I'm actually digging in something that comes from Syria... How does Chlorine cause metal to get rusted? It's full of pictures of rockets (allegedly filled with chlorine gas) that are all rusted. In my mind i find it pretty obvious that a strong acid like HCl can cause rust, but what's actually the precise mechanism? Does HCL even matters oj simple Cl(g) can cause rusting?

PS: consider all the reactions taking place in open air

REPHRASED: Is it possible for Cl(g) to cause some metal to get rusted in a matter of minutes? What are the chemical reactions that make it possible/impossible?

  • $\begingroup$ Chlorine and HCl are quite different things, though I guess both cause rapid rusting. $\endgroup$ – Ivan Neretin Feb 6 '18 at 14:09
  • $\begingroup$ I know, but since i knew Cl + H2O made HCL in our lungs and "corroded" them, i thought something similar may happen. $\endgroup$ – Simone Pennacchi Feb 6 '18 at 15:06
  • $\begingroup$ It may happen, but this is a relatively unimportant pathway, both in our lungs and on metal containers. Chlorine is toxic and corrosive in its own right, not because of HCl. $\endgroup$ – Ivan Neretin Feb 6 '18 at 15:11
  • $\begingroup$ I mentioned HCL cause (studying medicine) i knew the mechanism Cl causes damage to our body: CL(g) + H2O(from our tissues) = HCL (that causes damage). Do you think that CL2 liberated in the air can cause rusting by itself? $\endgroup$ – Simone Pennacchi Feb 6 '18 at 15:14
  • $\begingroup$ Yes, I am pretty confident it can. $\endgroup$ – Ivan Neretin Feb 6 '18 at 15:17

Neither hydrogen chloride ($\ce{HCl}(g)$) nor hydrochloric acid ($\ce{HCl}(aq)$) are capable of oxidising iron to the ferric state ($\ce{Fe^3+}$) but in the presence of oxygen (air) the ferrous ions initially produced can then be further oxidised to ferric ions:

$$\ce{4 Fe^2+ + O2 + 2 H2O -> 4 Fe^3+ +4 OH-}$$

For reasons explained here, that process is slow in acidic conditions, though.

Dry chlorine does oxidise iron to $\ce{Fe(III)}$ but at room temperature this is likely to be a very slow process.

However, due to the equilibrium:

$$\ce{Cl2 + H2O <=> HCl + HOCl}$$

moist chlorine is likely to be far more aggressive towards most metals including iron, because $\ce{HOCl}$ is a very strong oxidiser, easily capable of oxidising iron (or $\ce{Fe(II)}$) to $\ce{Fe(III)}$. Due to $\ce{Fe(OH)3}$'s extreme insolubility, the hydrated ferric oxide, i.e. rust, would then precipitate.

  • $\begingroup$ So, dry Cl would cause rusting but quite slowly. Instead the addition of water produces HOCl that is a stronger oxidizer that speeds the process up. Then we have also HCl "producing" Fe(II) that with Oxygen is further oxidized to Fe(III), thus more rust. Now, are these three processes simultaneous or one excludes others? $\endgroup$ – Simone Pennacchi Feb 6 '18 at 22:05
  • $\begingroup$ Also, when you say "moist", does it mean in aqueous solution achievable with something like condensation of water on cold metal or you mean "dripping wet"? THANKS $\endgroup$ – Simone Pennacchi Feb 6 '18 at 22:06
  • $\begingroup$ Probably all three together, depending on circumstances. Moist here means something like water vapour containing gas or air. $\endgroup$ – Gert Feb 7 '18 at 19:10

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