Chemical conversion coatings/pretreatments for aluminum

[mcgyvr]

Is there a difference between specifying a "5 stage chrome phosphate" pretreatment versus "clear chemical conversion coating per MIL-DTL-5541F Type 2 Class 3 (ROHS compliant)"

Is the later just more specific? I guess the question is if I specify the MIL spec am I essentially getting a 5 stage chrome phosphate coating?
I always see a chrome phosphate pretreatment referred to as a chemical conversion coating.
This will be a pretreatment for powdercoated aluminum with some areas masked for grounding reasons.

 

[IRstuff]

But what is your intent? MIL-DTL-5541F isn't about chemistry; it's about meeting certain performance requirements after processing. What you do during the processing is essentially irrelevant, so long as the performance requirements are met, and the processing was according to "manufacturer's" requirements.

So, regardless, does "5 stage chrome phosphate" pretreatment mean anything at all? Is that your acceptance criteria? Do you specify what each of the 5 stages is supposed to be? Is that something you really should be caring about? Does the end product meet the corrosion resistance, adhesion, and electrical resistance requirements that you've specified?

TTFN

FAQ731-376

 

[mcgyvr]

Powder manufacturer specifies the "5 stage chrome phosphate or equivalent" in their documentation which is generic and does not have performance requirements associated with it. I guess I just wanted to make sure that by using our typical MIL spec callout that I would meet or exceed their suggested requirements/equivalents for proper preparation of the metal prior to powder coating.
The MIL spec on unpainted/grounding surfaces meets my requirements but wanted to make sure it is an equivalent to their requirements. In the case of issues with the powdercoating or whatever I want to make sure that I know I at least did my part and used an equivalent pretreatment.

 

[IRstuff]

But, I don't know that they are equivalent. With the Mil Spec, you know what you're getting, i.e., something that resists corrosion for a known time in a specific environment, adhesion, and electrical resistance. If these are important to you, then you need to specify accordingly. If not, then machts nichts.

TTFN

FAQ731-376

 

[mcgyvr]

Thanks IR.
I was planning on using the MIL callout anyways. I was just hoping someone would say that yes that "should" be a suitable "equivalent" to the generic 5 stage chrome phosphate or equivalent call out the powder manufacturer recommends.

 

[IRstuff]

Well, obviously, the converse is to get the manufacturer to certify that his process can meet the requirements of MIL-DTL-5541F. Which, I assume, he won't, probably because he doesn't know.

TTFN

FAQ731-376

 

[KENAT]

I'd ask your powder coat supplier. I'd guess it was but can't be sure.

Seems you make a point of calling out "RoHS", if this is important to you then I'd be even more inclined to call out the relevant version of the mil-dtl. Who knows what "5 stage chrome phosphate" has in it, except presumably chrome (which may be a bad thing) and phosphate.

Are you really asking if the RoHS version is equivalent to the non RoHS version for your application? Again, I'd check with your powder coat supplier.

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