7075-T6 Impact

[rdcmc]

We are Chromate Conversion coating a part that is made of 7075-T6 al. Recently the parts started showing black spots on the machined areas after cleaning. This area will develop a white "fuzz". Once wiped off the area is left blackened and will not chromate efficiently. We have ruled out any stray currents. any thoughts as to what could cause this.

The "spots" will not occur on all parts but are sporadic from batch to batch within the same heat lot.

 

[kenvlach]

7075 is well-known for having Cu- and Fe-containing intermetallic constituent phases (from the initial solidification, not the heat treatment). These undesirables cannot fully solutionize (high melting point compounds with low solubilities & low diffusion coefficients). Formation should be minimized by rapid chill casting. Then any remaining must be broken up and disssolved by hot working (making the wrought material).

When present, the intermetallics can greatly interfere with anodizing. Less frequently a problem with chemfilm, as nitric acid deox. will remove the exposed crap.
thread367-175504
thread332-189729
One workaround is shot peening to break up intermetallics near the surface.

http://www.finishing.com/429/61.shtml

thread90-93774

The University of Illinois developed an electrolytic process for removing intermetallics from the surface of 2024. Don't know any commercial users. Should work for 7075.

http://www.otm.uiuc.edu/attachments/T98002-092.pdf