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Mil-C-5541 Corrosion Pitting Evaluation

Q. 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 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 there a certain depth that a pit must have to constitute a pit? We found some pit-like crevices in our samples but they were not deep as though they were 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 standard G46 but have not found a precise way to identify pits. Thank you for your time.

Lisa Orona
Plating Shop - Palm Bay, Florida, USA

A. 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 probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

Q. I would like to ask anyone who is familiar with Salt spray testing of Aluminum panels for class 3 using 6061 T-6, how do you analyze?
I have heard it is supposed to be done by vision only, but when I read MIL-DTL-5541F I do not see that fact referenced.
The reason I am asking is I've seen marks on my panels not always a tail and when I look at them under 10x magnification I do not see any sign of a pit. If magnification is not used is it not just a judgment call?

Micheline Forth
- Rincon, Georgia
March 13, 2014

A. Hi Micheline, as I read it (section 3.6.1) any spot larger than .031" is a failure, but for the smaller spots/pits that you are counting (section 6.9):

"Determination of a corrosion spot or pit. As a general rule, a corrosion spot or pit usually displays a characteristic tail or line, however, any visible corrosion or pitting except scratches or substrate surface defects is to be counted."

Personally, I feel ("visible") unambiguously means that the unaided eye is to be used.

Yes, it can be a judgement call, but to improve and align our "judgement", several readers over the years have recommended Amiel G. Forshee's "SALT SPRAY TESTING : PROBLEMS WITH AND SUGGESTIONS OF IMPROVING CONFIDENCE" from October 1991 issue of Metal Finishing. It is available from abstract services if you can't find a copy of the magazine. Best of luck.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
March 2014

Q. Is it true that any tail is considered a corrosion spot even if no pit is evident after 168 hours?

Micheline Forth [returning]
- Rincon, Georgia
March 14, 2014

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