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Polyolester lubricant and anodized finishes

One of my client's products has a gear box machined into its 6061 body. This part and its mating parts are anodized to milspec Type II Class 2 with Sanodal Deep Black MLW. The gear box is 2/3 filled with a Nye Lubricants polyolester oil. We do a burn-in to tune gear backlash. After burn-in, we drain the oil, flush with mineral spirits [on eBay or Amazon] , and refill with the same polyolester oil.

It was recently brought to my attention that nobody knows why the fluids enter the unit clear and leave opaque black. After researching many different avenues, I have come to the question of the anodize sealant and whether it is allowing the ester oil to leech out the dye. I have contacted our machining vendor's finishing house and they want me to re-explore oil degradation and suggested I look into "pyrolysis gas chromatography mass spectrometry of the used oil."

Anybody here know about ester oil and anodizing compatibility especially with respect to sealants? Anybody care to suggest another inquiry angle? (Incidentally, a past composition analysis of the used lubricant differential to fresh lubricant revealed only the presence of aluminum, copper, tin, and organic debris. I am guessing the organic debris is the dye since the copper and tin are from the bronze gearing.)

Thank you all for your time.

Paul Stuart
Mech Engineer Consultant - San Jose, California, USA
December 2, 2011

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I cannot address your organic chemistry. But I can suggest that very very few anodize shops do a complete seal, and even fewer test the seal to see if it is complete. The seal is so concentration weak that after the first day it is almost impossible to completely seal, then add that to the problem of waste treating nickel and you understand why they run the "seal" as long as they can. Have your anodizer make up a new seal in a five gallon bucket and see if that meets your requirements. Then have him do a daily ASTM seal test, record the results, and show you.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

December 5, 2011