I have always used metal as a standard but is aluminum ok for a TPR pipe? Have never seen it used until yesterday. Was bent across tank to drain into open lower drain along with AC and humidifier pipes. See pics.
I would say no, due to aluminum will harden easier and corrode with the hard water that will leak from this valve. It also has to be able to handle temps from super heated water that is above 212F and 150PSI.
Then why is PEX and CPVC allowed?
I’ve not seen AL drain pipe before either, but as shown in your pic’s, the REDUCTION in diameter is NOT ALLOWED.
hmmm… good point. I just never seen aluminum used. I would think it would be more of a hardening issue. Plus don’t they rate the pipes for continuous use and constant pressure.
Better known as “Steam under pressure”!
I was also thinking of this from looking at the photo better.
The 1/2" pipe is the real problem.
Would you rather I hit you over the head with a length of AL, PEX, Copper, Lead, or ???
When a TPR discharges pressurized steam… do you really believe an unrestricted, relatively short length of AL pipe will change (on a molecular level, ie harden) to even be an issue?
Thanks for the comments. Was going to call it out for reduction but wanted opinions about aluminum.
Ok so than when you come across this and I want to know because I am sure I would run into this, there would be no issue with using AL??
I am not saying your wrong I just want to learn;)
I have never seen AL used for a TPR discharge pipe.
Not worth worrying about IMHO.
Your right 
be constructed of an approved material, such as CPVC, copper, polyethylene, galvanized steel, polypropylene, or stainless steel. PVC and other non-approved plastics should not be used since they can easily melt.
Aluminum of proper size, no issue.
Is it draining into a sewer clean out???
Thomas, thanks for your reply. Was not sure if pipe is reduced and not just a bushing to change threading. The drain is in a ground level condo and cannot be traced. It is not near any other plumbing and could even lead directly to the exterior.
Definitely reduced. That alone is enough to call it out for replacement. Also, if the termination point is not visible, then that is also a red flag.
I am wondering if that is not aluminum but EMT, (electrical conduit, galvanized steel). Someone went through a lot of fittings, first a galvanized bushing, then a brass bushing or nipple and finally a flare nut?
Looks like aluminum hydraulic tubing.