Hi,
The hard black anodizing of aluminum greatly reduces the thermal conductivity but greatly improves the emissivity. Overall, the effect is beneficial.
The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edn., pp. 1092-1094 (2001) cites a number of studies:
Burr and Hay,
Canad. J. Research, Sect. A, vol. 28, p. 282-286 (1950), compared the heat loss from aluminum cylinders as a function of anodic thickness (up to 20 microns) from 23 to 100
oC. Heat loss from anodized aluminum was 43 to 79 % greater than from polished aluminum.
Thermal resistance (
oK/W) as a function of anodic thickness is given by T.R. Ogden et al.,
Materials Letters vol. 5, no. 3, p. 84-87 (1987). Values increase linearly from 0.8 at 25 microns to about 1.15 at 50 microns. The plot extrapolates to 0.5 at zero thickness, which I presume is due to surface effects and the anodic structure itself (which has a barrier layer below the cellular structure which makes up the bulk of the thickness).
The relative coefficient of heat emissivity was studied by N. D. Tomashov,
Light Metals, vol. 9, 429-438 (1946). Some values, with a perfect black body at 100:
Polished aluminum 4
As-cast aluminum 18
Anodized surface 80
Other studies show that about 90% of the emissivity is reached with an anodize thickness of 5 microns, and that there is little improvement above 15 microns thickness.
Jamie, is there a need for 50 microns thickness (such as abrasive environment)? If not, I suggest reducing the anodize thickness by a factor of 4, to 12.5 microns (0.5 mil). The only finned aluminum HXers I have ever processed with 50 microns of hard anodize were for submarines, where the environment was seawater. For air-cooled fins, 0.4-0.8 mils (sufficient for dyeing black) of either conventional or hard anodize is commonly used.
sreedhara, if the fins are inside an outer tube, all I can suggest is applying 10 microns anodic thickness prior to assembly.
P.S. Most of the above info is also in
The Surface Treatment and Finishing of Aluminum and Its Alloys, 5th Edition, which is clearance priced at $199 from ASM Int.