Bonding aluminium to epoxy

[gary1]

Evening,
I'm just after opinions on what would be the better adhesive to use to bond an aluminium strip along the side of my boat, it is going along the chines.The aluminium strip isn't structual but it will get the occasional hard knock and it will be bonded to epoxy.
The 2 adhesives I hear the most about is Plexus methacrylate and 3m 5200 I'm after a permanent bond if i can get it.
Thank's
Gary

[Jimbo1490]

The trick with bonding to aluminum is the preparation of the aluminum, more so than the selection of the adhesive. The thing is that aluminum is always corroding. You have to remove the corrosion, then you have a few minutes or hours to STOP it. Then you can make a permanent bond to it.

You can remove the corrosion mechanically or chemically, or a combination of the two methods. If there is visible corrosion, you probably should start with mechanical methods. Scotch-Brite non-woven 'surface preparation' discs are a favorite for this, as well as the 3M bristle discs and the nylon cup brush. All are available for various size hand grinders to match the size of the work.

After the grinding step you may be able to go straight to the corrosion stop step, which is the chromate conversion (Alodine), but most likely you will need to use an acid cleaner/etchant first. Apply the etchant by either a spray bottle, garden sprayer or a chip brush. Scrubbing with scuff pads helps. Never let the etch dry out! If it does dry before rinsing, etch again! After etching, rinse thoroughly and make sure water 'sheets out' over the entire surface to be bonded with no 'breaks' or little pits or dimples in the film of rinse water on the surface. This means the surface is truly clean and corrosion free. Then you can apply the chromate conversion coating. For a long narrow surface, a clean white rag is the ideal applicator, but you could also use a spray bottle or a garden sprayer or chip brush.

Let the conversion coating dwell on the surface for a few minutes. Again, never let it dry out! Then rinse off thoroughly. A visible straw yellow to tan coating should remain. You now have about three days to bond to this before it hardens up too much and nothing will stick to it.

Now I prefer to apply a coat of Mil Spec yellow epoxy primer, let that dry for 12-24 hours and then do any bonding over the primer while the primer is in 'green cure' and accepts epoxy very well. But you could just bond right after the aluminum drys off from the last rinse.

The etch will burn you and the conversion coating is toxic, so wear gloves and a respirator, especially if you spray it on .

Jimbo

[TerryKing]

Jimbo, thanks, this is a very nice and complete explanation; I'd like to add it to the Materials part of the WIKI.

Can you give us a couple of Names / Brands for the Conversion Coating? Or is "Alodine" a trade name? Where in the average community can you find this?

Can anyone add what this is called in different places worldwide?

If you were to paint exposed aluminum would the steps be the same, painting after the primer?

[Jimbo1490]

Alodine 1201 is the trade name for the surface treatment from Henkel. The Etchant from Henkel is Alumiprep 33. These are the products featured in the Awl-Grip Application Guide.

Eldorado Chemical (now PPG aerospace) also sells one called Doradokote. Eldorado offers several etchnats, my favorite being their AC-5.

PPG automotive offers a conversion coating as DX503 and the etchant as DX533.

DuPont sells them as 225S for the cleaner/etchant and 226S for the conversion coating.

DuPont has published documents detailing use of these products here:
http://www.performancecoatings.dupon...19290_225S.pdf
http://www.performancecoatings.dupon...19291_226S.pdf

You can use these instructions for the other company's products since they are all similar, with the exception that some company's etchant may be stronger and need more dilution.

The dilution ratio of the etchant is not at all critical and can be varied according to need. On surfaces that have been recently well abraded, a weaker etch is preferred over a strong one. Conversely a dull, tarnished aluminum surface can be brought up to readiness for the conversion coating by using stronger etchant, also known as brightener.


Jimbo

[TerryKing]

Jimbo, take a look at:
http://www.boatdesign.net/wiki/Mater...lding#Aluminum
and make any edits that you see fit.

Thanks for the info; I'll handle aluminum better from now on.

[mongo75]

Hey Jimbo- Alodine doesn't burn, it just makes it look like you've been smoking some wacky weed LOL. I love the stuff (helo mech)- we used to do parts in a zip lock bag and pour right out of the bottle, let it sit for a couple of minutes, and then rinse. EPA changed it so we have to use a stupid marker type applicator. It's a real big PITA to do a panel....

[gary1]

Thank's jimbo,
Mate sorry I havent replied sooner I've been away fishing, anyway thank's for the advice I'm not sure on the avaliabilty on the chemicals over here but I'll look into it and see what I can come up with. Thank's again for the help
Gary