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Anodize Cracking

I have several AL 7075 parts of different shapes and sizes coated with a clear anodize 20 microns thick. After receiving the parts, tape tests were done to check if the coatings appeared good. All of the parts seemed fine and then were cleaned in an ultrasonic bath of isopropyl alcohol and alkaline degreaser(Deconex). After integrating several of the parts, a piece of tape would remove the coatings on some parts but not others. It appears to be random which parts the coating is easily removed. Some parts that are identical have variation with the coatings. These parts have been at room temperature since being coated. Looking under a microscope the coating appears to be cracking. Any clues as to why this is happening or what I can do to find out?

Paul Oppenheimer
mechanical engineer - Washington, DC, USA

First of two simultaneous responses --

Paul. Your alkaline cleaner is removing the coating. Can you just use the alcohol cleaner? Tape tests are not used on anodizing. Anodizing converts the aluminum surface into an oxide. There is no plating involved.

Robert Henry
Brooklyn, New York

Second of two simultaneous responses --

You should never use an alkaline cleaner on an anodized part.

James Watts
- Navarre, Florida

simultaneous replies 2004

If the metal had been blasted (bead, sand, and grit), and/or bent or hammered so that the surface was "work hardened" then the anodizing would likely flake off. I

f very thin parts were anodized, like in your case where you are going to a thickness of 0.0008, well 1/2 of that or 0.0004 is "in" the metal (under the original surface), then if 0.0004 is close enough to the other side the anodizing would crack on occasion.

The aluminum oxide is hard and brittle, the base metal is soft and malleable.

If you started with clad material and anodized all the way thru the clad, then something will flake off.

Come back with more information and let's have another go at it,

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

Thanks for the comments. We will no longer be using the alkaline cleaners. After posting the original question, it was brought to my attention that some of the parts described above were cleaned with the alkaline and alcohol while others were cleaned with just alcohol. Both types of parts have some cracking when examined under a microscope. The alkaline cleaner may have been part of the problem but doesn't appear to be the only one. Could the ultrasonic cleaning the cause of the problem? What is a good method to check if the coating is done properly? These parts are for a spacecraft and are required to meet stringent outgassing and contamination specifications.

Paul Oppenheimer
- Washington, DC

In the semiconductor industry, Ultrasonic cleaning has been shown to affect/damage the anodic coating. As far as your coating flaking off, my bet would be a poor coating due to overheating of the part/area during the anodizing process (I've seen this frequently on 7075 alloy).

Marc Green
Marc Green
anodizer - Boise, Idaho


The anodizing was defective; the cleaning and tape test (apropos in this case) merely led to its early detection. Possible causes and contributing factors:

Overheating due to insufficient cooling during anodizing. Alumina formation creates 1670 kJ/mol, plus resistance heating occurs as the anodize current passes through the film. Normally noticeable from a powdery appearance, the overheating can result in disintegration of the oxide-metal interface.

Sharp edges and inside corners. Anodizing grows vertically into and outward from a surface. At an edge, the 2 growth interfaces intersect at a right angle and the corner gets starved (and this is the area of fastest dissolution, too). At inside corners, the outwardly growing surfaces intersect, resulting in a line of defects. This is the reason that greater radii of curvature are required with increasing anodize thickness per MIL-A-8625 / MIL-PRF-8625 [on DLA]F and ALCOA studies; see photos in The Surface Treatment and Finishing of Aluminum and its Alloys, 6th Edn., vol. 2, p. 787-788 (2001). For a 20 micron (0.78 mil) anodize thickness, a radius of curvature of at least 0.64 millimeter (0.025 inch) is required.

Anodize cracking known as crazing can occur due to temperature change, as the anodic coating has a lower thermal expansion coefficient than the Al alloy. Common if hard anodize formed at 0 °C is sealed or otherwise heated. Visible with a stereo microscope at 20X magnification.

A contributing factor is very limited ductility in age- and work-hardened Al alloys. In one case, where T6 temper material with very sharp edges had been hard anodized and then subjected to a 15

5 F thermal shock, an interfacial stress of about 70,000 psi contributed to flaking off of the anodize. Were any parts examined prior to cleaning? What was the anodizing specification, and were the parts sealed or subjected to a hot rinse? The following portion of MIL-A-8625F was not met: "3.13 Workmanship. Except for touch up areas in accordance with 3.3.4 and as noted below, the applied anodic coating shall be continuous, smooth, adherent, uniform in appearance, free from powdery areas, loose films, breaks, scratches and other defects which will reduce the serviceability of anodized parts or assembles."

Ken Vlach [deceased]
- Goleta, California

contributor of the year 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, and we at, continue to benefit from.

July 6, 2012

Q. Hello,

I have a question regarding red and black anodize flaking.

We are currently sending parts for anodizing (Aluminum 2024-T4 per MIL-A-8625F, TYPE II, Class 2.)

Once we receive the part from anodizing, we clean them using the Ultrasonic cleaner preparing the surface for the bonding process. After applying the bonding material, we oven cure the parts and then during wiring process, the parts are placed once again thru the Ultrasonic Cleaner.

After reading the previous responses, I do understand that ultrasonic can be a possible cause of the flaking problem we are seeing. However, We have tried several different batches of the parts that we get from anodizing and some batches go thru the process mentioned above and Flaking does not occur and on some batches the flaking occurs.


Thank you in advance for any advise or feedback anyone can provide regarding this.

- IRVINE, California, USA

A. Hi Asutosh,

I see a couple of possibilities that may be giving you problems.

Firstly, you are right that the ultrasonic cleaning may well be giving you a problem. Knowing that if you use the wrong frequencies that you can actually "punch" holes through thin aluminum sections, it would be no surprise to learn that the ultrasonics are contributing to a damaged anodise layer.

The other thing that may be contributory is your bonding process. What is the curing cycle? Remember that aluminum has approximately 5 times the co-efficient of thermal expansion of aluminum oxide. If you have a long high temperature curing cycle the aluminum will expand at 5 times the rate of the coating. This may not actually cause flaking, but may cause cracking of the coating. The subsequent ultrasonic clean now has even more chance of flaking the anodising off.

Brian Terry
Aerospace - Yeovil, Somerset, UK
July 12, 2012

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