Originally Posted by Mike Swearingen
If the stems are so corroded that they just snap off, you can almost bet that the seats that seal against the washers are too.
I agree that Mike's suggestion to replace the valves with ball valves is a good idea, but theres good reasons why I can't agree with the above statement.
Small valves like this are cast out of BRONZE, not brass. The stem and screws will be made of brass, but the valve body itself (including the seat will be cast out of bronze).
Bronze is much more water resistant than brass. Bronze is an alloy of copper and tin, whereas brass is an alloy of copper and zinc, and zinc is a highly reactive metal. It's that zinc content that results in brass corroding more rapidly than bronze or pure copper. And, depending on the amount of zinc in the brass, it's properties will change. Brasses with a high zinc content machine well, and it's these kinds of brasses that will be used to make the brass washer and handle screws. Brasses with a lower zinc content will be used to make the valve stem.
It's the different zinc content in the brasses used to make screws and valve stems that's the reason why the screw on an old water valve will often crumble under the force of the screw driver when you try to remove it, but the lower zinc content spindle will still be strong.
And, it's the fact that the valve body is made of bronze that's the reason why the valve body itself is never deteriorated or crumbly like the screw or spindle. And, the copper piping is the most corrosion resistant of the lot.
PCampbell: I think your best bet is to take that valve down to some of the homecenters and hardware stores where you live and see if you can find new valves with the same thread on the bonnet nut so that you can just replace the cartridges and handles in the existing valve bodies. Look on the handles for any indication of who made these valves.
PS: The "word" for copper in ancient Egyptian heiroglyphics is the same "Ankh" symbol that was used for eternal life, or life after death. The ancient Egyptians were undoubtedly well aware of copper's natural resistance to corrosion.
PS2: That natural resistance to corrosion of copper comes from the fact that copper "rusts", but the oxide film it forms is highly impermeable and as it grows in thickness, it better and better protects the underlying copper from further oxidation. This is why new copper piping is kinda orangy gold, old copper piping is brown. Copper oxide is brown in colour.
Ya gotta know this stuff to get your DIY'er armbadge in plumbing.