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Letter 4013
Grey colour in Natural Anodized
Aluminum
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Recently, we had rejected a batch of anodized aluminium (natural
anodized colour). The reason why we rejected that batch was because
one end of the profiles was grey in colour. After examining the film
thickness at the grey side, we found that the film thickness at grey
side was 14 micron whereas at normal natural anodized colour side was
8 micron. We also noticed that the hardness along the profile was
also different, approximate 5 units (measured with Webster Hardness
Instrument). Does anybody knows the actual problem of this
phenomenon. Please reply.
T.Q
Abbe Lim
- Selangor, Malaysia
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Reference, Canning Handbook
1989, E & F.N. Spon Ltd, London, p. 798, "the anodic film is
electrically resistive, therefore, if a variation of coating
thickness occurs, there is a redistribution of the current flow over
the surface. This results in an increase in the current density at
the area of thin coating or low resistance, and a corresponding fall
in current density at the area of thick coating or high resistance."
I would guess that there is a problem of stratification of the
anodizing solution, and the parts are racked vertically in this
solution, or there is a really bad arrangement of cathodes in the
tank.
Best regards,
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In the aerospace business, we always look with suspicion upon an
area of a part that anodizes more darkly than the rest. This can be
caused by areas of differing hardness along the part due to an
improper heat treat. You might want to evaluate the material in
different areas of the part using Rockwell hardness, conductivity and
perhaps even running a tensile test or two, if the component is
structural.
Mark McGlynn
- Oklahoma City, Oklahoma
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