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