Dissimilar Metal Corrosion (Galvanic Corrosion)

On every project, there are a wide variety of choices to make concerning the materials for the job. One important factor to consider is whether or not the items you are using are compatible with each other to ensure the long-lasting life of the products.

Corrosion has a key impact on the life of those products. While oxidation can be dealt with in many ways, galvanic corrosion due to the use of dissimilar metals can accelerate corrosion on key components. But what is galvanic corrosion?

When two dissimilar metals are in contact with each other, and when there is the presence of an electrolyte, a less-active metal will become a cathode, resulting in the transfer of electrons from the more-active metal (anode). This is similar to the way a battery works to power electronic equipment. The differences in “electrical potential” will determine the severity of the accelerated corrosion that takes place.

But how do we determine if there will be an issue or not? We need to look at the “Galvanic Series” and see what the electro-potential differences are — in other words, how close various metals are on the table. Galvanized steel, zinc, uncoated steel, and aluminum are very close together and in most cases can work well when combined in assemblies. This is fortunate because galvanized steel conduit is compatible with the vast majority of outlet boxes and connectors in use today. This very activity of dissimilar metals is what allows the zinc galvanization to protect the underlying steel products from corrosion, thereby providing reliable service life.

On the other hand, less-active metals such as stainless steel (passive), copper and brasses are much farther away from galvanized steel and aluminum — when these metals are combined, severe corrosion can occur in the presence of an electrolyte. While pure water isn’t a good electrolyte, the various minerals and contaminants in it make water a good conductor of electrons, and salt water is even more severe. It is important to note that in addition to direct metal-to-metal contact, water and other liquids that may drip from less-active metals, such as copper, can also lead to localized corrosion of the more-active metals. This corrosion can occur from condensation or rain that may drip from a copper water line, copper bonding wires or copper contacts.

You can avoid the serious effects of galvanic corrosion by considering the compatibility of the metals you use together, as well as by minimizing the presence of an electrolyte where possible.

Read about the UL and NEC requirements for corrosion protection in our Tech Talk.

August 2017