why is rust sometimes considered bad and sometimes good? isn’t all rust eventually bad?


why is rust sometimes considered bad and sometimes good? isn’t all rust eventually bad?

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Rouge (red iron oxide) has been used for centuries as a pigment for cosmetics and paints – even by cave men! It is also used as a polishing agent, for which a very fine particle size is required.

It is used as a corrosion inhibitor and sealant in primer or undercoat paints. When painted directly on bare iron or steel, the iron oxide reacts with the metal surface and “bites in” to the surface, giving a very strong bond. It has low porosity to moisture, and is used to paint bridges.

Red iron oxide of a very high purity is used to color pharmaceuticals.

Iron oxide is also half of the active ingredients of thermite. You mix iron oxide and aluminum powder together and when you ignite it, you get the thermite reaction which will weld steel together.

Sometimes rusting is done on purpose in a controlled way that only affects a microscopic layer of the metal, which protects the metal from rusting in the bad way. This is most notably used on older guns, and is called “bluing” due to the slight blue tint given to the metal.

I suppose you have to consider the properties of rust. For starters, we know that it never sleeps.

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.

Your traditional rust peels, because it “swells” (it’s crystal structure is larger than iron is ot increases in volume), bonds poorly to the iron underneath and is brittle. This is bad, because all it does is expose more iron to oxygen, leading to more rust, until all iron is eaten away.

But it’s possible for an oxide layer to not change volume a lot (there are acceptable limits) and be well bonded to the metal underneath. It’s still brittle, but it’s really hard (as ceramics do) and it’ll “heal” if damaged (sacrifice a bit of exposed metal). Because it’s strongly bonded, it won’t peel off, and it’s hardness and chemical inertness (oxides are really stable) it acts as a protective barrier instead.

Aluminium has this naturally. Steels can be treated to have it (blueing for example). Some special steels naturally create a better rust layer (it’s still your traditional orange) like this, that doesn’t peel.