Corrosion of Iron and Steel
I'm just going to ask (and answer) a few basic questions.
The answer, for simplicities sake first for pure iron only, is simple: Iron rusts, meaning it oxidizes because iron atoms like to be surrounded by oxygen atoms (or sulfur atoms or...) far more than to be surround by brethren iron atoms. Being oxidized is closer to nirvana than being pure. That's why we find only iron ore and not the pure metal in nature.
Now we are a the heart of the problem. The answer above applies to all these elements just as well as to iron - but they do not corrode! At least far slower than iron.
- Equilibrium thermodynamics ("nirvana seeking theory") tells us what some material wants to be, what goal it has in life. Iron and aluminum, silicon, and so one most certainly want to be oxidized if there is oxygen around at ambient temperature and normal pressure. No doubt about this.
- Kinetics tells us how far it is to the goal, how long it will take to get there. You can take this literally. If something is to change, atoms need to move some distance, And that takes some time, on occasion about forever.
- It prevents oxygen and about everything else to reach the material - oxide interface. If no more oxygen can reach the silicon, aluminum, and so on, no more oxidation can occur.
- If that extremely thin (typically a few nm at best) and therefore invisible layer is damaged, e.g. because it is scratched or fractured, it immediately heals itself again.
The next question is obvious now:
That is a tough one. In fact, the quality of iron oxide layers as rust protectors vary quite a bit. If one makes the right oxide carefully, it does protect, or to use the proper word, passivate the surface nicely. The process of "bluing" does just that. The right oxide is magnetite (Fe3O4), the black oxide of iron. Haematite (Fe2O3), the red oxide, is bad. It reacts with water to some kind of hydroxide (like FeO(OH)) and that cause a large volume change with unavoidable cracking, blistering and flaking off that doesn't stop as long as there is some iron to consume. You have seen that. It's called rusting.
But let's go on:
 | |
| Still erect but rust will win |
Easy in principle. Not so easy if you look a details. The easy part is that as soon as you "look" (with a microscope) at the surface of carbon steel, you see (slightly oxidized) ferrite and cementite. Here are pictures to that.
The cementite cannot be covered with an oxide. If you could oxidize it, the gas CO2 would be produced and no solid layer. At the edges where iron and its oxide meets the cementite, mechanical and "chemical" stress is produced that offers points of attack for oxygen and water molecules from the air.
The answer to that has two parts.
The first and easy one concerns stainless steel. A typical stainless steel needs to have enough chromium alloyed to it (at least 11 %) to enable the formation of a closed chromium oxide layer on the surface. Since chromium oxide (Cr2O3) meet both requirements from above, the steel surface is now passivated and resists corrosion attacks.
The second and more difficult part to the answer concerns Corten steel. That's a special steel with weird alloy elements that manages to produce a stable and rather thick oxide. It's so thick that the steel actually looks very rusty. But rusting does not continue after the initial formation of the oxide.
You must answer that yourself. Going beyond what I have given you is best done by reading good books, possibly preceded by a few years of studying the necessary basics in chemistry, materials science, and physics.
With frame
Overview of Major Steels: Scientific Steels