Water heater corrosion

I have a Bradford White water heater (2 years old) and I noticed a problem.
The hot water coming out of the heater is connected to a copper pipe using a copper/brass? union. There are two nipples coming out of the heater on top, one connects to hot water out and one connects to cold water in. There is a valve on the old water pipe.
The union that connects the hot water nipple and the copper pipe is corroded. It has turned party green and it has also caused part of the "top" cover area around the nipple to be a little rusted. I don't know how much time I have before it breaks. The problem is, the union will be hard to remove, the part you can apply an adjustable wrench to turn it is the corroded part and will not take any stress.
I assume all I need to do is to shut off the cold water valve. Then try to unscrew the hot water union (if possible), then take that union to a plumbing hardware store and get a replacement? or is there more to it? Do I need to shut it off?
MC
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miamicuse wrote:

    I had an almost identical problem with the installation of my hot water heater. That connection point was corroding and slowly leaking. I used Teflon tape on the threads and never had another problem with it.
    As for shutting off the water, that should work to a point. You might however need to drain some of the water out of the water tank to lower the level so that there is no water at the connection. This might require you to open up a hot water faucet above the level of the tank to allow the line to drain. Always turn off the hot water heater when working on it.
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