Corrosion process ionic electrode potential

Electrocatalysis in metallic corrosion may be classified into two groups Adsorption-induced catalyses and solid precipitate catalyses on the metal surface. In general, the bare surface of metals is soft acid in the Lewis acid-base concept and tends to adsorb ions and molecules of soft base forming the covalent binding between the metal surface and the adsorbates. The Lewis acidity of the metal surface however may turn gradually to be hard as the electrode potential is made positive, and the bare metal surface will then adsorb species of hard base such as water molecules and hydroxide ions in aqueous solution. Ions and molecules thus adsorbed on the metal surface catalyze or inhibit the corrosion processes. Solid precipitates, on the other hand, are produced by the combination of hydrated cations of hard acid and anions of hard base forming the ionic bonding between the cations and the anions on the metal surface. 

A related study on the spectroelectrochemical behaviour of a copper electrode in [C4mim] [BF4] containing benzotriazole (BTAH) has been investigated by cyclic voltammetry and SERS [64]. A considerable decrease in the anodic currents in the presence of BTAH, as seen in the cyclic voltammograms, suggests that BTAH inhibits the oxidation of copper in the ionic liquid. The SERS results have shown that at potentials positive to the PZC for copper in the ionic liquid, a [Cu(I)(BTA)] polymeric film is formed as BTAH interacts with the copper(I). The polymeric film is responsible for the corrosion inhibition process. However, at potentials negative to the PZC, BTAH is found to adsorb chemically on the copper surface. This phenomenon is determined by the observation of the v(Cu—N) stretching mode and by the... 

Figure 9-3. The schematic shows electrochemical corrosion processes associated with ionic contamination on circuit boards and other devices with copper conductors. In the presence of atmospheric moisture, contaminant residues like sodium chloride form sodium ions (positive) and chloride ions (negative). Current flows between metal lines due to migration of ions to electrodes of opposite polarity. For many metals, the anode corrodes when the potential becomes sufficiently positive. The anode metal dissolves and metal ions migrate towards the cathode where they may be redeposited.

Polarization curves. The rate at which the anodic or the cathodic process takes place depends on the potential ( ). The corrosion behaviour of the reinforcement can be described by means of polarization curves that relate the potential and the anodic or cathodic current density. Unfortunately, determination of polarization curves is much more complicated for metals (steel) in concrete than in aqueous solutions, and often curves can only be determined indirectly, using solutions that simulate the solution in the pores of cement paste. This is only partly due to the difficulty encountered in inserting reference electrodes into the concrete and positioning them in such a way as to minimize errors of measurement. The main problem is that diffusion phenomena in the cement paste are slow (Chapter 2). So when determining polarization curves, pH and ionic composition of the electrolyte near the surface of the reinforcement may actually be altered. 

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