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topic 4379

Phosphoric acid anodizing per BAC5555 and ASTM D3933-98


Q. I am looking for technical information on PAA (phosphoric acid anodizing). I have searched the web and cannot find much technical info. Do you know where I can get the info? I am trying to set up a small bench top operation. I run a small job shop and I'm just seeking to help a customer out while he does some samples for adhesive bonding tests.

Michael Broussard
plating & metal finishing shop - Albuquerque, New Mexico

Handbook of Aluminum Bonding Technology and Data


A. Phosphoric acid anodizing is porous and provides a substrate for mechanical locking, so I suppose that is why you need it for the adhesion test. The specification for the adhesion test should have the parameters you need for applying the anodic coating.

tom pullizzi monitor
Tom Pullizzi
Falls Township,


A. It is reference book time. Take a look at this sites "must have" book list. I would take a look at AESF's book (three ring binder) by Grubbs and Montgomery. Very straight forward and the cheapest good reference. There are better, but cost a lot more.

James Watts
- Navarre, Florida

Ed. note: James' recommendation was from 2001, and even then the "Light Metals Finishing Process Manual" was quite hard to find. If you see an affordable copy anywhere these days, grab it :-)


A. There is very little information on this specialized process. I know of two methods:

The first is used by Boeing for adhesive bonding. It uses 10-12 % by wt phosphoric acid at 70 °F. Anodize at 5-8 ASF for 20-25 minutes.

The second method is 30-50% by volume Phosphoric acid at 80-90 °F. Anodize at 3-10 ASF for 5-20 minutes.

Best of luck to you.

Ira Donovan, M.S.F.
Kansas City, Missouri


A. Hi Michael. You may be looking for Boeing's BAC5555 process. You may find something on the web under PAA or BAC5555, or you can do a literature search which will give you some data, or consider contacting Boeing's commercial development office about licensing BAC5555.

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

A. The definitive PAA spec is probably BAC5555 from Boeing although if you want a desktop method there are PANTA ["phosphoric acid non-tank anodizing"] or PACS ["phosphoric acid containment system"] systems commercially available.

Ciaron Murphy
- South Wales

3M Acrylic Structural Adhesive

A. If you can track down ASTM specs, you might try D3933-98 [link by ed. to spec at TechStreet], Preparation of Aluminum Surfaces for Structural Adhesives Bonding (Phosphoric Acid Anodizing).

James Davila
- Dayton, Ohio

March 4, 2009

A. PAA specs would be found in Boeing BAC5555, if you are looking to set up a little bench operation, I set one up using a solution makeup of 15 oz/gal of 85% H3PO4, running at 77 +/- 5 °F, 15 amps per square foot for 25 minutes, I used a small piece of metal for my cathode. Good luck

stephen clark
- weatherford, Texas

(2001) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I am trying out Phosphoric Acid anodizing for a new customer. I'm using 260 gm/l Phosphoric acid, 4 A/dm sq c.d., time 15 mins, but was unable to get any microns. Can you help me with a process for Phosphoric acid anodizing. My customer requires this process as they need to go for another bonding process.

Thank you,

Johnny Goh
- Singapore

Ed. note: To make more info available with less searching, we've combined together onto this page a number of previously separate threads about phosphoric acid anodizing. Apologies if there is therefore some repetitiveness to the Q&As, or if the posters seem to be ignoring previous postings (they may not have been there at the time).

RFQs for PAA and related processes . . .


RFQ: Looking for a certified shop to complete F600 (PAA and Bond Primer) with Cessna approval. Please send contact information, I'll forward drawings for an RFQ.

Tim G [last name deleted for privacy by Editor]
- Wichita, Kansas, USA


RFQ: Need a source for PAA etch to D3933-98 [link by ed. to spec at TechStreet], mask, BR127 prime and chem-film. Parts are type 7075-T6 aluminum 1/2" x 5" x 23".

Fred V [last name deleted for privacy by Editor]
- Bohemia, New York


RFQ: Request for quotation. Need a PAA treatment according to ASTM 3933-98 for 72 plates 0.04 X 3 X 6 of type 2024-T3 aluminum
Can anyone help me farther?

