Yellow/green stains on our anodized aluminum
An ongoing discussion from 1996 through 2015 . . .(1996)
Q. I have a tough problem and my travels have taken me to your hotline. Others in my company who are the technocrats are doing everything they can think of ... so I thought I would seek your help.
The Product: Aluminum tube from both ingot and recycled source metals is anodized and then formed into products that are bent, punched and assembled. Lots of models. These are then placed into corrugated packaging and trucked to distribution centers where they are stored until ordered by customers.
The Problem: After an undefined period of time, boxes are opened and aluminum has pale yellow to nearly lime green colors - spots, randomly on the product- but not under fittings or closely covered areas. Surfaces of the darkest versions feel sticky.
What we know: We have all physical evidence - we have a lot of product in a lot of warehouse and we have not looked at every thing yet - coming from one of our two anodizers. The spots are harder to see in sun light. The anodizing is both satin and bright - both have the same symptoms. Very random. We have never seen it in our production areas or in our tube storage areas. MEK / methyl ethyl ketone will remove it, also Acetone [linked by editor to product info at Amazon], Also dishwashing soap. One part that traveled in the trunk of a car for an hour (hot, dark) became much much darker yellow /green.
What we do not know: What the yellow substance is - sent to chem lab - report not back yet, How long it takes to exhibit itself. Some examples found this week (June) from Feb production - probably from Jan anodizing - all the way through to 1st week in May. How to remove it safely or if it will come back, assuming we could remove it.
by Robert Probert
The Chromating - Anodizing - Hardcoating Handbook
What we are doing: Using a consultant immediately at the anodizer - next week; talked with retired QA manager (12 years ago) who had a similar sounding condition - but it occurred immediately in raw tube. Talked to other authorities - everything from bad sealing to free sulfur to phosphorous (electro spectrogram shows phosphorous in yellow sample not in clear sample although at the time this was done we did not understand that an "aging" process seemed to be involved and the clear one may have been too new). The seal seem to be poor - but it seems unclear whether something is migrating out and destroying the seal or the poor seal is causing the problem- and why yellow/green and not oxidized white?
Lots of questions here to answer - would deeply appreciate any help you or your readers can provide. I browsed a lot of the web today and did not find enough. If you can point me to a web reference that would helpful as well.
Best regards,Leah Loyd
A. Hello Leah,
"Stickiness" usually is an indication that the anodizing is not properly sealed.
But, yes, sulfur from kraftpaper or corrugated paper has been reported to stain finishes.
The anodizing shop may use yellow or green dyes on other parts, and a small amount of discolorization from these could be dragged into a rinse tank or the seal tank even if a clear finish was spec'd.
Another thing that needs mentioning (so I'm glad you didn't identify the shop!) is that some shops will 'repair' scratched coatings and rack marks with a magic marker, or they will lacquer the parts so they perform better in salt spray tests, and so on. I would not dismiss the possibility that the shop is chromate dipping, lacquering, touching-up, or in some fashion applying a material to the surface that you weren't expecting--and that this, rather than the anodized finish itself, is what is discoloring.
But assuming the problem is not packaging, or an extra "feature" the shop added, and is not a result of poor sealing, then potential cures directed at improving the rinsing include:
-- alternating hot and cold rinses,
-- neutralizing the acid with sodium bicarbonate before sealing, or
-- displacing the sulfuric acid by immersing the parts in dilute nitric acid before sealing, or
-- a combination of two or all three of the above.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
May 2, 2012
have you found the reason?
We had a similar issue, and don't know how to proceed.
- Giengen, Germany
Sealing Pits on anodized aluminumApril 16, 2015
Q. I am getting greenish deposits that look like pits on my finished parts. Seal tank is good, right pH and concentration.
I have looked at my rinse tanks for TDS, pH, and time. My rinse tanks are good as I consider them.
I don't see these "pits" prior to seal.
What am I missing?
GM - Keene New Hampshire
April 25, 2015
? Could you send a photo which includes the problem? It will be helpful for better explanation.alaattin tuna
akpa aluminium - turkey, sakarya
May 4, 2015
A. Hello Robert,
What is the seal you are using? The discoloration could be a dirty tank or smut. You could try pumping out all of the tank but the very bottom and cleaning it out and see if that fixes it. If it doesn't the other option is you could try a additive to remove the smut off parts, but from what I've read don't use an additive if the parts get painted.
- Fort Walton Beach, Florida
July 9, 2017
Q. Hello sir.
Sir my question is --
Why stain mark come on gold colour during Aluminum Anodizing process?
Alupco - saudi arabia
A. Hi Zulqarnain. Sorry but that question is too vague. If you post pictures of the stains and describe your anodizing process though, people may be able to help. Thanks.
If you'd rather just see defects and try to compare them to yours, you can try these two references:
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"