Summary for clear anodized aluminum staining Discussion
Clear anodized aluminum staining is discussed as a likely sealing issue rather than a CNC oil problem alone. Mike Braun reports that anodized 6061-T6 extrusions from one vendor occasionally show clear, glossy stains where cutting oil sat, even though the line was checked for caustics and the supplier said the anodize and seal were done correctly. Ted Mooney explains that anodizer “sealing” means a hot-water-based hydration step that closes the microscopic pores in anodized aluminum; if those pores are not fully sealed, the surface can absorb soils such as cutting oil. He cautions that he would not claim anodized aluminum is universally impervious to oils, but if staining happens only sometimes, the affected batches were probably not sealed as well. He suggests asking the anodizer to periodically verify sealing with the ASM/ASTM B136 seal test and certify the results.
Q. My company builds products out of anodized 6061-T6 aluminum which we purchase from several vendors. With one vendor in particular we occasionally find that an entire batch of clear anodized aluminum is becoming stained by the cutting oil used in our CNCs. The stain is clear in color but affects the gloss of the extrusion allowing you to see exactly where the oil was sitting. We have repeatedly checked our line to ensure nothing caustic was hitting the aluminum and our supplier has verified that the anodize and sealant processes were properly followed. Has anyone else encountered problems such as this or have an idea of why this would occur randomly and only very occasionally? I have had a difficult time researching what type of variables could affect the sealant on anodized aluminum.
Mike Braun
manufacturer who purchases finished aluminum extrusions – Hartford, Connecticut, USA
April 14, 2016
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A. Hi Mike. When the word “sealant” is used, most people envision some sort of organic clearcoat like a lacquer or a polyurethane. But when anodizers speak of “sealing” they are actually talking about an immersion in a warm or hot water-based process (sometimes with additional additives like nickel acetate, nickel fluoride, or sodium bichromate) that hydrates and swells the aluminum oxide coating to seal the “pores”.
A natural and desirable part of the aluminum anodizing process is the formation of millions of microscopic drill holes that extend from the surface of the part almost to the aluminum base. When color anodizing is done, these pores absorb and hold dye. But whether the anodizing is clear or colored, as a last step the parts must be immersed in that warm or hot seal long enough to swell the tops of the pores closed or they will absorb soils of all sorts.
I don’t think I would claim in the abstract that anodizing is impervious to cutting oils, but if the cutting oils you use, under the conditions that you use them, usually do not stain the aluminum, but sometimes do, I would say that the aluminum is probably not as well sealed in those cases. There is an ASM seal test, ASTM B136, and perhaps you can ask the problematic anodizer to periodically run it and certify the results.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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April 2016
Q. Thank you Ted! Out of curiosity would you say that the sealing process a delicate or highly technical procedure or is it pretty basic? I am asking because I have been working with the supplier on this and they are confident in their product which is proving difficult in finding a solution to this issue. So now I am wondering if it is easy to make mistakes during this process as is bound to happen with any manufacturer who supplies in large volumes. I’d imagine that a poor seal could result in “leaks” or possibly crazing which might allow a foreign substance into the pores.
Mike Braun [returning]
– Hartford, Connecticut
April 20, 2016
A. It’s fun to say that something is easy or basic when I’m not the one who has to do it, but, yes, sealing is a basic step in Type II anodizing; it’s always done, and the seal quality should be frequently checked. If parts are “sticky”, that would be an indication that the sealing is poor and must be checked immediately. True, a problem with poor sealing will be parts that get dirty and are impossible to clean.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
April 2016