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Anodizing is Flaking Off of 7075 Aluminum Parts
2004
Q. Some 7075 parts were type II anodized 20 micron thick (.0008") per mil-spec. They were subsequently cleaned in a cleanroom, in ultrasonic cleaning stations with D.I and (supposedly) anodize-safe detergents, dips in isopropyl alcohol and D.I., and a vacuum bake approaching 100 °C.
When the tape was repositioned, the anodized finish came off. Based on a lit search and preliminary theories, the client blamed ultrasonic cleaning for crazing the parts, and alcohol of dehydrating the parts. Either or both processes may theoretically be deleterious, but subsequent study, described below, seemed to me to point away from either being the real problem.
The parts were chromic-phosphoric acid stripped (the shop doing the stripping was surprised how fast they stripped; this may have been due to the large surface area of the defective coating).
The anodizing was redone in thicknesses of .0003", .0007", and .001", finishing with an hour and a half of nickel acetate sealing. The coating still cracks and comes off at thicknesses greater than .0003" as platelets before baking, and as "mica-like" multiple layers of platelets after baking.
A higher power photo of the .001" coating, however, (400x) revealed other than what I expected to see:
I admit to very little experience looking at anodized aluminum at 400X, but this doesn't look like crazing or dehydration to me. This "ceramic tile floor" with "grout" between the tiles looks more to me like oversealing that caused so much expansion from growth of boehmite / hydroxides that it broke the film and continued to grow? When we tried sealing for very short times, these "grout" lines become very thin.
Then again, I may be misreading the micrograph, and I don't think we're down to root cause either. Anyone have comments on what they see, or a recommendation on mechanical prep before the chemical processing, or what to study? Thanks!
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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.
Tim KnoxMetal Finishing - Springfield, Vermont, USA
2004
"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby
on
AbeBooks
or
eBay
or
Amazon
(affil links)
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,
Richard Guise- Lowestoft, U.K.
2004
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.
Asyong Salungga- Toronto
2004
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.
Mussadiq Hussain- Canyon Country, California, USA
2004
2004
Thanks for all the feedback, folks!
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Heating/Baking causes 7075 Aluminum Anodize to flake off
2004Q. The material is 7075-T651 aluminum to be anodized per MIL-A-8625 / MIL-PRF-8625 [on DLA] Type II Class 2. When we receive the parts they look fine, but one of our first processing steps bakes the part at 150 °C for 1 hour. After that, there are specks all over the part where the anodize has removed itself from the part and the rest is easily removed with tape. Any tips?
Jeffrey Mandl
Electro-Mechanical Manufacturer - Twinsburg, Ohio, USA
A. TRY SHOTPEENING PRIOR TO ANODIZING. GOOD LUCK.
Khair Shishani
aircraft maintenance - Al Ain, UAE
2004
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.
Khozem Vahaanwala
Saify Ind
Bengaluru, Karnataka, India
2004
"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby
on
AbeBooks
or
eBay
or
Amazon
(affil links)
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.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
2004
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.
- Columbia, Maryland, USA
Ed. note: it is our sad duty to advise of the passing of Paul Yursis in August 2005.
Here is a brief obituary by Mike Caswell.
Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)
2006
Hi,
Just a question, has anyone ever experienced pitting corrosion in the 2024-T4 material. EDS shows that chlorides are present in the pits. Is it the process line, or something else.
Your help would so greatly appreciated.
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.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
2006
7075 MIL-A-6825 Type 2 Class 2 flakes after heating to 180 °C
Q. We have machined 7075 chassis. We have them black anodized using MIL-A-8625 / MIL-PRF-8625 [on DLA] Type 2 Class 2. We have heated them up to a temperature of 180 °C. After cooling, the anodizing starts to "flake" off of the chassis. Is this a problem with the anodizing process or is the alloy of the 7075 changing properties after reaching these temps? We have done the same testing to 2024 & 6061 & have had no problems.
Jack D. McCormickOil well navigation tools - Twinsburg, Ohio
2006
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.
- Navarre, Florida
2006
A. Like he said, the aluminum expands 5 times the aluminum oxide. Something has to give. Also the dye cannot stand that temperature.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
2006
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.
Steven EinfeldtSpace products - Chatsworth, California, USA
June 25, 2012
Multiple threads were merged: please forgive repetition, chronology errors, or disrespect towards other postings [they weren't on the same page] :-)
Q. I have a part made of 7075-T75311 .It has been anodized per MIL-A-8625 / MIL-PRF-8625 [on DLA] type 2 class 2. Square pieces of the anodize approx. .003"" square are flaking off the surface. I have examined some of the flakes in a SEM EDS; I find 35.6% oxygen, 46.8% aluminum, 7.2% sulfur, 2.8% copper, 2.7% zinc. I realize this is sulfuric anodize. I did not expect the sulfur to be present, it is on both sides of the flakes.
Was the anodized part poorly neutralized and rinsed? Any ideas what causes the flaking and cracking?
Steven Einfeldtengr. - Chatsworth, California, USA
February 20, 2008
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.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
February 23, 2008
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.
Drew Nosti, CEF
Anodize USA
Ladson, South Carolina
August 1, 2008
Q. 7075 is a high strength alloy, but it is more susceptible to corrosion. I just started anodizing and have had much better luck with 6061. I noticed also severe pitting in some of my 7075 parts and was wondering if the coating growing from the sides of the pits is causing stress fractures as it grows against itself in addition to the other factors mentioned. I also wonder if lowering acid content in the anodizing process would help prevent pits and too rapid a growth.
Benjamin Craig- Eighty Four, Pennsylvania, USA
April 18, 2011
Copper particles found between anodize layer and substrate
Q. We have recently run into a huge problem where our customer has found that anodize is coming off of our 7075 aluminum parts after a tape test is performed. We are only a machine shop and send our parts out for MIL-A-8625 class II type I with a dichromate seal.
Our sub has sent out the parts to be tested and the conclusion was that there was copper particles found between the anodize and the substrate. We have confirmed that our customer cleans the parts and then puts them into an oven @ 250 degrees for 2 hours. After baking when they perform a tape test the anodize comes up in small pen point size areas. I'm at a point where I don't know which way to go. My sub is NADCAP approved and all of there routing shows that parts were plated to spec.
We just machine them and have not changed our procedure for 25 years. Never seen this before. Please help
Bobby B- Tampa, Florida
December 5, 2013
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A. Any thing over 80 °C (176 °F) and the (1) the zinc comes out of alloy into the grain boundaries, and (2) the anodic oxide cracks - this occasionally results in "flaking". Try ramping up slowly for 15 minutes. If that dos not work, there is nothing you can do with this particular batch of 7075, especially at 400 °F or °C. Robert H Probert Robert H Probert Technical Services Garner, North Carolina December 16, 2013 A. Copper migrates to the surface of the metal with temperatures over 140 °F. Also, the deoxidizer they're using prior to the anodize could have high copper contamination. - Seattle, Washington, USA December 17, 2013 |
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