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topic 46468

Problems in anodizing 7075 (7xxx) aluminum




A discussion started in 2004 but continuing through 2018

2004

Q. We observed, recently, after the sealed (dichromate) chromic acid anodising of 7000 series the presence of a significant layer of "dust" on the surface of the parts. Does someone explain the reason for that? Could it be caused by an excessive hydration of the anodic layer?

Is it necessary to strip and re-anodize again the part or this dusty oxide layer can be removed in some way? After wiping the panel with water was not very efficient but the presence of the anodic layer was confirmed by electrical measurements.

TAMIRO Salvatore
aerospace - Turin, ITALY


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

from Abe Books
or
info on Amazon

2004

A. Yes, according to Pinner in his latest book with Sheasby, "perfectly" sealed anodizing will have a loosely adherent powdered hydration on the surface. Elimination of this problem can sometimes be had by slightly lowering the pH, lower the temp, lower the time, and/or all of the above.

However, you may have a zinc salt film in the form of zinc acetate, zinc sulfate, or even a calcium salt film as precipitated calcium, or calcium acetate or calcium Oxide.

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


2004

A. To paraphrase what Bob said: 7xxx series and 2xxx series alloys (more pronounced) have down-sloping anodization curves (Ohms per sq. ft. vs. time). Dissolution problems occur more readily with these alloys than for 1xxx or 6xxx series allows (up-sloping). Reducing temperature, electrolyte concentration, or time will all reduce dissolution effects.

If this is your case, you will still have the "dust" effects even if you sealed a sample in boiling water or steam, since it would have nothing to do with the sealing method. If this is what happens, you will need to use one or more of Bob's remedies to fix this.

Paul Yursis
industrial electronics
- 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
by Mike Caswell.



Bright anodizing of 7075 (7xxx) aluminum

2007

Q. A supplier is claiming he cannot bright anodize 7075-T6 due to its copper content.
He says putting it in a caustic etch will make it black in colour.
Being relatively new to anodization I don't have all the facts but shouldn't a brightening dip be acid based, sulphuric or other.
And isn't it possible to bright anodize 7075-T6?
Would really appreciate some help with this really general query.

Jack Judge
Engineer - Ireland


2007

A. It's possible to bright anodize 7075 for shininess although the anodize will be colored, unlike that on the purer 5xxx alloys commonly used for reflectors and automotive trim. Brightness also decreases as anodize thickness increases.

That caustic etching leaves a black smut on 7075 is hardly relevant, as a) the smut is easily removed by deox/desmut solution and b) caustic etching should be omitted or at least minimized prior to bright dipping.

Most anodizers do not have bright dip (chemical polishing) solutions, and not all bright dip solutions are suitable for 7075. Some phosphoric-sulfuric-nitric acid bright dips (including some Alupol & Phosbrite versions) can be used on alloys up to 8% Zn and 5% Cu. See The Surface Treatment and Finishing of Aluminum and Its Alloys, pages 110-116, 6th Edn.

Ken Vlach
- Goleta, California
contributor of the year

Finishing.com honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.



2007

A. Surely it's possible to bright anodize 7075-T6. There are other etchants besides caustic. Even castings can be brightened for anodizing.

John Hu
- Singapore



Inconsistency in Black Dye Appearance on 7075

October 8, 2008

Q. We have noticed an inconsistent appearance on many of our 7075 parts after pulling them from the black dye. We're using new titanium racks, freshly charged solutions in all of the prep tanks, new cable running from the rectifier (and even a new rectifier), and we still get some parts coming out with a nice, semi-gloss appearance, while others have a matt look to them and the dye rubs off. The only thing we haven't yet done is to replace the dye tank and we still haven't tried a strong, ammonium bifluoride / nitric desmut. It doesn't have the characteristics of a chemical problem, however, as chemistry cannot tell the difference between one part and another. I'm starting to think my customer has substandard alloy.
Thanks for your ideas.

randy fowler
Randall Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA


October 9, 2008

A. Randy

If some parts dye well and others don't, the dyestuff is not suspect. The powdery coating (wipes off black on a fingertip) suggest over-anodized/burned. Possibilities include inadequate solution agitation in your anodize bath; excessive dissolved aluminum; poor anode:cathode ratio; and poor electrical contact/racking.

If you perform a caustic etch prior to anodize, you might be able to detect variation in the aluminum by the residual smut. Play with the etch times to evaluate.

Willie Alexander
- Colorado Springs Colorado


October 10, 2008

A. Long time no see, Randy. You didn't mention if the difference you are seeing is in the same load, or in alternating loads. It sounds to me that it's possible the parts that are substandard are overheating in your tank, causing a very soft and worthless anodic coating. Any major difference in the alloys (parts) should be evident after your etch, I would think.

Is there any difference in the coating thickness between the good, and sub-standard parts?

Marc Green
Marc Green
anodizer - Idaho


October 10, 2008

A. For dye ruboff, check your seal quality and temperature.
If the part is sticky, we found the cause to be in the desmut tank.
7075 has a high amount of alloy material, particularly copper, so a good desmut step is really needed.

