Metal can withstand pressure but it can't withstand moisture. Moisture can rust any type of metal and for this reason; pieces of metal needs protective covering against moisture. The only way to protect metal from moisture is to paint it.
Put a layer of wet paint on metal that you want to protect from atmosphere. The wet paint would make a protective covering on the metallic surface and it would remain on the metal for long time. When you see that that paint is peeling from the metal, you can again paint it with another coat of wet paint.
For durability and good results, you should use dry powder that is a kind of powder. The dry powder is sprayed with a gun and the metal is heated to a certain temperature where the dry powder starts melting. The paint melts to make a protective covering on the metal. For best powder coating, use batch process.
Batch process is manual process in which the painter deals with the metal pieces in question manually. He prepares metal pieces, paints and heats the metal pieces manually and then leaves the heated metal pieces to cool. Batch process has two advantages that are it produces little waste and it is affordable.
If you have thousands of pieces of metal and all the pieces of same size and dimension then the painter would choose automated line painting that is automatic painting by machines. Advantage of automatic painting is it saves time. It is best powder coating for metal pieces of same size and dimension.