Reworking a chromated & painted part?

A. David, We routinely brush chromate discrete areas of parts as part of a repair process. I think this would work in your application if you remember a couple of points: 1) The chromating solution will need to be more concentrated than a tank solution- check with your chemical supplier (Macdermid offers a brush application "kit").
2)The chromating chemical may stain or discolor the paint on the surrounding area. Test the solutions on an inconspicuous area or a scrap part to determine if masking will be required. Brush chromating would be far cheaper than stripping and re-coating. Keith

A. I believe that Keith's method sounds fine. But to have more confidence in the outcome, I would do the rework procedure, then test the parts in some accelerated corrosion test (like neutral salt spray or whatever you presently test), to see what happens to that touched-up edge.

A. Hi, Skip. I think that many shops use the slang term 'chem film' when referring to MIL-C-5541 chromate conversion coating of aluminum. In other words, I don't think there is any difference between chromating and chem filming. We have a FAQ on where to get mil specs, including a source where they are free.

But note that you must comply with the spec completely, including using products on the Qualified Products List; you can't say: "I see it refers to chromate conversion coating, so I'll slap some chromate on it" :-)

A. Hi Rebecca,

Alodine 1201 ⇦this on eBay or Amazon [affil links] contains hexavalent chromium compounds, which are very good at staining, especially light colored topcoats. If you are concerned about staining you should mask around the rework area with a suitable masking tape and then treat the area, that way the Alodine will not come into contact with the topcoat.

A. Hi David. I have seen production situations where parts which needed to be partially powder coated & partially chrome plated were selectively powder coated first; then the finish powder coating was used as the maskant for the entire chrome plating process, which included electrocleaning, cyanide-based strikes, and hexavalent chrome plating. So some powder coatings can certainly easily stand up to immersion in a chromate conversion coating tank.

Unfortunately, the issue is that we don't know whether your powder coating will. The only real answer is that it is not hopeless, so you should not dismiss it without trying it :-)

Because it is generally okay to selectively "touch up" chromate conversion coating, in which case you would have chromate on top of chromate in the overlap zone, I doubt that you need to remove the earlier chromate in the absence of a spec that says you must.

Regards,

A. Well this is a different question.
A true Chromate process will involve multiple steps:
(1) Caustic wash (this will degrade the existing powder coating)
(2) Rinse (no problem)
(3) Acid etch (this will degrade the existing powder coating)
(4) Rinse (no problem)
(5) Chromate ( I cannot see a problem)
(6) Rinse (no problem)
(7) Overflowing DI rinse (no problem)
(8) Halo DI zero microsiemens spray rinse (no problem)
Hope this helps,
Regards,
Bill

Dear All,

Thanks so much for sharing your inputs. I've done the followings and sharing my results on both A360 (die-casted) & AL6061.

1) Re-chromated with Trivalent: Only rinsed with water and soaked entire product without removing 1st layer of chromate or pre-treatment (acid wash) for about 8 minutes (pH @ 4.5).

2) Salt-Fog Tested: Both chromate and powder-coat passed 48 hrs salt-fog test. Do not see any reaction on the powder coat. No fading/peeling/bubble on surface. So far the tested product has been more than 72 hours and it still looks good.

Hope this helps with those who need to rework the same.

Regards
David

A. Hi Arlind,
I have never seen it called out, nor been challenged on it by any auditor (Nadcap, AS9100, miscellaneous aerospace inspectors).
That being said, the sooner the better, and a good DeOx step using a formula that is designed specifically for conversion coatings, immediately prior to the Chem-Film tank, is crucial. There are many on the market and my personal favorite is a blend of Nitric acid, Sulfuric acid, and Iron sulphate. It makes a royal rusty mess of our nice white tanks but oh boy does it do the job! Just don't get any into the non-hex/RoHS compliant conversion tank. Iron is a big no-no for the green stuff :)
The fresher the surface, the more successful you will be passing salt spray testing, too.

  Rachel,
Thank you for your response. I haven't been able to find a basis for this procedural requirement either but some of the veterans here think it might be a legacy requirement for an old customer.

I've got half a mind to conduct my own study to see if there's a relationship between exposure time and Chem Film bonding.

Thanks again,

Arlind

silly :-) Gotta love those types of origin stories for requirements ... where whoever fist carved them into the stone tablets of your procedures manual has long since retired, the customer doesn't have any idea what engineer (probably also retired) decided to put it in the purchase orders to begin with, or why (we all know how much information is passed down to eager young platers on cocktail napkins at convention center watering holes), and no one wants to be the guy to tell the Emperor to put on some pants, already!

I hope you have your very own salt spray chamber to play around with! If so, I'm jealous :) I'd love to do that experiment but oh HECK NO I don't want even IMAGINE the bill for the amount of testing to get a statistically significant sample size!

  Thanks for the cocktail napkin memories, Rachel. Being older than most of you I was a young plating equipment engineer in the days when 2 or 3 martini lunches were an everyday affair for the salesmen and their visiting customers (there were zero saleswomen or female customers). Sometimes the notes on the salesman's cocktail napkin describing what the customer wanted were runny; sometimes there would be fairly credible sketches on the back of paper placemats. Every month or so I'd be invited to one of those lunches.

Regards,

Hi Ted, maybe we will see you in Falmouth in November for the NENASF Regional! I'll bring extra napkins ;)