Corrosion in the Seawater Manifold

williammelbourne <melbourne@...>

I have a general question about corrosion in the seawater manifold.
In May this year the seawater intake manifold on our Super Maramu 2000 (7+ years
since new, 5 years at sea) developed a leak. The leak occurred at the "tee" junction
between the main manifold and the smaller pipe that connects to the hose leading to the
A/C pump. I removed the manifold, and I found serious corrosion inside of that junction (I
presume from electrolysis). The corrosion appeared to follow the original solder band at
the junction. However, the corrosion only occurred at the A/C junction, none was evident
in any of the other four junctions for the two toilets, the anchor wash and the desalinator.
I repaired the manifold by soldering a slightly oversized short copper sleeve over the
original A/C pipe. The sleeve fits against the main manifold but it is short enough to leave
enough room to reconnect the A/C hose onto the original A/C pipe, which was in good
shape at its downstream end. It seems to work OK, but I worry about the future.
My question is: Why did this corrosion happen? The manifold has always been well-
bonded, exactly the way that Chantiers Amel rigged it originally. Our grounding strap in
the bilge appears to be intact. I haven't done any stray current search yet, but I will after
hurricane season. It is true that the A/C line gets the most use (a factor of at least 10
more than the others), and perhaps here is a clue about the cause.
Maybe after 5 years in saltwater, I'm expecting too much from copper. But the contrast
between the lack of corrosion everywhere inside the manifold other than at the A/C
junction is at least suspicious. The inside of the A/C junction looked like Swiss cheese in
miniature, whereas the rest of the manifold looked corrosion-free. I also note that other
Amels of similar vintage have developed this same leak in the same area of the manifold.
I have already searched through the Amelyachtowners forum, in particular, Messages
No. 3814, 3813, 3811, 3810, 3809, 3782, 3781, 3780, 3779, 3778, for possible clues to
this corrosion problem. Although these messages are definitely informative, I couldn't
find a definite answer to my question.
Any comments? Thanks!
William Melbourne

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