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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Problems with Clear Anodizing
Q. I've currently 3 vendors supplying me with clear anodized aluminium (6082 T6).
Recently the parts I'm being supplied have shown signs of dark pitting and other marks that tend to be dark lines etched into the material.
One part in particular has a machined cut out on the front face and all this face (with the exception of the cut out) is pitted.
Could the problem be down to the aluminium skin?
Would it help the finish if I had all the faces of the components machined?
Thanks in advance
Buyer - Leeds, U.K.
March 31, 2009
First of two simultaneous responses --
A. If all three anodizers are having similar problems, there is an extremely high probability that you have a bad lot of aluminum. If it is from certain countries, the probability becomes a virtual certainty.
Step one is to get a certificate of analyses and heat treat from the original source, through whomever you bought it from.
I will guess that the silicon content is too high.
- Navarre, Florida
April 1, 2009
Second of two simultaneous responses --
A. We do not recognize the 6082, so please come back and tell us the ingredients. If the problem comes back from all three vendors, then we suspect that the alloying ingredients have come out of alloy and migrated near the surface due to the heat treatment, then they fall out leaving a pit under electrolysis.
If the problem only comes from one vendor, then we suspect chlorides in his line.
It would be helpful to know the free acid content and the dissolved aluminum in each of the three vendors solutions.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
April 1, 2009
A. 6082 is not a guaranteed anodising quality alloy, if you require a consistent good quality finish you should be specifying 6063. If however you need 6082 because of it's structural qualities (i.e., strength) it can produce a suitably decorative finish but the key is to the way it's produced and packed, because it is generally only used in applications where the surface finish is not important it it extruded faster and cooled by water quenching (spraying with water) which leads to excessive die lines & light corrosion.
Steve Ives- London
April 7, 2009
Q. Dear Sir,
We are anodizing a particular section which has a groove, generally what is happening a straight line is occurring at the back side of groove. The line is visible only after anodizing and not before it. I tried hard in buffing but was unable to remove the line.
I told that it was some extrusion problem and can't be possible in anodizing.
Is it possible to remove that line? One of my competitor was able to do so and I was really surprised over it.
Please guide fruitfully. Your replies are always appreciated and thanks for it.
Regards,
- Ahmednagar Maharashtra India
March 12, 2012
Hi, Sumit.
I do not have experience in that particular problem, so I cannot offer personal guidance.
But heaven helps those who help themselves, and we have numerous threads about extrusion lines in anodizing: not just this one, but also letters 42019, 36082, 23085, 54954, 26941, 37711, 57456
(Patient application of the search engine will probably find more). Good luck!
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 16, 2012
Q. Dear Sir,
I am aluminium anodizer from India. I have a profile (Skirting architectural section) for anodizing whose one side is plain and other side is having groove for fitting purpose. I am caustic etching the section for 2-3 mins after mechanically blasting it to give it a matte finish. The section requires 18-20 microns.
The problem is that I am getting a straight blurred dark line on the backside of the groove. Its may be a streak line or die line.
The customer is not ready to accept the goods in such condition and says that it should be removed. The line is only visible after anodizing and it's not visible in mill finish section.
I appeal to all, can you please pour in some of your productive suggestions and advice to solve this problem?
Your replies are a great help to me and are really appreciated to the core.
- Ahmednagar, Mahrashtra, India
July 2, 2012
A. Hi Sumit,
It was difficult to see the actual dark line before anodizing (under MF), But you can try to etch some short cut pieces during extrusion process but also cannot give a guarantee. Mostly it's related to die design creating non-uniform flow during presses. I'm not sure what is the shape of your profile. If the design has a welding chamber (or any bridge port of the feeder) then try to shorten depth of the die web and taper radius exit side to allow confluence of exit metal sooner to reduce weld line appearance.
cheer...
- Surabaya, East Java, Indonesia
July 13, 2012
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