Sad bas*ard time for me :(
As far as I remember not all metals can be anodised, or rather anodised and produce worthwhile results.
Aluminium, Titanium and Niobium are the best ones to use.
Niobium is the nicest with Titanium a close second (have a look at
http://www.coldsteel.co.uk/webstore/webstore.html for the finishes which can be acheived), as there is a multitude of cool colours and finishes available.
I doubt that steel can be plated with Titanium or Niobium as they are very non reactive, and therefore appropiate salts may not be available.
Niobium is very soft and even if plating where possible may not last very long.
Ti would make great machine heads as it's tough (& light), but it's strength makes it hard to work with.
Aluminum is shitee for anything like machine heads.
Theres a link here for Ti and Ni jewellery making, these folks may be able to answer questions better than me
http://www.mcsj.co.uk/titanium.htmAnodised surfaces also need to be protected from wear as I seem to remeber that it's a very thin surface layer which sets up an interferance pattern (like oil on water). Obviously Titanium would be best in this respect.
Kilby...