Anodizing is just a surface thing, it doesn't harden the base material, just puts a hard coating on the surface. The two wear points would be where the spring rubs around the perimeter and mostly where the collars are located.
I wouldn't think bare aluminum would last long, but hard anodizing can be harder than steel. If they are silver it must be clear anodized, they would never leave them bare. Regular anodizing is always clear and then dyed to get a color. Hard anodizing is a bit different, thicker and harder, and I think its always a grayish color like you see on many aircraft parts,.. uses a different acid I think?
With fatigue life the number of cycles depends entirely on the deflection. below a certain deflection, its essentially infinite, but its a sharp curve. as the deflection goes up the number of cycles drops fast.
I just don't see this stout shape having deflection problems. It never sees any loads above the spring pressure unless the springs coil bind. I think the biggest concern would be wear in the valve collar area but I haven't seen any of that at all.
Seems like the best material for this job if anodized and especially hard anodized? The only thing better would be metal matrix composite (MMC), its an exotic aluminum that's way stiffer and used in F1 pistons.
