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. I am interested in building sundials using such alloys with the goal that the surfaces of the materials of construction will stay pretty much new after one to two centuries of continuous outdoor use. This requires an extremely corrosion-resistant alloy combined with high hardness to resist the long-term abrading and pitting effects of wind-blown particulates. It is likely that plating with some hard, ultra-corrosion-resistant precious metal (such as rhodium or iridium) will not hold up over time (probably depending upon the substrate). Surface chemical treatment (e.g., anodization of aluminum) is certainly a possibility. For now I prefer to consider simple uncoated/unplated/untreated machined metal surfaces for various reasons I won't elaborate further here. There are two areas of consideration regarding using such alloys: 1) Cost and fabrication. 2) The real-life long-term corrosion-resistance in an outdoor environment. Regarding 1), obviously material cost is significantly higher for these alloys (but still not like buying solid gold <laugh/> ), and machining is much more difficult since these alloys are usually harder than hell and have other machinability issues such as work-hardening (I have personal experience machining Inconel, so it is not as bad as one would expect if you have the right cutting tools, but it's not a walk in the park like machining brass or a hard aluminum alloy such as type 6061.) Casting is not an option for some of these alloys, so one has to machine all the parts from stock, which for various types of sundials will be difficult and sometimes require a lot of material removal (wastage -- although proper design can minimize wastage.) Welding is usually difficult if not impossible, requiring heliarc or other exotic kind of welding technology. Regarding 2), the outdoor environment is nasty. We have air pollution, salt spray near the oceans, acid rain, bird and animal droppings, tree sap and other corrosive organic substances, windblown dust, etc. Some brass/bronze alloys are actually quite resistant to continuous corrosion since they form a fairly passive layer which resists rapid corrosion (e.g., the "patina"), and that's why they have been used in sundials for millenia (such materials are inexpensive and easy to cast and machine.) However, where near-zero long-term corrosion is desired, so the surface will look pretty much new after a century of outdoor exposure, this definitely requires exotics. (And even if a tiny amount of corrosion/pitting after a century does occur, it is usually so thin that cleaning up the surface is usually as simple as a quick buffing of the parts -- material removal to restore the surface to new is very minimal.) And note that not all stainless steels are actually this corrosion- resistant. For example, the DeLorean car with a stainless steel body developed noticeable (and ugly) corrosion after only a couple years so they had to distribute a chemical cleaner to the owners to remove this corrosion. Obviously, DeLorean did not choose the right stainless alloy to use for the car body (I would guess they used a magnetic stainless rather than a 300 series with a higher nickel/chrome content. Fabricability of the sheet metal was probably the most important criterion in alloy selection for the DeLorean body. Of course, stress induced during sheet metal fabrication probably contributed to the corrosion.) Anyway, I'm curious to know if any of the dialists here have looked into using exotic alloys in sundials, and specifically with regards to corrosion-resistance in an outdoor environment. The difficulty is that the exotic alloys are relatively new, so there aren't many 100 year old sundials (or any other outdoor object) made out of Inconel or titanium that we can learn from. <laugh/> Obviously, because of the color/appearance of these alloys (most are typically silver-white to steel gray), the classic sundial designs which usually use bronze would look sort of silly (except maybe for a few of the exotic alloys which I believe have a slight gold cast, alloys I plan to look into.) Thus, these exotic alloys are better used in more modern designs. I apologize for the length of this message, since I'm trying to anticipate the replies had I made this a brief two paragraph request. Thanks. Jon Noring -