James Watts
- Navarre, Florida



August 15, 2012

Q. Our products have two aluminum materials: 6061and 7075. We've chosen the 7075-T6 to anodize the surface (black, and we don't care about brightness), but it's always having problems. Sometimes had bubbles, and sometimes had paint peeling off; we cannot find the reason. Is the cause the suppler or material?

LEO SUI
- TAIWAN TAICHING


August 17, 2012

A. Hi Leo,

It is difficult to make judgements about your problems as you have given us so little detail, but let me give it a go anyway.

If you are not overly worried about material properties I would swap from 7075 to 6061. 7xxx series alloys are high magnesium^zinc alloys and can often suffer from pitting, white spots and various other problems, especially if the wrong current density is used. 6061 is a low alloy material, which is easier to anodise.

Your problem with paint peeling may be something to do with the anodising, but is more likely to do with the paint application. When parts are delivered for paint there are many factors that can affect adhesion, from contaminated surfaces to poor paint application to poor stoving. If you can give us a detailed breakdown of your process we may be able to advise you better, but at the moment all we can do is generalise.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom


August 19, 2012

A. I think you will find that the major component in 7000 series aluminum after aluminum is zinc. White spotting is a common problem whit this alloy. Some have found that a 50% nitric plus fluoride helps in the deox stage. As far as bight dipping, it is nasty stuff and most try to avoid using it.

drew nosti
Drew Nosti, CEF
anodizeusa1
supporting advertiser
Ladson, South Carolina


August 21, 2012

A. Hi Drew,

You are perfectly correct that 7xxx are zinc containing alloys (where was my brain?!)

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom



April 17, 2013

Q. I have machined parts from aluminum material 7075 T651 AMS QQ-A 250/12, ASTM B209 that had a clear anodize finish applied per MIL-A-8625 type 2 class 1. My customer is rejecting the parts because the finish is not clear.

discolored clear anodize
discolored article on right

The finish has a brown/goldish tint. The finish vendor states the copper in the material will cause this color to appear. I need some more information on what is the appearance of a clear finish . Thank You.

Don Ullrich
- Illinois, USA


April 22, 2013

A. Nowhere in MIL-A-8625 is a Type II Class 1 anodize described as "clear", only non-dyed (Ref. Para. 1.2.2). In fact, Paragraph 3.5 Class 1 states,"Any natural coloration resulting from anodic treatment with the various alloy compositions shall not be considered coloration."

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.
supporting advertiser
Wadsworth, Ohio

luke engineering banner


April 22, 2013

A. Don

Show your customer section 3.5 of the specification. It clearly allows for natural coloration. The 7XXX series will show a fair amount of color (gold) as the coating thickness increases.

If your customer requires a colorless finish, the anodizer will need to cut back on coating thickness.

Willie Alexander
- Colorado Springs, Colorado


April 23, 2013

A. There is enough "slop" in the range of ingredients in 7075 for a color variation to occur. Also the brown,bronze, tint is very typical in 7075 that is within the range of ingredients

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


April 24, 2013

A. Hi Don,

I think you are being told a few half-truths about why the color has changed. 7075 only contains 1.2 to 2% copper; its main alloying constituent is zinc at 5.1 to 6.1%. I can't imagine that the level of copper in the alloy would cause this problem. Now, if the anodising tank is heavily loaded with copper, then I could understand it a little; even then I'm not convinced it has anything to do with copper.

Brian Terry
aerospace - Yeovil, Somerset, United Kingdom



Type II Anodizing: Partial discoloration of 7075-T73 forging

March 4, 2014

Q. Hi, Experts,

This is Chuck from China, I am an anodizing engineer; I'm have trouble with an anodizing process.

After type II (MIL-A-8625 type II, Class 1) anodizing on a forging 7075-T73 alloy part, we found that some areas of the part have discoloration (the discoloration area was penetrated from inside to outside). By the way, these part's raw material was re-heat treatment and re-aging during the forging process since the Tensile yield strength and Tensile ultimate strength cannot meet specification requirements. And we also anodized some normal forging process part without this discoloration issue, also some other parts were anodized at the same time, no discoloration issue, so we excluded the discoloration being caused by the machining or anodizing. We suspected this is a forging raw material issue, and we have did Metallurgical testing, can't find any difference between the normal area and the discoloration area.

46468-2

My questions:
1. What is the root cause of the anodizing discoloration?
2. What analysis method can we use to find the root cause?

Thanks a lot.

Chuck

Chuck Huang
- Shanghai China



Want to get brighter finish on 7000 series aluminium

May 11, 2014

Q. Hi, any advice on getting a brighter finish on 7000 series aluminium? More like 6000 series.
I polish very highly and desmut then anodise for 75% of the time of 6000 series. I run at 12 ASF.

Chris Pocock
- Manchester, UK



Clear Anodizing 7075 Aluminium

September 12, 2015

Q. I know, reading the title, many of you will go, "oh no, not another one...!"   :) but yes, after going through a lot of stuff on 'our' dear website as well as lots of others, I still decided to post! So here goes...

