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Blisters in Hard Coated 7075


We are experiencing blisters in 7075 after hard Coat. Does anyone know why this happens, and how to avoid/eliminate them.

Joel A. Lipof
- Paterson, New Jersey

First of two simultaneous responses -- ++++++

Blisters aka pimples are caused by intermetallic particles which block anodize growth into the metal. A particle holds up an area of anodizing which becomes detached from the metal as anodize growth continues in surrounding areas. Looks like an incipient volcano. Stresses create cracks radiating outward from the center into the uplifted anodize (actually, surrounding anodize has moved downward) through which acidic electrolyte enters and creates white sulfate corrosion. The undercut anodize & corrosion products can detach & get swept away, after which the bare metal area some 500 microns in diameter can begin anodizing anew.
In SEM analysis back in 1997, I found particles of primary solidification phases FeAl3 and Cu2FeAl7 at the core of developing blisters. These are coarse particles up to 25 microns in diameter which should not be present in wrought microstructures; they are not the sub-micron particles formed by heat treatment.

Re preventing blisters: Shot peening breaks up the intermetallics within the near-surface Al matrix to a small enough size so as to not blister the subsequent hard anodizing, and is a common procedure for helicopter manufacturers. personal discussion, Leah Markowitz, Senior Engineer, HR Textron, 1998 SUR/FIN conference. See also "Hard Anodic Coatings on Aluminum Alloys: Evaluation and Control of Porosity," L. Markowitz, Metal Finishing, p. 19-24 (December 1992).

As Mil-A-8625 [link is to spec at TechStreet]F, paragraph 3.3.2 could be interpreted as prohibiting shot peening, note: "Aluminum parts which have been peened with metallic shot shall be chemically cleaned" prior to anodizing. MIL-S-13165C [ed. note: or AMSS13165 [link is to spec at TechStreet]], paragraph and MIL-P-81985, paragraph Typically, this cleaning is a light etching followed by a nitric acid-type deoxidation.

The problem can also occur with Al 2024. The University of Illinois has developed an electrochemical process for removing intermetallics from the surface of aluminum alloys. Probably useful for applied coatings and chem film but not hard anodizing.

Ken Vlach
- Goleta, California  

Ken received a special
"Contributor of the Year" award
from for his numerous
helpful and well researched responses

Second of two simultaneous responses -- ++++++

1. With time and temperature, and especially with heat treating, the zinc migrates to the grain boundaries, when you anodize over these grain boundaries, it flakes off.

2. Work Hardening from dull tools or tumbling or blasting. When you anodize completely through a work hardened surface, stress flake the coating off.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note:    
   Mr. Probert is the
   author of

September 12, 2012appended

Q. I am having a problem with 7075 aluminum blistering after black hard coat anodize. We have run several loads with no problems and all of a sudden the last 2 loads have had blisters. I have checked the parameters of my tank and everything is where it should be. We rack only on Aluminum racks, no titanium. Does anyone have any suggestions?

Paula Stablewski
- Batavia, New York, USA
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