Electrical Metallic Tubing is definitely the most-used of all raceways because of the range of applications it can be used for.  EMT can be used both indoors, and outdoors and has 3 different options of material.  The most common is Galvanized (Zinc-coated) steel. Galvanized EMT is what you’ll come across at Home Depot or in most jobs you work on.  The next is Aluminum EMT.  Aluminum is 1/3 of the weight of steel so it is often used for a cheaper solution, as well as a way to reduce the weight of an overall heavy install.  Finally, there is Stainless steel EMT available for a much higher premium, but with a guarantee that it will not rust or corrode, when installed correctly.

Anytime different metals come into contact with one another they will corrode one another over enough time.  Most often this means over several decades, however depending on the materials it can be a matter of months or years.  Aluminum and galvanized steel are reasonably compatible with one another unless a corrosive agent is introduced which will accelerate the corrosion process. The closer the metals are together on the dissimilar metals chart, the slower the corrosion process will be when an electrolyte is introduced. It’s ok to mix aluminum and galvanized fittings for this reason.

Stainless steel however, is pretty far up the chart when determining the corrosion potential of dissimilar metals. Because of the greater difference in potential between stainless steel and aluminum or stainless steel and galvanized (zinc) steel, stainless steel EMT shall only be used with stainless steel fittings and approved accessories, outlet boxes, and enclosures.

The 2017 National Electrical Code mentions this in 358:

358.14 Dissimilar Metals. Where practicable, dissimilar metals in contact anywhere in the system shall be avoided to eliminate the possibility of galvanic action. Aluminum fittings and enclosures shall be permitted to be used with galvanized steel EMT, and galvanized steel fittings and enclosures shall be permitted to be used with aluminum EMT where not subject to severe corrosive influences. Stainless steel EMT shall only be used with stainless steel fittings and approved accessories, outlet boxes, and enclosures.

 

EMT is installed with couplings and fittings that are secured with setscrew or compression-type fasteners. The tubing itself is not threaded like RMC and IMC. Common sizes of EMT include 1/2-inch, 3/4-inch, and 1-inch. It is commonly used for exposed indoor wiring runs in residential and light commercial construction. If installed outdoors in exposed locations, it must be assembled with special watertight fittings.