Corrosive Influences

Code Applications August 2020

July’s article provided a preliminary discussion on cathodic action and dissimilar metal reaction with deterioration of various metals, such as steel water and gas lines. Many of the old metal underground natural gas lines and metal water piping have been replaced with fiberglass for gas lines or Schedule 40 PVC for water lines.

Inside the building, the water piping system might be copper piping that was very common in the 1970s and 1980s, and it was later installed using cross-linked polyethylene plastic tubing (PEX), polybutylene (PB) plastic tubing, Schedule 20 or 40 PVC. Based on the information in last month’s article, take care to know and understand what type of gas and water system is installed inside and outside the building to ensure a safe electrical installation and connection for bonding and grounding.

Sections 250.50, 250.52 and 250.53 require a connection of metal underground water piping as part of the grounding electrode system. Section 250.104(A) requires bonding of metal water piping inside the building. Either of these two requirements in Article 250 could cause an inadvertent difference of potential and result in current flow that could affect steel or other similar metals and cause deterioration. As I stated previously, there are solutions, such as dielectric unions and cathodic protection, that can counter the deterioration effect on the metal piping. The definition of galvanic action is an electrochemical action that generates electrical current between two metals of dissimilar electrode potential, as mentioned last month as an anode and cathode.

The dissimilar metal issue and galvanic action does not stop with water and natural gas piping systems. We have also recognized that dissimilar metals also affect metal raceways, such as rigid metal conduit and intermediate metal conduit. Prior to the 2017 NEC , Section 342.14 for intermediate metal conduit and 344.14 for rigid metal conduit stated the following in the 2014 and previous NEC editions: “Where practicable, dissimilar metals in contact anywhere in the system shall be avoided to eliminate the possibility of galvanic action. Aluminum fittings and enclosures shall be permitted to be used with galvanized steel RMC, and steel fittings and enclosures shall be permitted to be used with aluminum RMC where not subject to severe corrosive influences.”

The 2014 NEC handbook explained the permitted use of aluminum rigid conduit with steel fittings and steel enclosures since tests show that galvanic corrosion at steel and aluminum interfaces is minor compared to the natural corrosion of steel or aluminum.

A few new words and a new sentence were added to 342.14 and 344.14, with similar text in 358.14 for EMT in the 2017 NEC dealing with dissimilar metals. The new text, underlined, reads as follows: “Where practicable, dissimilar metals in contact anywhere in the system shall be avoided to eliminate the possibility of galvanic action. Aluminum fittings and enclosures shall be permitted to be used with galvanized steel RMC, and galvanized steel fittings and enclosures shall be permitted to be used with aluminum RMC where not subject to severe corrosive influences. Stainless steel RMC shall only be used with stainless steel fittings and approved accessories, outlet boxes, and enclosures.”

The new word “galvanized” is meant to clarify that this application applies to galvanized versus other types of rigid and intermediate metal conduit. The last sentence recognizes that stainless steel conduit can only be used with stainless steel fittings, approved accessories, outlet boxes and enclosures since there is an increase in galvanic destruction of the stainless-steel components due to the dissimilar metals.

This stainless-steel deterioration due to galvanic action has become such an issue that further change has occurred to each of the aforementioned sections in the 2020 NEC for intermediate metal conduit, rigid metal conduit and EMT. The new text is as follows: “Stainless steel IMC (stainless steel rigid metal conduit or stainless steel EMT) shall only be used with the following: (1) Stainless steel fittings; (2) Stainless steel boxes and enclosures; (3) Steel (galvanized, painted, powder, PVC coated, or similar application) boxes or enclosures when not subject to severe corrosive influences; (4) stainless steel, nonmetallic, or approved accessories.”

An example of severe corrosive influences or environments would be chemical-laden pool areas as covered in 680.14 of the 2020 NEC . In addition, sections 342.10(B) for intermediate metal conduit, 344.10(B)(1) for rigid metal conduit and 358.10(B)(1) for EMT deal with raceways permitted in corrosive environments.

These sections require raceways, elbows, couplings and fittings installed in severe corrosive influences to be protected by corrosion protection approved for the condition.

Protection for locations involving corrosive materials and severe corrosion conditions will often require stainless steel protection for the electrical equipment. Take all of these issues into consideration when installing these systems.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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