Roni S [last name deleted for privacy by Editor]
- Ramat HaSharon, Israel

Cathode material and other requirements

Q. I have just quoted a large phosphoric anodize job. While very familiar with Mil-A-8625 [link is to free spec at Defense Logistics Agency,] Types I, II, and III, I am not very experienced with this process. I have obtained ASTM D 3933-98 covering this subject. My main question is what is the recommended cathode material for this chemistry? Thank you in advance for your help.

Best Regards.

Rick Richardson, M.S.F.
Dayton, Ohio

A. Phosphoric acid anodizing is done with stainless steel cathodes. If the anodizing tank is stainless steel, the tank itself can be the cathode.

Phosphoric acid anodizing is usually requested by the aerospace industry. It is used as a surface preparation for bonding. There are very stringent requirements for application and inspection of phosphoric acid anodized parts [for aerospace application]. Usually the customer requires a certification before parts can be processed.

If you have further questions, please feel free to follow up.

Ray Handwerker
- Bensalem, Pennsylvania

(2000) -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi,

I'm an engineer attached to Goddard Space Flight Center's Advanced Manufacturing Branch. People in our composites group are pushing the electroplating facility to install a phosphoric anodizing process to prep aluminum for adhesive bonding.

Can anyone tell me what equipment is needed to phosphoric anodize aluminum? I need info like tank liners, cathode material, voltage and amp requirements, bath temp, etc.

Thank you in advance.

Joel Mitchell
NASA - Greenbelt, Maryland


A. Hi Joel. We appended your question to a thread which should answer many of your questions without having to wait for replies. Phosphoric acid anodizing (PAA) is used to give aluminum a good tooth for adhesive bonding. See D3933-98 [link by ed. to spec at TechStreet].

There is also a Boeing spec for it, I believe it's BAC 5555. My understanding is that some of Boeing's competition does not use this process for adhesive bonding, but rather a variant of chromic acid anodizing.

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


A. I am currently setting up a phosphoric acid anodize (PAA) line. PAA is used to prepare the aluminum surface for acceptance of the primer before bonding. You should note that PAA specifications call for the anodized parts to be stored in a "clean room" environment prior to application of the primer. Parts must be transferred to the clean room within two hours of removal from the PAA line and be primed within 72 hours. After priming, parts can be stored before bonding.

Specifications you might want to obtain for reference include:

Ray Handwerker
- Bensalem, Pennsylvania


A. Greetings Mr. Mitchell,

If I may so inquire, why would your company lean towards a phosphoric acid anodize over a chemical film/chromate conversion coating finish. Sure it has hexavalent chromium content in it, but you're still dealing with a product that has less regulation (you are exempt from the chromium standards, because it is a non-electrolytic bath) and the coating is a very inexpensive finish that is geared for adhesive bonding (I have a customer that uses it for 100% this purpose and it works great). Now maybe there are some issues that are addressed inside of NASA that I am not aware of obviously, but I don't see the reasoning behind phosphate coating over chromate. Chromate can withstand 336 and in some cases (mostly material dependent) I have seen in cases it pass 500 hours. I'm not saying phosphoric cannot do this, but why open up a potential can of worms if you can achieve your goal and avoid some headaches.

Matthew Stiltner
plating company - Toledo, Ohio

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


A. Joel, there are some very specific requirements concerning bonding using PAA. There is an engineering document you need to be qualified to for Boeing work. I would suggest starting there with some of the engineering requirements then go to the PAA spec. mentioned above, BAC5555 You will also need BAC5514 for the adhesive primer requirements. Prior to engineering your line you may wish to also think about production flow logistics as Ray mentioned which are critical for best results, as well as meeting most aerospace specs. I cannot talk about certain Boeing processes on an Internet website for obvious reasons but would be glad to speak with you in person if you wish.

Mr. Stiltner, the answers to your questions are partially in the two volume set by Wernick, Pinner and Sheasby, The Surface Treatment of Aluminum and its Alloys, and a number of books on pretreatments of aluminum for bonding purposes. If you are interested, one I might suggest is Surface Prep. Techniques for Adhesives by R. Wegman published by Noyes publications. Another would be Handbook of Adhesives edited by Skeist.

