In an ideal world metal would simply be smelted and set into whatever shape we wanted and then left to maintain that form forever. Sadly, this is not the case as there is the small matter of rust. Rust is the death of metal. It ends the use of metal over time. The metal needs to be replaced or, at the very least, cleaned up and protected with special paint. In many cases this is added first and foremost before the metal is even installed.
Rusting and tarnishing occurs when there is a combination of the metal being exposed to both air and water. A metal object exposed to just air will not rust as quickly. This is why, in the Arizona desert there is a vast airplane graveyard where the planes are stored for spares and scrap.
If metal is underwater then it also has a slower rate of rusting. As soon as there is a combination of the elements then rust is sure to start. This is why it is a good idea to look at the best metals for the job you are undertaking and take preventative measures to help reduce the risk of rust in the future.
There are ways to combat this. One is to use protective paint. Another, more expensive, method is to use nickel which is rust resistant. If the metal is to be joined together then a Metal bonding adhesive, like that from https://www.ct1.com/product-applications/metal-to-metal-adhesive/ can be applied. Riveting, the original form of connecting metal can cause huge rust issues over time.