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Yellow-brown staining on AL6061-T6 suddenly started
Q. I have a customer who has run into a strange yellow-brown staining problem on AL6061-T6 alloy. They have been making these parts and using the same cleaning process for almost 2 years when trouble suddenly reared its head. Here are the details:
The aluminum parts are machined on a lathe-type of machine. The lubricants are straight oils (not self-emulsifying/cut with water) and do not contain active sulfur (they do contain sulfonated additives). There may have been changes in the lubes recently but this information is not clear yet.
The cleaner is a mildly alkaline, low-foaming, silicated cleaner. No changes have occurred to the cleaner or the raw materials used to make the cleaner since it was commercialized several years ago. The cleaner has been independently tested and proven to be safe on AL6061-T6 (and several other aluminum alloys) under similar operating conditions.
The parts are cleaned in a Bowden Spinning Basket Washer that has one wash and two rinses. This is one of those cleaning machines that holds the wash and rinse solutions in reservoirs and each solution is then shuttled back-n-forth to the wash tank. Each step runs 10-minutes at 150°F. The rinse water quality is deionized water but I have not verified how they control the quality once it is in the reservoir. Preliminary checks indicate there are no problems with the control valves that regulate transport of the solutions from the reservoirs to the tanks (no excessive cross-contamination occurring). The parts basket is made of fiberglass-reinforced polyester that is chemically inert to the cleaning environment.
The yellowing only occurs when the parts are exposed to the cleaner followed by the rinse; exposing parts to the cleaner followed by a fast cool spray rinse does not cause yellowing nor does soaking the parts in the hot rinse water. The parts appear to be sensitized to silicate-based cleaners in general but they are more sensitive to the one cleaner that they have been using than our other offerings.
Does this sound like something subtle has changed in the aluminum alloy? Slight tempering defect? I've seen only one similar incident in 10 years and that one involved a machining process that severely overheated the aluminum which caused the part to discolor when exposed to an alkaline cleaner.
Formulating Chemist - Indianapolis, Indiana, USA
2007
A. A different customer e-mailed me ~ 8 years after this original post to ask what if the problem was ever solved. The answer is YES, the problem was solved but I forgot to update this post. Here is what happened:
We determined that the sudden onset of yellowing was caused by the customer changing the alloy they were using in manufacturing. The customer was originally using AL6061-T6 but that alloy was giving them poor surface finish because the chips were not readily breaking away on the lathe. They changed to AL6061C to solve the surface finish / chip breakaway behavior. The customer was sourcing their aluminum from Bon-L located in Richmond Hill, Ontario, Canada. The only significant difference between the two alloys was the 'T6' contained ~ 0.25%(w/w) iron whereas the 'C' version contained ~ 0.5%(w/w) iron. This subtle increase in iron content gave the alloy increased hardness, tensile strength and improved breakaway behavior. Our testing found that this AL6061C alloy would discolor in any alkaline cleaner (Brulin's and a few select competitor products we had in-stock) that was high enough in pH to require silicates as the corrosion inhibitor to normally suppress corrosion on aluminum. The fix was changing to a very weakly alkaline cleaner with a pH no higher than 8.7 at use-dilution (we did not test any acid-based chemistries).
This was one of the most frustrating problems I ever had to troubleshoot. The customer had sent me plenty of aluminum samples to work with and all the cleaning performance testing suggested that something had changed with the alloy. I kept asking the customer for several weeks if they had changed the alloy and they kept saying "No". After about two months of troubleshooting, the customer readily admitted that an alloy change had occurred. In the end, changing to a slightly alkaline, silicate-free cleaner eliminated further staining. There were ~ 10,000 parts that needed to be recovered; recleaning in 11% (w/w) citric acid [on
eBay or
Amazon] solution at 150°F for 12 minutes recovered the parts. The original cleaning was done in a Bowden rotating basket washer but I don't know if that was what they used for the citric acid stain removal step.
- Indianapolis, Indiana USA
July 30, 2015
August 2015
Thanks so much David for the rest of the story.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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