Anodizing of A357 sample for metallograpy

[kamran1]

Hi all,

I tired to anodize an A357 sample for metallographic observation usng perpendicular polarized light. I used Barkers solution (195 ml H2O + 5 ml HBF4) at room tempearture with 20V supply using Al cathode for 80 sec. But I was not able to get any contrast between the grains. I also tried a wrought alloy and pure Al but failed to get ay contrast. The samples were mechanically polished before anodizing. I also checked whether the surface is anodized or not using a multimeter and there was no current across the samples.
Can any body help me in this regard.

Thanks

Kamran

 

[SMF1964]

Weird. Try a bit longer? According to the latest Microscopy Today (November) Vander Voort has an article on color etching that uses just that: Barker's, 30V dc for 2 minutes.

Another option Vander Voort presents is 100 ml H20, 4g KMnO4 and 1 g NaOH [Weck's reagent for Al]. Whether this will work on your grade of aluminum is unknown. Let me know.

I've used 10% Molybdic acid on pure aluminum, but had difficulty getting a decent photograph due to the fact that you've got to have almost 90° cross-polarized light, which didn't give me a lot of light to work with for the photograph.

 

[TVP]

The following article from Struers may be useful regarding preparation and etching of Al:

http://www.struers.com/resources/elements/12/2431/38art1.pdf