Real world corrosion control for mast fittings - SailNet Community
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post #1 of 9 Old 05-26-2014 Thread Starter
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Real world corrosion control for mast fittings

Hey folks, I'm replacing my standing rigging, and while the mast is down I'm also doing a bunch of stuff I would rather do on the ground than in a bosun's chair. That includes new wiring, lights, antenna bracket, and windex.

I'm finding myself going down a rabbit hole with corrosion control. With all the brackets, fasteners, and mounts I'm working with, every time I put two pieces of metal together I wonder exactly what they're made of and how much they're going to corrode in the coming years. I used aluminum angle for a consolidated bracket at the top of the mast, and I believe the sheave box it's mounted on is aluminum also, though probably not the same type. I'm using all stainless fasteners from a marine supply store. The mount for my light claims to be 316 stainless; the mount for the VHF is an unidentified metal.

Sorry for droning on, but the point is, I didn't see any corrosion control anywhere on this 35-year-old mast when I was taking stuff apart, and the mast looks like it's in fine shape to me. There was a little white dust here and there on things that I removed. I understand the theory of bi-metallic interaction and I've read a ton of stuff about various coatings and special materials. But what do real people do on real boats? My rigger and I stood and had a conversation about the stainless tangs I had just reattached to the aluminum mast, and he didn't say anything about any isolation measures like I'm reading about in the rabbit hole.

Are theory and reality matched up at all here?
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post #2 of 9 Old 05-26-2014
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Re: Real world corrosion control for mast fittings

Stainless fasteners in aluminum will corrode in a salt water environment. The longer they are in contact the more likely you'll never get them apart.

Tef-gel or lanacote will stop or limit the corrosion.

If you prevent corrosion of fasteners on one cleat you will be happy you used tef-gel or lanacote on them all.

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post #3 of 9 Old 05-26-2014
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Re: Real world corrosion control for mast fittings

Other things you can do:

--Use anti-seize on all threads (see above) and other thru-fasteners, such as rivets
--Install fasteners with a coating of caulking as an insulator (BoatLife LifeCaulk is a good choice, or 4200)
--As a backing, you can also install anti-corrosion insulation pads. These are commercially available from Sparcraft, but I bought thin sheets of ABS plastic from McMaster-Carr (ABS WHT .020x60x6 size is what I use) and cut to size
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post #4 of 9 Old 05-26-2014
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Re: Real world corrosion control for mast fittings

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post #5 of 9 Old 05-26-2014
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Re: Real world corrosion control for mast fittings

There is an anti seize product specifically made for aluminum. It is copper colored, unlike normal anti seize which is grey. Available in any auto parts store it is inexpensive and works well. Five years and counting on s/s screws into the mast at boom height (not aloft above most salt spray) w/o any corrosion at all.
"I used aluminum angle for a consolidated bracket at the top of the mast", If this is not anodized aluminum, you may have a corrosion problem with it down the road. Your mast and all the alloy fittings on it should be anodized, which is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish.
A good metal etch primer and polyurethane paint will go a long way to protecting your mast from corrosion, if the anodizing is coming off.

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post #6 of 9 Old 05-26-2014 Thread Starter
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Re: Real world corrosion control for mast fittings

OK, I think I'm seeing that:

1. Simply coating the threads of any screw that goes into the mast is a decent preventive measure, and

2. I am less worried about this in areas that won't be seeing a lot of salt spray, like the top of my mast.

How about the lower tangs, just under the spreaders? These are 316, and I had them electropolished. Would these generally be separated from the mast by a layer of something else? There was no such separation, or any other treatment that I could see, when I took them off.
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post #7 of 9 Old 05-26-2014
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Re: Real world corrosion control for mast fittings

Some pictures from a boom that is 29 years old, have been used on saltwater all its life (the boom was replaced with a new one last summer)
The traces of Duralac is only one year old


This lazy jack attachment probably installed by the PO - 10 years ago.


This is what i looked like underneath..


New boom - used liberal amounts of Duralac


I use Duralac wherever stainless is in contact with aluminium.
Buy it from my rigger who also use this stuff on every job i does.
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post #8 of 9 Old 05-26-2014
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Re: Real world corrosion control for mast fittings

The white dust used to be aluminum.

I think all the other posts cover what to use and where to get it from, but if you want to get comprehensive instruction about this kind of corrosion, look online or find a local general aviation facility (a seaplane terminal if you're really lucky) and ask an aviation mechanic (or marine aviation mechanic) about it. They have all the same problems with this that boats have, and in the case of the marine aviation guys, if something breaks off in the middle of the ocean, they're really going to have a bad day. So, they tend to be a little rigorous about preventing corrosion.
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post #9 of 9 Old 05-26-2014 Thread Starter
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Re: Real world corrosion control for mast fittings

Thanks everybody.

One thing I think I learned out of this is that the problem isn't just metal contacting metal, it's metal contacting metal when an electrolyte (like salt water) works its way between them. Is it fair to say that a big part of corrosion control, aside from choosing the proper metals, is keeping the electrolyte from getting between dissimilar metals?

I finished my masthead with naked fittings as originally planned, but will be getting one of the suggested anti-corrosion agents to keep on hand for stuff that's lower down.

As usual, I really appreciate the help.
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