs of any issue at all....until now, this past season. If the cause is as you suggest, wouldn't it have shown up long before now?

[fstbttms]

Originally Posted by sailcrazy

If the cause is as you suggest, wouldn't it have shown up long before now?

No idea. I could be wrong. It's been known to happen.

[senormechanico]

Another vote for stray current.
I'd check for + voltage on the bonding system.
A Silver Chloride half cell would be advised to check, but it wouldn't surprise me if you saw a bunch of + voltage on the bonding system with the negative lead of your multimeter hanging in the water.

[sailcrazy]

senormechanico..I just bought a silver-silver chloride reference electrode, but I've never used one before. Can you walk me through how to connect it and use it to check the bonding system, as you suggested? I'm in over my head here, and appreciate any "how to" offered to try and unravel this. I've made arrangements to have the affected areas taken down to gelcoat, reprimed, and then repainted before we resplash.....and I'm assuming the checks with the reference electrode are to be done after we're back in the water(?) But just how to go about it is very unclear. Also... with an isolation transformer installed on the incoming shore power line (all 3 wires-hot, neutral, and ground) am I correct in understanding I do not also need to reinstall a galvanic isolator?

[smac999]

you do not need an galvanic isolator with a transformer. the transformer is better.

the silver cell won't help you untill you are back in water.

[GordMay]

Originally Posted by sailcrazy

senormechanico..I just bought a silver-silver chloride reference electrode, but I've never used one before. Can you walk me through how to connect it and use it to check the bonding system, as you suggested? ...

See ➥ http://assets.fluke.com/appnotes/ele...r/B0269b_u.pdf

And ➥ http://www.cruisersforum.com/forums/...sion-2469.html

[BambooSailor]

I do not also need to reinstall a galvanic isolator?

No, you do not need galvanic isolator if you have isolation transformer, just make sure there is no electrical connection between boat's ground and shore power ground (which allows galvanic current to flow).

[foggysail]

I am with the stray current guys. Check a galvanic series chart for flowing seawater and notice how close the voltages are for copper and silicon bronze. I doubt there is enough difference for reaction between the copper bottom paint and the bronze anythings IMHO for any galvanic corrosion

[Corrosion-Eng]

Hi sailcrazy,
This is electrolysis created by stray current being conducted via water to the Earth. The CAUSE can be permanently eliminated. To do so we must measure current flow to the Earth then disconnect equipment until you have Normal (green) illuminated. Then correctly insulate the offending equipment. Please look at Google search for marine electrolysis then click on sea bis. Or contact me direct.

[fstbttms]

Originally Posted by Corrosion-Eng

Please look at Google search for marine electrolysis...

Or Google "electrolytic corrosion", since electrolysis is a different (and unrelated) electrochemical process altogether.

[fstbttms]

Originally Posted by Corrosion-Eng

Hi fstbttms,
Please study. I guarantee my results with 8 years money back.
Corrosion-Eng

I didn't infer you don't do good work. I inferred that you used a term incorrectly. And considering that you are apparently a marine electrician, I would think that using correct terminology would be important to you. But maybe not so much.

[boatpoker]

Originally Posted by fstbttms

I didn't infer you don't do good work. I inferred that you used a term incorrectly. And considering that you are apparently a marine electrician, I would think that using correct terminology would be important to you. But maybe not so much.

You are absolutely correct. I'm an ABYC Certified Corrosion Analyst and "electrolysis: is as you say a different and unrelated process that is irrelevant to corrosion on boats.