Anodizing on 7075 Aluminum Flaking, Blistering, Pimpling

A. Hi Peter.

Just one more thing that Robert Probert has mentioned on other threads but which didn't happen to have come up on this thread, and might possibly apply to your parts: sharp inside corners are a problem when anodizing. Because of the way that anodizing grows from surfaces, the anodizing on the two perpendicular sides 'crash' into each other which can cause crushing and flaking even without heat, but worse with it. Radiused corners always help.

Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Dave,

We too, have seen this problem a few years ago. The article you found is interesting. We assumed the problem was due to overheating of the parts (one of those "shot in the dark" guesses. We reduced our tank temperature to 30 °F, and it seemed to solve the problem. If the article you found does in fact state the real cause of this problem, I can't answer why lowering the tank temp solved it. As this is not SOP for us, I'm also wondering if stripping and reworking the parts had anything to do with our results...perhaps getting past the "pore, or void" that the article mentions?

70xx series aluminum alloy?

A. OBVIOUSLY YOUR VENDORS SURFACE PREPARATION WAS NOT GOOD. Had the parts been mechanically abused by time saving, sanding, bead blasting or such, in which case non-aluminum and hence non-anodizable matter, would have been imbedded into the surface and left to "bubble out" later. Or he did not clean, or he did not deoxidize and remove all of the aluminum oxide before applying the "controlled oxide".

A. WE HAVE HAD THE SAME PROBLEM, THE BUBBLES COME FROM TRAPPED HYDROGEN THAT FORMS IN THE MANUFACTURE OF THE ALLOY (7075). AS THE HARD COAT FORMS IT TRAPS THE GAS IN THE ALLOY,WE ONLY HAD THE PROBLEM WHEN WE DYED THE HARDCOAT AFTER PREPPING IT FOR 3 MINUTES IN NITRIC ACID DEOX BATH. I THINK THAT THE DEOX WEAKENED THE COATING AND ALLOWED IT TO BUBBLE WHEN IT WENT INTO THE 140 DEGREE BLACK DYE. IF THE PARTS WERE NOT PREPPED AND DYED THERE WAS NO PROBLEM WITH BUBBLES.

GOOD LUCK,

A. We are Mil spec certified for type 2 anodizing and we seal our parts in nickel acetate at 175 °F for 10 minutes. Never failed a seal test.

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Aluminium and
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by Wernick, Pinner
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A. An interesting one, we've experienced a similar problem with delamination of the anodic film from 7XXX series alloys, on hard anodising, not quite in the platelet form seen in the pictures -- more of a gross de-lamination. Subsequent SEM and EDAX analysis showed evidence of contamination at the interface (when cross sectioned & mounted). So we off-line hand cleaned, the problem was minimised but still came back intermittently.

Consulting the "Surface Treatment of Aluminium" bible, the sealing section in vol II does give some details of the relation between stress and film thickness, with lower CD's giving more compressively stressed deposit. So we modified the voltage ramping cycle to incorporate a 10 minute dwell at a lower voltage giving a lower CD approx. 1/2 of what we would normally hard anodise this material at (reasoning maybe incorrectly that a lower growth rate initially would give the film chance to form a more coherent layer). Since then it's seemed ok. We still get crazing with this material (depending upon thickness), but no de-lamination.

Might not be anything to do with your problem, but perhaps something to try. Also 90 minutes sealing seems a long time? Does it de-laminate if it's not sealed ?

Best of luck, Regards,

A. Could it be possible that thermal expansion/compression may be contributing to this effect? From cold anodizing solution to hot seal then cold alcohol then hot vacuum. That's a lot of extreme thermal differences.

A. I think that ultrasonic cleaning or heating has nothing to do with this problem. It is most likely coming from pre cleaning. Experiment with 6xxx Al and see if same problem arises. If you don't have the problem with 6xxx Al, than you should look into your cleaning process.

A. TRY SHOTPEENING PRIOR TO ANODIZING. GOOD LUCK.

A. With limited knowledge of anodising, I dare say that Cair Shishnani is on the right track; A glass bead peening; or a light polish using a silicon carbide emery belt may improve adhesion of the anodise.

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A. 1. Hydrated Aluminum Oxide has a different coefficient of expansion than 7075 aluminum. Sheasby and Pinner address this in their book.

2. Steel shot peening will imbed fine iron particles into the 7075 aluminum which will not anodize and will contribute to worse loss of adhesion.

A. I agree with Bob.

We have seen a similar condition with Al alloy heatsinks, Type II anodized to 17µm coating thickness, dyed and sealed in boiling water. Gross overheating was allowed to occur in the power MOSFET that was generating the heat. The onset of cracked anodize was 145 °C. case temperature maintained for 30 minutes. The cracking (like an egg shell) occurred in a pattern that neatly shadowed the MOSFET package. When cracked, the anodize could be removed in small pieces. Thicker anodic coatings seem more susceptible to this than thinner coatings, since cracking can occur at somewhat lower temperatures with thicker coatings. The cause is no doubt the large differences in thermal expansion between Al alloys and aluminum oxide structures.

A. 1. With time and temperature, and especially with heat treating, the zinc migrates to the grain boundaries, when you anodize over these grain boundaries, it flakes off.

2. Work Hardening from dull tools or tumbling or blasting. When you anodize completely through a work hardened surface, stress flake the coating off.

