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topic 59820

Anodize did not penetrate scratches of brush finish

March 11, 2015

Q. 6063 aluminum extrusion with brush finish was anodized using two-step electrolytic anodizing process whereby the colorant is an inorganic metal salt, not an organic dye. This is a sulphuric acid process that includes sealing using nickel acetate. No etching was done beforehand, although the part was cleaned beforehand, as is always done before anodizing. As can be seen in the photo, the anodizing did not penetrate the scratches that constitute the brush finish. What went wrong? 59820-1

Thank you.



Robert Eidschun
Customer of local anodizing shop - Victor, New York, USA

March 26, 2015

A. We used to have similar problems while anodizing brushed panels and later dyeing in black or any other color. Our solution is rather low tech. We do a light degreasing in a caustic solution, anodize for 3-4^minutes at room temperature, do a light caustic wash again, neutralize and then proceed for either Type II or Type III anodizing and dyeing. Never faced a problem again. I hope it works for you too.
Only make sure the aluminum surface is brushed using an Aluminum Oxide belt. Brass or steel buffs are a strict no-no as the particles embed into the soft aluminum surface and are difficult to remove if not impossible unless the surface is milled again.
Anodizing and re-anodizing might slightly vary surface dimensions and luster.

Winston DSOUZA
- Mumbai, Maharashtra, India

March 27, 2015

Q. Hi Winston,

Thank you for your very informative reply.

One question though: what do you mean by "anodize for 3-4 at room temperature"? Anodize for 3 to 4 minutes at room temperature?



Robert Eidschun [returning]
- Victor, New York, USA

simultaneous March 28, 2015

A. Hi Robert, let me educate you a little bit about anodizing. It's aluminum oxide that is "grown" under a controlled process. Like most coatings, sharp edges are detrimental to coating continuity and uniformity. In a sense, what you're creating with a bushed finish is long grooves with very sharp edges. Anodizing grows perpendicular to the surface substrate, so in the bottom of those grooves, you are not going to get a very good coating. If you were to put a cross section of your part under a S.E.M microscope, you would see that the light colored grooves that appear not to be coated are going to be deeper that the ones that did coat.

Anodizing not only grows on top of the substrate, but also erodes the substrate as well, much akin to rust growing on steel. The method that Winston suggests is doing little more than widening, smoothing, and micro deburring those grooves, to make them more conducive to anodizing growth. The same might be accomplished by having your anodizer increase his pretreatment etch time, that would be something you'd have to work with your anodizer to have them play around with.

Most decorative anodizing (your process) is done at a controlled temperature of around 70 °F (some processes temps vary, depending on the end result the coater is trying to achieve), that is what Winston means by room temperature, albeit good anodiziers will control the temperature very tightly to insure a repeatable end product. The anodizing process generates heat, so the bath must be chilled so that the temperature remains constant. And yes, I'm pretty sure he's talking about 3-4 minutes.

Marc Green
Marc Green
anodizer - Boise, Idaho

March 28, 2015

A. I am sorry for the confusion. Yes, I meant 3-4 minutes of anodizing time in Type II.

Winston DSouza
- Mumbai, Maharashtra, India

March 29, 2015

Q. Hi Marc and Winston,

Thanks for your messages.

By the way, at the factory that produces the extrusion in question, the extrusion is brushed and then either: (i) painted, which is what I started out with; or (ii) bright dipped, then anodized. In the case of (ii), the anodizing looks great, i.e. it seems to go all the way down to the bottom of the grooves, and so there are no white silvery lines. I suppose that the bright dipping rounds the edges of the grooves sufficiently.

By the way, I wonder if the grooves' sharp edges result in high current density such that the electric field is altered so that very little electrolysis (and thus very little anodizing) takes place in the bottoms of the grooves...


Robert Eidschun
- Victor, New York, USA

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