Anodized heat sink changing color - Page 1
Forum section: electronics / general

author: abraxa
author_rank: Frequent Contributor
timestamp: July 08, 2022, 10:24:30 pm
content: 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?

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2022, 11:34:53 pm
content: 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!

author: Cerebus
author_rank: Super Contributor
timestamp: July 08, 2022, 11:43:13 pm
content: 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.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: July 09, 2022, 12:08:43 am
content: Some liquids or chemicals can also affect the color.Jeff

author: abraxa
author_rank: Frequent Contributor
timestamp: July 09, 2022, 06:40:27 am
content: Thank you all for your great insights, I've learned something today!

author: Cerebus
author_rank: Super Contributor
timestamp: July 09, 2022, 10:35:38 am
content: Just for completeness, some illustrations for how anodized aluminium is dyed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 09, 2022, 01:11:37 pm
content: Want to say I've seen some heatsinks like that, faded (oxidized?) and most likely from heat; anyone's guess how high, and for how long, that takes though. Other factors might include ambient contaminants/oxidizers (especially NOx, ozone, etc.).Tim

author: HwAoRrDk
author_rank: Super Contributor
timestamp: July 10, 2022, 02:35:29 am
content: I have some AN hose fittings on oil cooler lines on my car that started off red and blue, but after several years are now more like pink and light blue, so aluminium anodizing dye can indeed fade.

author: rdl
author_rank: Super Contributor
timestamp: July 10, 2022, 12:28:53 pm
content: In general, as far as color for coatings, dyes are the least stable. Inorganic pigments are best with organic pigments somewhere in between. However, there is much variation depending on the specific type.

author: Cerebus
author_rank: Super Contributor
timestamp: July 10, 2022, 12:34:39 pm
content: I've noted that red is is probably the least 'fast' anodising dye and is particularly prone to fading in sunlight.You can get anodising dyes specified for "architectural" use, for window frames, door frames and the like that have particularly good light fastness but are limited to rather muted colours - blacks, 'bronze' and so on. There are post anodising passivation processes that are highly durable that involve active chemistry as opposed to dyes; there's one that produces a drab dark green that's much used on military equipment that is chromium based.

author: NoMoreMagicSmoke
author_rank: Regular Contributor
timestamp: July 19, 2022, 02:11:50 am
content: Black requires a UV stabilizer to keep it from fading (often to purple). If the anodizing wasn't stabilized well enough it will fade just like you are seeing.

author: coppercone2
author_rank: Super Contributor
timestamp: July 19, 2022, 02:13:15 am
content: I see alot of black purple heatsinks, its a common sight. The shitty ones rub purple off with a alcohol wipe. I noticed this when I was cleaning thermal compound. I thought it was a reaction of some kind with the goop at first then I realized the dye was coming off. For instance the HV power supply heatsink near a CRT.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 19, 2022, 02:39:51 am
content: Another factor is that there are different grades of anodization. Normal cosmetic anodization is a fairly thin layer, and relies on the dye being extremely opaque. Higher spec anodization used for optical or high wear applications create a much thicker layer that can absorb more dye and more likely to stay looking black even if it fades a bit.