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Blueing steel is a process of intentionally rusing steel as a treatment designed to prevent the rusting of steel.
The key is if you control the type of rust you can create a hard protective coating as opposed to natural rust which is more brittle.
@freemo And the color is caused by the thin-film interference effect! A form of structural color that appears not due to pigments or dyes, but due to (Bragg, iirc) refraction! This is the same mechanism that butterflies, tarantulas, and certain birds use, by constructing protein crystals, to color themselves with those particularly striking iridescent colors. Great examples include the blue Morpho, the Ephrbopus cyanognathus, and ruby throated hummingbirds (and opals, but they're just compacted silica spheres, so cool, but not nearly as cool as animals that do this sort of stuff). 😁
@johnabs You are thinking of aluminium oxide or titanium oxide coatings... in this case the color is just always a solid back.
@freemo No, it's called "blueing" for a reason. The metal passed through multiple colors before eventually reaching one as shown here (I think, I know he showcases it in at least one of his videos) https://www.youtube.com/watch?v=uST7iJgC_gs
It's the same basic principle as titanium anodizing, but the layering is done with iron oxide rather than some alternative, but fine control over metal oxidation through heat or electricity are both possible 😁
@freemo Edit, if you're referring to gun metal blueing, we may be talking past each other. The black layer you're referring to is made chemically with etchants rather than via heat accelerated oxidation, which would likely take the process straight to black (or may be a different form of iron oxide than what is created during heat blueing, though I suspect the gun process is named after the heating process).
@johnabs @freemo Gents - you can form an a thin layer of magnetite by heating or by electro-chemical means and it has the blue colour. due to interference. The relevant bit is that it is a barrier to oxygen diffusion so the underlying metal doesn't oxidise any further in air. Oxides on aluminium and stainless ( Cr oxide) act in the same way without needing any intervention ("self-passivation") but not on iron, which forms increasing amounts of (hydrated) oxides ("rust") if left alone.
@SteelFolk @freemo Thanks for the clarification! I couldn't recall if it was exactly the same thing (magnetite vs standard rust) anymore 😅
I used to do a lot of surface chemistry before a big career switch, so I couldn't recall the exact details, but I was mostly commenting on the structural coloring caused by the technique (thus, the technique's apparent namesake), even though the passivation is the intended application 😁
@johnabs @freemo You'll remember that there are nearly always layers of different oxides due the differing oxygen activities. It's also affected by iron activity too if it interacts with alloy elements, so pure iron rusts like like hell, which was why car bodies did at door bottoms when they were made with 'rimming' steel aimed at superior surface quality.