Conduit rusted through

This conduit was completely rusted through, and the feeder conductors were still supplying a panelboard. The probable cause here may have been from the outside underground run where the rusted conduit allowed water to find its way into the raceway. In either case, the conduit will no longer provide the necessary protection to serve its intended purpose — equipment grounding and protection against physical damage. I always call attention to the rule in Section 300-6. This section covers protection against corrosion for metal raceways, elbows, couplings, fittings, supports and support hardware and requires that they be of materials suitable for the environment in which they are to be installed. In Section 300-6, paragraph (a), a ferrous raceway is required to be suitably protected against corrosion inside and outside (except threads at joints) by a coating of approved corrosion resistant material such as zinc, cadmium or enamel. The exception “permits” threads at joints to be coated with an identified electrically conductive compound. Shouldn't this be a mandatory requirement instead of an exception?