Mechanism of Rusting under Different Conditions

Abstract

Rust 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 is found to be connected with the formation of ferrous sulphate; when once this has been produced, the presence of sulphur dioxide in the gas phase is not needed for the continuation of rusting. Here again there is evidence of an electrochemical mechanism. The anodic reaction is Fe = Fe²⁺ + 2e, whilst the cathodic reaction is the reduction of FeOOH to Fe₃O₄, which is then oxidised by air to give a larger quantity of FeOOH than previously existed. Since the fresh rust is formed at a sensible distance from the site of anodic attack, the rust, although adherent, is not protective. This action will continue only if an electrochemical conducting limb (FeSO₄ solution) and an electronically conducting limb (Fe₃O₄) are provided. If the humidity is so low that the Fe₃O₄ solution dries up, rust-formation by this mechanism will cease—explaining the principle of critical humidity, as developed by Vernon and Buckowiecki. If the metal does not form an elect ronically conducting intermediate oxide, a mechanism providing rapid atmospheric attack will be impossible; this explains why zinc, which under immersed conditions is corroded more quickly than iron, suffers atmospheric attack more slowly; indeed a zinc coating is commonly used to protect iron under atmospheric conditions.