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topic 33933

Masking/stripping when chromate conversion and anodizing on same part

A discussion started in 2005 & continuing through 2017


Q. We manufacture aluminum parts in various alloys, 1050, 5083, 6082, 7075 and Casting 357. The parts are sulphuric acid anodized and partly chromate conversion coated per Mil-C-5541 [link by ed. to spec at TechStreet] Class 3 for low electrical resistance in some areas (A).

Generally parts are supposed to be chromate conversion coated (Alodine 1200 or 1500) all over, then masked in Areas (A), and coating stripped from unmasked areas. Finally unmasked areas are anodized and masking removed.

One of our subcontractors claim that they have problems with masking (leakage, using lead tape) and stripping without using strong alkaline solutions like sodium hydroxide (due to long masking time) They want to mask and strip in deoxidizer within 10 minutes after drying of the chromate conversion coating.

Please recommend materials/procedures for:
1.Masking (usually flat surfaces around grounding points and mounting surfaces)
2.Stripping bath (type and immersion time).
3.How long (minimum) should the chromate "cure" before masking, and what is the maximum delay from chromating to stripping for a given stripping bath.
4.Our subcontractor suggests to "reverse" the process, i.e., mask the Areas (A), anodize the part, remove masking, and finally chromate the complete part (also on top of the anodizing) What will the effect be on the anodizing in this case? (corrosion resistance, etc.) Is this an acceptable practice in the aerospace industry?

Ola Saeter
- Kongsberg, Buskerud, Norway


A. Dear Ola

I have a better idea, if it is possible on your parts.

Anodize the whole part and then machine the areas where you want electric conductivity. After this you chromate the part.

Best regards

Anne Deacon Juhl
Anne Deacon Juhl
- San Diego, California, USA


A. Chromate is still a gel at 10 minutes and masking is going to remove parts of that coating. Look at the tape when you take it off. If it is properly masked, the chromate can be removed in a caustic cleaner tank or some acid tanks without much problem. If they are having leakage problems with lead tape, they are not burnishing the tape edges with a wood or plastic stick. Leakage should be minor with lead tape. I would expect more problem with vinyl tape. There will always be a trace of leakage, but that tiny amount should not be a factor on an aerospace part.

I am not in love with chromating over anodizing as even a non etch cleaner will slightly attack the anodize. The acidity of the chromate tank is not going to help it either. If it passes salt spray on an every lot basis, it might be OK. The OEM procedure must be followed. What do they say?

James Watts
- Navarre, Florida


A. I think your supplier is correct. His suggested process is easier, and will likely produce better quality. There are plastic film tapes which will hold up well in anodizing. Chromate conversion coating can be done after anodizing with no effect on a properly sealed anodize coating.

jeffrey holmes
Jeffrey Holmes, CEF
      South Carolina


A. Ola

1 If the part is properly chromated, most plating tapes will adhere properly if not excessively aged. Lead tape (3M #420 or #421) adheres best with its' natural rubber adhesive but does NOT produce sharp edges as it deforms when firmly pressed down. Polyester tapes will produce cleaner edges. Once again, the surface must be clean- perform an Acetone [linked by editor to product info at Amazon] wipe before taping to be sure.

2 & 3 If the chromate is not subjected to excessive heat during the drying process, it usually can go at least over night without interfering with the strip process. Process some coupons and age them, 1, 2, 3, days and then strip to test. Most chem films will 'cure' in 24 hours.

We have used NaOH (a readily available commercial blend) to strip chromate as well as anodize. The bath must be in good shape- not a lot of dissolved Al. We run ~10 oz/gal @ 125 F (removes Al at the rate of .0001" per surface per minute).

4 We would expect the 'reverse' process to produce more failures. The chromate is what allows the 'mask' to adhere. An alternative would be to anodize all over, machine off areas to be chromated, and then chromate. We do this on Type III Class II SEALED(!) anodize with no effect on the anodize (aerospace application). Not sure the effect on unsealed coatings.

Hope this helps.

Willie Alexander
anodizer - Colorado Springs, Colorado

Chromate, mask and anodize without stripping chromate first?


Q. On many jobs we chromate (aka Iridite or Alodine), then mask, then strip off chromate & then either hardcoat or anodize.

For a specific job, we are questioning what would happen if we anodize directly over a chromated surface, without stripping the chromate first. What effect would the chromate have on the resulting clear anodize?

