If two metals are to be used adjacent to each other in a particular design a ‘galvanic cell’may be set up. One option open to the designer in this situation would be to place a thirdmetal in the vicinity to protect the other two metals from corrosion. Explain how this mightwork to reduce or eliminate corrosion. In particular, make reference to the relative positionof the metals in the electrochemical series. State other things that a designer could do tolimit galvanic corrosion
Question
If two metals are to be used adjacent to each other in a particular design a ‘galvanic cell’may be set up. One option open to the designer in this situation would be to place a thirdmetal in the vicinity to protect the other two metals from corrosion. Explain how this mightwork to reduce or eliminate corrosion. In particular, make reference to the relative positionof the metals in the electrochemical series. State other things that a designer could do tolimit galvanic corrosion
Solution
The process you're referring to is known as "cathodic protection." This involves placing a third metal, known as a "sacrificial anode," in close proximity to the two metals in question. This sacrificial anode should be a metal that is more reactive (i.e., higher in the electrochemical series) than the other two metals.
In a galvanic cell, the more reactive metal will corrode preferentially, protecting the less reactive metals from corrosion. This is because the more reactive metal will give up its electrons more readily, thus becoming the anode of the cell. The less reactive metals will accept these electrons, becoming the cathode and thus avoiding corrosion.
In addition to using a sacrificial anode, a designer could also limit galvanic corrosion by:
- Using metals that are close together in the electrochemical series, as these will have similar reactivity and thus less tendency to form a galvanic cell.
- Isolating the metals from each other using a non-conductive barrier, such as a coating or gasket, to prevent the flow of electrons that drives the galvanic cell.
- Designing the structure so that the less reactive (more noble) metal has a larger surface area than the more reactive metal. This will slow the rate of corrosion of the more reactive metal.
- Controlling the environment, for example by reducing the amount of electrolyte (water, humidity) present, as this can also slow the rate of galvanic corrosion.
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