New Standards Boost Corrosion Protection for Steel Conduits

In electrical engineering, selecting the appropriate wiring method is crucial, with lifespan often being the primary consideration. The corrosion resistance of materials directly impacts the reliability and durability of the entire system across various installation environments. While no material is entirely immune to corrosion, the process is controllable. Steel conduit systems are highly favored for their exceptional mechanical strength and long-term protection of wires and cables. However, due to the diversity of installation environments, accurately predicting the lifespan of steel conduit systems is nearly impossible. Therefore, a thorough understanding of corrosion protection requirements in steel conduit product standards, adherence to the National Electrical Code® (NEC®) , and the judicious application of supplementary corrosion protection measures are key to selecting the optimal system for specific environments.

U.S. steel conduit manufacturers, including members of the National Electrical Manufacturers Association (NEMA) 5RN Section and the Steel Tube Institute of North America (STINA) American Conduit Committee, produce steel conduit products that meet the highest standards. These products include rigid steel conduit (RSC), intermediate metal conduit (IMC), electrical metallic tubing (EMT), and associated elbows, connectors, and couplings. To ensure compliance with NEC requirements, all raceways must be certified. NEMA/STI member products are certified to Underwriters Laboratories (UL) standards: UL 6 for RSC and associated components, UL 1242 for IMC, and UL 797 for EMT.

While UL standards don't include explicit lifespan testing, they do establish rigorous testing and performance requirements for protective coatings on steel conduits, EMT, and related components. Typically, the outer surface (OD) coating is zinc, while the inner surface (ID) may feature zinc or organic coatings. UL employs the copper sulfate test (commonly known as the Preece test) to evaluate zinc coating quality, ensuring adequate corrosion protection. A sample passes if no bright, adherent copper deposits appear after four 60-second immersions in copper sulfate solution.

The process of applying zinc to steel surfaces, known as galvanization, has protected steel from rust for over 200 years. Zinc's unique properties make it ideal for steel corrosion protection. First, it creates a physical barrier between steel and the environment. Second, it provides sacrificial (galvanic) protection. Since steel has a more positive potential than zinc (attracting electrons from zinc), current flow from zinc to steel reduces steel's corrosion rate. Thus, the zinc coating "sacrifices" itself to protect the steel. Even with localized damage, galvanization continues to protect the steel. White powder on conduit or EMT surfaces indicates active zinc protection (zinc oxide), while red indicates steel rust (iron oxide). UL permits supplemental coatings over primary corrosion protection (e.g., zinc), which most U.S. manufacturers apply for additional protection.

For RSC and IMC, UL requires protective coatings on conduit threads until installation. These products ship with a connector on one end and a thread protector on the other, often color-coded by size: blue for even sizes, black for 1/2, and red for 1/4 in RSC; orange for even sizes, yellow for 1/2, and green for 1/4 in IMC.

In 1965, the NEC added a requirement that "raceways must be suitable for the corrosive environment they're exposed to." With no clear method to demonstrate suitability, UL conducted investigations, field tests, and lab tests, resulting in supplemental corrosion protection guidelines published in UL's General Information for Electrical Equipment (White Book) and Electrical Construction Equipment Directory (Green Book).

Concrete Environments: Both concrete and soil present high corrosion risks. UL guidelines state that galvanized rigid steel conduit (GRC) or IMC in concrete typically requires no additional protection. For EMT in above-ground concrete slabs, additional protection is usually unnecessary, but below-ground slab installations may require it.

Soil Environments: UL notes that GRC in soil contact generally needs no extra protection unless soil resistivity falls below 2,000 ohm-cm (measured by local utilities). The authority having jurisdiction (AHJ) determines if additional protection is needed. EMT in soil contact typically requires supplemental protection.

Concrete-to-Soil Transitions: Severe corrosion can occur where steel conduit or EMT transitions from concrete to soil. NEMA/STI manufacturers recommend at least 4 inches of additional protection on both sides of the transition point. In coastal areas, the same approach protects EMT transitioning from concrete to salt air.

Understanding NEC rules is essential for determining additional corrosion protection requirements. NEC Article 344 covers rigid metal conduit (including steel, aluminum, red brass, and stainless steel), Article 342 addresses IMC (steel only), and Article 358 covers EMT. Article 300.6 (Protection Against Corrosion and Deterioration) also contains critical information.

Steel and stainless steel rigid conduits are permitted in "all atmospheric conditions and occupancies," including concrete, direct burial, and severely corrosive areas if "provided with corrosion protection and approved for the condition." UL-listed steel conduit meets this through its zinc coating (typical), with the AHJ approving the installation. Aluminum rigid conduit requires AHJ-approved additional protection in concrete or direct burial.

IMC requirements mirror those for rigid steel conduit. EMT is permitted in concrete, soil, or severely corrosive areas if properly protected and approved. Galvanic action between aluminum and steel is negligible, allowing their combined use if not exposed to severe corrosion.

NEC Article 300.6 includes requirements for field-cut threads: where corrosion protection is needed, threads must be coated with an approved conductive, corrosion-resistant compound (typically zinc-rich paint or UL-listed alternatives).

While steel conduit and EMT coatings provide excellent protection, highly corrosive environments may require supplementary measures like paint, tape wraps, heat-shrink wraps (all requiring AHJ approval), or factory-applied PVC coatings over the primary coating.

Paint: Acceptable options include asphalt coatings, zinc-rich paints, or acrylic, polyurethane, or weather-resistant epoxy paints (avoid oil-based or alkyd paints). Surface preparation is critical—clean, rinse, and dry without abrasion that might damage the zinc layer. Compatible primers or two-coat systems enhance protection.

Tape Wraps and Heat-Shrink Wraps: Specialty high-adhesion tapes must overlap to fully cover conduits and fittings. Heat-shrink wraps don't require a heat source. Manufacturers provide installation guidance.

PVC-Coated Conduit: UL standards (UL 6, UL 1242, UL 797) address supplemental coatings, which needn't meet primary corrosion protection requirements. Non-metal coatings are evaluated for flame spread, impact on primary protection, coupling fit, and electrical continuity. If PVC is listed as a primary method alongside galvanization, it must also pass salt fog, humid CO ₂ -SO ₂ -air, and UV/water tests. NEMA RN-1 provides PVC coating specifications to aid selection.

Field-cut threads on PVC-coated conduit require the same NEC-mandated conductive, corrosion-resistant coating, available from manufacturers or as UL-listed products.

Corrosion is complex, influenced by factors like galvanic potential/resistance (between anodic/cathodic areas), dust, chemicals, pH, temperature, and humidity. Proper product selection, maintenance, and environmental control can slow corrosion.

Beyond concrete and soil, dust can be highly corrosive (e.g., PVC-coated conduit isn't suitable for Class II locations due to static charges). Chemicals affect both metal and non-metal conduits—manufacturers can provide chemical resistance data, but local experience remains the best applicability gauge.

In liquid chemical environments, pH affects corrosion. According to the American Galvanizers Association, galvanizing performs well in solutions with pH above 4.0 and below 12.5, while aluminum suits pH 4–9.

Galvanized steel conduit and EMT feature excellent corrosion protection coatings that ensure long service life. In severely corrosive environments, supplementary protection can further extend system longevity.