What is the best etchant for Al-6.6Mg-0.5Mn (wrought aluminum) alloy?

I want to see grain boundary in annealing and (cold work +annealing) conditions. I used (5 ml HBF4, 48% +200 ml H2O); electrolytic etching (20 V, DC) & Polarization condition, but no result...

10 May 2017 650 7 View