Where the
world gathers for metal finishing
Q&As since 1989


-----

Verification of class 3 CCC on aluminum alloys using SEM EDS

2007

I work for a materials lab in Tucson and every so often I have a customer who receives several parts (combination of 2024 and 6061)that appear (visually) to not have the required coating. The MIL spec states that colors can range from yellow to clear and references an ASTM spot test (B449) to look for the Cr. I get positive results for both known "good" parts as well a suspect parts. The same spot test performed on bare alloy samples also shows positive, suggesting interference from the alloy.

There seems to be some debate (among the more senior scientists) on whether SEM/EDS can be utilized to look for Cr due to the thickness of the coating. I have data supporting the use of this technique on the hardware (yellow part had Cr, the suspect part did not)at low kV.

Do you have any advise?

Dan Feeney
Materials Analysis Laboratory - Tcson, Arizona, USA

---

Good afternoon:

You haven't told us what type/thickness coating is on the aluminum. What you'll want to consider is, at the exitation voltage you're using, how large of an exited volume you're generating on your sample when doing EDS. Then compare that to the coating thickness, taking into account the detection limit of Cr in your sample. The Cr peak might be difficult to see if the background is too high.

You said you tried some low kV analysis. How low?

If the coating is too thin, you'd probably have to use Auger or some other thin film technique to get an analysis, although that might be a bit pricey.

And maybe the only test that matters is the one referenced in the specs.

Steve Bizub
- St Louis, Missouri
2007

I've been able to verify the presence of Alodine 1000 on aluminum using a lab XRF instrument.

Terry Tomt
- Auburn, Washington
2007