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 Duralac was originally made for aerospace applications, but is today mostly used in sail boat applications where they often use stainless rivets on aluminium, just like Sonex does.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.

H-stab riveted

Got my new #30 drill bits on Saturday, drilled up the H-stab skeleton and riveted it together (still waiting for the 31 and 32 drill bits to be used on the skins). I know there is really no need to prime the internals of 6061 aluminium, but I just can't resist the urge to prime... Primed the thick parts with an aluminium primer from Duplicolor. That primer really feels nice, with a smooth and hard surface even though it is only 1K. Then I primed the mating surfaces using a brush and Jotun Yachting Vinyl Primer. That primer is supposed to be watertight and good, but the resulting finish wasn't all that good, it became thick and spotty.

The plan is to polish the airframe and use some color on the fiber glass parts and some of the aluminium. I would really like to use Jotun Yachting products, since they are available everywhere and they have this new system with high gloss 2K paint that that can be rolled on and is extremely durable. But the experience with the primer was not good, it probably has to be very thick to give a good finish, and then it becomes very heavy. Jotun also have a 2K epoxy primer, but then I am better off using my real aviation 2K Strontium primer (and that is just too much work and hazzle). Duplicolor automotive spray cans seem to be a better alternative right now, but will that paint last? Then I probably can use Biltema automotive paint at half the price (Biltema use some OEM paint from an unknown, but large supplier similar to Motip-Duplo). Both Biltema and Duplicolor is readily available and they have primers for plastic, steel and aluminium to be used under their color system.

Also tried my Duralac. Each rivet has to be dipped in that yellow high viscous stuff. Actually it's not that much work, because when dipped I can insert lots of rivets at the same time, they stay in place due to the viscosity. After that I just go over with my pneumatic rivet puller. For the external surfaces this yellow goo will not look nice, but the excess Duralac can be wiped off easily with paper and acetone, and still seal and protect the rivet and aluminium internally.

Materials starting to arrive

The first stuff from Sonex arrived today. It was a back order of a few parts. The Duralac also arrived. It was ordered from Southern Cross Spars, a Norwegian sailboat hardware shop/rigging facility. LAS Aero has problems shipping paint in Norway, but I'm working on some alternative shipping.