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Green and white corrosion on through hull

4473 Views 6 Replies 4 Participants Last post by  overbored
After months of forced isolation very far away from our boat, we came back and found a poor looking through hull for our sink/freezer drain. The boat is an Oceanis 41, 7 years old, and sat at the dock, with a galvanic isolator. Anodes have been changed regularly and bottom cleaned by a diver this winter. Inspected most other seacocks and they look normal with no deposits or coloration (like the one on the left).

1) how bad is this? Do we need to drop everything until this is resolved? And miss our only week of cruising this year?
2) what is the likely cause? Leak, corrosion, galvanic problem? Is this a surface issue and the sea cock should be functional?
3) it seems that Beneteau uses duplex brass, which is problematic. I can't fix that, so reminding me that their quality can be questionable is not particularly helpful, but it seems like something we want to address in the next haul out.
4) what is the best way to clean this up? So I can inspect the actual fitting underneath? What products/Substances/tools?
5) any other help and comments are welcome, as this is a new problem for me.


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If that was bronze, the green is perfectly normal, and the white is salts from a small leak. However, yours is brass, and appears like it may have a serious problem that should be addressed immediately. I base that on the pink spot on the threads connecting that T fitting into the valve. Pink means the brass is corroding by loss of zinc, and the water looks like it may be leaking from there. If that is the case, that fitting is in danger of snapping off. I'd keep the valve shut.

It's possible that pink is a trick of the light, and the hose above it is also leaking, so maybe the fitting is fine and the leak just needs to be addressed.

If it was mine, I'd shut the valve and remove the fittings for inspection. The valve is stainless and should be fine. I can't tell what the thruhull material is. If also brass, then that could have an issue too.

Looks bad to me. In a pinch, you could pull the valve off the thru hull fitting itself and see how bad it looks. If not bad, perhaps twist on a new valve and get your week in. Then deal with the thru hull later. These typically have mismatched NPT to NPS threads, so addressing it correctly, at some point, would be wise. This requires replacing the mushroom fitting and is a bit more time consuming.

I’ve replaced every one of the thru hulls and valves on every penetration (8) below the waterline. The rest are drains at the waterline to be done eventually.
Thank you. So white is salt (Small leak), green is verdigris (ok) and pink is bad (zinc from the brass being eaten away). I’ll look carefully for the pink as I clean it. I won’t mess with a fitting below the waterline, so finding someone to look into it will be fun.

does it seem that the problem is on the valve, the t fitting or the hose/clamps, or the through hull? no way to know for sure?
From the picture, it looks like the problem is the T fitting where it goes into the valve, and possibly where it connects to the horizontal hose (and maybe the hose above that, which is out of the picture). If you shut the valve, and are careful not to wrench it around on the thruhull, you should be able to take off that T fitting for inspection. My bet is the connecting nipple and barb threads are shot, and may just be dimestore brass and not DZR.

From the picture, it looks like the problem is the T fitting where it goes into the valve, and possibly where it connects to the horizontal hose (and maybe the hose above that, which is out of the picture).
Thanks for everyone's help. I got the lead mechanic here to inspect it and the valve is indeed mostly fine and the T Joint is shot. His recommendation is to keep it shut and replace both ASAP with the boat out of the water. Apparently Beneteau uses loctite on these joints and he does not recommend trying to remove it on the water. Keeping that valve open at the dock is what caused this, the t joint is brass and susceptible to corrosion and electrolysis.
just a friendly bit edit of boat education, Electrolysis is a process of making Hydrogen and Oxygen gas using electricity. Galvanic corrosion is the result of dissimilar metals in contact with an electrolyte ( sea water ) this what you have on a boat. Also might want test the Galvanic isolator to make sure it is working
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