Electrochemistry is a science at the interface between chemistry and electricity. It is based on reactions involving both chemical species and electrons. The provoked charge transfers are accompanied by the modification of the oxidation states of these species, thereby changing their physico-chemical states.

This presentation is limited to reactions which develop on a metal surface in contact with a solution (electrolyte). The reactions depend on whether or not the metal is connected to a power supply which artificially changes its potential.

- The potential of a metal in contact with an electrolyte is called the corrosion potential (Ecorr), or open-circuit potential. It represents all electrochemical reactions occurring at the surface of the metal. Since no current is supplied to the metal, all anodic and cathodic reactions balance each other. Ecorr is measured with a multimeter and a reference electrode[1]. Ecorr is not a stable value. It varies according to the metal and the electrolyte used. If Ecorr is increasing with time, the metal passivates. When it decreases the metal corrodes. In practice, changes in electrochemical behaviour are observed during the monitoring of Ecorr over time due to the particular material (polished metal or covered with a corrosion product layer interacting with the chosen electrolyte).

- By connecting the metal to a power supply, the metal will have its potential moved towards negative (cathodic polarization) or positive (anodic polarization) values.

The polarization of a metal is carried out thoughtfully. The objective is to provoke electrolytic reactions which cannot occur electrochemically during immersion and are characterised by their cathodic or anodic potentials. To define the values of these potentials a potentiostat (power supply with 3 terminals) is connected to an electrolytic cell comprising:

          - The metal under study (working electrode - WE),

          - The electrode which ensures the continuity of the current (counter-electrode - CE),
             generally made from platinum,

          - The reference electrode (RE).

The computer-controlled potentiostat is driven by software which enables the following operations: monitoring of Ecorr versus time, plotting of current versus applied potential (linear voltammetry), plotting of current versus time at a constant potential (chrono-amperometry).

Cathodic linear voltammetry enables monitoring of the electrochemical reactions occurring at the surface of a metal, M, which is cathodically polarized in an aqueous electrolyte from Ecorr (see the plot on the left below).  At the start of polarization, only the reduction of the water with dissolved oxygen is observed. The corresponding current is small and constant. A cathodic peak appears when the Mn+ specie (from a superficial corrosion product) becomes reduced. This reaction occurs at Ecp (intersection between the tangent of the peak and the axis of the potential), but it is only at the maximum of the peak where the reaction is fastest. For more negative potentials, water is reduced further, thereby producing hydrogen gas in addition to hydroxide ions. Here the reaction is also denoted by a characteristic peak: EH2, which depends on the particular metal. The same approach is followed when studying anodic reactions (see the plot on the right below). This time the metal becomes oxidized to produce Mn+ before the water oxidizes (producing oxygen gas and hydrogen cations).

If the species dissolved in the electrolyte provoke some electrochemical reactions, this electrolyte is said to be electrochemically active. It is essential to first check whether or not the electrolyte is active before determining whether electrochemical reactions are attributable to corrosion products.

Linear voltammetries applied on a metal M (oxidised) in the presence of a non electrochemically active aqueous solution : cathodic (a) and anodic (b) polarisations.

Once a reaction has been identified (e.g. a reduction reaction), the reduction process can be monitored at the potential corresponding with the maximum of the peak (chrono-amperometry); as shown by the plot below.

In practice, the end-user who wishes to artificially induce an electrochemical reaction does not use a potentiostat; which is considered laboratory equipment. Instead, a  power supply is used. The potential of the material is monitored using a multimeter and a reference electrode (connected in parallel). In the schematic below the metal is connected to the negative terminal of the power supply. It therefore becomes the cathode.

The material of the counter-electrode depends on the electrolyte being used. If it is alkaline, the counter-electrode is made of stainless steel (316) mesh. Lead sheets are employed in a pH neutral electrolyte when lead is being polarized. Platinum, which is considered a non-polarizable electrode, is preferred for any other polarization in neutral electrolyte.

Localised treatment

For the instances where immersion of the metal is not possible (as for composite artefacts made from metallic elements and organic materials which cannot be separated), the techniques presented above can also be applied; but locally using an electrolytic pencil such as the Pleco.

The working electrode is wet by the pad in the nozzle of the electrolytic pencil. The figure below shows the way the electrolyte is continuously renewed within the pad.

[1] An electrode with a predefined potential. Several reference electrodes are available (Ag-AgCl, calomel, mercury sulphate, etc). Their potential is specified versus the standard hydrogen electrode – SHE (0V).