Ashok: Clear conversion coatings(probably chromate)are very thin and have
limited protective properties for the base metal. When subjected to heat
from the wash process in combination with caustic or high pH conditions from
the saponifier they can be easily stripped. Once stripped the base aluminum
metal continues to react with the caustic saponifier. Depending on the type
of aluminum alloy a black smut will form when exposed to caustic chemistry.
This is what I believe you are seeing.
I don't know what saponifier you are using but you may want to try it in a
lower concentration and/or lower wash temps. You may also want to explore a
more buffered saponifier that operates at a lower pH.


Regards

Michael Barmuta

Staff Engineer

Fluke Corp.

Everett WA

425-446-6076

-----Original Message-----
From: Dhawan, Ashok [mailto:[log in to unmask]]
Sent: Wednesday, December 17, 2003 1:12 PM
To: [log in to unmask]
Subject: [TN] Tarnished Surface


Technetters

Anyone who has experienced and resolved tarnishing of Aluminium surface upon
cleaning with DI water + saponifier. The aluminium surface is protected with
conversion coating (clear) which is turning black when exposed to saponifier
solution.



Ashok  Dhawan P.Eng
Manufacturing Engineer
Solectron
Snailmail:1455 Mountain Avenue
Winnipeg MB R2X 2Y9 CANADA
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*       Phone:  (204) 631-7208
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*       I-net:  <http://www.solectron.com>
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