Mil-C-5541E 3

[rubepierre]

Class 1A has no color stated
Class 3 has none
so if there is no color specified,is clear assumed??

 

[kenvlach]

No, clear is never assumed. To my knowledge, it is an option only available for Class 3 coatings, which have a lesser salt spray requirement than Class 1A for QPL-81706 listing.
Color is not specified in MIL-C-5541E nor its material qualification spec. MIL-C-81706. It is most commonly yellow-to-gold for Class 1A, depending upon the Al alloy. The color arises from the chromate formed by a chromic acid solution, which is the long-established, most efficient means of meeting the salt spray requirements. [as-yet, nothing meets the EU's directive re carcinogens that is nearly as cost-effective.] The only non-gold Class 1A I am aware of is a specialty product used to touch-up black anodize.
Class 3 coatings are commomly produced in the same process solution by using a shorter time, and hence are a lighter color (pale yellow). 'Clear' is a customer-specified option. It is produced either in a solution formulated to give a clear coating, or by using hot water to leach out the color (bad for corrosion resistance).
A customer specification of 'chemfilm' defaults to Class 1A and hence, gold. If you want 'Clear,' it must be explicitly specified, i.e., as Class 3, Clear. Note: production workers frequently rely upon color for process control, obviously problematic for clear.
ASTM B449 discusses colors of chromate coatings on aluminum.

 

[Kenneth]

Kevlatch has provided you with some great background on chemfilm. If you’re interested in additional information on MIL-C-5541 (including a link to the document), see thread332-29529. As noted by kevlatch, the document states (Section 3.5) that: "Clear (colorless) coating shall only be used when specifically authorized by the procuring activity." Section 6.6 attempts to describe the range of coating colors produced by the products qualified under MIL-C-81706 (kevlatch’s description is better).

While you may wish assume that a "customer" specification of "chemfilm" defaults to a "Class IA" coating (and be right the vast majority of time), a review MIL-C-5541E will show that there is no "Default", or "Unless Otherwise Specified", "Class" where MIL-L-5541 is invoked -- Class must be explicitly specified.

 

[kenvlach]

Kenneth, I beg to differ.
The spec. uses language like "should specify the Class 1A or 3," not must, and remember that all of Section 6 is helpful guidance, not mandatory information.

MIL-C-5541E, Section 6.3 [final sentence at bottom of p. 8]
"If the coating class is not specified, Class 1A is recommended."

Ken V.

 

[Kenneth]

I see your point. My "must" is not lifted from the specification, and your point regarding the actual "should" language in the spec is well taken.

However:

Common-Sense Rule #1: When in doubt, ask. Why risk assuming?


The non-common-sense part: (my emphasis)

"1.2.1 Classification: The chemical conversion coatings shall be of the following classes, as specified (see 6.2)"

"6.2 Ordering data: Acquisition documents should specify the following:"

"b. Class of coating (see 1.2.1)"

and some more of Section 6.3:

"6.3 Interchangeability." ... "Detail drawing of parts of requiring treatment in accordance with this specification should specify either Class 1A or 3 and any paint finishing systems required to meet the performance desired."

and, finally, followed by the statement you referenced:

"If the coating class is not specified, Class 1A is recommended."

IMO, this language falls well short of establishing a default, or unless otherwise specified, Class. Personally, I'll take an "as specified" and two "shoulds" over a "recommended" any day. And the "recommended" that you want to follow is in the same Section 6, "non-mandatory", information that you seem to be asking me to "ignore"...

 

[kenvlach]

I agree, a specification should specify rather than recommend.

Re Common-Sense Rule #1: When in doubt, ask. Why risk assuming?
Of course, standard procedure is to ask. It is only an issue when some little machine shop has a know-nothing driver drop off a rush job at the end of day for next 6 AM p/u, and we can’t contact anyone. Purchase order says simply 'C/F.'

Re: "If the coating class is not specified, Class 1A is recommended."
This is sufficient to establish a default, legally speaking.

Thanks for your link to earlier thread. Seems like the same questions keep popping up.
By the way, I have been following Sanchem for years. Excellent articles in Metal Finishing and also in Products Finishing

http://www.pfonline.com/articles/090203.html

.
However, their environmentally-friendly, chrome-free conversion coating seems energy- and time-unfriendly. Prior to their proprietary process solution, need to produce a hydrated alumina basecoat by boiling the cleaned aluminum in DI water. Maybe they have something new, but I haven’t seen any of their major CC products that are electrically conductive per MIL-C-5541 Class 3; final product is a permanganate-alumina; maybe you were referring to their pretreatment Sanchem 1000 deoxidizer/desmutter?

 

[kenvlach]

Please see answers to a similar question on finishing.com:

http://www.finishing.com/193/42.html