Pretreatment of aluminum and problems in chromate conversion

A. Hello Guy. Using the Qualicoat pretreated product should be fine. Time between chromating and powder coating should not be critical as chromate conversion coating can be a final finish as well as a pretreatment so it's quite corrosion resistant in storage. Steam cleaning is a little iffy, as the rating for chromate conversion coating is 140 - 150 °F until it is covered with the powder. But I wouldn't say it won't work.

If you are going to powder coat, then anodizing would not be as good a pretreatment as the chromate conversion coating.

But just so there's not a misunderstanding, anodizing can be dyed in a wide variety of colors as an alternative to powder coating. For outdoor high UV exposure on gates, the reliable color spectrum is much more limited though. Although I don't know of industry marketing reports on the subject, I tend to look at aluminum railings on my hotel room balconies and everywhere else, and a lot of it is anodized rather than powder coated. But for the specific situation of coastal areas, sticking with powder coating is probably the way to go.

Luck & Regards,

A. Dear Manish,

I have to hit this real quick, as I can't type quick enough for e-mail!

A few high points:

• Six tanks not enough unless using heated de-ionized spray rinses
• Do not alkaline etch aluminum parts to be chromated for aerospace applications ... it destroys surface integrity.
• I had good results with Oakite (Chemetall) cleaner, and Oakite LNC, a nitric/iron combination deoxidizer/desmutter prior to chem film (as per Boeing aircraft spec.)(chem film is what we in the U.S. call chromate conversion coating))
• pH and concentration of chromate conversion coating baths needs to be maintained in strict tolerance; avoid contamination with iron or alkaline drag-in (use HOT DI spray rinse prior to, plus a warm 1-3 percent nitric dip or spray if possible (helps prevent drag-in of contaminants and maintains pH). I was hoping to try this before I got "downsized" from my last "job" ... you need somebody in India? I'd love to move there, seriously, it would be a wonderful experience for my family, and I'd like to live there as well!
• Anyway, back to the questions at hand ... I always used either Alodine or Iridite products for chem film as they were "Boeing approved" -- I am sure there are others and this is not meant as an advertisement for either. The critical thing is to maintain pH and concentration; check at least once per 8 hour shift. Use HOT DI rinses to avoid contamination.

As to your yellow appearing waste stream, you have a problem; you must reduce the pH of your chromate rinse stream to less than 2.4 (combine it with Deox tank drag-out rinse overflow and/or with spent deox solution to start with, add supplementary acid, iron and/or sodium metabisulfite in a 2.4 or less solution if necessary. Your goal is to yield a nice deep blue color., followed by a 20 minute dwell time, then raise pH to 9.0 plus, and it will turn green and yield a precipitate, especially with appropriate polymers The effluent will then be CLEAR once the precipitate settles, and you can then neutralize, test and discharge it ... I would love to go on to waste minimization and treatment.

Good luck,

"High Performance powder Coating"
by Bob Utech

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A. Hi Ashutosh. I don't know if 1700 hours will prove possible even with proper pretreatment (chromate conversion coating) and proper powder coating; you may need e-coating before the powder coating especially if the powder coating can't cover well enough due to the shape of the parts. Rather than having people kick the ball all over the field with no specifics, what are you doing now and how many hours are you getting from the process? Try to be specific, remembering that if you must be secretive about an unsuccessful process, people will certainly feel a need to be secretive about their successful one :-)

Thanks and good luck.

Regards,

A. Richard,

Aluminum surfaces develop an oxide layer when exposed to the atmosphere. Abrading the surface removes the oxide layer and allows paint to adhere. The oxide layer can be removed chemically as well. Once the oxide layer is removed, the surface has to be painted (powder coated/primed) or chemically treated to prevent the regrowth of the oxide layer. Aluminum treated with a chemical treatment (pretreatment) remain paintable with good paint adhesion longer than abraded aluminum.

A. For Aluminum painting a chem film is the best way of getting paint to adhere. I suggest Sanchem cc 3500 series material since it is environmentally friendly and will give you the salt spray resistance.

A. CC-3500 is dipped, sprayed or brushed. Is not effected by heat and can be painted over most anytime that is practical. It is also available in the UK.

