by Ron Joseph
Chromate Conversion Coating "Alodine" MIL-C-81706/MIL-DTL-81706
Q. I have question about MIL-C-81706/MIL-DTL-81706 CONVERSION COATING.
The statement, "The final proctective primer or primer/topcoat system shall
be applied only on a completely dry surface and within 48 hours after completion
of the MIL-C-81706/Mil-DTL-81706 conversion coating application." in our manual.
The 48 hour means the period after stripped skin paint (bare metal) then primer
must be painted.
A. First, let me clarify that "Alodine" is a trade name for Henkel's
conversion coatings. I assume that your question relates to any chromate conversion
coatings, regardless of the manufacturer?
I don't have a copy of the MIL-C-81706/MIL-DTL-81706 specification, but I have
read some trade literature that recommends that one apply the primer coating
within 48 hours after applying the chromate conversion coating. If you are following
the specification you should apply the conversion coating within a few minutes
after you have stripped the aluminum to bare metal. By this I mean that you
have removed the naturally formed aluminum oxide coating by means of the deoxidizing
chemical that forms part of the specification. The primer coating that I assume
you are referring to is an organic primer, whereas the conversion coating ("Alodine")
is an aqueous-based chrome oxide solution that is electrochemically applied
to the bare aluminum. I am talking about two different coatings; the conversion
coating followed by the organic primer, and the need to wait no longer than
48 before applying the organic primer.
Since I do not have a copy of the specification with me, I recommend that you
confirm this with the vendor of the conversion coating.