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topic 33933p2

Masking/stripping when chromate conversion and anodizing on same part

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A discussion started in 2005 but continuing through 2020

February 18, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Is Chem-film ever under the anodizing?

I seen a few requests for chemfilm and anodize processing without any masking requirements. Supposedly, there is a new MIL spec requiring this. Does this make any sense? Can a chemical conversion layer exist under anodize? Does this improve corrosion resistance?
Any information would be appreciated. Thanks.

Robert Bosio
- Oxnard, California, US

February 2014

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.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

February 20, 2014

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 [link is to free spec spec at Defense Logistics Agency,] F spec as a touch-up procedure. Perhaps that's what you're referring to.

Marc Green
Marc Green
anodizer - Boise, Idaho

February 20, 2014

thumbs up signYeah, 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.

Robert Bosio
- Oxnard, California, US

March 25, 2014

Q. We also do a lot of parts requiring chem film mask and anodize type II or III. My question is, if parts are chem filmed prior to masking and anodize, how do you dye and seal in the secondary operations without exceeding the 140 degree temperature limits for chromate coatings? Is this an area of concern for Nadcap. Also, if chemfilm is done after anodize, what can be used to prep the surface prior to masking?

Puzzled in CT

David King
aluminum anodizing - Oxford, Connecticut USA

March 26, 2014

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's even allowed as a touch up on 8625F, 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 Green
Marc Green
anodizer - Boise, Idaho

March 28, 2014

thumbs up signMarc, 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?


David King [returning]
- Oxford, Connecticut USA

April 19, 2014

Q. Hi,
I work in the aircraft treatments department.
On certain parts it calls for Alocrome and anodise on same part. We mask with aluminum tape then chromic anodise . The problem we have is the leaching of anodise under the tape, and having to orbital the face to be Alocromed. This is both costly and time consuming, and a pain in the neck to all concerned. Any help would be most helpful.

Thank you -- rod

rod astle
- wigan england

April 27, 2014

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

rod astle [returning]
- wigan england

May 2014

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.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

June 4, 2014

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.

robert probert

Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

June 13, 2014

thumbs up signHi 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

rod astle [returning]
- wigan england

May 28, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I have an anodized aluminum part that we had to add a machined chamfer on one of the ID holes on the part (which removed the anodizing). Can I re-anodize the part to cover the machined surface without stripping the anodizing first? Can I Alodine the part over the existing anodizing?

Kelley Posey
- St. Louis, Missouri, USA

June 2014

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).


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

June 4, 2014

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.

Marc Green
Marc Green
anodizer - Boise, Idaho

Anodizing over Chem Film?

August 20, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I have received a specification from an aerospace customer that requires a 2-step finishing sequence on a machined Aluminum chassis:
A. Chem Film (5541, Type II, Class 3)
B. Anodizing (8625, Type II, Class 2), interior surfaces optional

Exterior mounting surfaces (i.e. the "feet") are to be Chem Film'ed, but no anodizing, so I'm interpreting that those areas must be masked prior to anodizing.

My question is can anodizing be applied over top of Chem Film or must the Chem Film be stripped prior to anodizing?

My first thought was that the specification required the final finished coating stack-up to be Aluminum/Chem Film/Anodizing. An alternate interpretation is that they are only specifying the process sequence, not the finished coating stack-up. Chem Film first, mask off of the exterior mounting surfaces (and optionally the interior surfaces), then proceed to anodize, including any necessary preparation steps (such as stripping of the Chem Film from unmasked areas). The resulting finished coating stack-up would be Aluminum/Anodizing on all exterior surfaces and Aluminum/Chem Film on the exterior mounting surfaces. Interior surfaces could be either.

Any thoughts? Thanks for your help!

Mark Owens
OEM - Cranbury, New Jersey, USA

August 2014

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.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

August 22, 2014

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.

Marc Green
Marc Green
anodizer - Boise, Idaho

Chromate conversion then anodising?

