netneut
finishing.com -- The Home Page of the Finishing Industry
A website for Serious Education, promoting Aloha,
& the most FUN smiley you can have in metal finishing


Finishing.com has been free for 22 years,
but without net neutrality we could soon
cease to exist. Do us a solid, click on
the banner, and contact congress today!
HomeFAQsBooksHelpWantedAdvertiseForum
topic 5192

Blisters in Anodic Coating


(2000)

Aluminum alloy 7075 parts with about 0.003 in. thick anodized coating have small blisters in the coating after sulfuric acid anodizing . The blisters appear to be randomly located on areas with flat surface topography and are about 0.008in. in diameter.

Microscopic examination and cross sections show a fracture in the coating propagating from near the substrate surface at an angle out through the coating. The fractures appear to start from a small bump remaining on the substrate surface.

I suspect residual stresses in the coating are a factor, with local stressed areas due to anomalies in the coating from the intermetallic inclusions in the aluminum. Likely? Unlikely? Any suggestions for possible causes for these blisters would be greatly appreciated?

larry hanke
Larry Hanke
materials testing laboratory
Minneapolis, Minnesota




(2000)

Hi Larry, I had a gentleman from Lockheed-Martin contact me about this same problem. I asked for pictures of the bad areas and I could have figured it out. It basically ended up being that the coating would "bubble up" in areas. I basically have to agree with you on that aspect because they said the parts were very clean surface wise.

If that was not it my only other guess without seeing the parts was some sort of localized burning. I get this sometimes with smaller 7075 (smaller than a fingernail) and trying to "stuff to much current into their little throats :-)". Its a blistering that has to be mechanically polished away because just stripping the parts does not solve this and it could be attributed to the constituents moving closer to the surface and causing this problem. Because actually now that I think about it, 2 guys do this same part, one company has this problem, the other doesn't and it has to do with the way they polish the surface to a shine.

Matthew Stiltner
- Toledo, Ohio


(2000)

Larry, The microscopic examination you performed on the cross sections showed a fracture in the coating propagating from near the substrate surface at an angle out through the coating. This fracture may have been caused by corrosion beneath the substrate or on its surface. A few questions for you, was the material used certified? where was it stored and for what length of time? are your rinse tanks clean? i.e., pH's neutral. What amperage was used and was it stepped up slowly? What is the concentration of your Sulfuric bath? Do you use any additives?

Michael A. Von Rembow
- Auburn, California



This public forum has 60,000 threads. If you have a question in mind which seems off topic to this thread, you might prefer to Search the Site

ADD a Q or A to THIS thread START a NEW THREADView This Week's HOT TOPICS

Disclaimer: It's not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations may be deliberately harmful.

  If you need a product/service, please check these Directories:

JobshopsCapital Equip. & Install'nChemicals & Consumables Consult'g, Train'g, SoftwareEnvironmental ComplianceTesting Svcs. & Devices


©1995-2017 finishing.com     -    Privacy Policy
How Google uses data when you visit this site.