Yup. What Tim said.

Steve

Tim Williams wrote:

"Steve Smith" wrote in message
...


Leo, the film thickness does not track the temperature. The oxide grows
faster at higher temps as you say. The oxide stays around--it doesn't
go away as the temperature drops.



Here's the deal. Take a piece of iron. It has a perfectly clean surface, I
mean the oxygen and nitrogen molecules of the air are bouncing directly off
the pure iron (and occasionally iron carbide and others) surface. Now at
room temperature, a few of these do bust up on the iron surface and oxidize
it. Like aluminum, this layer is invisible, but because it's somewhat
indifferent, it doesn't change the chemistry of the iron and you don't
notice it. This layer is only as thick as it is because the oxygen
molecules can't get past it at this temperature.

Ok, so let's raise the temperature. Radiation heats the air molecules near
the steel to the same kinetic energy, that is to say, hot air's molecules
move faster. So they hit the iron surface with more energy, and
occasionally one will pass through the oxide layer and oxidize more iron.
(It probably passes by diffusion, where the surface oxidizes to magnetite -
Fe3O4 - or rust - Fe2O3 - which is then passed backwards to the metal, which
reduces Fe3O4 and Fe2O3 back to FeO, at the price of more Fe metal being
burned.) Just as carburization can diffuse hardening carbon (or nitrogen in
some cases) only so far, likewise the oxygen only goes so far through the
oxide. It's always passing through, even at room temperature, so the
response of oxide growth is probably logarithmic - it tapers off quite
quickly as thickness rises, but never stops completely. It's just that
thermal response is exponential, so it'll take about two million years to
eat a tin can, while at orange heat, your tin can will hold molten aluminum
for only about fifteen minutes!

A temperature of 350°F for a few minutes produces a nice light straw color
(hard to spot because the oxide takes time to grow, and you can't anticipate
it because this is the first interference layer, around 80 nanometers
thick?), but the same temperature extended to an hour gives a purple
coloration.



One factor confusing this issue is that the colors don't keep going.
Once the oxide gets thick enough, the colors go away, but the oxide
keeps getting thicker.



Actually, they come back several times, but each time harder to see because
the light has to travel through more oxide thickness. If you heat a shiny
bar from one end in plain air, you'll see the first layers, yellow, purple,
blue; then a darker run of yellow, purple and blue, and so on for maybe
three or four total modes. What's happening is light is spending 1/2, 1
1/2, 2 1/2, ... wavelengths inside the thickness of oxide (1/4, 3/4, 1 1/4,
... wavelength thick layer, for the wavelength of light *passing in the
medium* (light slows down per the index of refraction), of the *canceled
frequency*). Thicker layers attenuate more, so it quickly (500nm?) starts
looking black. Fe3O4 is a wonderful electromagnetic-absorbent material,
after all. No wonder the military uses it...

Tim

--
Deep Fryer: a very philosophical monk.
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