Some of them I had to replace because they leaked. Is this something that happens to copper pipes after time or is this because the solder they used was poor.
Green corrosion is due to formation of copper niterate on the brass taps. Copper niterate formation happens only when hard water containing nitre contents flows through the taps. Since the brass is made of copper zinc alloy, the nitre reacts with the copper content of the brass and forms copper niterate, which by and by corrodes the tap. Such water can also affect the health of the users unless it is sofned by boiling or filtered through the water purifier. You can clear the greenish substance by rubbing wet salt on that and cleaning from time to time to avoid more formation.
Let me add to jogimo2 explanation. The flux used to solder is an acid and most likely wasn't wiped off after the joint began to cool. This is done while still warm with a wet rag. You can sand paper (emery cloth) the joint, flux it and re-solder but the pipes must be blown empty of moisture. Also if you have any steel touching the copper anywhere (nails brackets etc) it will create electrolysis and somewhere, anywhere will cause a leak. For this reason a dielectric union is used when copper is coupled to a steel pipe. I don't like using plastic pipe in my house.
Have you ever seen pictures of the Statue of Liberty when she was green? Her condition and that of your copper pipes are caused by the same thing: the interaction of water and air. Rain falls on the Statue, then air (oxygen) causes oxidation, or the green tint. A few years back they had to replace the much of the Statue's skin, due to this corrosion having bored holes through it. In the case of your copper pipes, the culprit is sweat (condensation) on the outside of your pipes. When the pipes are warmer than the surrounding air, water from the air condenses on the outside of the pipes. The surrounding air (oxygen) causes corrosion, with the same end results as the Statue's skin. This corrosion doesn't happen on the inside of the pipes, where there is a lot of water, because they are never empty, being constantly under pressure. Since they never empty, air (oxygen) cannot get to the inside of the pipes, hence no corrosion within. There are two ways to correct this problem. The first is to replace all your copper with CPVC. Plastic pipes don't corrode under normal conditions. This can be costly if done all at once, so you might just replace the copper pipes with CPVC as leaks occur. The second way to fix your problem is to insulate and ventilate. Since the condensation is caused by a temperature difference between the interior and exterior of the copper pipes, reducing this temperature difference will reduce the amount of condensation. Either insulate the pipes, reducing the amount of air that surrounds them, or insulate the crawlspace/basement. Ventilation (moving the air around) will help what condensation does occur to evaporate more quickly.
