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 Duralac was originally made for aerospace applications, but is today mostly used in sail boat applications where they often use stainless rivets on aluminium, just like Sonex does.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

Rivet binding strength

Spent this evening on investigating different ways of drilling for the countersunk rivets. Some info can be found here and here. The issue is mainly what drill size to use, #32, #31 or #30. I made three samples that I wanted to tear apart in the vice. It all went well until I found out the very hard way that my vice is very good for pushing things together but terrible at pulling things apart :-) (when pulling, the big bolt just comes out not taking the rest with it).

Then I tore them apart using a flat screwdriver and found that the aluminium sheet broke before the rivet fastener. Doing this I mounted them flat standing in the vice, so that one part was in the vice and the other part including the rivets standing on top. I then got the idea of using a hammer sideways, parallel to the surface, to get a torsion (and shear) load around the rivets, that will then be felt as shear on each rivet. So I took what was left of the samples and made new rivet connections. This time I used straight forward 30, 31 and 32 drill bits. I used my squeezer on the 30 and 31 holes and the "simple" pulled dimple die on the 32 hole.

The result was that I had to bang from side to side and totally bend the pieces out of shape to get them apart. The #30 drilled eventually separated because the shop end of the rivet got pulled through the hole. The #31 drilled did the same, but the plate also got torn up. The #32 drilled separated because the sheet broke while the rivet bonding was intact.

So what is the strongest? For all the samples the aluminium plates had to be hammered way past yield strength back and forth several times. The pieces also got bent, which means there was tensile stress in addition to shear stress. So who knows? The #32 sample seems to have the strongest bonding, but it was also the easiest to brake apart, so it could very well already be fractured from the previous abuse with the screwdriver. Besides, #32 holes are totally impractical because a 1/8 cleco won't fit and the 3/32 cleco is too loose. The only possible way is to match drill to #40 or slightly larger, then un-cleco, drill up to #32 and dimple in one go because the 1/8 clecos fits the 32 holes after dimpling. Using solid rivets is also an option of course, they are not nearly this sensitive regarding exact hole size.

All in all, when the pure pull test didn't work out, I didn't really get that much more knowledge other than knowing that the rivets are not likely to pop out of the aircraft unless the aircraft is bent and twisted beyond recognition, no matter what drill size is used. However, the ball on the stem is never pulled through when using #32 drill bits for whatever that is worth.


Also tried to polish the Topgloss BR with Biltema rubbing by hand. The result was surprisingly good. Really nice and shiny and smooth.

I then tried to polish it with Jotun Hardwax and also some old car polish I had laying around, but then it got all milky. The tech sheet says that polishing the Topgloss BR may cause milkyness, but not Hardwax, the tech sheet is obviously wrong.

If I can rub/polish this polysiloxane paint this easily, I probably also can roll on some polyurethane just as easily and hopefully be able to polish it without getting this milkyness. I begin to suspect that this polysiloxane Topgloss BR has been rushed out into the market before it is fully usable.