Reworking a chromated & painted part?
Q. I have a rework project coming up where I need to drill some additional holes into an already finished piece of 5052-H32 Aluminum sheet metal. The finish consists of a Mil-C-5541 [link by ed. to spec at TechStreet] Class 3 chromate conversion followed by a Polane-T (Sherwin Williams) paint job. If I just drill and touch up the paint, will the paint just flake off in short order? I'm more concerned that by breaking through the original conversion coating, the part will get real ugly, real fast if I don't strip and re-chromate the entire part. What's your opinion?David Seebauer
cardiopulmonary - Milford, Connecticut
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
avionics - Minneapolis, Minnesota
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.
Falls Township, Pennsylvania
Q. I am trying to find a commercially acceptable equivalent of Chemfilm per MIL-C-5541, Class 3. I will be plating over Zamak #3 and would think a yellow Chromate will do the job, but I need to obtain a copy of the Mil spec. Can you point me in the right direction and provide any technical details that will support my plan?Skip Cook
mfg. - Cranston, Rhode Island
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" :-)
Ted Mooney, P.E.
Pine Beach, New Jersey
February 11, 2014
Q. Our company sometimes touches up bare metal with conversion coat. Right now we're using Alodine 1201. Sometimes if the parts have a topcoat, especially a light color, the alodine will leave a yellow stain. But the stain may be in a localized area or on a small number of treated parts. We have tried changing out the conversion coat and rinse water, but it still happens inconsistently on occasion. Is there a reason this happens? Can it be prevented?Rebecca Fischer
- St. Louis, Missouri USA
February 13, 2014
A. Hi Rebecca,
Alodine 1201 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.
Aerospace - Yeovil, Somerset, UK
September 9, 2015
Q. Dear All,
I have aluminum parts (both A360 diecasted & 6061), already chromated and partially white powder coated. Now I need to rework the surface with chromate.
1) If I soak the entire part for chromating, will it affect the powder coat when put into 48 hours salt fog test? Is there a chemical reaction that might weaken the powder coat finishing?
2) Can I do above without grinding off the first coat of chromate? What will happen?
Thanks so much for your advice.
- Western Australia
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.
Ted Mooney, P.E.
Pine Beach, New Jersey
September 10, 2015
A. Well this is a different question.
A true Chromate process will involve multiple steps:
(1) Caustic wash (this will degrade the existing powdercoat)
(2) Rinse (no problem)
(3) Acid etch (this will degrade the existing powdercoat)
(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,
Trainer - Newcastle NSW Australia
September 14, 2015
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.
- Western Australia
This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site