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When stainless steel is welded, or just heat treated, it will develop a thicker oxide layer, which in some cases is is rainbow colored, but can become so thick as to appear almost black. From the colors (diffraction effects, I assume), the nicer-looking layers must be on the order of a wavelength of light. Is there any more numerical information on their thickness?

Follow-up question: The colored oxide layers can be removed with a pickling paste (based on HCl) or a passivating cleaner (based on HNO$_3$ plus HF), but the resulting surface is much rougher than the original. Well, it is observably rougher if the original surface before heat treating was mirror-smooth. We call the new surface satinized. Is there any simple way to estimate surface roughness/surface area of the satinized surface?

Further follow-up: would the surface chemistry be expected to change due to the surface area increase (perhaps like shiny Pt vs platinized Pt (~340 mV): https://en.wikipedia.org/wiki/Platinum_black

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    $\begingroup$ You may have already found this reference: core.ac.uk/download/pdf/288376834.pdf $\endgroup$ – Andrew Apr 16 at 21:46
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    $\begingroup$ @Andrew 's source, Figure 2, seems like it might be compared to an ultramicrotome section reference chart from Radboud University's EM class. But I don't know which nodules or layer(s) you're comparing, or whether other factors like refractive index affect the spacing distance for structural color. $\endgroup$ – Mike Serfas Apr 17 at 0:28
  • $\begingroup$ @Andrew Thank you very much!! Great reference! $\endgroup$ – James Gaidis Apr 17 at 15:17
  • $\begingroup$ @Mike Serfas: Great color chart! Thank you! $\endgroup$ – James Gaidis Apr 17 at 15:17
  • $\begingroup$ I understand that they are related issues, but you might want to break up your question into one about structure and another about reactivity. $\endgroup$ – Buck Thorn Apr 19 at 14:26
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Not very thick. Carbon steel develops the same colors at much lower temperature/ times . Gold to pink to blue to purple , stainless shows the colors better because the base metal is brighter . It is caused by the thickness of oxides , a matter of a few layers of oxide molecules ; unfortunately I cannot give you a number in microns and can't remember which book it is in.

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