Yikes! Thru-Hull Corrosion!

[GordMay]

I suspect you may have a problem with Stray Current corrosion, the result of a ground-fault on your own boat, or perhaps a hot marina.

Stray current corrosion involves the same electrochemical reactions as simple and galvanic corrosion except the reactions are driven by higher DC voltages from sources such as batteries, battery chargers and alternators. Stray currents (or interference currents) are defined as those currents that follow paths other than their intended circuit. The conditions causing this type of corrosion require an electrical fault, along with a problem somewhere in the bonding system of the vessel (the system designed to keep all underwater metals at the same potential to prevent this form of corrosion). The externally induced electrical current, that attempts to flow between the objects, causes one object to give up ions to the other object, and corrode. Stray current corrosion can strike hard and fast, often resulting in damage in weeks, days, or even hours.

Galvanic corrosion occurs when dissimilar metals are placed in the same electrolyte (like water) and are connected electrically. The transfer of current from one metal to the other, in the completed circuit, results in the corrosion of the more anodic metal and the protection of the most noble of the two. This form of corrosion takes on the order of many weeks to months to manifest itself in a visible manner.
A common example of galvanic corrosion can be seen when a stainless steel fitting is attached directly to an aluminum component, such as a spar. Over time, the aluminum will corrode in the area around the fitting.

Check out some of our earier discussions:

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

http://www.cruisersforum.com/forums/...odes-8773.html

http://www.cruisersforum.com/forums/...tion-2043.html

http://www.cruisersforum.com/forums/...rina-8445.html

[Badsanta]

If you replace it use a flange adapter. Makes life so much easier

[delmarrey]

Originally Posted by rhunton

Yes it's a factory install (except for the bonding system).

No, it's nowhere near the batteries.

Fuzz built up overnight after a thorough cleaning with a wire brush. Don't know how that could be soap.

Sooo, what was the last thing you cleaned with the wire brush before this valve? If one puts an electrical charge to acid, it will form crystals.

[rhunton]

Thanks Gord. Yes, it's corroding and that's normally galvanic or electrolytic, but the way it's doing it is unusual. I haven't found anyone who has seen one doing this in this way before. The rate is unaffected by the thru hull being bonded or not, there's no large measurable ac or dc current (only seeing about 6 milliamps) and the boat is galvanically isolated anyway. Also, the other through hulls aren't acting up and i've verified good continuity throughout the bonding system. I'm also not burning up my zinc, which further confuses me.

Also - the residue being white instead of green suggests something other than typical bronz de-zincification right?

In any case, I've decided to swap out all 12 of the thru hulls and ball valves with Marelon and not worry so much about corrosion anymore. The 93 series units look really nice and I'll convert over to flush headed thru hulls while I'm at it just to satisfy my "inner racer" and make me feel like i'm doing something of an upgrade instead of just spending megabucks to replaced stuff that already sorta works. I'd rather be buying toys.

[rhunton]

Badsanta - I'm going to use the forespar 93-series integrated seavalves/thru hulls. Nice units, no more corrosion issues. I work the valves freqently enough for seizing to not be an issue.

delmarrey - used several different brushes, one may have had some unknown residue (although it was dry & clean), the others were new. Coated everything thoroughly with corrosion block (drenched might be a better term) after cleaning. Fuzz came back (albeit at a reduced rate).

[dp3]

I'm curious, did you have a chance to haul your boat and look closer at the thru hull? I have a Beneteau which has a thru hull that looks similar to yours. I'm wondering if I need to haul my boat right away or if it can wait until next scheduled haulout. In general, I would stay away from bonding thru hulls. The bonding completes an electrical circuit where the thru hulls act as anode/catode and the water is the electrolyte.

[rhunton]

Yes - we hauled the boat last May and replaced all the through-hulls. The most seriously corroded one actually tore in two as i loosened the internal nut. In retrospect, the fitting was frighteningly close to spontaneous failure. It was that thin. It also had a large accumulation inside the fitting. I cut many of the others in half to examine their cross-sections, and none were as bad as the one in question, although quite a few of them showed some signs of electrolytic action.

The replacement process went fairly smoothly. Some careful thought has to go into the handle placement since there can be interference problems. I also had to do a bit of grinding and filling in a few locations as the forespar 93-series units have a much wider base and require a smooth mating surface to form a seal. Otherwise all went well and no leaks on splashdown.

[Cheechako]

Just curious, have you tried your Marelon valves lately... are they still working?

[rhunton]

Yes and yes. These are the valves that a number of builders use, j-boats among them.

[Joli]

Glad to hear it's worked out for you. Gotta imagine some of the valves are in a position that is tough to get too.

I was suprised when on a big Bene recently to see all the through hulls were nothing more then skin fittings with a ball valve spun on. If others have corrosion problems like you did it may not be long before we hear about Bene's sinking at the dock.

[Solitude]

My through hulls have green starting on them. Bronze corrosion?

[Solitude]

Hit go befor finished...

They are not connected, bonded to anything is it normal to turn green? When do you know to replace them? I have read a lot on the site about zincs and corrosion but I dont see how shaft zincs would protect anthing not connected to it.
Do people do anything other that bonding or replacing.

How much green is ok? None?

[goboatingnow]

To my knowledge all the major production producers ( certainly in Europe) use skin fittings and ball valves rather then traditional thru hulls. THey seem to last the distance, I have them on a boat 22 years old.


Bonding is very uncommon in Europe. It tends to be done for the US markets as thats the general customer sentiment ( rather then all real science).

[Cheechako]

Green is typical and not a problem on Bronze. Fuzzy green and whitish that crumbles off when touched is a problem. General opinions on bonding vary. If you tie things together then current is allowed to flow from one to another. That is what you DONT want! However if you have a fitting with electrolysis issues, then bonding could help. Each boat is different.... I've had boats that came bonded and were great, didnt use zincs much etc. ....and I've had unbonded ones that didnt either!

[Solitude]

I will look at the thru hull again tonight. Its both my engine intake and stuffingbox that are green and damp. I have propper zing donuts on the shaft and was not intending to bond.

There are alot of homeade dock cables around me.

I have been thinking of buying a grouper anode just in case and attach it to the stuffing box.

You would think that not being plugged in would lessen the problem? Maybe not.