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I'm building a 750 Cruzer, which will live most of the time within a couple of miles of the ocean. I'm doing the usual things with the aluminum surfaces and not too concerned about the 6061, but the various attachments to the cabin frame are a puzzle. Sheet metal screws and aluminum rivets in 4130 sound like a recipe for rust in a few years. I am considering using a cosmoline-type amine wax/grease like NoOxId in the holes, and something that will migrate over the whole inside surface (LPS3, Boeshield, Corrosion-X etc.) over the years to keep moisture away from the steel as long as possible. Anyone with better ideas for preservation in a coastal environment?
Drilling all those holes in the cabin frame does seem like a bad idea. I have no answers, but I will add that Wikipedia says that Cosmoline melts at 113–125 °F.
I don't live near the coast but it's humid here year round and things really sweat when the temp changes. I use Randolph or Poly Fiber epoxy primer on most of my stuff and when have to put some fastener in a blind hole I dip it in the same primer before setting it in the hole. That has done a good job for me for 40 years of building flying and maintaining these things. And linseed oil all the inside of the steel tubes; That's the traditional treatment for steel tubing inner surfaces and has worked pretty well for decades now. Boiled linseed oil is the one to get. It semi dries and forms a thick coating on the tubing walls. For a while I went to filling tubes like lift struts with epoxy primer then pouring it out but went back to linseed oil...it looked like most of the primer just ran off the walls when it was poured out.
I guess there's probably some space age type stuff out there now for corrosion treatment but the old way still works pretty well too. I realize you have something of the extreme situation there and might want to go further. That may be necessary; I don't know. I would be concerned about it too.
Until I built my 701 I never drilled a hole in a fuselage steel tubing member...welded a tab on it usually for the fastener to mount. But then I saw the 701 has all kinda holes drilled in the steel cabin frame and realizing how many are out there and how long they have been operating often under rough conditions I started drilling some holes in some structural steel tube members. I never have gotten completely comfortable with that tho.
I've been wondering about that too. Especially things like the strut fittings interiors and such. I don't like drilling through the tubing either... seems to me like it would make more sense (and more work) to weld tabs or a length of steel that goes a bit below the tube then you could actually secure things with nuts and screws instead of selt-tapping junk... I'm still not sure what I'll do to the interior of fittings that have a lot of exposure. Since I'll be taking off my wings for storage in the winter, I'll probably just keep the rust at bay with a light coating of oil on the interior of anything I can't paint...
There are good reasons you don't want to weld things to that tube -- no way to control how strong it is once you're done unless you heat treat the whole assembly. There is an interesting discussion on this topic at the Panther web site, where this is a much bigger problem -- they have lots of holes drilled in the steel tubing to attach aluminum skins. Dan and his dad have been in the tube and fabric airplane business for decades, and report that (as I had heard from my bush pilot friends) corrosion in the tube interior is not really a problem. I can report that corrosion in Mooney tube insides is a problem *only* when water is allowed to sit. The 750 tubes remain open, so I think that linseed oil is probably a better approach than thinned cosmoline.
I am more concerned with the inner surfaces of the holes themselves. Weseman suggests dipping the rivets in Proseal (2-component polysulfide fuel tank sealant aka gorilla snot) if you are concerned with corrosion, they aren't (and they live in Florida.) ZInc chromate (old fashioned mil spec, still available in an alkyd vehicle from Spruce for $40 a quart) or epoxy primer as Joe suggests might be better. I will also research using threaded inserts rather than sheet metal screws. We have had good luck in another industry with Timeserts, these installed with zinc chromate in the threads might be the right answer.
I think I will be much more concerned about the steel/aluminum interface where the engine mount brackets attach, and put a rust check on the requirements for the annual condition inspection.
A hole here & there in the tubing is not a major structural problem. It's when you have several in close proximity. The hole however creates a place that more easily allows oxidation. I would keep hole drilling in wet areas down to a minimum. I agree with the guys on linseed oil. I filled all my 4130 tubes with linseed drained it and then proceeded to fabricate. Being alone the Gulf coast we have our share of moisture threats.