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 Galvanic Corrosion????		  New Topic  Topic Locked
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John J.
Navigator

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USA
157 Posts
Initially Posted - 04/30/2005 :  15:40:19  Show Profile
Today I received a catalougue update from CD. In their cover letter, they talk about galvanic corrosion at the point where the spreader bracket stainless bolt passes thru the mast compression tube. This week I plan to install new standing rigging which means I'll have to loosen, (hopefully), the very hardware they're refering to. I'm sure many you have replaced your lower shrouds. How common of a problem is this? Also, should I use a torque wrench when re-tightening?
1987 C 27
#6387
Edited by - John J. on 04/30/2005 15:44:22

Dave Laux
Captain

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318 Posts
Response Posted - 04/30/2005 :  20:46:00  Show Profile
I just had to take a small drill and enlarge the hole in a Cat 25 mast to get the bolt and tube out. Two days of soaking in penentrating oil did not loosen it. On my mast a few years ago I was able to get the bolt out but this one is older and lived in a saltier environment. No hope. Mast makers in general do not do a good job of dealing with galvanic corrosion. They put them together with no sealants and the mix of metals is bound to cause trouble. Several posters here recommend antiseize compound but it has copper in it and causes more trouble than it saves. There are compounds that work. The bolts that hold the mast head and the spreader brackets should be just tight, not torgued, no preload. Use a nylon stop nut. Dave
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John J.
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USA
157 Posts
Response Posted - 05/01/2005 :  08:58:10  Show Profile
Dave, it already has nylon lock nuts. It seems to me, I have heard that nylon lock nuts should be replaced rather than re-used, is that right?
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Dave Laux
Captain

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318 Posts
Response Posted - 05/01/2005 :  20:25:32  Show Profile
Yes, especially ones that have been exposed to sunlight for a while. Dave
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Stu Jackson C34
Admiral

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844 Posts
Response Posted - 05/02/2005 :  15:06:02  Show Profile
The corrosion comes from the SS bolt in the aluminum mast. It's a real killer. Catalina also used SS pop rivets in the aluminum spars. Go figure!

Either the compression tube is aluminum which won't react with the mast but will with the bolt, or it's SS which won't react with the bolt but will with the mast. Either way, there are two dissimilar metals making contact.

When we had to install compression posts in our boom recently at the tangs, we enlarged the holes and used SS rod to put the bolts through. I keep using Lanacote on the walls of the boom where the compression tube comes through to keep the two metals apart as much as possible. I had a choice of getting aluminum rod, but the bolts would have had to have been removed regularly.

Either way, it's a trade off.
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Chemist
Deckhand

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USA
16 Posts
Response Posted - 05/02/2005 :  21:27:47  Show Profile
I never cease to wonder how competent marine engineers insist on putting dissimilar metals in contact with each other. Years ago I worked for a company that made water pump impellers for OMC and Mercury Marine. In particular, the Marcury Marine design was a rubber impeller molded on a bronze hub. That was fine, but the shaft that ran through the hub was stainless steel. Now you have bronze in contact with stainless steel in a water environment. In warm salt water, i.e. the Gulf of Mexico, the bond between the rubber impeller and the bronze hub was the point of galvanic attack. I don't think that I would have been to happy to have the cooling water pump for my outboard fail a few miles off shore.

Bob Imes
1980 C25 #1544 sr,sk "Prime Time"
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.