Corrosion: What it is, what it does, and how to address it
The 1965 edition of the National Electrical Code (NEC) added the requirement that “a raceway be suitable for the corrosive environment to which it is exposed.” Section 300.6, Protection Against Corrosion and Deterioration, states, “Raceways, cable trays, cablebus, auxiliary gutters, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports and support hardware shall be of materials suitable for the environment in which they are to be installed.” Article 300, Wiring Methods, generally applies to all wiring methods unless modified by other articles.
Corrosion, simply stated, is the atmospheric oxidation of metals: iron + oxygen + water = rust. Limit the factors contributing to the corrosion rate of metal, and you extend its service life. The galvanizing processes used on steel rigid metal conduit (RMC), intermediate metal conduit (IMC), and electrical metallic tubing (EMT) have demonstrated that the zinc coating provides long-lasting protection. However, the requirement “shall be of materials suitable for the environment in which they are to be installed” refers to a specific environment.
NEC Article 344, Rigid Metal Conduit, and Article 342, Intermediate Metal Conduit, state that galvanized steel RMC and IMC “shall be permitted under all atmospheric conditions,” and Article 358, Electrical Metallic Tubing, has a similar requirement. The exception is limiting atmospheres (excluding some hazardous locations). But, “where protected by corrosion protection and approved as suitable for the condition” charges the contractor and authority having jurisdiction (AHJ) with the responsibility to employ other methods of corrosion control when deemed necessary. Options include changing to a more suitable material; modifying according to the environment (such as increased ventilation or drainage); and supplementary PVC coating, paints, or wraps.
Lacking other options, factory-applied zinc- and PVC-coated products are the top choice. The requirements for corrosion-resistant coatings and the methods by which they are evaluated are detailed in the UL and ANSI standards to which the product complies. The UL and ANSI C80 standards that cover these products align on the subject of corrosion-resistant coatings. The UL standards’ requirements for exterior and interior corrosion-protective coating are basically the same for steel EMT, IMC, and RMC, and provide for both barrier and cathodic protection.
Paints and wraps are not addressed by the product standards, so we’ll briefly review them here. They’re basically applied in the field, have limited application, and only address exterior protection. They provide barrier protection only, by isolating the metal and inhibiting the formation of elements needed for the corrosion process. When the barrier is compromised, the once-protected metal is at risk. Cathodic protection has the added property of sacrificing itself (corroding) for the metal it is protecting.
While the standards are similar in the types of permitted coatings, they are silent on the process of achieving the desired protection. The standards similarly require that the “coating of zinc shall cover completely, shall adhere firmly at all points, shall be smooth and free from blisters and other defects that can lessen the protective value of the coating, shall be in metal-to-metal contact with the steel…” (UL 6, clause 188.8.131.52, Zinc coating). The evaluation process described is a copper sulfate (Preece) test, but alternate tests contained in ASTM A239-95(2004), Standard Practice for Locating the Thinnest Spot on a Zinc (Galvanized) Coating on Iron or Steel Articles, are acceptable.
All products with a zinc primary coating must pass a minimum coating thickness for both the exterior and interior coating. The interior coating may contain zinc, but common practice among U.S. manufacturers is to apply a zinc-free organic coating to the interior of IMC and EMT. The standards do not stipulate the application process of the galvanized coating, but the market has a preference for a hot-dip steel RMC. The hot-dip process requires submerging the conduit in a bath of molten zinc, which exposes the interior to the zinc. Therefore, all U.S.-produced steel RMC has a zinc interior coating.
The standards recognize installations for which the zinc corrosion protection may not be ideally suited and have provided for the use of other coatings. Because these coatings are nonmetallic, they must undergo a battery of tests in addition to those required for zinc. Two very important properties of the coating are adhesion and abrasion. Abrasion is not covered in the UL and ANSI standards but is in NEMA standard NR-1 2005, Polyvinyl-Chloride (PVC) Externally Coated Galvanized Rigid Steel Conduit and Intermediate Metal Conduit. At least one manufacturer, Thomas and Betts, provides barrier and cathodic protection through its listing of both the PVC and zinc coatings as primary. Additionally, the coatings have hot-dip galvanized threads with a urethane that still permits electrical continuity when joined. Electrical metallic tubing is not available with a factory-applied PVC coating.
Recognizing an environment as a potential corrosion problem and understanding the mechanism of the corrosion problem are important preliminary steps in determining the suitability of available options. For a more comprehensive review of corrosion protection of steel conduit and tubing, read the Steel Tube Institute’s Conduit Committee Tech Talk, “UL and NEC Requirements for Corrosion Protection of Steel Conduit and Electrical Metallic Tubing.”
Brett is the director of codes and standards, electrical division, at Wheatland Tube. Brett currently serves on the NFPA Code-Making Panel 5 of the National Electrical Code Committee; as chair of the STI/NEMA 5RN Barcode Committee and Building Information Modeling Committee, Electrical Product Sub-Committee of the buildingSMART Alliance Specifiers’ Properties Information Exchange (SPie); as co-chair of the STI/NEMA 5RN Codes and Communications and Technical Committees; and also serves on the International Code Council’s Industry Advisory Committee.