Summary for black anodizing 125 c Discussion
Black anodizing of parts for a 125 °C bake is the focus of this thread. Sam Rearick asked whether switching from 2024 aluminum to 6061 would help keep black anodize from coming off during baking, and whether there were any special process secrets to improve adhesion. Robert H Probert replied that most black dyes begin to break down around 225–250 °F, so the problem may be the dye rather than the anodizing itself; he suggested testing a sample coupon and checking with the anodizer’s supplier for a more heat-resistant product. Marc Green added that 2024 and other high-copper 2xxx alloys anodize more poorly than 6061 and can leave smut that hurts appearance, but he also said the “coming off” may be due to poor or missing sealing after dyeing. He noted that 125 °C is not especially high for many applications and warned that a cheap or exhausted dye bath could also be the cause.
Q. I have been having parts black anodized for some time for my customer BUT recently he has objected to the anodizing coming off in his 125 °C baking process. The print calls out for 2024 material can you tell me if I would have better results if I use 6061 Alum?
Is there any special secrets that I can suggest to my plater to make the plating adhere after being heated to the 125 °C?
Thanks
Sam Rearick
tooling – Apollo, Pennsylvania, USA
May 19, 2011
| A. I believe most dyes will start to break down around 225-250 °F. It’s not the anodizing that is failing, but the dye. Not too sure if changing alloys will make a difference or not, you could always have your anodizer run a sample coupon for testing. Your anodizer should be well aware of the limitations of dyed coatings, and should contact his supplier to see if there is a product that will meet your needs.
A. Is this hard anodize, or is it black dye? Either way, 125 °C should not be a problem. Have your anodizer seal the product in boiling water and you should not have a problem. James Watts A. The thermal coefficient of expansion of the oxide is 0.2 that of the underlying aluminum metal, therefore the coating starts to crack above 80 °C. 2024 is worse than 6061. I can send you a xerox of that page the Sheasby/Pinner Book. If the dye is fading, that is probably a sealing inadequacy.
May 19, 2011 A. Could be a couple of issues here. First, 2024 and high copper 2xxx series of aluminum do anodize poorer than other alloys like 6061 as you’ve suggested. 2024 comes out of the black with a smut which can be wiped off but detracts from otherwise a good black anodize finish. Secondly, the “coming off” you’re alluding too is likely due to poor or no sealing of the anodize following the dye step. If it’s fading at 125 °C, that’s pretty low temp as many connector manufacturers require upwards of 550F with no visible degradation. Also, your anodizer could be using a cheap dye with poor heat fastness – ask your anodizer what the black dye’s heat fastness rating is? Lastly, the dye could just be spent – low on effective dye concentration, high in aluminum, sulphate, phosphate, etc. Last comment, you’re getting a pretty bad anodize job and you might be best off finding a more reliable source!
May 19, 2011 |
Boy, I sure am lousy as of late. Granted I haven’t been doing dyed work for quite sometime…but as I recall, didn’t most dyes start to break down around 250 °F
Perhaps I’m getting old and memory is failing…or maybe the dyes have gotten better in the last 10 years?
Marc Green
anodizer – Boise, Idaho
I highly recommend Robert Probert’s book, “Aluminum How To”. His book simplifies the frustrating questions and problems we all run into. Thanks again Robert!! And thanks, Finishing Dot Com!
Scott Frazier
– Spokane Valley, Washington
May 25, 2011
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