fittinggalvanic-corrosionpipeplumbingrust
This water heater was replaced by a plumber 4-5 years ago. Within about a year, all the new connections were rusting/corroding.
Did the plumber not sufficiently tighten the fittings?
Is this perhaps an electrolysis problem?
Notes:
A few things to check;
Did you place the expansion tank on the cold water side?
Did you pressurize the expansion tank to the same pressure that the water line is at after the regulator?
Is the expansion tank size rated for the size of your hot water heater?
With brass connected to galvanised pipe you can get some galvanic corrosion. There is something called a dielectic union which I've never needed to use. That may help. You'll need to clean up the rust. That may require cutting out the rusted part or at least scraping it off.
Here's a picture that describes the dielectric union joint.
Best Answer
Looks like minor leaks, compounded by mixing galvanized pipe and copper/bronze valve without using dielectric couplings between dissimilar metals.
Honestly, any plumber installing NEW galvanized pipes (rather than just making repairs to existing) in the past 5 years is doubtful. If you really want threaded pipe, stainless is available and lead-free, which galvanized iron pipe is not.