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Blisters on Anodized 7075
June 7, 2009
I have been having problems with blisters forming on 7075. This is very rare I know and I have torn our operation apart and can't seem to find anything that we may have missed. I have seen delamination before but this is a new one. I have done a matrix of experiments to prove out our process. I keep coming back that there is something that is not right with the metal that we are trying to anodize. The problem is that it is very intermitant. I expect it to come back because we have not done anything to end the problem. What I can say is that we have processing 7075 for many years and consider it not that hard to anodize and have a long track record of success. I have been told by many that this could be a hydrogen outgassing problem. A problem that is created when the billet of aluminum is 1st made. What I would like to do is prove this with the data and move on with life. Any ideas how I could go about that?
Mike RobergAnodizing Operations Manager - Minneapolis, Mn, USA
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First of two simultaneous responses -- June 15, 2009
In heat treated or "old" 7075, the zinc has migrated out of alloy and into the grain boundaries as molecular zinc. If it is near the surface and you anodize though to it in some spots, the anodizing flakes off.
With work hardened surfaces, near bends, milling, drilling, etc, when you anodize the hardened surface, it flakes off.
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Robert H Probert Robert H Probert Technical Services  Garner, North Carolina
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Second of two simultaneous responses -- June 16, 2009
Mike,
I have experienced similar 'blisters' on 6061 extruded material in the past and came up with the same kind of answers.Even met up with some experts at the primary aluminium producing plant we have here in our city to consult-although the material was not made by them-they were of the same opinion.Problem was intermittent,so much so that plates cut from the same rod lengths also did not all display the blisters, one rod length being made up of more than one billets in the mill.
If you strip the coating and even machine the blistered portion,you will still faintly see the marks on the material-it will not go away even if you machine off a huge layer!
Hope that is some consolation to get over the sleepless nights!
- Belgaum, Karnataka, India
January 17, 2012
I work at an manufacturing plant where we make parts for aircraft that require many sealing surfaces. We have been running a part made of 7075 (more specifically 7075-T73511) for years now. The part has a large sealing surface that has been continually rejected by our customer for scratches that don't even have a depth to them. The problem we are having is the scratches only appear after anodizing but never before. 100% visual is done at all steps throughout the process even at the anodizer. We have gone through our process as well as our anodizing supplier and have not found where the scratches are originating from. We lap the surface before anodizing and suspect that something is being impregnated into the material that doesn't show up until the anodize process brings it to the surface.
Josh Bugni- Valencia, California, USA
January 18, 2012
Hi Josh,
It will help if you can post some pictures of the part with the defect.
- India