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topic 45936

Problems & Solutions in Chromic Acid Anodize of 7075


A discussion started in 2003 and continuing through 2020
Adding your Q. / A. or Comment will restore it to the Current Topics page

2003

Q. I do a fair amount of Chromic Anodising on 7075 T6 material. From time to time I find that a tapped hole burns badly, destroying the threads, a very expensive problem. It has occurred with two different shaped parts. It always occurs in the hole located the highest. It normally only occurs on one part in a batch but once on 3 parts from a batch of 10. Round titanium jigs are used and the anode is circular and at the same distance from all the parts.

Willem Kleinschmidt
- Strand, South Africa



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


affil. link
"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
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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 obituary by Mike Caswell.




Anodizing problems with 7075T6

2004

Q. We chromic anodize 7075 t6 aluminium on a irregular basis and we are finding that the 7075 tends to pit very badly, especially when we double dip long lengths of it. We end for end the 7075 to get the coverage and find the pitting especial bad around the solution surface point of the material.

John Greed
plating - Adelaide, South Australia, Australia


2004

A. After assuming that the alloy is right and that the zinc is not precipitated in the grain boundaries, then let's recognize that chromic acid anodizing of 7075 is very difficult. Our Lockheed specifications do not allow chromic anodizing of 7075 except for adhesive bonding reasons. Our Boeing says to only use 22 volts when anodizing 7075 T 73. So, after checking to see if there is any pitting caused by the pretreatment, then try 22 volts.

robert probert



Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
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Chromic anodizing 7075-T7351 aircraft propellers

2006

Q. In order to manufacture a propellor for a light aircraft, I need to know if there are any problems in chromic anodizing 7075-T7351 plate in accordance with Mil-A-8625 [link is to free spec spec at Defense Logistics Agency, dla.mil] Type 1B, Class 2, Black, or will it provide a satisfactory surface protection.

Victor Shribman
- Kiryat Ono, Israel


2006

A. Para 3.4.1 eliminates 7075 "unless otherwise specified" because at 11.13 % non-aluminum it exceeds 7.5 % non-aluminum. Lockheed list 7075 as unacceptable, but acceptable for adhesive bonding. Boeing says to anodize 7075-T73 by the 22 volts process.

robert probert



Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
probertbanner



2007

Q. Recently we have been having a problem with Type I chromic acid anodize on 7075 alum alloy.
Certain areas on the parts after processing display small burnt hollow spots where the metal has been displaced - They are not rack marks. We have no problems with any other alloys such as 6061 or 2024. Could it be the distribution of magnesium or zinc in the composition of the alloy creating hot spots and buring?

Tony Caroselli
Anodizer - Kenilworth, New Jersey


simultaneous 2007

A. Tony

I have NO experience with Type I anodize, but if the aluminum content in a Type III bath gets too high, it will cause localized burning on heavily alloyed aluminums.

Chlorides could be an issue but I would expect you to have problems with all alloys if that were the case.

Are you having the problem with just one customer's 7075 or all? If just one, the alloy could be suspect. You could try having a competitor buddy run some parts to see if he has the same problems.

Willie Alexander
- Colorado Springs, Colorado


2007

A. No experience with chromic acid anodizing, but I've traced blisters in sulfuric acid anodized 7075 to Cu- and Fe-containing intermetallics; see letter #42961.

These higher melting phases aren't dissolved during the solutionizing heat treatment and don't anodize. The only workaround, other than procuring a different batch of material, is shot peening prior to anodizing in order to knead the surface layer of the 7075.

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.



simultaneous 2007

A. Sulfate is a problem child in type 1 anodizing.

James Watts
- Navarre, Florida


2007

A. The zinc in 7075 with time and/or temperature, migrates to the grain boundaries, then that left on or near the top, will "de-plate" when made anodic in sulfuric acid, and leave behind "pits". Find out the heat treat, heat, and time history of your metal and come back to us. Meanwhile, try ramping up slower.

robert probert



Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
probertbanner



January 17, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. What would cause SOME parts in a load of 7075 forgings to get attacked in a chromic acid solution and others not? The parts that are attacked are not attacked in the fixture areas and they look like random areas of the surface just exploded and have a black appearance.

Bob Bramson
- Kenilworth, New Jersey


January 19, 2013

Q. Does anyone know what the high limit of sulfates would be in chromic acid anodize? If sulfates were the problem, would it not affect all the parts?

Robert Bramson
- Kenilworth, New Jersey, USA


January 22, 2013

A. Boeing specs say then upper limit is 500 mg/L, that is 1/2 gram per liter. Experience is that if you are over this limit you fail salt spray.

robert probert



Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
probertbanner



July 7, 2020

Q. I had sent some work pieces made of aluminum 7075 for chromic acid anodising but the finished product which I received were all of different shades. Some were darker than the others and some were darker at certain regions then the rest.

The Vendor tells me it's because of buffing manually and it wouldn't happen if it were completely machine buffed.

Prithvi
- Bangalore, India

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