We have used Union Carbide/Citgo 5046 Water glycol for ovr 20 years.
We have used sealed pressure/vac tanks with external KleenVent bladders to preven tloss of water in high heat conditions, and keep dust out. In our applications, the sealed tank is essential to making the fluid work well.
Earliest tanks were of stainless steel because of the unknowns of the new fluid.
Any wetted surface is well protected from rusting.
We found the vapor phase corrosion protection of the fluid was quite good and the top of the tanks did not rust even with carbon steel.
However....... since it is water based, any storage or high humidity or long periods without use can wash off the protection and allow lid rusting, especially in the center where the temperature are the the coolest.
We have done some electroplated tanks. Added a few hundred dollars to the tank cost of 3000-5000 so not cheap but not unreasonable. Biggest drawback was access for electrodes, meaning the tank have to have an open top which added a lot of fab cost and difficulties in sealing with the pressure vacuum system. Also, baffles and suction boxes and return diffuser boxes designs were very restricted by the plating process. Hidden spots didn't get plated.
Now, we mostly use welded tanks with some JIC type removable dome covers for access for fabrication, cleaning, and service. They are plated with electroless nickel, so all interior boxes and surfaces are plated. Design can't have any trapped air pockets, but a few clipped corners or vent holes between compartments is very easy and cheap to do.
