Letter 42567

Corrosion Pitting Evaluation [Florida] 

October 2, 2006

Hi my company is trying to certify our chem-film process but need some help in determining the evaluation of corrosion pits. We are going by Mil-C-5541 [link is to spec at TechStreet] which states that for one panel no more than five isolated pits, none larger than 0.031 inches in diameter can be found on AL 2024 panels. Our question is what is the correct way to examine the corrosion pits…..visual examination, microscopic examination, or both? Also the mil spec is not clear on the depth of pits. Is their a certain depth that a pit must have to constitute a pit? We found some pits-like crevices in our samples but they were not deep as though they we probably the start of a pit. Would this be considered a pit? One last thing is there a minimum diameter a crevice can be to be called a pit. I have also looked at the ASTM stardard G46 but have not found a precise way to identify pits. Thank you for your time.

Lisa Orona
Plating Shop - Palm Bay, FL, USA

Leaving the specific answer to your questions to panel evaluators, here is some related advice. The single most frequent reason for failing salt spray, particularly with 2024, especially with old 2024, is the condition of the panel before you ever start. In old or heat treated 2024, the copper migrates to the grain boundaries, then causes salt spray failure. 1. Use fresh 2024 metal, and 2. before processing examine with a 10X loop, looking for pits before you start. I have seen 2024 panels processed by experts from three sources fail, then the panels from the fourth source, processed by the line operator passed !!!

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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