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There is a certain video which I just watched about 2 large cast iron balls. If the balls are struck together, no reaction happens, but if one is covered in aluminium foil and then struck, a large quantity of sparks are produced. This flies in the face of my metalworking experiences with these metals. When cut, steel reacts with oxygen in the atmosphere producing large quantities of sparks, but aluminium will not produce any sparks, despite being cut with an abrasive wheel or grinded with an abrasive wheel.

So why does the aluminium spark when the balls are grinded together? Could the temperature be higher forming a reaction with oxygen? Is there pure aluminium in the center of the foil which does not have the typical Alumina composition on the exterior and is being exposed to the atmosphere and is being oxidized?

Video: (CrazyRussianHacker): https://www.youtube.com/watch?v=w7yWoO7vqvI

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Physics comment: thermal conductivity combined with strength - Al is softer and has a much higher TC than steel. On Al you tend to rip bits out quite easily while on steel, frictional heating has a much greater effect. So the steel particles are much hotter. Whether they get red-hot from this alone is another matter, which a quick google couldn't answer. – Chris H Mar 29 at 11:03
    
So are they steel or are they cast iron? – Mason Wheeler Mar 29 at 15:33
    
@MasonWheeler I believe that cast iron is a type of carbon steel. – Sarah Szabo Mar 29 at 22:50
    
Nope. Cast iron is an iron/carbon alloy, but its carbon content is too high to be steel. Steel is silvery in color and malleable; cast iron is black in color and brittle. – Mason Wheeler Mar 29 at 23:07
    
@MasonWheeler Yeah, I just read the Wikipedia page on it, and apparently it contains $Si$ as well. – Sarah Szabo Mar 30 at 20:13
up vote 18 down vote accepted

This is actually related to the thermite reaction, where the rust on the balls reacts with the pure aluminum in the foil. Here is a pdf detailing the experiment.

The activation energy for the reaction is reached through the friction and pressure, and the rust on the balls is actually very important, since it is the exchange of oxygen between the iron oxide and aluminum that releases the heat as iron and aluminum oxide form.

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1  
Oh my god, that's so dumb of me, I forgot the balls were rusted and in my head I thought that they were just regular steel balls. – Sarah Szabo Mar 29 at 1:02
3  
While the thermite reaction between the iron oxide of the rusted balls and the aluminium foil is certainly the major factor in the resulting lightshow, even unoxidized steel balls could potentially ignite a small amount of the aluminium foil, in a similar fashion as how the piece of paper is burnt in this video. – Nicolau Saker Neto Mar 29 at 7:13
2  
Here's what happens without the rust: youtu.be/JTcE9wbbgZ8?t=323 – Tejas Kale Mar 29 at 10:51

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