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In case of rusting of iron the chemical reaction is not fast enough. The oxygen used is not molecular oxygen from the atmosphere but it is the oxygen from water molecule. The reaction is not rapid and appreciable amount of heat is also not produced. Also we do not associate any ignition temperature with this reaction. So can we say rusting of iron is not a combustion reaction?

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    $\begingroup$ that part of classifying rusting as being combustion or not really depends upon the definition of combustion you're using. $\endgroup$
    – khaxan
    Commented Feb 23, 2023 at 10:22
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    $\begingroup$ Would Chemistry be a better home for this question? $\endgroup$
    – Qmechanic
    Commented Feb 23, 2023 at 11:49
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    $\begingroup$ @Qmechanic generally combustion is more about thermodynamics and fluid dynamics, even if the underlying processes are chemical reactions. I agree though that this question is more about chemistry... but only when one knows the answer (that is when one knows that the difference is on the chemical rather than physical level.) $\endgroup$
    – Roger V.
    Commented Feb 23, 2023 at 13:45
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    $\begingroup$ This question is fine right here where it is for the reasons stated. Related but different in Chemistry SE: Why and how is rust forming on moon? which is related but different than Why can't rust form without water? who's title is sort-of the answer here. At room temperature, to proceed at a reasonable speed, the reaction requires more than just O2 gas. $\endgroup$
    – uhoh
    Commented Feb 24, 2023 at 2:12

3 Answers 3

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Rusting of iron is an oxidation reaction, but not combustion. Although the reaction's equation looks the same as the equations for combustion (e.g., of hydrogen and oxygen mixture, $2H_2+O_2=2H_2O$), the reaction mechanism is different: combustion reactions are chain reactions, where intermediate reaction products (e.g., various radicals of hydrogen, oxygen and water molecules in the example of $2H_2+O_2=2H_2O$) serve to trigger more and more reactions, resulting in positive feedback / avalanche process, which is the reason for the rapidity of combustion and explosions in comparison to iron oxidation, which can be essentially treated as encounter between two molecules.

The figure for a chain reaction below comes from here enter image description here

References:

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    $\begingroup$ I like to think that combustion consists of many complex oxidation reactions. Am I right? $\endgroup$
    – HEMMI
    Commented Feb 23, 2023 at 11:33
  • $\begingroup$ @HEMMI I'd say that it is one reaction, but involving simultaneously many molecules (in fact, involving more and more molecules.) $\endgroup$
    – Roger V.
    Commented Feb 23, 2023 at 12:05
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    $\begingroup$ Nanoparticles of iron are considered to undergo combustion (see pubs.acs.org/doi/10.1021/acs.energyfuels.6b00121 for example). Storage hazards need to be carefully evaluated depending on the size(s) of your iron nanoparticles. $\endgroup$
    – Jon Custer
    Commented Feb 23, 2023 at 13:33
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    $\begingroup$ @JonCuster I suspected that under suitable conditions Iron may undergo combustion as well. Thanks for the reference. $\endgroup$
    – Roger V.
    Commented Feb 23, 2023 at 13:42
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    $\begingroup$ Actually now I'm not so sure that ticks all the boxes to classify as combustion. It's a rapid oxidation reaction, but even with excess oxygen it doesn't produce enough heat to be self-sustaining $\endgroup$
    – llama
    Commented Feb 23, 2023 at 21:08
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We can call rusting of iron a combustion reaction. Water is needed in the reaction to carry the ionic current in the short circuited electrochemical cell reaction.

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Rusting of iron does not fulfill the definition of combustion, and is not combustion, just exothermic oxidation. For instance, combustion reactions are characterized by high temperature, usually several hundred to several thousands degrees Celsius, whereas rusting of iron usually does not result in any noticeable change from room temperature. Even in the near optimal case, like iron-based heat packs that generate heat from powdered iron reacting with air, the reaction temperature does not even reach 100 degrees Celsius. I guess you could still call rusting a combustion reaction, you do you. However, calling it combustion does not make it a combustion reaction. Much like you can have a man dress in a skirt, put on some lipstick and start talking in falsetto voice ala Mickey Mouse: one can humour him by pretending that he is a woman and by calling him a woman, but everyone knows he is really a man.

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