Basically there are some particular surface phenomenon characteristics with PAA which are not acquired elsewhere other than possibly FPL etches, and that could be arguable. Ted is right that some adhesive bonding is performed using CAA as an alternative to PAA, but my understanding is the lap shear test comparisons do not show the same results over time.

Ward Barcafer, CEF
aerospace - Wichita, Kansas


A. For background, you might want to check out MIL-HDBK-83377 which replaces Mil-A-83377 [link by ed. to spec at TechStreet] (ADHESIVE BONDING (STRUCTURAL) FOR AEROSPACE AND OTHER SYSTEMS, REQUIREMENTS FOR. When we wrote it some years ago, it contained just about everything we wanted for aerospace structural adhesive bonding.

Surely, since the original document was written, there have been advances in structural adhesive bonding. However, many of the basic tests and ideas for quality control, etc., MAY still make sense. If Boeing's BAC-5555 spec is still active, then it would be prudent to consider it's use. The Mil spec was written to get people to use stressed, environmental testing for adhesive bonding quality control instead of relying only on plain lap shear tests and to use good overall practices. At that time, Phosphoric Anodizing was best for most structural adhesive bonding. Chromic acid and other anodizing methods, while good for corrosion protection, seldom proved in environmental stress tests, e.g., salt spray wedge tests, to be best for adhesive bonding. In addition, clad aluminum bonded parts corroded in the clad layer, thus releasing the bond.

If you want a quick and dirty demonstration, make standard peel test specimens from candidate surface preps. Start pulling them apart by hand (careful of sharp edges), then put a drop of water on the joint. You MAY be surprised by the result.

Weldon Scardino, P.E. (ret)
- Dayton, Ohio


Q. It is not clear to me if Phosphoric Acid Anodizing is still patented or not. ASTM D 3933-98 process is, in practice, equivalent to BAC5555 process but no reference to BOEING process is present. So, is everybody allowed to use ASTM D 3933-98 without the necessity to ask the license to BOEING Company?

TAMIRO, Salvatore


A. Hi Salvatore. I don't know if the patents have run out on BAC5555 or not -- you'd have to do a patent search. Sorry for the non-definitive answer. But I do know from other cases that just because something is a MIL, ISO, ASTM, or other standard does not guarantee that it's not a patented process or that you can use it without royalty. Good luck.

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

Fatique Issues


Q. Currently we build Aluminum Structure for the Aerospace industry and use Chromic Acid Anodizing (CAA). We would like to switch to Phosphoric Acid Anodizing (PAA) for the corrosion protection of our parts. I need to know what the fatigue behaviors of PAA are in comparison to the fatigue behaviors of CAA or sulphuric acid anodizing (SAA)?

Krag Rodewald
Aerospace OEM - Dallas, Texas


Q. What are the surface phenomenon characteristics of PAA that make it so desirable for adhesion? Also, are there any alternative platings that can produce the same results?

Jeff Navarro
- San Diego, California, USA

March 16, 2011

A. Hi, Krag and Jeff.

I am not an aircraft engineer and cannot give you an authoritative answer; but maybe this help you until someone more knowledgable steps forward. PAA and Chromic acid anodizing are thin coatings which I wouldn't expect to have nearly the bad effect on fatigue strength that much thicker coatings like sulfuric anodizing have, yet they are somewhat porous and adhere to the substrate well, while being corrosion resistant.

We have a good thread on the site, "Anodizing's Impact on Fatigue Strength", Letter 15354, and also an FAQ: "Introduction to Anodizing" which you may find helpful for this subject.


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

Where to get spec BAC5555?

November 5, 2011 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread


I am working on a Epoxy joint on Aluminium.
I have found some info regarding a Boeing procedure BAC5555 (Boric acid anodizing) on the internet. First tries with a 10 % boric acid solution at room temperature / 20 min / 15 V showed a big improvement. So I am now looking for the BAC 5555 to get more information.

Who can help?

Bernd Pretzlaff
Developer - Husum, SchleswigHolstein, Germany

A. Hi Bernd. I believe that BAC 5555 is Phosphoric Acid Anodizing rather than Boric Acid Anodizing. The spec can be obtained from Boeing commercial division or from some specification clearing houses like


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

Can sulfuric acid anodizing be substituted for PAA


Q. I am working at a US Air Force base. It uses sulfuric acid anodizing at many sites, and has a phosphoric acid anodizing operation in one building. This operation pre-cleans the thin aluminum aircraft skin/panels (with an alkaline cleaner), DI water rinses, etches (with ferric sulfate and sulfuric acid) and then phosphoric acid anodizes (with 30% HPO3). It is then sealed using DI water in a series of two dip tanks. The phosphoric acid anodized sheets are then pressure bonded.

