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| Anodized heat sink changing color |
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| abraxa:
Hello folks, I couldn't find a proper forum for this, so I figure I'll use general chat. I have a device where a TO-220 IC is attached to a heat sink that appears to be way too small. The device is malfunctioning after some time of operation and the heat sink doesn't appear black - it looks faintly purple. I suspect that the change in color comes from too high a temperature. Does anyone know if this can be? And if so, at roughly what temperature would the oxide layer change color in this way? |
| Ian.M:
The aluminum oxide layer produced by anodizing is basically clear, or a bit milky shading to white if its very thick. *ALL* other colors are produced by dying the oxide layer in the sealing bath or during the oxide layer formation in the electrolyte bath. Therefore its all down to the thermal stability of the mixture of dyes used to get your black color. As you and I don't know what the anodizer used, your guess is as good as mine! |
| Cerebus:
The black isn't the oxide layer, it's a dye adsorbed into the pits that form the oxide layer. So what you're seeing is the dye fading or changing for one reason or another. The oxide later itself has a melting point higher than the base aluminium and is pretty close to inert (as far as everyday environmental substances are concerned) below that temperature. If the temperature had got high enough to affect the oxide coat you'd have a little puddle of molten aluminium before you got to that point. Exactly what is affecting the black bye is anybody's guess. It may just have been poorly applied, or the sealing operation that closes the oxide pores over the pocket of dye may not have been done adequately well. And yes, heat is a possible cause of the dye changing and while I don't know what sort of temperature that would happen at my instincts are that you'd notice the problem from malfunction of the IC well before you got to that sort of temperature. When anodizing aluminium myself I've seen a sort of deep purple/black sheen when parts haven't been well enough washed/decontaminated while transferring to the black dye bath from the anodizing bath, or if the concentration of dye in the dye bath has fallen too low. I suspect that this may be an original defect that you simply failed to notice before now. |
| Jeff eelcr:
Some liquids or chemicals can also affect the color. Jeff |
| abraxa:
Thank you all for your great insights, I've learned something today! |
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