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topic 3738

Trouble Clear anodizing 6061


We assemble watches and have encouraged a supplier making Aluminum cases for us to switch from 1100 alloy to 6061 to get a stronger part. They just reported that they have trouble clear anodizing the 6061; they say it goes yellow. Before I visit them, what should I be looking for as the cause? I understood 6061 should be very suitable for all kinds and colors of anodizing. Is contamination the most likely cause ?

Hamish Low
- Hong Kong


It is an industry wide problem that tables produced which state that certain alloys anodise OK do not distinguish or define what constitutes aesthetic acceptability, what is acceptable on an industrial machine tool may not be acceptable on a piece of jewelry.

The 'problem' you refer to is not at all unusual, indeed almost predictable. The yellowish hue you are seeing is almost certainly associated with the copper content in the alloy and not with anything that the anodiser is doing incorrectly. Aluminium alloys with copper in them are usually used because the copper makes them easier to machine, however, when anodised the copper atoms become incorporated in the anodic film and give a yellow hue to the finished article since they are not translucent in the way the aluminium oxide is. Generally the thicker the film the more noticeable the colour change as more copper atoms are incorporated. From a technical view-point the product is probably fine, (corrosion resistance, abrasion resistance etc) however with a product such as a watch I assume that there are aesthetic requirements as well and this is actually were your problem lies.

Possible ideas to help, ask your aluminium supplier to supply alloy at the low end of the copper content range, (allowable range for Cu is 0.15 to 0.4) and if the product allows anodise with a thinner film, but note that with a thinner film you will probably sacrifice technical performance for improved aesthetics!

Good luck!

Peter Hirst
- UK

January 26, 2012

Q. I have seen this question raised several times on this forum (letters 19453, 33304), and the response seems to be a lack of coating thickness and/or poor sealing. We are having this problem now on all of our aluminum components. This is a new problem for us as we have had material coated for years. The problem is - it is not an issue we can direct to 1 coater or material type. We have machined parts, parts formed from sheets, and extrusions which are turning yellow/green. Finish doesn't seem to matter either - be it tumbled, bead blasted, brush finished, or no prep at all. There are several different platers being used, included parts purchased complete from overseas - (China).

It appears that the discoloration takes place once the parts are exposed to plant conditions. We have a clean environment, nothing out of the ordinary as far as atmospheric conditions. Lighting is your basic florescent fixtures and metal halides. We are stumped, and our platers are stumped. It appears to me that the lighting is causing the anodize to turn - has anyone heard of this before? We can have the parts stripped and replated, only to have them to turn color again. The yellowing can also be removed using a mild paint stripper - (just testing - probably not a solution..)

We really need some help here - we are trying to protect our interest here - our customer will surely reject our product if it is received or turns yellow at their facility.

David Vargo
Manufacturer / Assembly - Cleveland, Ohio, USA

January 27, 2012

1. Is heat involved ? 2. On some incoming parts, perform an ASTM seal test, if they pass, then leave in the plant the normal time for the fading to happen, if they fail the seal test then you know why. Most job shops do not control their seal and do not perform seal test, so YOU do the seal test and determine whether sealed, then come back to us.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como

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