For functional purposes, conduit is open to the surroundings if you don't use particular Conduit Fittings. While the initial intention of a metallic raceway has been the security of cables from mechanical damage, the metallic raceways of today serve another function. These days, we expect them to take fault currents that are potentially deadly. That means we have to pay particular focus on component selection.
Fittings come in many substances, such as cast aluminum, cast wrought iron, cast magnesium, and manufactured steel. Because all recorded fittings meet minimal performance standards, the Code does not specify which sort of material you need to use for all those fittings. For this reason, you need to base your choice on design factors or personal preference. In addition, Your local Conduit Fittings Manufacturers will understand which substance most installers use and might inventory only fixtures of the stuff.
If you have any doubts, contact one of the Conduit Fittings Suppliers with the particulars of your application and get an engineering recommendation.