Understanding Dissimilar Metal Corrosion A.K.A Galvanic Corrosion
Contact between dissimilar metals occurs very frequently but it is not often a problem. The aluminum head on a cast iron block, the solder on a copper pipe, galvanizing on a steel purlin and the steel fastener in an aluminum sheet are common examples.
Causes of Dissimilar/Galvanic Corrosion
For galvanic or dissimilar or electrolytic corrosion to occur, three conditions must be met:
- The metal join must be wet with a conductive liquid
- There must be metal to metal contact
- The metals must have sufficiently different potentials
When dissimilar metals come into contact with electrolytes such as condensation, rainwater or other sources such as oil, dirt and airborne particles, it can produce an electrochemical reaction. This event allows electron flow from one metal to another. The byproducts of corrosion such as hydroxides, sulfates and oxides take the position of the original matter, but these byproducts can be carried by an electrolyte.
Metals in the galvanic series that are close to each other have little effect on one another. This is due to the fact that when the separation that occurs between two metals in the series elevates, the effects of corrosion are greater on metals that occupy a higher position in the series.
The surface areas of dissimilar metals in contact are also significant in identifying which metal is more prone to accelerated corrosion. It is not a good idea to have a large cathodic surface that is in contact with a small anodic area.
Dissimilar/Galvanic corrosion can only occur if the dissimilar metals are in electrical contact. The contact may be direct or by an external pipe or wire or bolt. If the dissimilar metals are insulated from each other by suitable plastic strips, washers or sleeves then dissimilar corrosion cannot occur. Paint is not a reliable electrical insulator especially under bolt heads or nuts or washers or near edges of sheets of metal.
All metals dissolve to some extent when they are wetted with a conductive liquid. The degree of dissolution is greatest with active or sacrificial metals such as magnesium and zinc and they have the most negative potential. In contrast, noble or passive metals such as gold or graphite are relatively inert and have a more positive potential. Stainless steel is in the middle although it is nobler than carbon steel.
Measures to Prevent Dissimilar/Galvanic Corrosion
- Insulate the two metals electrically
- Metals should be kept away from ionic compounds such as acids, bases & salts.
- Usage of protective layers like Plastic, Varnish & Paints.
- Selection of metals with same electro-potentials.
- Usage of Electroplating & Cathodic Protection System