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topic 33933

Masking/stripping when chromate conversion and anodizing on same part

A discussion started in 2005 but continuing through 2018


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 is to free spec at Defense Logistics Agency,] 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
Spartanburg, 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 your searching efforts and to offer multiple viewpoints, we've combined some 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
supporting advertiser
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"
by Wernick, Pinner & Sheasby

from Abe Books
info on Amazon

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 is to free spec at Defense Logistics Agency,], 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
and co-author of The Sulfamate Nickel How-To Guide

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

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