Corrosion Theory Summary

Corrosion DOE-HDBK-1015/1-93 CORROSION THEORY Rev. 0 CH-02 Page 9 Corrosion Theory Summary Ionization is the process of adding electrons to or removing electrons from atoms or molecules which creates ions. Conductivity is a measure of the ability of a substance to allow electron flow. Corrosion is the deterioration of a material due to interaction with its environment. Electrolysis is the decomposition by electric current. General corrosion is the process whereby the surface of a metal undergoes a slow, relatively uniform, removal of material. Corrosion is electrochemical in nature because the corrosive chemical reactions involve a transfer of charge. The metal ions go into solution causing the metal to become negatively charged with respect to the electrolyte. The difference in the charge causes a potential to develop and produces a voltage between the electrolyte and the metal. The oxidation step of the oxidation-reduction process is where an atom (in this case a metal atom) releases electron(s) and becomes a positively-charged ion. The areas where oxidation takes place become electrochemical cells made up of two different substances. The oxidation step results in a chemical transformation that is destructive to the metal. The positive metal ions may go into solution, or they may combine with any available negative ions or water to form ionic compounds. An example of the oxidation step is: Zn Zn + 2e +2 - The layer of absorbed atomic hydrogen is said to polarize the cell. This type of polarization is called activation polarization and is sometimes referred to as hydrogen polarization, or cathodic polarization, because the polarizing reaction occurs at the cathode. Both concentration and activation polarization decrease the net oxidation-reduction reaction rate. In corrosion processes, activation polarization usually has the greater effect. Summary The important information in this chapter is summarized below.