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Grinding steel produces sparks. Grinding aluminium does not. Why?

I found this article about why grinding steel produces sparks but would like to confirm if it is true.

I would also like to know why does it behave so on a molecular level if anyone can explain that.

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Some Answers/comments come quite close to the real reason, but stop at commonplace like "must be caused by the oxide layer (of aluminium)" The core reason why is simple: iron is the only metal where the oxide has a melting point lower than the melting point of the metal. This is reason for sparks and why iron/steel is the only metal You can cut with "oxy-fuel cutting". All other metals will develop a solid oxide layer which impedes further burning. Of course there is influence from the "bulkiness" of the steel piece and the oxygen concentration.

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  • $\begingroup$ To answer your question "for what reason did this old question show up today?" Old questions without accepted answers are recycled through the feed so that they are noticed and receive new answers. $\endgroup$ – Pranav Hosangadi Jan 24 '15 at 15:57
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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. First is the the tensile strength of the metal. This is how much stress the metal can take before it breaks. 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. 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. 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. While some aluminum pieces still may fly away from the grind stone they still will not spark. 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. 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.

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  • $\begingroup$ this being said almost anything will spark given the right conditions mostly depending on tempurature and available oxidizers $\endgroup$ – Terry Smith Feb 16 '17 at 3:55
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I'm not entirely sure what the answer to this question is. 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).

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.

The answer must have to do with some sort of chemistry, and in particular the rate at which oxide forms. 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 resulting iron powder bursts into flames in contact with any oxygen (although I don't have a reference for the particle diameter).

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.

So while pyrophoricity is probably the main culprit, I have no explanation for why. Aluminum is far more electronegative than iron, with a value of 1.5 versus 1.8, and is well-known to have a much higher enthalpy of combustion (both on a molar and on a per-mass scale). The mean oxide film thicknesses of iron and aluminum are also supposedly comparable, on the order of 3 nanometers, so unless carbon steel acts differently, it can't be due to the oft-cited thin-film passivation property of aluminum.

However, I'm willing to bet that someone who works in materials science might have something more to add.

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  • $\begingroup$ I'd reckon the difference has to do with the properties of the oxide film. $\mathrm{Al}_2\mathrm{O}_3$ sure is physically very stable; I suppose that's why it protects so much better than iron oxide can. $\endgroup$ – leftaroundabout Apr 1 '14 at 12:58
  • $\begingroup$ Though the oxide film might change the sparks in some way, the main cause of the sparks has to be attributed to friction. If we are cutting through an entire metal piece, the oxide film is only on the surface right? Why are sparks produced throughout? $\endgroup$ – user42733 Apr 1 '14 at 15:02
  • $\begingroup$ @ParthVader: I think you've got the role of the oxide film backwards: the oxide film prevents combustion, rather than promotes it. The properties of the oxide film, and the kinetics of its formation, presumably determine the pyrophoricity of the material. $\endgroup$ – DumpsterDoofus Apr 1 '14 at 17:54
  • $\begingroup$ Oh right. I understand it now, thanks. But I still feel the primary cause is friction. $\endgroup$ – user42733 Apr 1 '14 at 18:15
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Many years ago, when I was a technician apprentice in mechanical and production engineering, I did a course on metallurgy, and feel confident that I can answer this question. We know that steel burns in air. If you get a small enough particle hot enough, it will burn until it is completely oxidised, or the oxygen can no longer get to the surface of the material. The same is true of aluminium, however, when aluminium oxidises (rusts) the oxide (rust) forms an oxygen proof layer on the surface. This prevents further oxidizing. Aluminium actually reacts to the oxygen in the air very easily. If you are welding aluminium, it is necessary to wire brush the area to be welded immediately before welding (arc welding only on aluminium) so that the electrode can actually contact the metal instead of the extremely thin but strong layer of aluminium oxide on the surface. When aluminium is ground, it will become very hot, just as steel does. This means that the oxide layer will form almost instantly, and will seal out the oxygen from the metal's surface. This will prevent further burning. Aluminium oxide is an extremely tough and insulting material. It's the white ceramic material on the top of a spark plug for instance.

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  • $\begingroup$ Yeah... or you could just AC TIG weld it. $\endgroup$ – Sarah Szabo Mar 29 '16 at 1:00
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While friction plays a good role, the molecular structure of each metal plays a greater role. Just imagine how energy released by dry wood in the form of sound differs to the same wood if it is green when suddenly cracked! Aluminium is more malleable than iron. It is something to do with crystal structures, when the bonds are abruptly broken through applied force. While aluminium tends to absorb alot, iron resists to a yielding point with great released energy. In this case, the sound, the heat and the light from resistant iron. Presence of oxygen increases the oxidation to release light.

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I believe that I can contribute to this discussing, I have always believed it the combination of friction of the disk (it can reach as much as 7700 rpms) and hardness of the metal you're cutting. The metal you cut reaches a temperature of at least 900 Degrees Celcius that explains the color of the sparks. For aluminium the melting point is much lower only approx. 600 degrees Celsius so the sparks won't fly around like metal but actually adheres to the grinding disk so you won't see anything flying and will be harder and harder to cut as the disk will be filled more and more with molten aluminium, to prevent this people use candle wax so the aluminium adheres to the wax on the disk and will fall off.

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While grinding metal, sparks are produced because of Friction. The rotating grinder cuts through the metal molecules, rubbing against them and producing heat. Some particles get loose in this process and burn because of this heat.

From Wikipedia, "Steel is an alloy of iron, with carbon being the primary alloying element, up to 2.1% by weight. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocations that naturally exist in the iron atom crystal lattices. Varying the amount of alloying elements, their form in the steel either as solute elements, or a precipitated phases, retards the movement of those dislocations that make iron so ductile and so weak, and so it controls qualities such as the hardness, ductility, and tensile strength of the resulting steel."

In other words, Steel gets its strength because of the carbon impurities in its lattice. So obviously, Steel is much more stronger than Iron. Aluminium on the other hand is even softer and lighter than Iron, so there's even lesser friction and hence, lesser sparks. There is also the point about rusting and Aluminium does not rust. Though I am not sure if this is directly related to the production of sparks.

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  • $\begingroup$ What about the link I posted that marks iron as a pyrophoric material that ignites due to oxidation? Is really friction directly to blame for the sparks? $\endgroup$ – Marek Andreansky Apr 1 '14 at 11:01
  • $\begingroup$ Yes, friction is directly responsible. The other factors just determine how much heat is developed and how easily the particles burn. $\endgroup$ – user42733 Apr 1 '14 at 11:10
  • $\begingroup$ Re, rust: Aluminum almost instantly forms a thin passivating layer of $Al_2O_3$ on exposure to air. en.wikipedia.org/wiki/Passivation_%28chemistry%29 $\endgroup$ – Solomon Slow Jul 12 '16 at 17:35

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