David A. Kraft
- Long Island City, New York


A. Good question Dave. I'm not really sure on that one. I may have to try it myself. One concern I would have would be dissolving a lot of hex-chrome in my anodize tank.

I am a little curious as to why you wouldn't want to remove the chromate prior to anodizing ... is it a finish, or a tolerance issue? Like your shop, our S.O.P. is to remove the chromate prior to anodizing. We really haven't noticed any finish, or dimensional issues arise from this practice.

Marc Green
Marc Green
anodizer - Idaho

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

Alodine after Anodizing


Q. Are there any limitations to Alodining over anodization? My application calls for black anodization (which is non electrically conductive) over 90% of the part for cosmetics and abrasion resistance, but the remaining 10% of the parts surfaces must be electrically conductive. To accomplish this, I plan on masking these surfaces during anodization, then Alodining the entire part.


Sean Garry
- San Jose, California, USA

First of four simultaneous responses-- (2001)

A. Sean,

Not that I want to give away any trade secrets or anything but why don't you Alodine the entire part (better masking adhesion) and then mask, strip and anodize?

Good luck-

Bill Grayson
- Santa Cruz, California, USA

Second of four simultaneous responses-- (2001)

A. This operation is done very often. I would suggest a non chrome material that will do the same thing as the Iridite/Alodine products. Might be a perfect fit for California.

drew nosti
Drew Nosti, CEF
Anodize USA - Ladson, South Carolina

Third of four simultaneous responses-- (2001)

A. Sean,

It would be easier to chromate conversion coat the entire part. Mask the areas to be chromated. Strip the chromate, then black anodize.

Bill Park
- Palo Alto, California

Fourth of four simultaneous responses-- (2001)

A. I've done it both ways, and prefer to chromate (Alodine) first, mask, THEN anodize. Depending on the type of masking you are doing (taping, painting), the chromated surface helps the adhesion of the maskant, especially for paint masking.

Marc Green Marc Green
anodizer - Idaho

Does chromating aluminum first cause soft and powdery hardcoat?


-- this entry appended to this thread by editor in lieu of spawning a duplicative thread

We have an aluminum part that had a chromate conversion coating applied and then some surfaces were black hard coat anodized. Can the anodization be done properly over an Iridite coating? If so, how will that hard coat differ from anodizing bare aluminum? The reasoning was that the internal sealing surfaces needed a thin surface treatment and the external surface needed a more durable surface treatment because it will be in seawater. The sealing surfaces were masked for the anodize. The black coating (if it is a hard coat anodize) looks soft and powder like. At least not a typical hard coat I've seen.

Any info is appreciated.

Michael Hall
- St. Petersburg, Florida

Surface Treatment & Finishing of Aluminium and Its Alloys
Wernick, Pinner & Sheasby

February 2014

A. Hi Michael. We appended your inquiry to a thread which shows the overall sequence to be common -- if it's done right :-)
I'm not clear on why your anodizing is soft and powder like, but it's not an inherent problem in the sequence.


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Q. I have a machined aluminum 6061-T651 housing which needs a conductive coating (chromate conversion clear Iridite) for electrical purposes but also a hard anodizing to strengthen a few large fine tapped threads. Can I mask the part and perform both operations? I would prefer not to mask the part and perform a single finishing operation to both harden the threads and promote conductivity. Any suggests?

M8-1.5 stainless Heli-coils

Thanks very much,

Jon Gallant
- Ottawa, Ontario, Canada


A. Hard anodizing will not strengthen threads. It will increase the wear resistance. That said, hard anodizing would not rank high on my list for threads since there is a rotational crushing effect that I think will cause the hard coat to not last well. If this is internal threads, why not heli-coils? If it is external, why not electroless nickel? But, in answer to your question, you are going to have to mask at least once. Typical is to chromate, mask and then anodize. The exact procedure will depend on the shop and the part. There is no 100% answer.

James Watts
- Navarre, Florida

Can chromate serve as a resist for hard anodizing

November 25, 2009

Q. Our company has a customer that wishes to have selective anodize on an item that is chromated to Mil-C-5541 [link by ed. to spec at TechStreet], will chromate serve as a resist to the anodize process?

John Bradshaw
Process Eng. - Fort Pierce, Florida

First of two simultaneous responses -- November 25, 2009

A. NO, the anodic sulfuric acid will deteriorate the chromate. You must mask the chromate, or mask for anodizing and chromate afterwards. Tobler lists a long line of maskants.