October 22, 2014

Q. Hi
I have been asked to provide a finish on Aluminium of Chromate Conversion then Hard Anodising. There are no masked areas and there is no post machining. In other words no need for electrical conductivity.
I just don't see the reason for Chromatic Conversion at all. Am I missing something?
Thanks in advance

Johnathon Turpin
- China

simultaneous October 24, 2014

A. Does not sound to me like you are missing anything. Not even a threaded hole to plug? That would be the only thing that I can think of, but even then, it wouldn't make a lot of sense.

Marc Green
Marc Green
anodizer - Boise, Idaho

October 24, 2014

A. Hi Johnathon,

Are you saying that it is chromate conversion all over followed by hard anodize all over? Or do you really mean that there are areas that require conversion coating and other areas that require hard anodizing?

From my own experience the use of chromate conversion coating on certain areas is to give an electrical bonding point, especially true if the conversion coating is MIL-DTL-5541 Class 3

Brian Terry
Aerospace - Yeovil, Somerset, UK

October 27, 2014

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.

Marc Green
Marc Green
anodizer - Boise, Idaho

October 28, 2014

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

yehuda blau
Yehuda Blau
YB Plating Engineering and Quality - Haifa Israel

Anodizing and chromate conversion coating and lacquer all on the same part

August 31, 2015


I am new to posting here but thought I would throw 2 pennies into the frackus. I have worked in coatings for some years and have some hands-on experience with 5541 and 8625 and coating on top of them. Recently the company I currently work for got AS9100 and in the mix we got thrown a client to try to do some work for. The part that we sampled from them has three different processes, coating wise, performed to it. It has an anodize in most areas, a chemical conversion coating in others and a "varnish" for the sake of electrical insulation on others. I had used this coating before and was therefore somewhat familiar with it.

The client claims that they have a problem with the coatings flaking and peeling after they machine areas of the part adjacent to the coating. I am not a subscriber to ever machining next to a coating line unless the coating is designed for it, which this is not. I made it clear that after we applied the material, got adhesion and met the specs then it was there's to be done with as they so pleased but our job was done. They agreed and our coating process went off without a hitch ... UNTIL ... the coating started peeling after conversion coating.

I am familiar with conversion coatings as well and have a good deal of knowledge on the process and the chemistries. My first thought is that during the conversion coating process the part is being dipped in an alkaline cleaner, an acidic desmutter and an acidic hexavalent chrome based material, probably all at elevated temperatures and adequate times for effectiveness.

My thoughts are these:

1. This varnish was not meant to be used this way, it is probably being degraded by the chemicals.

2. During desmutting there are materials like silicone and copper being removed from the surface of the aluminum that was freshly exposed during machining. This fresh aluminum is right up against our coating so as material is removed from that edge so is our coating exposing an area to peel.

Anyone else have any experience in this?


Jeremy Rivera
- Chicago, Illinois, USA

July 27, 2017

Q. We have a supplier who chromate conversion coated the whole lot of parts (in this case it is a heatsink extrusion) after the part was masked and black anodized. The quantity is 600 each.
This resulted in a part with a blue tint instead of black.

black anodizing is blue

My requirements for anodizing are for electrical resistance mainly while the masked non-anodized areas maintain electrical conduction.

My question is, are these parts compromised by this process?

We know that the correct process is to conversion coat first, then mask the areas that do not need anodizing, then anodize, followed by removal of the masking.

Adrian Siqueiros
Astronics AES - Advanced Electronic Systems - Kirkland, Washington USA

July 2017

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.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

July 31, 2017

thumbs up sign Thanks for your response Ted.
In order to perform thickness measurements we would have to bring in parts from our China supplier which would be time consuming.
Our needs for these parts are immediate and dealing with testing the coating plus allowing the bluish tint is not an option for us at this time.
We have asked our supplier to strip the anodize off, and proceed with the correct process sequence of conversion coat, mask, anodize, de-mask.
My main concern after this is dimensional as the stripping operation will remove some material off the heatsinks. Hopefully our tolerances will accept the resulting part dimension changes.

