O2+ 4H2H2OCorrosionDOE-HDBK-1015/1-93GENERAL CORROSIONRev. 0CH-02Page 13Figure 5 Simplified Schematic Diagram of Oxide Corrosion Film on the Surface of a MetalRegardless of the exact diffusion mechanism, the oxide layer represents a barrier to continuedcorrosion and tends to slow the corrosion rate. The exact effect of this layer on the corrosionrate depends on the uniformity and tenacity of the film. If the film is loosely attached, developsdefects, or is removed, the metal surface is again exposed to the environment and corrosionoccurs more readily.FactorsAffectingGeneralCorrosionRateLike most other chemical reactions, corrosion rates increase as temperature increases.Temperature and pressure of the medium govern the solubilities of the corrosive species in thefluid, such as oxygen (O ), carbon dioxide (CO ), chlorides, and hydroxides. A rule of thumb2 2is that the reaction rate doubles with a 20?F to 50?F temperature rise. This linear increase withtemperature 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 protectiveoxide 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. Thereason for this increase is the rapid reaction between oxygen and the polarizing layer of atomichydrogen absorbed on the oxide layer. The following reaction rapidly removes the polarizinglayer.(2-11)
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