What is Corrosion?
The 1965 edition of the National Electrical Code® (NEC®) added the requirement that “raceways be suitable for the corrosive environment to which it is exposed.” Section 300.6, Protection Against Corrosion and Deterioration, requires that, “Raceways, cable trays, cable bus, 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 apply generally to all wiring methods unless modified by other articles.
Corrosion, simply put, is the atmospheric oxidation of metals: iron + oxygen + water = rust. Limit the factors contributing to the corrosion rate of metal, and you extend the service life of the metal. When steel is galvanized, the processes used on RMC, IMC and EMT have demonstrated that the zinc coating provides long-lasting protection. However, the requirement that it “…shall be of materials suitable for the environment in which they are to be installed” indicates a need to be more specific.
While Article 344, Rigid Metal Conduit, and Article 342, Intermediate Metal Conduit, states that galvanized steel RMC and IMC “shall be permitted under all atmospheric conditions” — and Article 358, Electrical Metallic Tubing, has a similar requirement, except limiting atmospheres (excluding some hazardous locations) — “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. This could mean changing to a more suitable material, a modification to the environment (such as increased ventilation or drainage), or the use of supplementary PVC coating, paints or wraps.
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 our Tech Talk article, UL and NEC Requirements for Corrosion Protection of Steel Conduit and Electrical Metallic Tubing.