How safe is it to keep and use a mildly corroded hot water valve?
There is a decent chance that the valve will begin to leak where the stem passes through the packing nut/gland. Not uncommon at all and you will need a bucket to contain the water until you can get the supply shut down upstream. Sometimes it sprays everywhere until you can wrap a rag around it. Sometimes the leak will stop when you either open fully and &quot;back-seat&quot; the valve, or when fully closed.
You already know the valve won't give a tight shut off, why not change it out right now while you are in there working. Install a high-quality 1/4 turn valve.