Electrochemical techniques have a wide range of application, but their use in corrosion and electroplating tends to be concerned with trying to find out about properties of the metal-solution interface: for example, the rate of reactions at the surface, the nature of films on the surface or the morphology of the surface. The basic tools available to us are voltage and current. The voltage across the interface can be changed and the current recorded or vice versa. From these two parameters, we must attempt to deduce everything we can about what is happening at the interface. When we immerse a metal in solution, there will be a tendency for the metal to react with the solution, either with metal atoms dissolving as cations or cations already in the solution depositing as metal atoms:
As a result of these reactions, the metal will tend to accumulate a negative or positive charge. The build-up of this charge on the metal will change its potential in such a way as to inhibit the reaction generating the charge until the potential reaches a value at which the rates of the two reactions are equal and opposite. This is known as the equilibrium potential, and is the potential the metal will adopt in the solution in the absence of any other reactions.
It is very important to appreciate that when a piece of metal is sitting in a solution at its equilibrium potential, this does not mean that the rates of the metal dissolution and reprecipitation reactions are zero. Instead it implies that the rates of the two reactions are equal. Since electrochemical reactions invariably involve a transfer of charge, we can define their rates in terms of charge/unit area/unit time or current density. When the metal dissolution and reprecipitation reactions are in equilibrium, we refer to the (equal and opposite) rates of each of the two reactions as the exchange current density.
In corroding systems other anodic reactions are possible, the two most important being the reduction of dissolved oxygen to hydroxyl ions and the reduction of hydrogen ions or water molecules to hydrogen gas:
The balance between one or other of these cathodic reactions and the metal dissolution reaction results in a rate of reaction given by the corrosion current density. One of the main applications of electrochemical methods to the study of corrosion is the estimation of the magnitude of the corrosion current density. Electrochemical techniques are also used to study the mechanisms of corrosion processes.
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