The common complaint is from the customers is that "we need a nice bright clear finish after clear anodizing 7075 aluminium" and My definite need is also a coating thickness between 10-20 microns to meet the standards, which, as we all know, is easier said than done. (I am one of those guys who never suggests compromising on the coating thickness just for the sake of the color shade but most of the time the customers remain unconvinced).
So, what I would like to hear from any and all of you experts out here is this...
What are the recommended pre-treatments according to you that would assist in reaching the objective?
What would be the most ideal process parameters to maintain for this alloy?
Please elaborate.

Rubbing salt into the wounds, so to say, was when my son came along and told me that the latest Apple iPhone case was made out of 7075 and it has come out in 3 beautiful pastel color shades as well! I am sure we all agree that they would have had to get a pretty decent 'bright clear' first, in order to get close to the pastel shades launched ... agreed we may not be blessed with the huge technological reach of a corporation like Apple ... still can't help but wonder...
So are there any tips and tricks for this particular alloy, will be highly obliged if you all can share them here, for frankly I am tiring of constantly answering the same questions from too many customers, who couldn't care less about either us or our 'technical' problems :(

I also take this opportunity to wish all our dear forum members out here, a very happy 'Ganesh Chaturthi' coming up soon on the 17th of September, the annual festival of the Elephant headed God who is the god of learning, knowledge and enlightenment and also known as a remover of obstacles and problems! (Now you all know what I will be praying for this time) :)

Ravi Rao
- Belgaum,India


September 2015

thumbs up signHi Ravi. It would be great to know the anodizing thickness on the new iPhone 6S if an early adopter gets one and has a thickness measurement gauge. "Pastel" is another way of saying "low-saturation", so as a guess supported only by this flimsiest of evidence, I suspect that Apple's thickness is well under 10 microns [see update below].

I'm not claiming that the pretreatment answer is in Robert Probert's "Aluminum How-To" book (I don't know), but I will say that it was certainly not a stone which "Apple's huge technological reach" left unturned :-)

Regards,

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


September 12, 2015

Q. Hi! Ted, Thanks! Letter no 46468 ... hmm ... is this going to be the most discussed in the anodizing forum? I wonder.

Iphone6s ... someone please test thickness and confirm coating (after the dye, even that wouldn't be accurate though.)
Ted, I think you are right, it would probably be less than 10 microns. But what surprised me is that they were confident enough to go to the consumers with color shades on this alloy, do you think the shades will be consistent enough? Will the 'rose gold' match perfectly all the time ?! If it is yes, and under 10 microns, well, OK...maybe it is not that difficult. The new plans to trade in the device after an year. along with other things, will also mean that we will not hear about the fades and patches ... am I right? Anyway we do not want this discussion to sidetrack to the new wonder device of the year :) All the best to its launch and success.
But still, an interesting alloy, and I suspect more and more applications are turning to it and we might as well take a serious re-look at what we anodisers can 'tweak' to the best of our abilities.

Ravi Rao
- Belgaum, India


September 2015

thumbs up signHi again Ravi. Yes, I'm trying to combine threads on similar topics instead of starting a new thread with each question for 3 reasons: short threads get no respect from Google, so nobody can find them once they're off the current forum index; the thread makes for better reference material if it includes a variety of replies and perspectives; sometimes good answers to a question have been posted already, and the questioner may want to read them and fine tune his/her questions :-)

The material is definitely 7000-series aluminum, and one cross-section microscopy I googled showed the thickness to be 10.5 micron.

Apple is quite careful; I'm confident that their colors will match. We all have a tendency to call difficult things "impossible", and the new iPhones should serve as inspiration to the rest of us that although color consistency on 7xxx is difficult, it is indeed possible.

Regards,

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


October 7, 2015

Q. As I scour through the innumerous chats on the 7075 clear anodising and its associated difficulties, I would like to specifically point out to a problem that has been presented in this very thread just a few communications above, by Chuck from China, which also appears to have been unanswered....the dark shadow like patches are the types that I am experiencing as well although they are not as symmetrical as the ones in the pics posted by Chuck.
Have tried the proprietary deoxidiser solutions,have tried the chromic phosphoric stripping composition as a pre treatment, have tried the sulfo-chrome composition etch, have tried the alupol V composition, have once again tried the caustic-nitric and a double nitric desmut....nothing seems to help.
Of course my bath is regularly tested and is compliant with the normal Type 2 ranges for sulfuric and aluminium content.
temperature between 20-22 °C - controlled.
Titanium racks on Al-Bus Bars
Good air agitation...
Frequent DM Water rinses and double rinses...
Although I am quite sure that the shadow like patches must be material related or heat treatment or forming related,
I still need to get rid of it as it is an aesthetic requirement for the customer...
I wonder if there is anything out there...

46468-3a  46468-3b  46468-3c

Am also attaching some pics for all to get an idea of what I am talking about, only hope that they are clear enough as it was under tube lights and taken in the night shift.

Ravi Rao
- Belgaum,India



Prep of aluminium 7108 before anodizing

September 15, 2018

Q. Hello.
I need to anodize aluminium of 7108 quality (Al and Zn alloy).
It tends to get a powdery surface.
Anyone who have exeprience with this and how to solve it ?

Geir Lexau
- Norway



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