Can Cl(g) cause quick rusting?
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:

$$\ce{4 Fe^2+ + O2 + 2 H2O -&gt; 4 Fe^3+ +4 OH-}$$ 

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:

$$\ce{Cl2 + H2O &lt;=&gt; HCl + HOCl}$$

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.