H3O+ eH + H2OCRUD AND GALVANIC CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 24Figure 10 Galvanic Corrosion at Iron-Copper Pipe JunctionFigure 10 illustrates that galvanic corrosion occurs when two different metals are in contact andexposed to an electrolyte.Figure 10 shows the junction of iron and copper pipes containing a solution of a copper salt. Theoxidation potential of iron is sufficiently greater than that of copper so that iron is capable ofreducing Cu ions to copper metal. In this case, iron corrodes near the junction, and additional+2copper builds up on the copper pipe near the junction.The solution to which the metal junction is exposed need not contain a salt of one of the metalsfor galvanic corrosion to occur. If the iron-copper junction were exposed to water without Cu+2ions, the reduction reaction would be as shown in Equation (2-4).(2-4)Again, iron would corrode near the junction, but in this case hydrogen would be formed on thesurface of the copper.PreventionofGalvanicCorrosionA method called cathodic protection, discussed previously in this module, is often used to retardor eliminate galvanic corrosion. One of several ways of accomplishing this is to attach a thirdmetal to the metals to be protected. This metal must have an oxidation potential even greaterthan that of the metal to be protected. The most active metal then tends to corrode in place ofthe protected metal. The metal that corrodes to protect another metal is called a sacrificial anode.This method is applied in the original design of structural materials. Zinc is a common sacrificialanode and is often used in cooling water systems that contain seawater.
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