Aluminum and iron (which is used to make steel) can be corroded because they are metal and all metals (except gold, and maybe 3 other extremely rare metals like platinum) are susceptible to corrision of some form. Since we build basically everything out of metal, we must protect them from eroding away.
Most protection methods are not hazardous. For example, its not hazardous to paint steel. But sometimes the chemicals we use to protect metals are hazardous. For example, nitric acid is used to create an oxidation layer around aluminum. This protects the aluminum from being corroded, but the nitric acid itself is hazardous. But if it is dealt with properly, the acid is not hazardous. It can be turned into fertilizer by adding ssodium hydroxide or other chemicals.