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

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como

Second of two simultaneous responses -- November 26, 2009

A. No it won't. Typically one would chromate the entire part, then mask the areas you wish to remain chromated, then anodize.

You could also mask the areas to be chromated, anodize, and then go back and chromate the areas that require it.

For costing purposes, I would base my decision of how to process based on the complexity of the masking of the areas you need chromated/anodized.

Marc Green Marc Green
anodizer - Idaho

December 2, 2009

Q. Follow up on selective anodizing -- will powder coat serve as a resist. Part is complex and difficult to maintain a uniform coating.

John Bradshaw [returning]
Process Eng. - Fort Pierce, Florida

December 4, 2009

A. Although there are probably some powder coatings that would not hold up, I'm confident that there are some that will. I've seen powder coating successfully used as a resist for decorative chrome plating in a process cycle that included electrocleaning, cyanide copper plating, and hexavalent chrome plating, so I certainly think it can withstand the anodizing process. But how will you get it off -- or are you implying that the powder coating would stay on as a permanent finish?

Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

Hardcoating first, then chromating?

September 20, 2011

Q. Does anyone have any experience with properly sealing a hardcoat anodize surface to protect it during a chromate conversion process? I am a mechanical engineer working on an aluminum (6061-T6) electronics enclosure. The inside of the enclosure is finished with a chromate conversion coating to provide a good ground path, and the outside is dyed (black) hardcoat anodize finished for abrasion resistance and aesthetics. We are currently having a "brick" hard coated and then machining the complicated interior features before sending the part back out for the chromate conversion coating.

The problem that is arising is the need to seal the anodize to protect it during the chromate conversion process. I have read that sealing the hard coat softens the oxide layer, reducing the benefit of having a hard coat in the first place, but have not been able to find to what extent it is softened. Since we are machining the parts after the anodize, I am concerned that the finish will be marred if softened too much. I've read that cold sealing will lessen the softening effects, but I haven't been able to find specific information on chemicals/temperature to use for this process.

Nathan Lowe
Mechanical Engineer - Hanover, Maryland, USA

First of three simultaneous responses -- September 22, 2011

A. Unless the areas to be conversion coated are of an extremely complex geometry, and difficult to mask... have you considered having the part conversion coated first, and then have the appropriate areas masked from the anodizing process?

The "softening" of sealed hardcoat (compared to unsealed) is real, but it is rather minimal. Is the reason for the hardcoat simply for abrasion resistance during post machining operations..or for use in the field?

Too add to that, seeing how it's a dyed coating, I would think you'd want to seal it in the first place, to prevent the color from fading over a period of time.

The conversion process normally requires some type of etch, which very well could effect an unsealed coating, especially a dyed one. If I were you, I'd process as follows:

Machine part to final dimensions
Conversion coat
Mask areas important for conductivity
Anodize (seal if necessary)

The masking step will indeed add cost to the process (especially if the masked areas are complex), but it's really the proper way to do it.

Or, I'd just seal the coating (if the masking step adds too much cost) and conversion coat afterwards. To me, when I think "enclosure", it sounds like just a cover for something that won't see much friction in the field, after it's installed. But I definitely could be wrong.

[rant]This is why all design engineers should really take some type of "coating feasibility/cost analysis" course in school. I've seen way too many products come through my door where engineers haven't thought "Finish First". [/rant]

Marc Green Marc Green
anodizer - Idaho

Second of three simultaneous responses -- September 23, 2011

A. Since you state that your hardcoat is dyed black, it would seem to me that your finisher is already sealing your part in order to keep the dye inside of the pores, rubbing the part with an acetone soaked wipe would prove this very easily. A proper seal should allow the hardcoat to survive through a very mild chromate cleaning cycle, and subsequent chromate with no problem. If sealing the part raises concerns over the hardness of the part, have your anodizer run a 4x4 panel through the process and have it Taber tested to ensure it will still meet your requirements.

Jordan Somerville
Finishing Jobshop - York, Pennsylvania

Third of three simultaneous responses -- September 22, 2011

Q. I have a couple additions to my question. I learned that some of our anodized "bricks" have already been made, without the sealing step. Some of these have also already had the interior features machined into them. Is it possible to seal the anodize without harming the bare aluminum on the machined surfaces so we can still do the chromate conversion step? I assume we can mask the bare aluminum, but the purpose of doing the steps in the "backwards" order (anodize, machine interior, chromate conversion) was to remove the need to mask the intricate interior features.

