Powdercoating pretreatment for alum and steel 2

[TJK1]

Would chromate conversion coating per Mil-C-5541 work ok as a pretreatment for powdercoating on alum?

Would Phosphate coating per TT-C-490 work ok as a pretreatment on Cold rolled steel?

What about Zinc per ASTM-B633, would it work ok as a pretreatment for powdercoating on Cold rolled steel?

If these are not good choices, what would you recommend?

 

[kenvlach]

1) Yes, Class 1A chemfilm per MIL-DTL-5541 is commonly used. But, it sometimes gets too thick (lessens adhesion due to greater porosity & moisture content), so I recommend class 2 per ASTM B449.

2) Yes.

3) Yes, with Type II (with colored chromate).

 

[ck1999]

It depends on the quality and type of paint. An iron phosphate can be used on both aluminum and cold roll steel as long as it is designed to be used on both metals.

The iron phosphate will not apply a coating to the aluminum but will increase adhesion and can usually get a few 1000 hours salt spray (depededing on paint and grade of aluminum).

ck1999

 

[kenvlach]

Phosphating and Metal Pre-Treatment by D. B. Freeman, ISBN 0-85941-299-7, has some advice & gives solutions on mixed metals pretreatment for painting.
For processing aluminum in an iron-phosphating bath, fluoride is added (as aluminum activator).
For steel + zinc-coated steel, a modified zinc phosphate (Type 20 in Appendix II). "This process can also treat up to 15% aluminum" in the work load.
For aluminum + zinc-coated steel, some of the aluminum chromating baths (Types 22 & 23) can be used.

 

[TJK1]

For the 6061-T6 aluminum panel, I think we will just use Chemical Film Treatment per MIL-C-5541, class 1A, clear.

For the steel bracket, we do not have anyone near by that can do the phosphate coating, so the powder coater we plan to use said that he could just sand blast the steel part and that it should be sufficient for a good bond of the powder coating on the part. He says he does that quite often. Does that sound feasible?

 

[kenvlach]

Skipping the chemical pretreatment of steel is maybe OK for indoors, but both adhesion and corrosion resistance will suffer. Depends upon the application and any standards.

Specifying "MIL-C-5541, class 1A, clear" will confuse your metal finisher. Class 1A is normally yellow or gold color (although some trivalents not yet on the QPL are colorless).

 

[ck1999]

Where are you located that you cannot find a custom coater with a phosphate line?

Depending on the size of the parts you could look at a high pressure heated spray wand. Which is not ideal but can be used in some instances, having lower start up cost.

ck1999

 

[TJK1]

There are no platers in town that do phosphating, but I found out that some of the powder coaters claim to use a product called Fremont 72 and Fremont 490. Although they don't know anything about it, they use it as some sort of a pretreatment wash for parts before they are powdercoated. I tried looking it up on the internet but couldn't find anything. Do you know anything about these products?

 

[kenvlach]

The Metal Finishing Guidebook & Directory lists Fremont Industries, Inc.
Try

http://www.fremontind.com/index.htm

You may need to contact them or have the powdercoaters do so (the products mentioned don't appear on their website; a corporate account is needed to access product bulletins & MSDS's).

 

[TJK1]

kenvlach, thanks for the info.

 

[ck1999]

Freemont 72 is a wipe-on or dip product. It is marketed to leave a phosphate coating on steel and clean and deoxidize aluminum. So this should improve paint adhesion but probably is not the best method. I know Freemont is now owned by Galaxy. I do not know what Freemont 490 is.

ck1999