Wednesday, July 03, 2013


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.