Can this operation be switch to sulfuric acid anodizing?

We could more easily recycle the waste anodizing solution is we could convert the phosphoric acid anodizing bath to sulfuric. Is there any literature about the advantages and problems associated with switching from phosphoric acid to sulfuric acid anodizing? Will it require a different sealer, such as with sodium chromate?

We have been told that the sulfuric acid process is more temperature sensitive, and we might have to install a chiller and a heat exchanger to control the temperature. Other changes? Thanks, Nick

Nick Conkle
Battelle - Columbus, Ohio

1 of 2 simultaneous responses(2000)

A. The main office, downtown, used to have one of the finest libraries going. Phosphoric anodizing is well known as being superior to sulfuric acid for bonding purposes. I rather doubt if the gain is waste treatment is worth the reduction in bondability. That is a management decision. since it involves the military, you may die of old age before a change is approved.

James Watts
- Navarre, Florida

2 of 2 simultaneous responses(2000)

A. Are you sure that the DI water tanks following the PAA are for sealing, versus rinsing? What is the temp of those tanks? Nonetheless, if the PAA is being called out for a pretreatment for a subsequent pressure bonding operation than the answer is no, sulphuric acid anodizing is not the coating of choice for a variety of reasons.

The only other anodize treatment called out for bonding prep work is CAA which now brings Hex Chrome into your facility. The surface morphology of PAA, and coating thicknesses produced are much better suited to bonding operations than SAA.

Ward Barcafer, CEF
aerospace - Wichita, Kansas


thumbs up signThanks you for your input regarding the suitability of sulphuric acid anodizing as a substitute for PAA. Nick

Nick Conkle
Battelle - Columbus, Ohio

Need an alternative to PAA


Q. I am working on a project to find alternatives to Phosphoric Acid Anodizing to actually try to eliminate the phosphoric acid. I don't think this is possible, but I thought I should at least try to post the question. One option was to eliminate the entire process and propose Ion Beam Assisted Deposition treatment to the components. Any thoughts.

Susan Brandick
- Johnstown, Pennsylvania, USA


A. The next best process to PAA would be any method of etching the aluminum surface to roughen the profile. Chromic acid etch, sand blast, bi-fluoride etch, sanding. Basically any of these processes would have to be tested for relative bond strength.

Michael G. Broussard
plating and metal finishing shop - Albuqerque, New Mexico


A. You might find the article "An Alternative to Anodization: Sol-Gel solutions for Metal Finishing" published in Metal Finishing, December 1998, to be of interest to you.

Bill Park
Space Systems/Loral - Palo Alto, California

Bonding aluminum to carbon fiber for race cars


Q. My company is co-molding forged 2014 aluminum inserts into a carbon fibre component for the auto racing industry. We are currently grit blasting, Acetone [linked by editor to product info at Amazon] washing and priming the material with BR127 from Cytec, before laminating with glass/carbon preimpregnated with epoxy based resins. We are researching either chromate surface conversion (CAA) or PAA for better bonding performance, but are getting conflicting information from various anodizing sources. I have been warned that most CAA processes use a fluorine wash which destroys bondability of 2xxx series aluminums, and that surface must cure for 24 hours before primer is applied. Others tell me that PAA requires primer application within 3 hours of anodizing and near clean room surroundings to avoid subsequent damage. Also, PAA anodizers say you can laminate carbon skins directly to the converted surface without galvanic problems, but other coatings still require a glass barrier. This is auto racing, not aerospace, so we have some flexibility, but still cannot tolerate problems.

My basic question is: what is the ideal corrosion inhibitor/bonding preparation for 2014 aluminum (forged) for subsequent bonding to glass/carbon using epoxy resins.


