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Alocrom 1200 bend test failures

Q. Does anybody have any experience of bend test failures on Alocrom 1200 test pieces primed with PR205.

GREG YATES
- Manchester, England

Q. We had some problems with epoxy primer applied on conversion coating (Alocrom 1200). I think that the loss of primer adhesion is caused by the contaminated, powdery Alocrom or by unsuccessful deoxidizing process. The paint surface was damaged by only the bend test. The adhesion test is good. Deoxidized surfaces inadequately rinsed can produce interfering films or previous conversion coating. What is the procedure for inspecting previous conversion coating?

Garbacia Stefan
painting- Bucuresti, Romania
2003

A. Hi Greg, hi Garbacia. De-oxidation is a bit of a misnomer; it's actually de-smutting (removing silicon, copper, and other non-aluminum smut-creating materials from the surface). It seems more likely to me that lack of proper chromate conversion coating is the problem than poor deoxidation. You could proceed per a specification like MIL-DTL-5541 [on DLA]. Compliance with the spec would be a good sign that you will not have subsequent epoxy adhesion problems, whereas lack of compliance with the spec will be a sign of coming trouble.

Practically speaking, overheating the chromate film before it has 24 hours to cure seems to be a very common cause of flaking chromate conversion coatings. You should not exceed 140 °F. It's just a theory but, since the curing of epoxy coatings generates significant heat, I think it could spell trouble to epoxy prime sooner than 24 hours after conversion coating. Good luck.

Let us know if you learn anything. Thanks.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 23, 2009

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