Masking/stripping when chromate conversion and anodizing on same part

A. Hi Robert. We appended your inquiry to a long thread offering you many perspectives on what is involved when a component is partially chem-filmed and partially anodized -- but I can't imagine any possibility of chem-film under anodize since anodizing is a process that converts the aluminum on the surface to aluminum oxide.

Regards,

A. Although it wouldn't surprise me that our government would come up with a spec that's not practical, no you can't do this. Basically for the reason Ted stated. However, chromate conversion is allowed under the MIL-A-8625 / MIL-PRF-8625 [on DLA] F spec as a touch-up procedure. Perhaps that's what you're referring to.

Yeah, it's just being flowed down from some of our customers more recently. It used to be a once in a year type callout, but I've seen it about 3 times this month. I thought the same as the thread responses but also considered maybe there's something that I might not know.

A. David, here's my 2 cents. Typically when dealing with duel coatings like you mentioned, the conversion coating with be used for electrical contact reasons, and to prevent the Al from oxidizing... not corrosion resistance. It's my understanding it's the corrosion resistance that becomes affected when subjecting the conversion coatings to higher temperatures.

You're typical dye baths operate around 150 °F, No worries there. I imagine you're sealing in a mid-temp Ni acetate, or similar, perhaps 170 °F - 180 °F? Of course you could use a "cold" seal that operates at lower temperatures, but in my opinion, it's an inferior seal, and quite honestly, I'm sure you don't want to add another bath, when it's really not necessary.

If you're certifying to 5541F, there is no requirement for it to be subjected to heat prior to sending your test panels. Apply the conversion coat, send your panels for testing, pass...boom, you're done. You've met the spec requirements. Same goes for the 8625F spec.

All you're doing is what's being asked for on the print, I assume. If the engineer calls out for coatings that could potentially fail due to his/her lack of coating knowledge due to subsequent processing, or even use in the field...well, to be blunt, that's not your issue. You've sent your panels, you've passed the test showing you're applying the coating properly, and you've received your certification.

Since the duel coating appears to be a common practice for you, and if you're not having failures in the field, and most importantly, the customer is happy with your product... then quite honestly, I'd just keep on keeping on with what you're doing, and not try to open a can of worms which could possibly create additional work and expense for you.

Conversion coatings and anodizing have been applied on the same part for many, many years...it's even allowed as a touch up on 8625 F, and I seriously doubt that any NADCAP auditor will ever ask you to show that your 5541F coating passes the test requirements after application of an anodized coating.

Let sleeping dogs lay, my friend...UNLESS you have customer complaints. Then you can bring up the concern of subsequent processing potentially affecting the coating properties. I don't believe that 180 °F in the seal tank is going to have a significant measurable impact on conversion coatings used in the field.

As to your question about applying the conversion coating after anodizing...I would avoid that one.

Marc, Thanks for your response.
After re-reading the mil spec I may have been over thinking this one. 6.11 refers to temperature limits on unpainted surfaces, so I should be fine if I consider masking as painting. Would you agree?

Dave

Q. Hi,

In my work it requires that we mask part with lead tape 1st, then anodise, remove tape and mask for prime paint. Remove tape and locally Alocrom area that has not been anodised after paint. We can't do this without making a mess of part, staining paint, and taking a long time and costly. Any advise please.

thanks rod

A. Hi Rod. We appended your question to one of several threads here that discusses the situation in great detail. After you have digested it, please get back to us with remaining questions. Thanks.

Regards,

A. Chromate first all over, mask the chromate area, strip and anodize. The 40 volt chromic anodizing attacks the edge of the maskant more than the 20 volt anodize. Use the best quality maskant. Do not use lead tape. Use plastic tape, well mashed down. There is one maskant leader in our industry but I cannot name him here.

Hi ted,

I have digested it as you said. I've only been doing job 6 months so not that much of an expert. But have tried different methods and I just deoxidize the component, rinse, and just Alocrom the bit that needs doing, mask with foil tape, strip, then anodize. Seems to be working , but I'm still learning. Thanks for advice given by all on here.

p.s.: Like someone said on here the planning people don't always think it through, do they?

Thanks Rod

A. Hi Kelley. We appended your inquiry to a thread which discusses the alternatives in pretty good detail. I would say, no, you can't anodize the machined area; and yes, you can Alodine it to repair this part (although it's probably not the best sequence for production work which involves anodizing and Alodining).

