Right now, iron will rust on Mars' surface like on Earth. Yes, even though its atmospheric carbon is already oxidised; carbon-dioxide can oxidise metals. On Earth at least it used to be that rust left on the surface was "reduced", back to iron. Our Oxidation Event, when the atmosphere changed, is marked 2.5 Gya. Joe Michalski's team reports a similar turnaround for Mars.
On Hadean Earth the reducing gases would I think start with methane, pure CH4. Probably the same on Mars. The release implies that the Martian 3.5 Gya water-age, called the Noachian there, was still reducing despite the H2O and perchlorates. The oxidation of the atmosphere came after that.
As for "after that" I don't know that there was ever free oxygen on Mars but, to be fair to Michalski, he doesn't say there was. Carbon monoxide and nitrous oxide have oxygen already; but do they take more oxygen, from iron rust? Carbon-monoxide is reactive to iron compounds (like haemoglobin), more even than oxygen. All these gasses will be pumped out by those volcanoes. Maybe a tipping-point was reached where the volcanoes sputtered out and CO2 swamped the others.
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