Chemical reactions are occurring all the time and whilst we may not notice most of them there are some that become visible over time. In the case of metals oxygen and water tend to be the two elements that cause the most damage in terms of the reactions that take place with the metal.
When some metals are exposed to the air it can cause rust and corrosion to occur. This is particularly true if the metal is damaged and the inner parts are allowed to meet with air. This is where joints become very susceptible to issues and why some construction engineers will use Metal bonding adhesives like the ones from https://www.ct1.com/product-applications/metal-to-metal-adhesive/ rather than drill holes into the metal. These adhesives can be just as strong as nuts and bolts and they don’t have the risk of allowing air and water to seep into the joints and cause rust and corrosion to occur.
Other ways in which metals can be protected from the damage that can occur when air and water meet the metal is to use a surface treatment that has stronger qualities than the metal itself. It is also one of the reasons that metal alloys are used as it allows for the benefits of more than one metal to be combined to make a super metal that will be more durable and, in some cases, more versatile as well.