Josh Poertner
- Speedway, Indiana, USA

Filtration of PAA solution



I work for a company that specializes in composite structures. We currently have a small PAA outfit and would like to get it Nadcap approved. Currently, Nadcap does not have a specific PAA spec, but one will be handed down soon after their quarterly meeting this month. We are trying to get BAC5555 compliant since it is going to be used as the "straw man" for the new Nadcap spec. I was hoping someone could suggest a filtration technique for the acid tanks to keep chloride and fluoride levels down to acceptable levels (35 ppm for chloride, 75 ppm for fluoride). Our current system needs to be improved upon. If you are curious about Nadcap, the meeting results should be available on their website at the end of the month.

Thanks for any replies!

Paul Bozzo
- Sacramento, California, USA

Ed. note: We have a long and interesting but highly charged thread about Nadcap as "Trouble with NAPCAP Audits", letter 27698.

Shop Problem: Is fungus/algae buildup causing white streaking?


Q. We have developed a white residue after our parts are removed from the Phosphoric Acid anodizing Tank which is causing streaking.

Is this the fungus that I have read about or something else?

David Storey
TTF, LLC - Breaux Bridge, Louisiana, USA


Q. I am looking for any information on the removal and control of algae that is in our Phosphoric Acid Anodizing tank. This solution of a 75% Phosphoric Acid at 16 oz / gallon of water is maintained at 77 F. The use of a Ultra Violet system has been mentioned as one possible cure?

Eddie Roach
- Tulsa ,Oklahoma

Ed. note: Hello, David and Eddie. We have an excellent thread about Control of Algae in PAA; please see letter 22828.

Shop Problem: Can't remove tape after adhesive bonding


Q. We are using a tape to hold Phosphoric Acid Anodised details together during adhesive bonding (350ºF) and the tape we are using is leaving a residue which we can't remove with a solvent. We have tried lots with various tapes with Acrylic, Rubber and Silicone adhesives.

Desperation is setting in. Can anyone recommend a really good high temp tape?


Ciaron Murphy
Aerospace Repair - Blackwood, South Wales, Britain

Shop Problem: Incoming aluminum is too smooth for bonding


Q. Is it possible for an aluminum surface to have too low of a surface roughness for phosphoric acid anodizing? Our PAA plater is getting hardware(no roughness specified) where the surface has a mirror like finish, probably a 2-8 RMS. They state that at this low roughness the plating has problems adhering. I am skeptical-any truth to this? Thanks!

William Pinzon
- Greenlawn, New York, USA


A. We have tinkered with PAA over the years typically applying adhesive primer after anodizing, but anodizing is a conversion coating. Specifically, little protruding oxide fingers rise right up out of the aluminum - I'm not sure that a smooth surface or a rough surface would have any difference - a clean and properly deoxidized surface will behave identically given the same electrolyte, current density, etc. Are you sure your anodizer isn't talking about adhesion of the primer to the PAA - this might have some merit - however, with PAA being only a fraction of 1 micrometer in thickness, I'm not sure it matters.

milt stevenson jr.
Milt Stevenson, Jr.
Anoplate Corporation
supporting advertiser 
Syracuse, New York

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Should PAA solution turn yellow after use?


Q. My company has a small phosphoric acid anodize set-up for prepping 5083 Al and 2024 Al. We would like to know if it is it normal for the solution to turn a yellow color after processing. We are using a 10% PA solution in a glass tank with aluminum cathode and aluminum racks, voltage 10-15 v per D3933-98 [link by ed. to spec at TechStreet]. We previously had not experienced this yellow color change until we recently improved the electrical contacts and the mechanical contacts holding the parts to the racks. Does the spent solution turn yellow with continued use?

David F [last name deleted for privacy by Editor]
- Oakland, California, USA

March 29, 2016

A. First of all, you probably want to use stainless steel cathodes instead of aluminum for PAA. The aluminum cathodes will be dissolved continually in phosphoric acid, shortening your bath life.

The discoloration could be related to some of the alloying materials dissolved in solution. For alloys with a lot of Chrome, the solution will become a greenish color. I would guess that the copper from the 2024 alloy could be somewhat to blame, along with whatever dissolved out of your cathodes.

I am not an expert, but this would be my guess. I wouldn't be too worried about it until the concentration climbs above what is allowed.

Jon McCulfor
- Zeeland, Michigan, USA

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