Jon, I'm no atmospheric chemist, but I've been looking into this. There is quite a lot of short term info available on preventing degradation of metal sculptures, and much less on long term. It's not a simple problem. For sculptures, protective coatings need to be renewed frequently, e.g. every 10-15 years.
There are actually several factors to be taken into account, including: 1) Corrosion resistance The atmosphere contains various corrosive agents, such as gaseous sulfur dioxide and sulfur and nitrogen oxides. They combine with moisture in the atmosphere and fall on our dials as acid rain. Once in contact with the surface, they react both chemically and electrochemically, so the material decomposes. The agents come from artificial sources such as pollutants and also from natural sources (volcanos). 2) Wear resistance Wearing of material is caused by handling by people and from particulates tossed by the wind (sandblasting). (handling by people also introduces additional chemicals). Corrosion resistance is a simpler problem than wear resistance. Gold is the best example of an inert material (which is why you can go out and pick it up off the ground where it's been laying for millions of years). It's a great coating except that it is too soft, and when you add other metals to harden it, it is no longer inert. Other options are to plate metal dials by vacuum deposition with materials both inert and much harder than gold. There are various ones available, such as titanium nitride, which has a nice gold color. You can now commonly buy drill bits with this coating. There are better, harder coatings such as titanium aluminum nitride but the colors may be less attractive. This type of plating is relatively thin, and although the materials are very hard it may be vulnerable to wear. It is relatively expensive, but for a dial designed to last indefinitely this is not really an issue. One option would be to put your dial in a glass enclosure that has an inert gas pumped into it. This would prevent both corrosion and wear. Exotic materials are an interesting possibility but they take equally exotic construction methods. I think there is a lot of potential in this area, but certainly it will not be easy. For extremely long term issues, there was an interesting study performed on how to post signs on nuclear waste repositories to warn people away. In this case, longevity is such an issue that it is difficult to predict what type of language or symbolism should be used. Tom Kreyche -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jon Noring Sent: Thursday, October 17, 2002 9:15 AM To: [email protected] Subject: Experience with corrosion-resistant/exotic alloys? Everyone, David's post today asking for information about fabricating a sundial using stainless steel brings up a more general topic I wanted to ask the group. What is the known experience of using corrosion-resistant alloys in sundials? These alloys would include various types of stainless steels and the "exotics" such as the family of nickel-chromium alloys (e.g., Inconel, Hastelloy.) Other exotics include titanium and tungsten alloys. I guess one should also consider the so-called machineable ceramics, an area I know zilch about. -
