Code Change Summary: Changes were made regarding the compatibility of Rigid Metal Conduit (RMC), fittings and dissimilar metals.

Galvanic action leading to corrosion can occur when two dissimilar metals are in contact with one another in an electrically conductive fluid (electrolyte).

For this to occur, the following conditions must exist:

• There must two dissimilar metals in contact with one another; an anode (the less noble metal that will corrode) and a cathode (the more noble metal that will be protected through the corrosion process).
• There must be different corrosion potential between the anode and the cathode (see illustration).
• There must be an electrolyte (electrically conductive fluid) in contact with the two metals.

During the corrosion process, the less noble metal (anode) deteriorates at an accelerated rate and deposits portions of itself onto the cathode as the anode tries to return to its original state before it was refined (ore).

In the 2017 NEC^®, Section 344.14 changed to recognize the significance of the differences in potential between the most commonly used materials for RMC which are galvanized coated (zinc) steel, aluminum, and stainless steel.

Aluminum and galvanized steel are reasonably compatible with one another (see illustration) unless a corrosive agent is introduced which will accelerate the corrosion process. The closer the dissimilar metals are together on the dissimilar metals chart, the slower the corrosion process will be when the two dissimilar metals are in contact and an electrolyte is introduced. Likewise, the further the two dissimilar metals are from one another on the dissimilar metals chart, the faster the corrosion process will be when the dissimilar metals are in contact and an electrolyte is introduced.

Relative surface area of the two dissimilar metals is also a factor. The rate of corrosion will depend on which of the two dissimilar metals has the larger surface area and the nobility of the two metals. Example: a small stainless-steel (more noble) screw or rivet driven through a large galvanized steel (less noble) surface will last significantly longer when introduced to an electrolyte than a small galvanized steel screw or rivet driven into a large stainless-steel surface.

The above concept drove the 2020 code change in this section. Installing a small stainless-steel fitting or coupling on a larger galvanized RMC is acceptable and the rate of galvanic action is minimal, whereas installing a stainless-steel RMC with a small galvanized steel fitting would be the reverse.

Similar code changes occurred in 342.14 for Intermediate Metal Conduit and 358.14 for Electrical Metallic Tubing.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC^® edition of NFPA 70.