CorrosionDOE-HDBK-1015/1-93CORROSION THEORYRev. 0CH-02Page 9Corrosion Theory SummaryIonization is the process of adding electrons to or removing electrons from atomsor 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 itsenvironment.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 reactionsinvolve a transfer of charge. The metal ions go into solution causing the metal tobecome negatively charged with respect to the electrolyte. The difference in thecharge causes a potential to develop and produces a voltage between theelectrolyte and the metal.The oxidation step of the oxidation-reduction process is where an atom (in thiscase a metal atom) releases electron(s) and becomes a positively-charged ion.The areas where oxidation takes place become electrochemical cells made up oftwo different substances. The oxidation step results in a chemical transformationthat is destructive to the metal. The positive metal ions may go into solution, orthey may combine with any available negative ions or water to form ioniccompounds. 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 iscalled activation polarization and is sometimes referred to as hydrogen polarization, or cathodicpolarization, because the polarizing reaction occurs at the cathode.Both concentration and activation polarization decrease the net oxidation-reduction reactionrate. In corrosion processes, activation polarization usually has the greater effect.SummaryThe important information in this chapter is summarized below.
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