Paper clips are ubiquitous in almost any office. They are simple items that are subject to physics and chemistry as anything else is. Thus, they often rust over time. But how and why?
Paper clips are usually made of galvanized steel, which is steel that has gone through a process to coat it with zinc in order to limit rusting. By submerging the steel into molten zinc, the two metals will be chemically bonded, creating more robust protection than other techniques of sealing will.
No, zinc is non-corrosive. Thus, so long as the zinc-steel bonded outer layer remains, paper clips will resist rusting.
Yes, because steel is made mostly out of iron. Iron is a corrosive metal, meaning that over time, due to chemical reactions, it will transform into iron oxide (rust).
Rusting is a form of corrosion that happens when iron and oxygen bond into iron oxide, which is what rust is. Iron is rarely found in nature on its own because of how easily this chemical reaction takes place.
Essentially water (H20) helps transfer electrons from the iron and the oxygen from the water bonds with the iron to form rust (Fe2O3). Thus, when steel and water come into contact rusting will happen over time. The more water, the faster corrosion will take place.
How fast the zinc layer of the paper clip will wear off is the major factor here, but the presence of humidity (water) in the environment will play a major part as well. Estimating exactly how long it will take is extremely difficult, but generally the estimation will depend on how much of a zinc layer there is from the galvanizing process, the humidity of the environment where the paper clips are stored, etc.
Galvanized steel will resist rusting longer than non-galvanized steel. The less paper clips are disturbed and the dryer they are kept the longer they will last without rusting.