So while pyrophoricity is probably the main culprit, I have no explanation for why. Meanwhile, a routine stroll through eBay or Firefox-FX reveals that aluminum powders with particle sizes on the order of 3000 mesh or smaller are readily available, and these are obviously not pyrophoric. The resulting iron powder bursts into flames in contact with any oxygen (although I don't have a reference for the particle diameter). Try the following experiment: mix equimolar ferrous sulfate and sodium oxalate solutions, filter the yellow precipitate, dry it, and then pyrolyze it in a test tube. The answer must have to do with some sort of chemistry, and in particular the rate at which oxide forms. Similarly, anyone who has taken a blowtorch to a sheet of aluminum foil knows that it won't ignite, even though its surface area is much larger than steel wool. It's probably not friction heat alone as Parth Vader claims, since you can ignite coarse steel wool with the flame of a match, and yet the significantly finer "atomized" 100-mesh aluminum will not ignite under the same conditions ("atomized" refers to the manufacturing method of molten metal expulsion with inert gas through a turbulence-inducing orifice into a cooling chamber, with the word symbolic of the microscopic morphology of the resulting particles). I'm not entirely sure what the answer to this question is. Now the steel will incandesce or glow as oxidation occurs because this is an exothermic combustion reaction as it has much more energy initially form the "shattering" and the oxidation of the iron into Fe2O3 gives causes the metal to incandesce before filly melting and oxidizing. This is because the energy released from the friction grind stone to the particle in the form of heat and velocity is not enough allow the aluminum to incandesce or glow.
While some aluminum pieces still may fly away from the grind stone they still will not spark.
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Now there is also another factor at play here and this has to do with the heat capacities, gibbs free energy of formation,and incandescence. So when the aluminum is touched to the grind stone it will simply deform and bend rather than shatter into small pieces, any pieces that do leave will have less energy than a steel particle would, as the deformation is less abrupt. Steel has almost twice the tensile strength of aluminum, steel is also much more brittle meaning when it does break it is more likely to snap or break than bend like aluminum. This is how much stress the metal can take before it breaks. First is the the tensile strength of the metal. I am a chemical engineer this exact problem was discussed in my sophomore materials science class, it has to due with a few different factors.
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