Dissimilar Metals

PDQ issues applicable across all PDQ Yachts (or if you can't find a place for something, it probably belongs here for now)
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jimlong
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Dissimilar Metals

Post by jimlong »

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Hi all,
I'm not quite sure what's causing this corrosion at the jib support cable where it attaches to the front cross member. I cleaned the area up a bit as there was a pile of white corrosive material all around the pin. I realize they are dissimilar metals but so are the ss stanchions that attach to the aluminum toe rail and there is no corrosion around any of those areas. Engines are one year old so zincs are still fresh. Curious if anyone else has run into this problem and what remedies are in order. Thanks, Jim
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Re: Dissimilar Metals

Post by thinwater »

Please, someone correct me if I am wrong. I think the problem is local and not zinc related.

I believe the bolt rope track is of a different aluminum alloy than the cross beam.
a. My boat does not have that track (the track is in the beam).
b. It does look like galvanic corrosion.
c. Look at the rivet head.
d. I have seen that pattern on other boats with that specific track.

(You can blow the picture way up--high resolution)
http://sail-delmarva.blogspot.com/2009/ ... -tack.html

I would suggest looking all over the boat, and if that is the only location, either find a different way to secure the tramp or remove and reinstall the track with an insulation strip (very thin polyethylene) and tef-gel on the fasteners. Not as hard as it sounds; rivets are easy to drill-out.
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Re: Dissimilar Metals

Post by jimlong »

This is the only area that I've noticed that kind of reaction. I'm not so concerned with the tramp slide track as I am with the jib cable support. Before I cleaned the area up there was a pile of corrosive material similar to what you would find on a car battery post inside the toggle swage and around the pin of the cross beam bracket. My friend says that I'm too paranoid and that this jib support pin probably wouldn't fail in my lifetime but I'm not so sure. I read that as long as dissimilar metals stay in a nice dry cozy environment that no reaction will take place but that when salt water comes into contract with the metals that the reaction process is stimulated. Well that particular area usually gets wet with breaking waves or spray so maybe I just need to be more vigil about rinsing off with fresh water after every outing. Thanks for the reply Drew.
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thinwater
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Re: Dissimilar Metals

Post by thinwater »

I should have been more clear. The tracks is doing the opposite of what zinc anodes do; it is CAUSING the corrosion at an adjacent location (the gull striker clip), by electrolysis. See the damage to the rivet heads?

Thus the track is the root of the problem and the clip has become the sacrificial anode.
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Re: Dissimilar Metals

Post by jimlong »

I understand what your saying now and yes looking at the rest of the rivet heads across the front there does seem to be corrosion on all of them. I hate to see the tramp track die an early death so that the gull striker/ jib support bracket can live on, haha!. Ideally I would like to prevent electrolysis from getting a death grip on either one of these components but am not sure how to prevent it. Should I be looking at an Isolator, or maybe mount a anode somewhere under the cross beam?
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Re: Dissimilar Metals

Post by thinwater »

No, and anode would do no good.
a. Zinc is less active than aluminum.
b. Wouldn't be reliably wet.

Is there a grove/track in the aft edge of the front beam? It looks like a mast section. If it is not the same size as the bolt rope you could use sail slugs or sew on a larger bolt rope.

This is why I suggested removing the track, placing a thin polyethylene strip under it (like milk bottle material) and resecuring with rivets covered with Tefgel. This won't stop the effect, but should greatly slow it.

It is also possible you have a lose wire in the mast somewhere.

You never said; is the corrosion the same on both sides?
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Re: Dissimilar Metals

Post by jimlong »

This last Summer I noticed 8-10 small holes about the size of a BB along the bottom side of the cross beam so I sanded them down, reamed them out and sprayed them with alum/primer.
Yes there is a slide track in the aft edge of the beam but I think I will just remove the rivets as you suggested and insulate the tramp slide track from the cross beam.
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Re: Dissimilar Metals

Post by amytom »

Jim,

I would seriously consider dumping the add on track and just using the built in track. That was the original design and reducing the points of failure is always a good thing. Worst case you have a canvas shop knock up a new strip with the larger bolt rope for a few bucks. I recently made a new one for ours to replace the sail slugs a PO had put in instead. Took about an hour to sew and maybe $75 for all new materials.
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Re: Dissimilar Metals

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If I used the slide track on the cross beam the tramp would be attached at point approximately 2-1/2" - 3" lower than than the existing track. Maybe I'm missing something here but it would seem that I would have to lower all the tracks if I elected to attach the bolt rope to the cross beam. Also maybe you can see that since I took that last pic white corrosive material is beginning to form again around the jib support pin.
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Re: Dissimilar Metals

Post by thinwater »

I didn't know about the holes on the underside. Makes me wonder what the inside looks like.

One thing I would do is take the wire loose and use a multi-meter to check for stray current. Does the voltage differential change if certain lights are on or off, or if shore power is connected?
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Re: Dissimilar Metals

Post by jimlong »

I checked the jib cable with a multimeter and it registered 12.6 volts almost the same as the battery level. Decided to test the wind gen that I recently installed and sure enough when I switched it off at the on/off toggle switch the reading on the jib cable went down to 0 volts. Obviously one of the 3 wires are incorrect coming from the wind gen. The red positive wire goes thru an amp meter and then thru an on/off kill switch to a fuse breaker to the battery. There are only 2 other wires which are the black (turbine negative) which goes to the negative post on the battery and a green wire which is referred to as the turbine ground (earth ground). I attached this green one to the battery negative post as well. I'm not sure what would be wrong with this setup but will try and ground the green one to another location on the boat tomorrow instead of the battery bank and then check the multimeter again.
Thanks for your help Drew, I'll keep you posted on the results.
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Re: Dissimilar Metals

Post by amytom »

It would be interesting to see if the voltage was still there if you turned off all the breakers. Maybe even the battery kill switch.
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Re: Dissimilar Metals

Post by jimlong »

Drew, Tom
It turns out that in fact the wind generator was the culprit. I borrowed a 12' ladder from the marina here in Green Turtle Cay and removed the turbine and sure enough the positive wire was chafed and a small amount of wire was exposed.
I now remember that when I installed the turbine I had to pull extra wire out the top of the pole so that I could rest the turbine in my lap while I attached the leads. It was hot, the turbine was heavy and the #8 gauge wire that I used did not want to go back in the hole at the top of the pole after I finished wiring it. So in an effort to get the wires back in I hung down in the aft locker and pulled on them from the opposite end of the pole and at that particular moment I probably pulled a bit too hard and stripped the positive lead on the sharp edge of the stainless tube at the top.
Anyway I checked the cross beam several times as well as other locations around the boat and am pleased to report that the current from the wind gen is now being deposited into the house bank instead of circulating around the boat. Thanks for your input, this forum is the best!!!
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Re: Dissimilar Metals

Post by jimlong »

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Thought I would show how the current was able to get to the front of the boat. The 1" spacer attached to the aft railing down to the toe rail forward to the pulpit and then down to the cross member.
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