Adrian Siqueiros [returning]
Astronics AES - Advanced Electronic Systems - Kirkland, Washington USA

August 2017

A. Hi again. If the parts are stripped with chromic-sulfuric acid rather than caustic soda, 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 :-)


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

Which one to do first on same component: Chemical Conversion or anodising?

May 22, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Our customer expects both anodising (MIL-A-8625, TYPE-2, CLASS-2) and Chemical Conversion coating (AMS-C-5541, CLASS-1A, CLEAR) on same component. We are confused about which process will be done first? Customer is also not clear in this case. Which process to take first in this case?

Tamil vazhudi
Engineer - Coimbatore, Tamilnadu,India
  ^- Privately contact this inquirer -^

May 2018

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.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

Conversion coating removal harmful to anodized parts?

May 22, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I must remove chromate conversion (MIL-DTL-5541) coating from Aluminum 7075 parts, which have been anodized as well. (Chromate in spots, anodize in others) Most methods to strip the conversion involve immersion in a caustic bath. What will the impact of the caustic soak on the anodized neighboring coatings on the same part?

Robert Hutchison
Program Manager - Pittsford, New York
  ^- Privately contact this inquirer -^

Anodise encroachment into chromated area

September 4, 2018

Q. I have an aluminum plate "Aluminum alloy 6061-T6511" that I have anodized with Sulfuric anodize MIL-A-8625 Type II, Class 2, black, on that plate I also have surfaces that can only be coated with Chem film MIL-DTL-5541, class 3. Is there a visual standard out there that would define the amount that the black anodize finish is permitted to bleed onto the surface finish with the chem film MIL-DTL-5541?

Chuck Jeschelnig
quality manager - Mentor, Ohio USA

September 5, 2018

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!

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

Chromatizing on anodized surface

August 16, 2019 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Already a component is anodized and some part of it needs to be conducting. Can local chromatizing be done on already anodized surface. If so suggest the product and procedure.

Dr. Deepashri Nage
Larsen & Toubro - Mumbai, India

February 28, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I want to do a protective coating "Sanchem Safegaurd cc3400" for protection against corrosion on my Al6063 alloy product. After protective coating, I want to anodize it to improve heat radiation. I will have to mask some areas to ensure its electrical conductivity. Tape masking remove cc3400 coating which is my issue. Is there any other conversion coating equivalent to cc3400 more compatible with Anodizing? Is there any way to stop peeling of protective coating?

Kuljeet Singh
Engineering design - India

February 2020

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.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

simultaneous March 1, 2020

A. Kuljeet

The fact that the cc3400 is being removed with tape suggests it is not being applied properly. The data sheet indicates it is a pre-treatment for paint application. If the cc3400 doesn't stick to the aluminum, paint surely won't stay put.

Your problem might the cleaning prior to application of the cc3400, or the cc3400 chemistry is not as it should be.

Willie Alexander

March 3, 2020

Q. Thanks Mr. Ted, So my idea is to improve radiation emissivity of electronic housing by anodizing. Meanwhile, I also need protection against saline environment(as in coastal areas) due to which I thought of using Sanchem cc3400 conversion coating.
If I can't do conversion coating with anodizing, what is possible solution to achieve both emissivity and corrosion protection in aluminium.

Kuljeet Singh [returning]
Vihaan Networks Limited - gurugran, haryana, India

March 2020

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.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

March 6, 2020

Q. Thanks for help Mr Ted. One last question, for connectors I need conductive surface at connector-housing interface. These areas will be masked while anodizing. So these local areas will have no corrosion resistance. Is there any treatment for these local areas to protect against corrosion?

Kuljeet Singh [returning]
Vihaan Networks Limited - gurugran, haryana, India

March 2020

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!


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
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