How does anodization increase emissivity? In articles devoted to the study of the contribution of radiation to heat dissipation, it is often mentioned that an anodized heatsink has better emissivity compared to a bare heatsink.
Since the emission occurs in the infrared range, the visible colour change due to anodization of the surface should not affect the emissivity.
I'm wondering what is the cause of the increases in the emissivity of an anodized heatsink? Is this a general feature of anodized surfaces compared to bare surfaces?
 A: Metals generally have a very low emissivity. The easiest way to see this is that emissivity and reflectivity add up to one, and metals generally have a very high reflectivity i.e. they are shiny. If the reflectivity is high the emissivity has to be low.
When you anodise aluminium you create a layer of aluminium oxide on the surface and aluminium oxide has a much higher emissivity than the metal. A quick Google found this NASA article that reports alumina can have an emissivity as high as 0.69.
So with an anodised aluminium heatsink the metal can heat the oxide by conduction, which is efficient because the metal is an excellent thermal conductor and the oxide layer is thin (c. 10μm). Then the oxide layer can radiate the heat efficiently because it has a high emissivity. The end result is that the anodised heatsink radiates much more efficiently than a clean metal surface.
We should note however that outside of the space industry the main mechanism for heat loss is convection and the anodisation has little or no effect on this.