A. Also in the USA Alodine makes a chem film and so does Iridite.

... and so do Chemetall and several others...

Gentle readers, please do your best to discuss pretreatment processes in generic terms like chromate conversion, trivalent conversion coatings, chrome-free replacements, etc., rather than promoting specific proprietaries ( huh? why?) Thanks.

Regards,

A. The type of pretreatment you want really depends on the performance requirements. If you just want good short term adhesion, a simple degreasing and etching may be all that's required.

If you need "real" corrosion protection, then a typical pretreatment may be needed. This could be a standard chromate process, though automotive and other manufacturers are getting away from that due to certain requirements. You could also use a non-chrome pretreatment. Some even use phosphate baths.

For a full blown treatment system, you're looking at 1 to 3 chemical tanks plus associated rinses, depending on your specific requirements. (Figure 2 to 5 tanks total.)

Specifics (i.e., cost, how difficult it will be, etc.) greatly depend on what type of performance you want, how many parts you'll process, what your environmental/discharge limits are, etc. I suggest you contact some vendors and provide them with some details. They should be able to recommend a specific process or products.

A. Dear Dave,

The easy answer is contact your local paint mfgr. or outlet supplier and purchase a "Two Pack Wash Primer".

This consists of: a) primer b) catalyst c) sometimes a thinner.

Mix a & b - leave for 15 mins - spray mix to aluminium and allow to air-dry (15-30 mins); if OK apply powder and bake.
Note on application of a/b mix - only apply a very thin film - but not dry spray and not too much that it sags. This is a simple cost effective approach to your problem.

A. In our line of business we coat aluminum sheet. We have processed some with chemical pretreatment, and some without. All our product is further processed through a series of stamping dies for the end product. We use an EPOXY coating stands up very well to the stamping process. I would say that with just a good degreasing / cleaning, a powder coating would hold up fine. As a check, and what others here have said is that your supplier of the powder coating should be able to tell you the limits of the coating.

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. Put on Mil Spec chromate conversion like Alodine or Iridite, but put it on very thin, almost down to Class 3, as the cure temp of powder coat will dehydrate the heavier chromate and the paint will peel.

A. Hi Manoj. Your question is rather abstract ... can you give us some details, such as what kind of thinner is applied and for what reason and under what circumstances? You are looking for a lacquer that can't be removed with lacquer thinner -- but one of the things that distinguishes lacquers from other coatings is that they can be removed with lacquer thinner ⇦this on eBay or Amazon [affil links] Warning! highly flammable! , so remaining vague can leave us chasing our tails :-)

There are certainly clear coatings available which lacquer thinner will not remove. Can we bake the coating? If so, a coating that will somewhat resist solvent thinner can probably be roll-coated onto the coil. Is there time for an epoxy to cure? Can we UV cure the coating?

I'd like to yield to someone with greater experience in this, but I think a chromate conversion coating will resist the solvent thinner. Thanks!

Regards,

A. Hi Cari. The most common cause of powdery chromate finish is drying at too high a temperature. These chem films are slow forming but stable gels which reportedly can take up to 24 hours to cure; heating them above 140 °F can dehydrate them.

We do not recommend one brand over another in this forum, but we do draw readers' attention to the fact that Buzzbond 1200 is apparently a hexavalent chromate that does not comply with RoHS and other reduction of hazardous substance program requirements. So you should investigate whether those requirements apply to your product and, if so, review MIL-DTL-5541 [on DLA] and select a non-hexavalent replacement from your vendor Malter International, or from others. Good luck.

Regards,

A. Hi Alex. Today's trivalent chromate conversion coatings for aluminum have very little if any color. See MIL-DTL-5541 [on DLA] and choose a trivalent product from the Qualified Products List. Simply cleaning and then clear coating aluminum doesn't really offer much corrosion resistance; the chromate conversion coating is quite necessary. Good luck.

Regards,

A. Hi Sanu. While it is certainly possible that the powder coating is improperly cured, without knowing anything further about your operation, a betting man might consider the possibility of the chromating not being done quite properly first. So we've attached your inquiry to some food for thought regarding chromate curing time, curing temperature, thickness recommendations, etc. Best of luck.

Regards,