Corrosion of Metals: Electrochemical Reactions

Metallic Corrosion

• Metallic corrosion is an electrochemical process involving both oxidation and reduction reactions.
• Oxidation occurs at the anode, resulting in the loss of valence electrons from the metal atom, which may form an insoluble compound or go into the corroding solution.
• Reduction occurs at the cathode, where the electrons are transferred to at least one other chemical species.
• The corrosion environment dictates the type of reduction reaction that will occur.

Galvanic Couple

• A galvanic couple demonstrates the varying degrees of ease with which metals oxidize.
• When two metals are in an electrolyte, one metal (the anode) corrodes, while a reduction reaction occurs at the other metal (the cathode).
• The electric potential established between the anode and cathode indicates the driving force for the corrosion reaction.

Standard Emf and Galvanic Series

• The standard emf series ranks metallic materials based on their tendency to corrode when coupled to other metals.
• The ranking is based on the magnitude of the voltage generated when the standard cell of a metal is coupled to the standard hydrogen electrode at 25°C (77°F).
• The galvanic series ranks the relative reactivities of metals and alloys in seawater.

Half-Cell Potentials

• Half-cell potentials in the standard emf series are thermodynamic parameters valid only at equilibrium.
• Corroding systems are not in equilibrium, so these potentials are not directly applicable.
• The magnitudes of these potentials do not indicate the rates at which corrosion reactions occur.