A3 Corrosion

Topic

When in contact with a specific medium, this metal degrades through an electrochemical reaction at its surface; it is this electrochemical reaction—rather than a simple, direct chemical reaction—that constitutes the metal's corrosion, as it involves a transfer of electric charge between the metal and the medium, rather than merely a direct exchange of atoms. Because this electrochemical reaction involves such charge transfer, the metal's mass loss rate is—in accordance with Faraday's law—directly proportional to the electric current generated by the reaction itself.

The consumption of metal at the surface during this electrochemical reaction simultaneously produces an oxide layer—the nature of which depends on the metal's composition—and the Pilling-Bedworth ratio predicts whether this layer protects the underlying metal. This ratio compares the volume of the newly formed oxide to the volume of the metal consumed to create it. If the Pilling-Bedworth ratio is close to one, the resulting oxide layer is dense and adherent, protecting the underlying metal and inhibiting the electrochemical reaction; conversely, if the ratio deviates significantly from this value—whether higher or lower—the resulting oxide layer becomes porous or cracks, failing to protect the underlying metal and allowing the electrochemical reaction to continue.

On the metal surface where the oxide layer fails to provide protection, the electrochemical reaction proceeds at a rate determined by the reaction's anodic overpotential—that is, the extent to which the metal surface's electric potential deviates from its equilibrium potential as the reaction progresses. The fact that this anodic overpotential is higher means—in accordance with the exponential behavior described by the Tafel equation—that the electric current produced by this electrochemical reaction is also higher.

Consequently, this model—based on the electrochemical reaction and mass loss rate initially established—determines the metal's actual corrosion current from the intersection of the Tafel curve for this anodic reaction and the corresponding Tafel curve for the complementary cathodic reaction occurring simultaneously in the same medium: at the potential where the two curves intersect, the current produced by one reaction exactly matches the current consumed by the other, and it is this shared current at the intersection point that constitutes the metal's corrosion current. The fact that this corrosion current—derived from that intersection—is higher means that, according to the same Faraday's law used to formulate the model, the metal's mass loss rate is also higher.

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