Does this look like electrolysis?

Ken Cross · May 19, 2013

I don't know, but I wouldn't expect electrolysis to affect the surrounding fiberglass.

captcoho · May 19, 2013

What yard are you in ? And what are their thoughts?

fstbttms · May 19, 2013

What you are seeing is called "burnback." It is caused by improperly priming the underwater metal. The copper biocide is eroded away due to electrolytic corrosion ("electrolysis" is a commonly (but erroneously) used term that describes a different electrochemical process) and no antifouling protection is left. This will continue on the fiberglass in concentric circles away from the metal. While this most commonly happens with paints that have an acrylic resin it can happen with any antifouling paint.

Charles Erwin · May 19, 2013

What your photos show is a very good example of a case where the underwater metals are bonded and well connected to adequate zinc - at least one years worth - usually more.

The deposits form because the underwater metals and bonded zinc(s) makes the underwater metal potential more negative. In turn - the anti foul is somewhat conductive because of the copper (black even more so because of the carbon) - thus the paint around the metals is made more negative. As a result calcerous deposits form in the shape of "halos." In fact "halos" is the most common phrase one hears.

The fix is to clean and coat the metals with epoxy - two coats minimum over clean metal and slightly onto the hull. Then you can overcoat with anti foul.

So it looks like your epoxy-over-metal step was not as good as you thought, there was not enough coats, or the through hulls were not spiffy clean at the start.

By the way - in any case - the condition is cosmetic but the the anti foul properties are a little bit compromised in the area. No way inverter, glow plug, or mast can be associated with what your photos show- but if you want to you can measure your hull potential and you will know for certain.

Edit. Are you in fresh water, are you using magnesium anodes, do you have sail drive?

Charles

Charles Erwin · May 19, 2013

Oh - I forgot -- is there a bright green color on any one of the underwater metals?

Charles

Bill Roosa · May 19, 2013

yes, unbond your thru hulls

Bob J. · May 19, 2013

Bill Roosa said:
yes, unbond your thru hulls

This is such a controversial issue. Several surveyors I've asked about this recommend bonding the thru hulls to the engine not the lightening ground or keel in the case of my boat. The surveyors I've discussed this issue with believe bonding the thru hulls together prevents an attack on a single device should an electrical condition occur by spreading the joy. Seem folks from outside the USA believe in not bonding thru hulls from what I've seen on other sites. Frankly I'm no expert & still not sure what the right thing is to do but I'm planning on bonding them to the engine.

When I saw the pictures Sefuller posted earlier today I wondered myself what the heck was going on.

Charles Erwin · May 19, 2013

One wonders how bonding could possibly be controversial since the consequences are a well understood electro-chemical process. The European notion seems to be "do not bond" but European historical results provide a history of unacceptable accelerated degradation of underwater metals.

Ask your "surveyors" two questions: (1) just exactly why "not bonding" presents any advantage in fiberglass hull cases and (2) just what threat bonding presents in a fiberglass hull case. Pretty sure you will come away more confused than you already are.

In any case - we stray from the OP question which is to explain why he has local deposits proximate to the underwater metals with VC 17? Do your "surveyors" opinions/assessments help us to address this question?

Charles

Bob J. · May 20, 2013

Charles Erwin said:
In any case - we stray from the OP question which is to explain why he has local deposits proximate to the underwater metals with VC 17? Do your "surveyors" opinions/assessments help us to address this question?

Charles

What kind of question is that to ask....
Kind of passive agressive are we?

Charles Erwin · May 20, 2013

A thousand apologies - that was never my intention.

Charles

Joe Blizzard · May 20, 2013

Sefuller said:
Any thoughts/suggestions?

My guess (and it's only a guess) is that it's galvanic corrosion, which has nothing to do with your boat's electrical system. You have bronze and copper electrically contacting one another and immersed in an electrolyte (water). Bronze, being more noble than copper, becomes the cathode and copper serves as the anode of a tiny little battery, and as a wee bit of current flows from anode to cathode through the electrolyte, the copper anode gradually loses electrons and corrodes.

