Corrosion and Galvanic Compatibility Knowledge
Corrosion Testing Equipment
Galvanic corrosion occurs when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte. It may also take place with one metal with heterogeneities (dissimilarities) (for example, impurity inclusions, grains of different sizes, difference in composition of grains, or differences in mechanical stress). A difference in electrical potential exists between the different metals and serves as the driving force for electrical current flow through the corrodant or electrolyte. This current results in corrosion of one of the metals. The larger the potential difference, the greater the probability of galvanic corrosion. Galvanic corrosion only causes deterioration of one of the metals. The less resistant, more active one becomes the anodic (negative) corrosion site. The stronger, more noble one is cathodic (positive) and protected. If there were no electrical contact, the two metals would be uniformly attacked by the corrosive medium. This would then be called general corrosion.
For any particular medium, a list can be made arranging metals sequentially from most active, or least noble, to passive, or most noble. See Galvanic Compatibility for design applicability.
Galvanic corrosion is of particular concern in design and material selection. Material selection is important because different metals come into contact with each other and may form galvanic cells. Design is important to minimize differing flow conditions and resultant areas of corrosion buildup.
In some instances, galvanic corrosion can be helpful in some applications. For example, if pieces of zinc are attached to the bottom of a steel water tank, the zinc will become the anode, and it will corrode. The steel in the tank becomes the cathode, and it will not be effected by the corrosion. This technique is known as cathodic protection. The metal to be protected is forced to become a cathode, and it will corrode at a much slower rate than the other metal, which is used as a sacrificial anode.