I purchased a Rheem Performance 40 Gal water heater (Model XE40M06ST45U1). I've removed the anode rod in the water heater and discovered there's substantial corrosion. The anode rod that came with the water heater was a magnesium anode rod. I've learned that an aluminum anode rod does better in areas with hard water (which I have in my area). I'm hoping to replace the magnesium anode rod with an aluminum one. The part number for the magnesium anode rod is SP11309T. But I can't find an aluminum anode rod with the same diameter and length (0.750 In. Diameter X 39-3/8 In. Long). Is there a way to find the aluminum anode rod replacement for the magnesium anode rod?
Anodes are usually universal. The thicker and longer they are the longer they will last. If you can find one that is a little bit shorter it won't be a problem, even longer will work if it will fit. If it's not as thick, not a problem either. The threaded part should be the same for most if not all anodes.
I found this for you
Aluminum Anode Rod - 0.75 In. Diameter, 42 In. Length
at : Rheem
The length comes in 3 (screw on) parts each 14 inches long, a solution for people who do not have 50 inch clearance above the water heater.
The anode rod is supposed to corrode. It is 'sacrificial': it corrodes first, in order to protect the other materials in the water heater. It's how galvanizing works. The same thing is done on boats with metal propellers and prop shafts.
When different metals are exposed to an electrolyte (water; for boats this might include seawater), they create a battery, and the more electronegative material will be eaten away. Perhaps you made a battery using zinc and copper strips pushed into a lemon or potato in science class? The zinc will have developed a coating of zinc compounds as it was corroded. You have seen disposable batteries that leaked because the outer shell had been eaten away by this reaction.
The simplest way to protect equipment from that corrosion (no external power source etc. required), is to add a sacrificial anode that corrodes first; it is sacrificed to protect the more valuable parts of the system.
In hot water heaters, magnesium, aluminum or an alloy is typically used; for galvanizing, zinc.
Without the sacrificial anode the steel water heater shell, pipes, etc. will corrode, and they cost more and tend to develop leaks, which can cause water damage.