Now I know what you're thinking, you primed the bronze thru-hulls before you put the VC-17 on them, so they should be electrically insulated from the copper. Here's my thought on that: You probably primed the outer surfaces of the bronze fittings, but not the insides, so it's possible that the VC-17 has an electrical path at the edges of the primer. It's also possible that the primer isn't all that good as an electrical insulator.

Bob J. · May 20, 2013

Charles Erwin said:
A thousand apologies - that was never my intention.

Charles

Struck me kind of funny. Apologie accepted!

Bob

Charles Erwin · May 20, 2013

If this was a local galvanic event - the finely divided copper particles in VC-17 were protecting the 85 555 through hulls and the paint was eroding as a result - then one should see the VC 17 coming off or a hint of green tinge in the paint layer. The through hull alloy and copper potential are 60-120 mv different so the erosion rate would be very slow.

The general notion is that white halos don't show up where (1) there is no bonding (2) the bonding circuit exists but does not lead to a submerged zinc (3) when hull potential is less than about negative 700mv.

White deposits (insoluble) being laid on does not seem to fit if galvanic activity were the culprit. Still, we are missing some facts: fresh water? bonding system intact and includes an adequate anode? what anode alloy is in play? whether (if sail drive) then is the sail drive anode the only submerged anode? hull potential less than about -700mv?

Charles

Maine Sail · May 20, 2013

That kind of effect can also be caused by a small on-board DC leak.. There are literally thousands & thousands of boats with 85-5-5-5 bronze thru-hulls and copper paint applied directly to them that don't have these issues.. In nearly every case of that type of event I have also found stray DC current. Fixed the DC leak and no more issues.

It could also be mixed metals in the seacocks or any one of the already pointed out issues from above.

You would be well served to check your bonding and earth grounding system for a DC leak, and as Charles pointed out, be sure your boat is adequately protected by its anodic system.

Charles Erwin · May 20, 2013

OK - The boat needs to be in the water to conduct the testing. Here is a sketch. The values in the sketch are relative to a zinc reference. Silver/Silver chloride cell is not so reliable in fresh water.

As to the fact that your strut and some (but not all) through hulls do not seem effected - the probable conclusion is that the bonding wires are not making good solid connection to the un-effected ones (one ohm or less.) Those items that are not well connected will not show symptoms.

Search for Charley Wing's book on electrical for a very good explanation. Not possible to explain every possibility here - but the effects you describe do not seem threatening

Otherwise - surely someone in Wisconsin knows the score. Consult ABYC for reference to a local person who has the training.

Charles

ps

Boatowner’s Illustrated Handbook
of Wiring; by Charlie Wing
International Marine
Camden Marine

http://assets.fluke.com/appnotes/electricalpower/B0269b_u.pdf

fstbttms · May 20, 2013

Charles Erwin said:
By the way - in any case - the condition is cosmetic but the the anti foul properties are a little bit compromised in the area.

The condition is somewhat more than "cosmetic." I can assure the OP and you that when burnback occurs, the affected paint is ruined and has zero anti fouling properties.

captcoho · May 21, 2013

Sefuller said:
I am at Southwind Marine, and the folks I have asked just kind of shrug.

I know I have at least seven coats of Interprotect (epoxy on the exterior surface of the thru hulls, but I suspect as one person commented the VC-17 made contact with the interior passage of the thru hull which was not coated. There are a couple other bronze thru hulls that show no signs of this happening. Being a little sloppy with last year's VC-17 application would explain why this never happened before.

I am using using a magnesium anode on the propshaft, as I am in fresh water.

How would I check for a DC leak?

My through hulls are all bonded with a thick copper wire attached to one of the keel bolts, stainless bolt, cast iron keel. The magnesium is the only submerged anode. There is a zinc (I believe) anode in the Universal M25 engine.

Three boats all of which had/have VC 17, bronze thru-halls. Two of which keep in a slip....Never seen that.
Need a Fluke, let me know....

Does this look like electrolysis?

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