Metallurgical Failure Analysis, Mark Douglas Bell, PE, Consulting Metallurgical Engineer, San Diego, California, USA Bell Metallurgy
Metallurgical Failure Analysis
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Mark Douglas Bell, PE
Consulting Metallurgical Engineer

Galvanic Corrosion



This corrosion can be used to our advantage. Galvanized steel, zinc electrodes in marine environment and sacrificial electrodes in tanks and water heaters are examples of using galvanic corrosion to an advantage.

The attached chart lists metals and alloys in order of their electromotive potential. Knowing the distance between metals, measured in millivolts, is the key to understanding galvanic corrosion. The noble metals gold and silver will have a large potential difference when placed in contact with steel or aluminum. The metals toward the anodic end of the chart will corrode to protect metals that are more cathodic, toward the gold and silver.

Three factors must be present for galvanic corrosion to occur:
  1. Dissimilar metals. The greater their distance on the chart will drive the corrosion of the more the anodic metal and likewise the more cathodic metal will be protected. In some metals, stainless steels for example, there may be a potential difference between areas on the same component. One area will corrode, one will be protected.
  2. An electrical circuit must be completed for a current to flow. As the current flows there is a loss of metal on the anode.
  3. Both metals must be in contact with an electrolyte. If the joint is dry then there will be no current flow and therefore no corrosion.
Galvanic corrosion can be controlled by removing one of the three factors. Don't use dissimilar metals, use an insulator to completely insolate the joint and keep the joint free of moisture, oil and debris, all electrolytes are practical rulers of controlling galvanic corrosion. There is an advantage of galvanic corrosion in that the cathode is protected by the anode. Therefore a large anode will protect the relatively small cathode. The loss of metal is spread over the large anode area. Galvanized steel when scratched will not allow the steel substrate to corrode. Instead the loss of metal is spread over the surface of the zinc and there will be no steel rusting as long as is some zinc remains in the immediate area of the scratch.


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