Mechanism of Rusting Under Different Conditions

AbstractRust formed from iron immersed in a salt solution is a secondary product precipitated where the soluble anodic and cathodic products meet; being mainly produced at a distance from the metallic surface, it cannot protect, and the corrosion proceeds at a nearly uniform rate; only if either anodic or cathodic product is sparingly soluble will corrosion be slowed down—the two cases being known as anodic and cathodic inhibition.The currents passing between anodic and cathodic areas have been measured and are found to correspond to the corrosion-rate in the sense of Faraday's law. During corrosion by drops of salt solution placed on a horizontal surface, the current passing between the central anode and the peripheral cathode has been shown up by a magnetic method due to Blaha. In these cases the electrochemical mechanism is not in doubt.Atmospheric corrosion set up by saline dust-particles can be explained by a mechanism like that of drop-corrosion. That set up by moist air containing sulphur dioxide i...