Regards,

A. Spot anodizing certainly would be an option, although the color may not match the original anodizing. Alodine (chromate conversion) as a touch up, is permitted under Mil 8625F, albeit the coating properties of chromate conversion processes are not the same as anodizing.

A. Hi Mark. We appended your inquiry to a thread which discusses the options when chem film and anodizing are required on the same part. A given area of the part is not both anodized and chem-filmed, in either order; rather, it's one or the other.

Regards,

A. Process steps should be as follows:

1. Alodine entire part
2. Mask area to remain Alodined
3. Anodize (strip Alodine in anodizing pretreatment) and seal.
4. Remove masking from surfaces to remain Alodined.

A. I did have one thought on this. Perhaps at one point there was some masking needing done for electrical contact, and that was later changed, but the print wasn't properly rev'd/amended.

However, as you describe it now, it makes no sense. Definitely worth a phone call to your customer.

A. From my experience it is a mistake in the drawing . Usually Chromate conversion coating comes under the masking area. As you mentioned no change made after changing coating requirements or masking requirements

A. Hi Adrian. To be clear you are saying that the process, which was supposed to be chromate, mask, anodize, unmask ... was actually done instead as mask, anodize, unmask, chromate? Although several different things can cause the bluish color, one of them is the anodizing not being thick enough. You should probably do a thickness test in the blue area and make sure it meets your minimum rather than assuming that the chromating caused the blue discoloration and did no other damage to the anodizing. The blue color could also have been caused by the chromating process bleaching/leaching the dye out of the pores, indicating that it is no longer properly sealed if it ever was, so a seal test is probably called for.

Even if you ignore the aesthetics, I think you are right to be very cautious about whether the finish will meet your needs when it wasn't done right -- so a range of tests should be done.

Regards,

A. Hi again. If the parts are stripped with chromic-sulfuric acid rather than caustic soda [affil links] , you'll lose no additional aluminum, only the aluminum which was consumed by the anodizing process, which is roughly 50% of the anodizing thickness. Since the parts are black, I'd guess that is about 0.0004" of anodizing, for a net loss of about 0.0002" per surface -- but measurement and/or specs is a hundred times as good as a guess.

Too bad there are no anodizers in the USA :-)

Regards,

A. Hi Tamil. We added your question to a long-running thread on the subject. It sounds to me like the ideal way to do it is to anodize the whole component, then do the machining, then conversion coat the freshly machined area.

If that isn't possible, other sequences have been described, but I think they all involve masking. Marc Green's reply of August 22, 2014 would seem to be the straightforward way.

Regards,

A. Hello Mr. Jeschelnig,
I am not aware of such a reference with visual guides (similar to the ones that exist for adhesion testing). This isn't to say that one doesn't exist, but we do double processing with masking as a MAJOR percentage of our overall parts throughput (it's one of our shop's specialties) and nothing like that has ever crossed my path.
The tolerances for edge bleed should be right on the customer print if it is important to them- sounds like they are not, or you wouldn't be looking for a standard.
If you're having issues with the customer rejecting parts for such, I'd get with their purchasing/engineering folks and get a written agreement as to what constitutes acceptable edge bleed. And of course they will say, "well, NONE!!!" And then the fun begins, and the next thing you know your inspector is going crosseyed from squinting into a Micro-Vu all day...
Good luck!

A. Hi Kuljeet. I think you realize that you cannot anodize on top of a conversion coating, but I'll make that clear for other readers who might misread your question.

We appended your inquiry to a thread on the subject of how to best achieve an aluminum surface with portions anodized and portions conversion coated. After reviewing the thread, feel free to ask clarifying questions. Good luck.

Regards,

A. Hi Kuljeet. Anodizing offers as much corrosion protection as conversion coating, and usually more. But anodizing is electrically non-conductive whereas conversion coating can be conductive.

Regards,

A. Hi again, Kuljeet. I'm not sure if we're having a language difficulty, and you've now decided that you don't want to do what is the subject of this thread, after all, or what :-(

But to take it from the top one more time, when you need corrosion resistance on aluminum, and you need part of the surface to be conductive and part of it to be insulating, you anodize the areas you want to be insulating and you chromate conversion coat the areas that you want conductive and corrosion resistant. And you do it as described in this thread. Good luck!

Regards,