Emissivity of Oxidised Steel

Oxidation transforms a metal's radiative behaviour. Bright-rolled steel emits at about 0.1; the same steel with mill scale or a rust layer emits at 0.7–0.85, and heavily rusted surfaces approach 0.9. Nothing about the bulk material changed — only a few micrometres of surface. This is why radiant heat transfer from a hot process pipe or a boiler shell is usually far larger than a "steel is shiny" intuition suggests, and why steel structures in a fire absorb radiation efficiently.

For anyone using an infrared camera or pyrometer, this is the number to get right, because indicated temperature scales with the assumed emissivity. Set the instrument to 0.95 and point it at bare bright steel and it can read a hundred degrees low; set it to 0.8 on a rusty flange and you are close. Standard practice is to apply matt black tape or high-emissivity paint at the measurement point and read that. Emissivity also varies with wavelength and viewing angle — the single "total hemispherical" figure quoted here is an average that a narrow-band instrument does not exactly see.