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Corrosion of Iron
Figure 6  Representation of Cathodic Depolarization by Oxygen

Chemistry Volume 1 of 2
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O2 +    4H     2H2O Corrosion DOE-HDBK-1015/1-93 GENERAL CORROSION Rev. 0 CH-02 Page 13 Figure 5   Simplified Schematic Diagram of Oxide Corrosion Film on the Surface of a Metal Regardless of the exact diffusion mechanism, the oxide layer represents a barrier to continued corrosion and tends to slow the corrosion rate.  The exact effect of this layer on the corrosion rate depends on the uniformity and tenacity of the film.  If the film is loosely attached, develops defects, or is removed, the metal surface is again exposed to the environment and corrosion occurs more readily. Factors Affecting General Corrosion Rate Like  most  other  chemical  reactions,  corrosion  rates  increase  as  temperature  increases. Temperature and pressure of the medium govern the solubilities of the corrosive species in the fluid, such as oxygen (O ), carbon dioxide (CO ), chlorides, and hydroxides.  A rule of thumb 2 2 is that the reaction rate doubles with a 20?F to 50?F temperature rise.  This linear increase with temperature does not continue indefinitely due, in part, to a change in the oxide film. When water velocity is extremely high, the impact of the water tends to remove the protective oxide layer and some of the metal under it (erosion), thus, exposing more metal to corrosion. Water velocities of 30 to 40 ft per second are usually considered to cause erosion. The presence of oxygen in water to which iron is exposed increases the corrosion rate.  The reason for this increase is the rapid reaction between oxygen and the polarizing layer of atomic hydrogen absorbed on the oxide layer.  The following reaction rapidly removes the polarizing layer. (2-11)







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