Strange corrosion problem

[Frank Langer]

Hi,

I would appreciate any insights or help on a puzzling corrosion problem on my E30+ Yacht Specialties steering pedestal. The part of the pedestal that holds my speed/depth instruments sits on two 1" stainless steel tubes (sorry, don't know how to post a pic). The part in question is made of aluminum, and is attached to the stainless steel support tubes with two stainless screws embedded in the aluminum so that they cannot easily be removed.

About every two months, without any advance notice, a small crack and slight paint blistering appears in the bottom two inches of the top piece that rests on the stainless supports--it seems to happen overnight! At first I thought it might be from me leaning on the pedestal for support, but I have been very careful to avoid doing that, and the problem has reoccured.

I have chiselled/drilled out the crack and filled the 1/8" by 2" crack with Marine Tex, sanded and repainted, quite successfully, only to have the problem reoccur.

The insulated electrical wires for the instruments are routed up the inside of the stainless steel support tubes--I don't know if electrical current inside the tube would increase corrosion or if that is irrelevant. But the problem is much worse on the side containing the electrical wires, almost non-existent on the other side (just very minor paint flaking and no cracking).

The only permanent solution I can think of is to try to drill out the embedded screws, remove the top aluminum piece, coat the inside where it sits on the stainless tubes with dielectric grease and reattach with new stainless steel screws, also coated with dielectric grease.

Does this all make sense? Has anyone else encountered this problem? Could there be a different cause to this problem, and hence a different solution?

Thanks for any tips, insights or advice.

Frank

 

[exoduse35]

more than you ever wanted to know

the reason you still have a problem is that you have not stopped the crack. I f you look closely, cracks start at a single point then expand in both length and width. Their causes are either from vibration or erosion. With vibration, the mass on either side of the crack fault line is moved at different rates.In erosion. there is pressure created in the gap between the inside edges of the fault line. All materials that crack will develop a grain at some level. As an example I guess a crack is like splitting wood. the wedge exploits the linear grain and as the pressure erodes the bond at a particular point the crack begins, as the crack expand in width the length also expands until finally a complete break occurs. An example of vibration is in folding a beer can back and forth several times, to fold it a line is developed then folding it moves either side until fatigue along that line ultimately causes failure. Therefore to arrest a crack you must interrupt the linear grain, stop movement, and relieve the internal pressure of the crack to stop its spread. Specifically in your case find the end of the crack (it must be the very end) and drill a small hole through it. that will interrupt the grain and as it opens the force will open slightly without folowing the grain. To prevent cracks tn anything remove any linear markings, polish the surface or sand in a random direction. and do not add excessive point pressure or flex to an item and it will never crack. This is probably long winded but study it and it will go a long way to prevent and understand how to eleminate any crack in the future. Edd

 

[Frank Langer]

Thanks, Edd. That makes sense to me, and I'll drill a hole like you suggest to stop the crack from spreading. But I think it's more than just the crack working it's way up, as the crack is reappearing in the same spot, not moving too much further up, and is usually preceded myb bubbling and flaking of the paint--that's what's making me think it might be corrosion/electrolisis at work. But I could be wrong....

Thanks again for your post. Other thoughts also welcome.

Frank

 

[Cory B]

Weld it?

Frank,

We have the same cast aluminum instrument housing that you have. When we got it, it was cracked on both sides where the tubing entered it. We took the whole thing off, and cleaned it up the best we could. We then took it to the local machine shop and had them weld/polish the cracks (he did warn me that welding cast aluminum may not be a permanent solution). I repainted it with white rustoleum, and retapped the setscrew holes, and put it all back together. Its been 3+ years now and is still going strong.

I think in our case the corrosion (and maybe the resulting cracking) was largely caused by the stainless set screws and aluminum housing. I think (hope!) when I put everything back together I used dielectric grease where needed.

 

[Frank Langer]

Thanks, Cory. That's very helpful.
Frank

 

[Sean Engle]

Editing & Attaching Images

..stainless steel tubes (sorry, don't know how to post a pic)....

Frank - Please see this thread for info how to edit and attach images:

http://www.ericsonyachts.org/infoexchange/showthread.php?t=5383

This thread assumes you have a camera and know how to get the images from your camera onto your computer. Let me know if you have further questions, etc...

Thanks,
//sse

 

[Touchrain]

I have this instrument pod in good condition if someone needs one. Replaced to mount the Raymarine E80. Contact me and maybe we can figure a way to get it to you.