As I continue to study the problem of finding a corrosion-proof metal
(or the closest thing to it) for constructing sundials which will last
indefinitely (read: centuries) in any natural outdoor environment, and
still remain "new" in appearance, I am beginning to get very excited
about titanium. I believe this is the metal/alloy which will best meet
this requirement and still be relatively affordable and machineable.
As I mentioned before, using gold, platinum or other inert precious-
metal alloy would be great, but at $400 to $2000 per troy ounce, only
a multi-billionaire would be able to afford a sundial made from such
metals.
Also, one could certainly plate such precious-metals onto a substrate
of a cheaper material, but over time the plating may come undone --
and corrosion of the substrate can still occur even if the plating
itself is inert (over time corrosive chemicals can find their way
right through inert platings, and anyway it is impossible to totally
seal the substrate from the outside world.) So using a corrosion-prone
substrate is not an option in my book. The substrate itself must be
very corrosion-resistant if any plating/coating is done.
Several of the nickel-chromium alloys look very promising (with
Hastelloy 'C' being the very best among them), and should be further
explored. Such alloys are very expensive (I got some quotes of Inconel
stock in the $15-20/kg range, Hastelloy 'C" would be, I guesstimate,
$20-30/kg!) But cheap compared with gold or platinum. These alloys are
also very difficult to machine because they are so damn hard and tough
(the hardness is actually good for resistance against windblown dust
abrasion.) And they are not totally impervious to corrosion (they are
still susceptible to certain types of chemical action which I can
envision occuring, albeit rarely, in the outdoor environment, such as
aqueous chloride ion.) But even if such corrosion occurs, it will only
occur in a thin layer on the surface, which can be easily buffed out
during a periodic (say once a century) cleaning.
I would not recommend using most stainless steel alloys if ultra-long
life is a requirement, since under some unusual conditions (which can
occur in the natural environment, especially in a marine environment
where lots of aqueous chloride ion is present) the iron in these
steels will rust, or "white corrosion" will form. However, if one is to
use a stainless steel, the best "available" one is 316L (the 'L'
designates "low carbon" which reduces the tendency for the iron to rust).
It is still expensive, but nowhere near that of nickel-chromium alloys,
and it is easier to machine, relatively-speaking. And like the nickel-
chromium alloys, any corrosion which does occur is likely to be very
thin and can be periodically removed without appreciably affecting the
dimensions of the part (only after many buffings will the part begin
to appreciably change in dimensions -- this does not include pitting,
though, which may occur fairly quickly.)
So this brings us to titanium.
What makes titanium so interesting is that it will, I believe, be
truly corrosion-proof in virtually all outdoor environments. A sundial
made out of titanium (pure or an alloy) should look the same two
centuries from now as it does today. It will still be affected by
wind-blown dust, though, but all that will do is to slightly
"sandblast" the surfaces over a long period time depending on how
dust-laden the air is (I will later discuss nitriding which should
greatly reduce, if not entirely eliminate, dust abrasion.) Because
titanium is an outstanding "self-healing" metal, any scratches are
instantly oxidized with titanium dioxide which forms a completely
passive and clear protective layer.
Titanium is still very expensive, about the same price per unit weight
as Inconel or Hastelloy C (this estimate is based on some online
prices of stock of various alloys -- finding this information online
is difficult), but because its density is about half that of the
ferrous metals (yet with the same strength as steel), means that it
should be in reality about half the cost -- but still much more than
316L SS. Machining of titanium is a bear, though, but the problems are
different than one encounters with nickel-chromium alloys. For
exhaustive details on how to machine and finish titanium to avoid the
well-recognized problems, see:
http://www.titanium.com/tech_manual/tech2.cfm
(Overall, machining most titanium alloys, including pure titanium,
should be comparable to machining 316SS, from what I've read, so it
appears not to be as bad as the nickel-chrome alloys, but it is not a
walk in the park when compared to brass, bronze and aluminum.)
Regarding corrosion resistance of titanium, about the only thing which
will attack it at environmental temperatures is fluoride ions, which
is not encountered to any significant extent in the natural outdoor
environment. At outdoor temperatures (even in Death Valley), aqueous
chloride will not attack it at all. That's why titanium is used in
marine environments (several former-Soviet submarines use titanium
hulls -- the hulls are totally impervious to the salt water corrosion
and abrasion.) Titanium alloys are also used in the industrial handling
of hydrochloric acid, for example. It is a "wonder" material in many
ways, the more I read up on its corrosion resistance and other
properties. See, for example:
http://www.corrosion-doctors.org/MatSelect/corrtitanium.htm
The color of titanium is naturally silver-gray. For some examples of
its natural color and the quality of surface finish obtainable, see
http://www.boonerings.com/ which is a company that makes titanium
rings (and in the past bicycle parts.) There are thousands of other
web sites with pictures of titanium parts and objects so one can get a
good idea of its natural appearance. But coloring titanium is easily
accomplished two ways.
The first way is the nitriding of titanium (there are several
commercial processes to nitride titanium parts) which not only greatly
hardens the surface (the surface becomes "damn hard" thereby greatly
increasing resistance to windblown dust abrasion), but will turn it a
rich golden color. Here's an image of a titanium clutch plate (having
a poor, "rough" machined finish, just to note) which was nitrided
following machining:
http://www.probascokarts.com/Used%20Parts/L&T%20Titanium.jpg
It's clearly not the same as the color of gold or brass/bronze, but
nevertheless it has its own character. (Titanium nitride coatings are
also applied to steel tool bits and other parts to greatly increase
their hardness. See: http://www.brycoat.com/coverlg.html for some
nice examples.)
Also, by certain anodizing/heat treatment of the parts (not the same
as nitriding!), one can precisely control the thickness of the light-
diffracting titanium oxide layer and thereby produce any color found
in the rainbow. Here's an example of a beautiful blue color achieved
using such an "anodization" process for one of the first titanium
coins ever minted (from the Pobjoy Mint):
http://www.pobjoy.com/images/products/2000pb1.gif
http://www.pobjoy.com/images/products/2000pb2.gif
For various interesting examples of "anodized" titanium art, see:
http://www.titaniumart.com/
(Note that "anodization" coloration due to regulating the oxide layer
thickness is very sensitive to the thickness -- and it is a very thin
layer corresponding to the wavelength of visible light, so as the
layer scratches/abrades away, the color will change and eventually
disappear. Only nitriding, which as I noted above is a different
process, imparts a "permanent" gold coloration, and this nitride
layer can be made relatively thick, from 100 microns to as much as a
half millimeter, depending on the nitriding technique used. The
nitrided layer apparently has the same corrosion resistance, and
maybe even better, than the un-nitrided titanium substrate. And it
also greatly hardens the surface. The only concern I have is the UV
resistance of the nitride layer -- will long-term exposure to UV
from the sun cause it to fade or crack over time? -- I'm not sure
if anyone has studied this since the primary motivation for
nitriding is surface hardness, not appearance. My guess is that the
nitride layer will remain fully stable under long-term UV exposure.)
>From an aesthetic point-of-view, I think that a metal sundial
instrument made with both finished native titanium parts (with a
silver-gray color) and nitrided titanium parts (with a gold color)
would be quite aesthetically-pleasing, more so than if it is solely
one or the other color. For example, the dial can be nitrided, but the
various screws and the base could be native titanium with a matte
finish.
Hopefully the information herein will be useful to those sundial
makers who wish to build very long-lasting metal sundials, and wish to
experiment with these candidates.
Jon Noring
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