In the true definition of a conduit yes a steel sleeve is in that category.
Steel conduit weighs more, provides more protection, and costs more than plastic conduit. Plastic conduit weighs less, is easier to work with, and costs less than steel conduit.
Conduit is piping for electrical wiring. Steel conduit is the most common conduit used. It is often left visible in industrial atmospheres and can be seen running to electrical outlets, lighting panels, electrical panels, etc.
One can find information about conduit fittings from various sources. Some of those sources are Grainger, Home Depot, Prime Conduit, Steel Conduit and Lowes.
steel and plastic conduits
it is stronger and live for a long time
PVC is cheaper but is used differently. PVC is usually baried in ground and emt usually runs exposed. You can use PVC exposed if it is schedule 80 gray PVC electrical conduit. Schedule 40 is for under ground
There are no advantages or disadvantages over which type of junction box to be used on electrical wiring. As long as the junction box is of an approved type (CSA or UL approved). It is cost or location which dictates which boxes will be used. PVC boxes for home wiring are the cheapest. Aluminium or steel are usually used in rigid or EMT conduit systems. PVC again when the conduit system involves underground wiring. In large electrical installations it is the electrical engineer that states what the conduit system type is and what type of electrical junction boxes that must be used.
One could purchase aluminum conduit at home improvement retailers such as Home Depot, Lowe's or Rona. One could also order it through a wholesaler such as American Conduit.
The old sleeve and cap were not stainless steel or perhaps the hardware used to affix the cap is not stainless steel.
In electrical terms it represents Rigid Steel Conduit. It is also known as Rigid Metal Conduit (RMC).This is not to be confused with Electrical Metallic Tubing (EMT) also known in the trade as thin wall conduit.
There are many types of conduit used for the containment of electrical wires. PVC is a non-metallic option. If you need something that provides mechanical protection, EMT is a steel conduit with a thin wall. Need even more mechanical protection, you can use rigid conduit. It comes in steel or aluminum, and must be threaded together.
A 770 Greenlee hydraulic pipe bender bend is used for rigid steel conduit from 2" to 4".