why is rust sometimes considered bad and sometimes good? isn’t all rust eventually bad?
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if by rust you are referring to the red oxide of iron, Fe2O3, then it is never good.
if you are referring to the blueing (a mixture of Fe(II) and Fe(III) oxides) or blackening (Fe3O4) process to passivate steels, then this is not called rust and is a way to create a more controlled and manageable form of iron oxidation to slow down the natural formation of rust.
rust is terrible because of the number of oxygen atoms per iron atom, essentially making a huge molecule that cannot possibly fit into the crystal structure of the original iron or steel surface and so eventually flakes off when a enough has formed. on top of that they also form hydrates which are literally supermolecules that have H2O molecules as part of the structure.
blue oxide is slightly better on the oxygen:iron atomic ratio, while black oxide which you get by just doing the same bluing process but more intensively and for longer, has the lowest O:Fe ratio. iron ore is essentially black iron oxide.
at the end of the day all iron oxides aren’t good rust protection because they are permeable to oxygen, which means they will naturally deconvert to rust, or worse, rust forms under the treated surface **and then** everything flakes off. anyway, in blued or blacked steels, typically the surface will be oiled after treatment and must be kept oiled to fight the atmospheric oxygen problem, like in gun barrels.
for the best protection of steels, the surface is treated with nitric acid to remove all traces of iron, and **then** passivated to create better corossion resistant metal oxides like those of chromium. and that’s why stainless steels are defined by their chromium content.
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