5200 and Aluminum

Eric Caswell

Eric Caswell

Active member
Anyone have any experience with 5200, Aluminum, and salt water. Lately I've noticed some of the aluminum bedded with Sikaflex has created a white putty like mat'l that has put dents in the wood it was bedded to. Will 5200 do the same. Someone said it has to do with the aluminum not being exposed to the air.

Any ideas.

Thanks,
Eric
 
I'm guessing that what your seeing is electrolysis due to moisture and salt. All aluminum will do that to one extent or another. Anodizing helps but there are no gaurantees. A good grade of marine paint will help put a barrier between the aluminum and the corrosive elements. Not familiar with 5200 but alloy 5052 offers good corrosion resistance and is used extensively in marine applications.
 
Sika's sealent for metal recommends the use of anodized aluminum or special priming. Try giving them a call 800-933-7452 maybe they can offer some recommendations.
 
I've used 5200 extensively with aluminum for sealing (caulking), bedding, and bonding, with no problems. Other than being very messy at times, I love the stuff. Bond strength with aluminum is excellent. Maybe the wood is reacting with the aluminum and causing corrosion. What alloy of aluminum are you using? Salt water - you should use 5086-H32 is best. 5052-H32 is easiest to get and nearly as good, not quite as strong. 6061 is acceptable and stay away from 2024 and 7071 sheet. 2024 and 7071 will corrode just spitting on it. Aluminum to aluminum bonds are strong as long as material is clean, but when bonding Aluminum to other materials, I always prime with Zinc Chromate primer first.

Hitch
 
Mike, Thanks for that. I have talked to the sales reps at the trade shows about Sika but never considered calling their tech line. I'll have to do that.

Hitch, Thanks for the feedback. I'm using 5052 plate. Where we see the most trouble is against softwood. I suspect we would see more against hardwood (Rail Caps, Guards) if the wood wasn't hard enough to resist denting.

One thing I have implemented (in the last few years) is coating both surfaces when assembling to prevent air pockets against the aluminum. I haven't taken any of those apart yet to see if it cured the problem. Priming would be an option for some of the work but some must be welded in place. We bed most of the welded stuff with "Ship's felt" but on this particular job the wooden deck was shaped from the covering board to the outside of the deck leaving a gap between the plate and the plank at the outboard edge.


88.JPG

The plate is 1/4 " so the gap is about 3/8" at the most tapering to nothing about 3/4" in.

Thanks, Eric
 
Hi Eric -

I guess I don't understand the problem nor can I see it in the picture you provided. I can tell you that many superyachts, sailboats, pleasurecraft have been built with 5200 and bonding of aluminum to other materials is usually not a problem. If there are problems it is usually one of a couple things: 1. Improper surface preparation (aluminum will oxidize if left out for awhile prior to using) and 2. Using the material to fill large gaps and then that area being sealed wher no air can get in. The former is solved in a number of ways and the later is solved by sometimes spraying some water (regular, not salt water) in the area prior to bonding and to make the gap as small as possible. The 5200 is a moisture curing urethane. If the area is sealed off and no moisture allowed into the bond, it will never cure. An application that comes to mind where this happened was on a well known east coast fishing yacht manufacturer was using the 5200 to bond the transom material to the fiberglass skin and then immediately fiberglassing over this whole composite. The 5200 would not cure properly in this type of situation.

Here is a SWAG on your problem. The white material on the aluminum could be aluminum chloride resulting from a reaction of the aluminum and salt water. Dents may come from the gas given off during this reaction. It may be a result of of the aluminum not being completely sealed with the sealant or the sealant used was somewhat porous.
 
Thank you Mark.

The picture is of the application I am faced with now.Thinking about it we will try etching it with wash primer then gunning in 5200 and use a butter knife to ensure both the aluminum and the wood are physically wet out and then fill the gap and tool. I hope that will keep things looking good.

Around here anything that is moisture curing is a good thing to use and usually doesn't require additional water. :^)

We did some close looking last night at everything we disassembled on this job and every thing points to keeping air pockets out of the picture.

Thanks,
Eric
 
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