I agree that something harder would probably be better, but 25mm steel tube is completely impossible to find here. It gets tricky to verify, as well, as I’ve had several companies tell me they have 25mm but when checked it was 25.4mm OD. There is a thread about this ‘Australian Tube Options‘ in the Assembly section.
I also agree with respect to the wear resistance, but I’m wondering if flat spots might end up forming due to deformation of the underlying material, rather than wear itself. The anodized layer is only a few um thick (12.5um on the regular anodizing that I’ll be going with, but potentially 25-50um for a lot more money). Obviously, that layer isn’t strong enough to have any structure by itself, so it’s still reliant on the base material to provide the overall strength (in the same way that you couldn’t use a tube made out of tinfoil, no matter how hard it’s surface was). I guess it will act to spread the load somewhat (after all, it must be somewhat stronger than the underlying aluminium) but my assumption is that this effect has limits.
My hypothesis is that with enough time and pressure, this base material would cold flow until there are flat spots on the rails. What I don’t know is how much actual force the plastic pieces exert through the bearings etc. I assume that the round tube will eventually end up with a flat spot that is wide enough that the force exerted no longer causes it to deform plastically. It’s all just a question of how wide that flat spot isGiven the lack of options here, it looks like I’m simply going to have to try it and see! I suspect this could be directly measured using something along the lines of one of the standard indentation type hardness tests.
A quick google has just shown that it seems that hardness testing of anodized aluminium is indeed tricky in this regard as the macroindentation hardness tests (high force, large deformation) tend to reveal more about the underlying material hardness rather than the surface hardness by breaking through the oxide layer. http://www.practicalmachinist.com/vb/general/anodizing-hardness-202779/ post #4 has a comment on it.