What is Galvanic Corrosion?

In previous posts, we have discussed the different types of corrosion and how detrimental corrosion can be to a conduit system. Corrosion can significantly weaken the structure, causing a need for maintenance or complete replacement. One type of corrosion, galvanic corrosion, can be reduced or avoided.

Galvanic corrosion occurs when two electrochemically dissimilar metals are in contact with each other in an environment where an electrolyte solution, such as saltwater, is present. A moist/wet environment or immersion in water creates a flow of current (conductive path) from one metal to the other. Every alloy or metal has a distinctive corrosion potential in a given electrolyte. The greater the difference in the metals’ nobility/corrosion/electrical potential, the more likely the couple is to experience galvanic corrosion.

A Galvanic Series table provides a ranking of metals and their nobility/electrical potential in an electrolyte solution. Metals listed at the top of the table are most anodic and least noble; the metals near the bottom are most cathodic and most noble. The anodic metals will corrode preferentially while the cathodic metals corrode little or remain intact.

Environments for Galvanic Corrosion

Galvanic corrosion occurs when the following conditions exist:

1. A direct contact between two dissimilar metals with different corrosion potentials,
2. A conductive electrolyte solution, such as saltwater, connecting the two metals on a regular basis.

If any of these conditions are not present, galvanic corrosion cannot occur.

Another factor that affects the potential for, and amount of corrosion, is the surface areas, both precise and relative, of the metals in contact. For example, if carbon steel or galvanized screws were used to fasten a stainless steel post base, significant galvanic corrosion would result in premature failure of the screws, whereas if stainless steel screws were used to fasten down a carbon steel or galvanized post base, little or no galvanic corrosion would be expected.

Preventing Galvanic Corrosion

Electrical raceway designers and installers can prevent galvanic corrosion by:

• Using a conduit system comprised of the same metal
• When joining dissimilar metals in a conduit system, choose metals that are as close together as practical on the Galvanic Series Table
• Insulating the joint to break the electrical connection
• Designing the joint so that the anodic area is large relative to the cathodic area, which will reduce the noticeable effects of corrosion

If coupling dissimilar metals within a stainless raceway is unavoidable, IMOA has a resource available (Avoiding Building & Structure Galvanic Corrosion) offering design advice and steps that can be taken to mitigate the risk of galvanic corrosion. 

Installing an entire system comprised of one metal, such as corrosion resistant stainless steel, avoids connections between dissimilar metals and eliminates the risk of galvanic corrosion. Gibson Stainless offers a full line of stainless steel conduit and fittings to build a rigid conduit system entirely in stainless.