Rule 33 of 40 · Chapter V — Water, Weather, and Place
Watch for corrosion at connections
Why this rule exists
Corrosion is a slow electrical fault. An oxidized or galvanically corroded connection grows resistance, resistance makes heat under load, and the joint degrades faster in a cycle that ends in failure, a dropped ground, an open neutral, or a fire. Outdoors and in damp or coastal air it is relentless, and dissimilar metals in contact corrode faster still. The insidious part is that it hides inside the enclosure and progresses for years while everything appears to work. A connection you made clean and tight is not permanent; the environment is always working to undo it.
In practice
In corrosive environments use corrosion-resistant materials, stainless hardware, tinned or listed connectors, 4X enclosures, and apply the specified antioxidant on aluminum and outdoor terminations. Keep moisture out with proper sealing and drainage. During service, open enclosures in exposed locations and look for green or white powder, discoloration, and heat marks; re-terminate anything corroded rather than just retightening it. Protect dissimilar-metal joints with listed connectors rated for both metals. Treat coastal, pool, agricultural, and industrial atmospheres as needing a higher grade of everything.
When it doesn't apply
Antioxidant compounds are specified for certain connections (notably aluminum) and not others; follow the listing rather than applying grease everywhere. Some corrosion is cosmetic surface oxide on a still-sound joint, but if it is at a current-carrying contact, do not gamble. In heavily corrosive or classified settings the material and method requirements are stricter; match them rather than improvising.