Corrosion of iron, commonly known as rusting, is not a simple direct chemical reaction but an electrochemical process in which different microscopic regions on the metal's surface behave as tiny anodes and cathodes, with the metal itself acting as the conductor and any surface moisture serving as the electrolyte. At anodic regions, iron atoms lose electrons and pass into solution as Fe²⁺ ions; at cathodic regions, dissolved atmospheric oxygen, with the help of water, gains these electrons and is reduced to hydroxide ions. The resulting Fe²⁺ and OH⁻ ions then combine and are further oxidised by atmospheric oxygen to form hydrated iron(III) oxide, i.e., rust:

4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (rust)

Because this is fundamentally an electrochemical cell reaction, its rate depends heavily on the availability of an electrolyte and on the presence of substances that make that electrolyte more conductive or more corrosive:

• High humidity supplies the thin film of moisture on the metal surface that is needed to act as the electrolyte, completing the circuit between the anodic and cathodic sites on the metal. In very dry air, this electrochemical cell simply cannot operate effectively, so rusting proceeds very slowly, if at all.
• Acidic pollutants, such as oxides of sulphur and nitrogen (which dissolve in atmospheric moisture to form sulphurous, sulphuric, or nitric acid) and dissolved carbon dioxide, lower the pH of the surface moisture film and increase its ion concentration and conductivity. This accelerates both the anodic dissolution of iron and the overall rate of the corrosion cell, which is why iron structures corrode noticeably faster in humid, industrially polluted, or coastal environments.

By contrast, cold temperatures generally slow down reaction kinetics, and low atmospheric pressure has little direct bearing on corrosion rate. Continuous sunlight combined with dry air does not supply the electrolyte needed to sustain the electrochemical process, so it does not meaningfully accelerate rusting. Low humidity together with the absence of pollutants removes both the electrolyte and the corrosive agents from the picture, making this the condition least favourable to corrosion — essentially the opposite of what accelerates it.