A. Blisters aka pimples are caused by intermetallic particles which block anodize growth into the metal. A particle holds up an area of anodizing which becomes detached from the metal as anodize growth continues in surrounding areas. Looks like an incipient volcano. Stresses create cracks radiating outward from the center into the uplifted anodize (actually, surrounding anodize has moved downward) through which acidic electrolyte enters and creates white sulphate corrosion. The undercut anodize & corrosion products can detach & get swept away, after which the bare metal area some 500 microns in diameter can begin anodizing anew.
In SEM analysis back in 1997, I found particles of primary solidification phases FeAl3 and Cu2FeAl7 at the core of developing blisters. These are coarse particles up to 25 microns in diameter which should not be present in wrought microstructures; they are not the sub-micron particles formed by heat treatment.

Re preventing blisters: Shot peening breaks up the intermetallics within the near-surface Al matrix to a small enough size so as to not blister the subsequent hard anodizing, and is a common procedure for helicopter manufacturers. personal discussion, Leah Markowitz, Senior Engineer, HR Textron, 1998 SUR/FIN conference. See also "Hard Anodic Coatings on Aluminum Alloys: Evaluation and Control of Porosity," L. Markowitz, Metal Finishing, p. 19-24 (December 1992).

As MIL-A-8625 / MIL-PRF-8625 [on DLA]F, paragraph 3.3.2 could be interpreted as prohibiting shot peening, note: "Aluminum parts which have been peened with metallic shot shall be chemically cleaned" prior to anodizing. MIL-S-13165C [ed. note: or AMSS13165], paragraph 3.3.10.2 and MIL-P-81985, paragraph 3.5.2.1. Typically, this cleaning is a light etching followed by a nitric acid-type deoxidation.

The problem can also occur with Al 2024. The University of Illinois has developed an electrochemical process for removing intermetallics from the surface of aluminum alloys. Probably useful for applied coatings and chem film but not hard anodizing:
http://www.otm.uiuc.edu/attachments/T98002-092.pdf

A. Chlorides from dragged in tap water and/or fluorides from proprietary products anywhere in an aluminum treatment process line will cause pits. This is aggravated by the galvanic action between titanium racks and stainless steel tanks or heaters.

In old 2024 or heat treated 2024, the copper has migrated to the grain boundaries, where it is more rapidly picked out by chlorides and/or galvanic action, thus leaving behind pits.

Do a laboratory line with beakers, D.I.water, and fresh make-ups, and see if you can confirm what I have said above.

A. Up front. I do not know, but I suspect that it is a function of the thermal coefficient of expansion. They are significantly different. On a small part, it would not make as much of a difference as on a large part. There is also a possibility that the center of the aluminum is retaining the temperature a lot longer than the oxide so that you would have the oxide shrinking much faster than the aluminum that is under it.

7075 has a relatively high non-aluminum content which does not anodize, so your adhesion of the oxide would not be as strong, which would also allow it to fail easier. Now, the important question. Why are you baking it to such a high temp. Lower would most assuredly be better for the oxide.
A third factor is you seal a certain amount of liquid in the sealed anodize. If you are baking relatively soon after seal, I would expect steam formed and cracking the pore walls of the anodize. Remember, anodize is not a solid. It is tightly packed hexagonal pores of a hard and brittle ceramic.

A. Like he said, the aluminum expands 5 times the aluminum oxide. Something has to give. Also the dye cannot stand that temperature.

A. Reference NASA-TM-104622. I have had the same problem with different parts made of 7000 series aluminum . The answer is do not hot water seal or seal at all. Keep the anodize thickness at .0006" max. The problem goes away. There is a little loss of corrosion protection, but it beats cracks. It was all attributed to thermal stresses. See the NASA technical memorandum.

A. The sulfur is from aluminum sulphate occluded in the pores. This probably has nothing to do with the flaking; however, it interferes with dye adsorption, and it can be reduced by
(1) cutting the anodizing solution down to about 8 gm/L dissolved aluminum and
(2) running the work thru a 5% nitric acid to flush out most of the aluminum sulphate before dyeing.

Regarding the flaking. How old is the 7075? Was the 7075 heat treated? Have you aged, heat treated and either tumbled or blasted the surface? Has the surface been work hardened?

In 7000's that have been aged or heat treated, the zinc migrates. If that grain boundary, containing the zinc, is close to the surface, then you anodize completely through and down to it, then it will (unplate) and flake right off.

A. 7000 series aluminum is a favorite of aerospace. It contains a sizable amount of zinc and flaking after anodizing and dye is not uncommon.Changing pretreatment can help with some of these problems. I would try staying away from an etch or limit it to a very short time. I would also try Nitric and fluoride for deox to help remove the zinc from the surface.

A. Hi Josh,
It will help if you can post some pictures of the part with the defect.

A. Typically, that is Zinc precipitate in this particular alloy.

A. First things first, is that if you have SEM capability at your facility, I would analyze one of the blisters to confirm that is indeed the issue. I only have one part that I anodize that is made from 7075, and to date, I've never had issues. I know we're very careful about using Alcoa aluminum in this part that we manufacture. We use a HNO3 based de-smut, which may, or may not help you out, depending on how bad the precipitate is (if that is your issue)

If you do a search on this site, I know this particular question has arisen many times. You should be able to find numerous posts about this problem, and perhaps a solution from others who anodize this alloy more frequently than I.