Also, a handful of the unsealed, machined pieces were sent out for chromate conversion. The results look like the color is washed out and splotchy (I can get some pictures if necessary). Did the chromate conversion just strip the color out of the anodize, or is it possible that the anodize has been harmed?

Nathan Lowe [returning]
- Hanover, Maryland

September 27, 2011

Q. Thank you Marc and Jordan for your responses, they have been very helpful. I added some more information above about my problem, but I'll further clarify and answer your questions.

The reason we are chromate conversion coating after the anodize is because the interior features are very difficult to mask. We've made these pieces in the past with the proper method (chromate first, anodize last) and when we could find a shop even willing to do it, the cost of the masking was the majority of the cost of the entire part.

The reason for the hardcoat is only to protect the finish during post machining. Use in the field should not require a particularly rugged finish.

For the existing pieces, although they were dyed, they were not sealed. I have been speaking with our finishing shop and they tell me that sealing a type III anodize will not set the color and will not protect the finish through the chromate conversion step but sealing a type II anodize will do both of those things. This is the opposite of what I've read and doesn't make particular sense to me, but I could be wrong. Is there a difference between sealing a type II and a type III anodize?

I have some parts that were hardcoated about a week ago and have been sitting on a shelf inside since then. Is there any problem with sealing those after they have sat like that? I assume parts that were hardcoated, machined, and sat with bare aluminum cannot be sealed due to the exposed aluminum, is that true?

Last, as I asked in my update above, the pieces that have been through the entire process (hardcoat (no seal), machine, chromate conversion) have definitely lost their color, but could the hardcoat have otherwise been harmed?

To respond to your rant Marc, I agree completely and would also say that during school I would have benefited from some time in a machine shop for similar reasons. But to defend myself here, I did not design this enclosure originally and have only been tasked with making it cheaper to produce. I did specify "cold sealed" for the hardcoat on the drawing due to my initial research, but that information was not sent on to the finishing house.

Nathan Lowe [returning]
- Hanover, Maryland

September 27, 2011

A. No worries on my "rant", Nathan...wasn't specifically referring to you. As a matter of fact, over the last several years I've had the absolute pleasure of working with a mechanical engineer who's one of the brightest guys I've met..and has helped me solve many a problem created by engineers that...shall we say...didn't think finish first.

Anyhow, about all I can say is this. I hardcoat, 10's of 1000's of parts a year, a lot of which either require masking, or a machining step afterwards, or both. All of them are sealed, as they are used in a plasma/corrosive environment. From your last post, it really sounds to me as if the sealing step would be more of a benefit, as opposed to a detriment.

If indeed your masking requirements are that complex, I would seal the coating, post machine, and then conversion coat. If your coater can't seal his hardcoat to withstand a simple conversion coat process, then find another coater.

The big key here is, to make sure your post coating machining process does not subject your parts to dings and the like. Your machinists have to be careful, especially if there are a lot of sharp edges/corners on the part. If there are, and that's what you're concerned about getting damaged, then it wont matter if the coating is sealed or not. Anodizing, while extremely abrasion resistant.. is a very brittle coating. It takes very little effort to knock the coating off of a sharp 90 degree edge.

I'll also add, that my company makes quite a few parts that are Type 2 anodized, that are also sealed, and require a secondary machining operation. No problems at all, and I say long as the machinists are CAREFUL while loading, and unloading the parts from the mills/lathes.

Out of curiosity..could you attach a picture of the part in question? Might help me a bit.

There are many different sealing processes available to anodizers. And it is possible that your current coaters sealing process for a type 3 Class 2 coating may not be adequate to prevent damage to the coating during a conversion coating process afterwards. However, it definitely can be done.

Properly sealing a type 3 coating to withstand a conversion coating process may require more time in the sealing bath, and/or variations in the anodizers operating parameters. Again, I can be done. And yes, there is a difference between properly sealing a type 2, and a type 3 coating.

To your other question "has the hardcoat been harmed". Probably not, just a simple loss of color, which is to be expected on a type 3, class 2 coating which hasn't been sealed. The actual coating itself should be just fine as long as aesthetics aren't a huge concern.

Marc Green Marc Green
anodizer - Idaho

Is Chem-film ever under the anodizing?

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

Q. 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

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

Marc Green Marc Green
anodizer - 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 - 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

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
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

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 - 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

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 - 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 - 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 - 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
Striving to live "Aloha"

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
Striving to live "Aloha"

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