Mix Oxygen with iron and you get what we call rust. When all the oxygen is used up, the reaction stops until more air is introduced to the system and then it starts all over again.
So why is then that a metal tends to rust faster when it gets wet as opposed to just being in contact with the air??
Hope this answers your question. We used to calling it "putting a skin on the piping" when they first started up a new system.
copied from gewater.com/handbook/boiler_water_systems/ch_11_preboiler.jspOxide Formation
Iron oxides present in operating boilers can be classified into two major types. The first and most important is the 0.0002-0.0007 in. (0.2-0.7 mil) thick magnetite formed by the reaction of iron and water in an oxygen-free environment. This magnetite forms a protective barrier against further corrosion. Magnetite forms on boiler system metal surfaces from the following overall reaction:
3Fe + 4H2O ® Fe3O4 + 4H2
iron water magnetite hydrogen
The magnetite, which provides a protective barrier against further corrosion, consists of two layers. The inner layer is relatively thick, compact, and continuous. The outer layer is thinner, porous, and loose in structure. Both of these layers continue to grow due to water diffusion (through the porous outer layer) and lattice diffusion (through the inner layer). As long as the magnetite layers are left undisturbed, their growth rate rapidly diminishes. The second type of iron oxide in a boiler is the corrosion products, which may enter the boiler system with the feedwater. These are frequently termed "migratory" oxides, because they are not usually generated in the boiler. The oxides form an outer layer over the metal surface. This layer is very porous and easily penetrated by water and ionic species. Iron can enter the boiler as soluble ferrous ions and insoluble ferrous and ferric hydroxides or oxides. Oxygen-free, alkaline boiler water converts iron to magnetite, Fe3O4. Migratory magnetite deposits on the protective layer and is normally gray to black in color.
There is a type of steel called weathering steel that is designed to rust on the surface, build a patena which protects it from further rusting. The first major usage of weathering steel was the John Deere world office in Moline Illinois. They now use weathering steel in bridge construction. It requires no painting and has very minimal maintenance.
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