Its due to the decrease in H+ ions at the site of rusting.  The hdrogen ion is involved in the reduction of O2. So decreasing the concentration of pH will be less favourable for rusting

it becomes apparent that the rusting of iron is primarily due, not to attack by oxygen, but by hydrogen ions.



In order that rust should be formed iron must go into solution and hydrogen must be given off in the presence of oxygen or certain oxidizing agents. This presumes electrolytic action, as every iron ion that appears at a certain spot demands the disappearance of a hydrogen ion at another, with a consequent formation of gaseous hydrogen. The gaseous hydrogen is rarely visible in the process of rusting, owing to the rather high solubility and great diffusive power of this element. Substances which increase the concentration of hydrogen ions, such as acids and acid salts, stimulate corrosion, while substances which increase the concentration of hydroxyl ions inhibit it.