(I also comment later on Roger Bailey's comment on using Type 316 stainless steel in a sundial.)
Tom Kreyche answered: >Jon Noring asked: >>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. >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. Of course, this assumes that metal sculptures are made out of bronze, ordinary steel or maybe aluminum, not any corrosion-resistant alloy. There are "cast stainless steels", but I suspect very few, if any, sculptures are made with such "exotic" materials. (However, I've seen stainless steel sometimes used for welded-type sculptures.) There's something romantic and "artsy" about a "bronze" sculpture, but if you said "this sculpture is made out of type blah-blah-blah stainless steel made by Acme Alloys, Inc.", then the romance is lost, and instead one gets a wholly different impression, best described as "industrial". To me, what's important is how it looks, and that it looks and works the same a hundred years from now as it does today. I'm an engineer -- what can I say? (To be honest, it is difficult for me to accept "patina" as beautiful. To me it is simply corrosion. Whenever I get anything made out of copper, brass or bronze, the first thing I do is shine it up, even if everyone says I should keep it as it is...) <laugh/> >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). There's also photochemical. The UV from the sun can initiate or accelerate various chemical reactions which can cause corrosion of certain metals and coatings. >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). Some of the quite hard alloys (including the more common corrosion- resistant stainless steels such as Type 316) are hard enough that ordinary silica particulates in the air (of typical particulate density) will barely scratch, if at all. Only when the surface is mirror-polished may one see a reduction in the polish over time due to wind-particle abrasion. Some of the exotic alloys are best described as "damn hard" (there's "hard", "very hard" and "damn hard" on the Jon's Scale of hardness), and they are very scratch-resistant. They are sometimes used in industry to transport abrasive and chemically- corrosive slurry and gas mixtures. >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. Gold is an excellent example which you brought up. Some of the precious-metal (and platinum-group) alloys are pretty much as inert as gold, and also can be harder than hell. Rhodium and iridium are two elements in these groups which are noteworthy of imparting extreme hardness and ultra-corrosion-resistance to such alloys. (An osmium-iridium alloy is best characterized as "beyond hard".) In any event, building a sundial out of a gold or other precious metal alloy is something only a multi-billionaire would even consider doing. <laugh/> (Btw, I believe one can alloy gold with iridium or platinum to harden it appreciably, yet keep its ultra-corrosion-resistance.) >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. Again, my original message focused on finished solid metals, not coated, plated, surface hardened, etc. To me, I just want to machine it, shine it up some and put it out in the field. No plating, coatings, surface hardening, etc. Just natural metal, as God (oops, I mean the metallurgist) intended. :^) I'd love to find a "super" alloy with a naturally golden or brass color but without the "gold cost". I'm not sure if there is one that meets the corrosion-resistance and hardness requirements. I do recall seeing golden-colored gas turbine blades, but this may be due to surface nitriding or similar treatment. Usually gas turbine exhaust blades use one of the very exotic nickel-chromium-cobalt alloys as a substrate, but even here may need further surface treatment to survive the extreme temperature conditions found in the expansion section of an aircraft turbine engine. >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. Depending on what you call "exotic", many of the nickel/chrome containing alloys (of which stainless steel is at the low end) have the necessary native properties to make a hard and very corrosion- resistant surface without need for plating, coatings or surface- hardening. In addition, one can use ordinary machine tools for fabrication (although one should use carbide, nitride or diamond cutting tools for the harder ones.) I previously noted that, while a graduate student building chemical reactors, I routinely machined Type 316 stainless and Inconel 600 on World War II era lathes, milling machines, saws and drills. I rarely worked with the common materials like brass, aluminum and ordinary steel, so I guess I am not concerned with machining such exotics on ordinary metal working tools. You just learn how to do it. It's slower -- you take your time -- you sharpen your tools a *lot* -- you learn patience -- that's pretty much it. Anyway, I read with interest Roger Bailey's comments on using Type 316 stainless for a sundial he made 20 years ago, and as he describes it looks as good as new. Type 316 stainless may be all that is needed for a one century life without any noticeable degradation in a typical outdoor environment, although if one goes that far, one may as well jump to the next higher level and choose one of the myriad of high nickel-chrome alloys, such as Inconel, Hasteloy, Monel, etc. They cost more, maybe triple or quadruple than that of high-grade stainless steel, but not that much more when considering the total cost of the final product, and the extra corrosion protection and abrasion resistance they impart. It all depends upon what you want to accomplish... Jon -
