Rusty pipes and bad parging

I had tried to post these first 3 photos before, but it failed for some reason. So here we go again. Much less text this time.

Here's the boiler. See the rusty wet spots below the boiler.
The biggest rusty wet spot comes from water leaking from this joint.

I think the rust is caused by bi-metalic corrosion, but the HVAC supplier is blaming the water. I find it hard to believe that water inside the pipe could cause that amount of rust on the outside of the pipe.

This is the pressure relief valve on the boiler. I guess we got a reject pressure gauge as it is banged up and there is no glass on the gauge.

The connector piece between the pressure relief valve and the gauge was replaced 2 weeks ago because it was all rusty. The valve is leaking from inside, not at the joint.
This is the parging. It looks pretty awful.