Matte anodizing of machined aluminium parts

letter 14545

Matte anodizing of machined aluminium parts

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Dear Gentlemen,

I am in the process of setting up a small lighting manufacturing firm here in Mexico.

For the production of the required parts I found a couple of job shops, among them an anodizing firm.

Now I encountered following problem:

Some of the required parts have to be machined in mills and lathes, the cutting process generating heat in the material.

Since I want the surface to have a matte appearance, I encounter the problem that with the anodizing process for a matte appearance, on the machined parts there appear the crystals as small shiny spots, which is undesirable.

Can anyone tell me the correct process of matte anodizing of machined parts which have suffered heating during the cutting process, so no spots will be visible?

Thank you very much,

Hans J. Preisendanz
- Mexico, D.F. Mexico

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Matte anodize can be achieved by matte etching the aluminum prior to anodize. Typically done with either warm caustic soda bath or warm fluoride acid bath.

Milt Stevenson, Jr.
Anoplate Corp

Syracuse, NY, USA

WHEN YOU SAY THERE ARE "CRYSTALS" ON THE ALLOY SURFACE, DOES THIS OCCUR AFTER MACHINING, OR AFTER ANODIZING?

MARK DEWITT
- ROCHESTER, NY

Dear Mark, according to my anodizer, these crystals form because of the heat generated during machining. However they are not visible after machining. They become visible after the matte anodizing process where caustic soda is applied to etch the surface for a matte appearance.

Hans J. Preisendanz
- Mexico, D.F. Mexico

Hi,

6061 tends to show a chrystaline grain structure when over-etched. Try mechanical surfacing (tumble,etc.) and then anodizing WITHOUT caustic etch.

Good luck,

Bill Grayson
- Santa Cruz, CA, USA

Dear Milt, thanks for your reply.

This is what my anodizer did. The result is that now there is a bunch of silvery shiny spots on the matte surface which are absolutely undesirable. My anodizer tells me that this comes from the crystalline structure showing up because the structure suffered some changes due to the heat generated during machining (turning). Also see my question letter No. 14750.

So for the moment I am still with the question on how to acheive an evenly matte anodized finish on machined (turned)parts. I am looking for a finish just like the one used on some of the trendy european watches.

Hopefully someone can help me resolve this.

Thank you.

Hans J. Preisendanz
- Mexico, D.F. Mexico

Hans,

If your machinist is generating enough heat to affect alloy properties, I would look for another machinist. Being in the anodizing business for almost 20 years, I think the problem lies with the matte etch process. I can tell you that it is a very difficult etch to produce desirable and consistent results . The "spackled" finish is usually the result of over-etching. While machining does generate some heat, the trick is to achieve a matte etched surface that is also cosmetic. I have used a nitric acid/ammonium bi-flouride/water mixture at low temperature to achieve a cosmetic matte surface for black anodized optical components. However, different alloys produce various degrees of "matte" than others. Obviously, the more aluminum content, the better results you will see. The other variable is experience. If your anodizer is new to producing a cosmetic matte anodize, it may take some time to research and test etching processes, alloys, machining operations and patience to finally get the results you want.

Good luck!

Maek DeWitt
- Rochester, New York

Dear Bill,

I did glass bead blasting with 200 mesh glass beads, obtaining a wonderful surface. Only new glass beads where used and the sandblast booth cleaned thoroughly before, to avoid contamination.

I then asked the anodizer NOT to etch the part, only deox and anodize it. The result is that the part turned out awful dark gray. We then put the parts in caustic etch for 20 seconds and again they came out matte silver. so far so good. Only after anodizing the cristalline structure became visible again, although only very slightly, but still visible.

My questions now:

1.- Does this dark gray/brownish color come from the residual silicon building up in the pores after glass bead blasting? If so, how can this be removed before bringing the parts to the anodizer? (Mechanically/Chemically) The anodizer would´nt install a deox line with ammonium bifluoride for small quantities of parts.
2.- Is it recommendable to further lessen the etching time? How much time would be minimum to avoid showing up these crystals after anodizing and removing any silicon from glass bead blasting?

Is there anyone having similar problems?

Any advise or information is very welcome.

Thank you in advance.

Hans J. Preisendanz
- Mexico, D.F. Mexico

Hans, during anodizing, the substrate is dissolved during formation of the oxide film. 1. A warm/hot anodize tank 2. too high current densities applied 3.mixing parts in a tank load to optimize production 4.using titanium racking vs. aluminum racking 5. if the anodize tank is lead lined or if cathode square footage greatly exceeds parts square footage 6. poor tank agitation ...all these factor in to exessive surface loss (and your bead blast finish). If your part looked good after a 20 second etch and acid de-ox, my hunch is one or more of the above condions exist. You never mentioned required coating thickness or if the anodize is functional or cosmetic. If it is merely cosmetic, a thin oxide film under strict conditions, your anodizer should be able "customize" his process to produce acceptable, fairly repeatable parts.

Mark DeWitt
- Rochester, NY

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