Corrosion tests

I made a rivet corrosion test plate just before winter. What I did was to spray it once with salt water and then just lay it on the veranda. It has laid there during the winter and spring in rain and snow and sun and I took some pictures of it now before fastening it to my boat to see how it handles marine atmosphere. It will stay on my boat the rest of the season.

The primer used is aluminum pigmented vinyl. The rivets are stainless steel and ordinary AN aluminum. The yellow "gue" is Duralac. Duralac is a special compound made particularly to prevent galvanic corrosion between dissimilar metals. Typically this is stainless steel and aluminium in marine applications. Duralac was originally made for aerospace applications, but is today mostly used in sail boat applications where they often use stainless rivets on aluminium, just like Sonex does. I use Duralac on all my stainless steel rivets for the Onex. I was a bit too generous with the duralac here, before I got used to it.

The aluminium rivets looks like new. Nothing is going on there. The SS and Duralac also look like new, primer or no primer, so this is good. Bare stainless rivets on bare aluminum shows signs of corrosion, particularly on the "shop" side. What is more troublesome is that bare stainless rivets on primed aluminium also shows signs of corrosion, even under the primer. The reason for this is probably that when the rivet is set on the dry primer, it widens and create small cracks in the primer that allows water to enter. This is obviously prevented when using duralac, but I hoped the primer alone would stop it. Maybe if the rivet was set "wet" with primer, all would be good, but I didn't make such a sample (didn't think of it).

Conclusion so far:

• Bare stainless on aluminium will make the aluminium corrode.
• Duralac stops all corrosion, primed or not primed, this makes me happy.
• Pre-primed (and dried) aluminium does not prevent galvanic corrosion.  Maybe another primer works better, or "wet" setting works ? 
• Aluminium rivets on aluminium does not corrode, primed or not primed.

Priming

6061 aluminium and SS rivets should be corrosion resistance enough, and usually is. However, in a coastal climate SS and 6061 aluminium is not the best combination. Ordinary SS (304) will rust and it will also cause galvanic corrosion to the aluminium due to the salty atmosphere.

Still, the Onex is an airplane, not a boat, so a bit of common sense has to be used. The galvanic problem is the most severe. Luckily it is easily fixed with something sailors have used for ages on aluminium masts: Duralac. Duralac is a zinc chromate jointing compound especially made to protect aluminium from galvanic corrosion. The process will be to coat the rivets with Duralac before insertion, and the problem is gone. Duralac works in two ways; the zinc chromate chemically prevents corrosion and the compound seals the joint so no air/water enters (no oxygen, no corrosion). It protects both the rivet and the aluminium.

The 6061 aluminium should itself be corrosion resistance enough in a coastal atmosphere. It is also very resistive against crevice corrosion, much more than 2024 clad (up to an order of magnitude in fact) and even more than pure aluminium. Crevice corrosion typically occurs between riveted aluminium parts.

All in all Duralac should do the trick. Any priming of the internals surfaces would be a waste of time. Still, I will apply a thin stripe of single component zinc chromate primer at the joints with a brush. 6061 is resistance against crevice corrosion, but not corrosion proof. This will add negligible weight and minimal hazzle, and the airframe should be completely corrosion proof for as long as I live at least. For bolts and screws I will also use Duralac or RTV depending on application, or maybe only some protective oil/grease. For the parts that receives mechanical weir, like the floor etc, I will apply a coat of strontium chromate epoxy, but only as a top coat on the riveted parts.

LAS Aero in the UK supplies everything needed. I have not used them before, but will try them and see how it goes.