How does a relatively small amount of chromium prevent steel from rusting?
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Oxide layer, but here is another explanation why using a little amount still works.
Imagine that the chromium is distributed evenly across the entire alloy, so that means in any arbitrary volume, you will roughly find one Cr for every 9 Fe (in reality, there are lattice defects and other things but ignore that for now). Now, corrosion of steel is mostly a surface phenomenon, so you only really need to consider the surface layer atoms. Examining the 2D surface layer, you will find a similar distribution of atoms, i.e. 1 Cr for every 9 Fe atoms. For simplification, let’s say that it only takes one layer of atoms to passivate the metal (i.e protect). So, when the surface atom is Cr, it oxidizes, which then protects the test of the metal underneath. What if the atom was Fe instead? It would rust and flake off to reveal the next layer underneath.
Now, when that Fe atom flakes off, the next atom can either be another Fe atom, or a Cr atom. If it’s Cr, then all is good. But if it is an Fe atom (it is also more likely), the process can now just be repeated again, and eventually you will hit a Cr atom (because encounter chance is roughly 10%, because the atoms are evenly distributed), which will then stop the oxidation process.
So, although the steel looks smooth from the top, microscopically you have a lot of irregularities across the surface where there are “valleys” and “peaks” until the chromium atom(s) were reached. In reality, the oxide layer is several atoms thick, the oxide itself has a slightly larger volume than the base metal, there may be other atoms, the alloy lattice structure also needs to be considered etc. But since atoms are so small, you don’t notice these sub-microscopic imperfections anyway.
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