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I had an idea today. I recall that borax is used as a flux for welding and heat treating. It could very well be used as a coating to prevent oxidation and flaking of metal components without reacting or burning in air-charged stirling engines. The melting point is 743 deg. C and the boiling point is 1575 deg. C. I remember using the stuff in chemistry for a base bead material that certain chemicals imparted a color to when heated. I never could burn the stuff.
Its high temperature properties plus the fact that it is a solvent for metallic oxides makes it ideal for stopping corrosion of internal hot side parts. You probably can't use it on any bearing type surfaces, but it leaves a nice glaze on the surface that also absorbs water to prevent further corrosion. It is a regular anti-corrosion additive in automotive antifreeze.
Has anyone tried this idea on any type of stirling engine? I suspect it would work on the tin can stirling engines as well. I am having trouble finding the thermal conductivity of borax, though. If anyone finds it, please post it. Oh, and I am suggesting using the anhydrous type of borax for the moisture absorbing properties.

Ok. Been digging into this a bit further. Lots of blacksmiths use "20 mule team borax" for fluxing their pieces of metal before they weld them together on the forge. The technique of soaking the metal in a cooled supersaturated water solution of borax and setting it aside was surprising. The alkalinity of the solution prevents it from rusting and precoats the metal, thereby protecting it effectively.
I revisited this borax material under heat. It becomes rather glassy when the moisture boils out of it. By itself, it cools too quickly, and cracks if there are any bubbles in it. It is as hard to work with as powdered glass, as I have little experience with glass. I will test out the supersaturated water solution on some steel brake line and see how well it stands up to the heat.

Ok. The water-borax solution just doesn't stick to the metal properly or evenly. It foams up and walks around sputtering when heated. What we need is a better coat. I may try anhydrous borax to coat the steel brake lines with. I am thinking using something tacky to coat the line with and rolling it in the powdered anhydrous form, then heating it up to glaze it properly. Maybe I might have to cook it in an oven at 300 degrees F or something. Hopefully, I will have more free time to experiment further.
Not sure if pottery glaze would work....but it is a second thought. Anyone ever mess with pottery glaze on metal? Could save me a lot of time on this.

If that dont work you could try VHT paint or ceramic coating the parts .I have thought about going this route ( VHT paint or ceramic coating )but both of my engines are low temp and also run on atmospheric pressure.

vile fly, you are unsure about pottery glaze, but how about the enamal used for metal jewelery? The crystals are placed on the metal, then its heated. You used to be able to get cups, plates, kitchen ware of enamaled steel. Ian S C