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Letter 32036
Interpretation of ASTM B117 Salt Spray
Corrosion panels
++++
I am having difficulty in evaluating salt spray failure on
conversion coated aluminum 2024 T3 panels per ASTM
B-117 [link is to spec at TechStreet].
Please define a ratable spot or pit based on size(what is counted
and what is not counted). I am seeing numerous spots of size barely
discernible using naked eyes if not for striations of light pink
identifying the spot locations, most spots would not be visible. In
some areas still the panel shows light yellow color. Measurement of
visible spots at 10X mag indicates perhaps 1-8 spots per panel up to
0.003" diameter).
I am allowed to have 5 spots or pits per panel or a cumulative
total of 15 spots for 5 panels, none of the pits may exceed 0.031"
and no specification mentions what is the smallest diameter to be
considered a spot. Discoloration is permissible.
Duane Praymayer
Aerospace - Winnipeg, MB, Canada
++++
ASTM D1654 [link is to spec at TechStreet] gives
some rating guidelines for decorative coatings, but they are useless
for what you describe. There is no ASTM method of rating pits in the
size range you describe, but there may be a mil spec or industry spec
for it.
This type of specific interpretation is usually (and best) left
between the vendor and customer. Depending on the application the
definition of a "pit" may cover a huge range. I would generate a
suggested set of standards using 10X micro-photographs, and present
them to the customer for interpretation as to what is acceptable.
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Jeff Watson
- Pearland, TX
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++++
What you describe is somewhat common with panels that are over a
year old. Try it in brand new panels from a new sheet of aluminum.
Bet your mini pits go away.
James Watts
- Navarre, Florida
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2024 is more difficult to pass salt spray. Best results if you do
not etch, and use a good quality desmut. The proprietary desmuts work
better than home brew. Allow the panels to air dry for 24 hours after
coating, and handle very carefully to avoid any scratches or damage
to the chromate coating.
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Jeffrey Holmes,
CEF
- Spartanburg, SC, USA
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++++
I agree with Jeff. However, I have found that better test results
are typical if you use a chromate containing desmut/deox as opposed
to the ferrous types.
Gene Packman
- Syosset, NY
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Although Mil-C-5541 [link is to spec at TechStreet]
(Chemical Conversion Coatings) doesn't give any significant guidance
on the matter,
Mil-A-8625 [link is to spec at TechStreet] (Anodize)
does define a pit as an area of localized corrosion with depth
greater than its width. 8625 also lists 5541 as acceptable for
touchups, so you may be able to use the same definition of a pit.
Good luck.
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