1. electro polishing in solution ( 820 ml H3Po4 + 160 gr Cro3 + 135 ml H2So4 + 100 ml H2O ) with use of AL cathode in 50 voltage for 5 min.

2. etch in solution ( 25 ml HNO3 + 75 ml distilled water ) for 30-60 sec.

good luck!!

According to ASM Handbook Volume 9 about metallography, Keller solution is not recommended for grain size and shape for 6xxx series aluminum alloys. Electropolishing using Barker reagent and polarized microscopy with colour tint is recommended. You can try Weck reagent if you want to avoid Barker solution/electropolishing

Dear Cheng-Yu Chiang , I would say that the darkest spots in your images are just holes or scratches on your sample´s surface. Scratch lines are probably caused by mechanical polishing, and may be the tiny holes too, or may be they were there in the original alloy.

Hope it helps

Md Abid Shahriyar

Thanks for the information, I would try it. But I want to know how good the electropolishing is. Do you have some images from your own experiment?

Waqas Muneer

Thanks for your information. I didn't rule out electropolishing. I just want to know how clean the surface is by using electropolishing.

Manuel Gómez

Thanks for your answering. But how can I avoid those pits/scratches? I have tried many methods.

Cheng-Yu Chiang , actually I don´t know, your polishing method looks like correct. We often use mechanical poishing, using a polishing machine and different slurries of alumina and/or silica. The main polishing plate has a rubber surface coating the heavy steel plate. The rubber function, I think, is to save the sample from these scratches, when some polishing particle find a strong resistance on the sample the rubber can act as a kind of buffer, it bends and avoid the scratch. Usually long times of polishing are needed for our samples, mainly glasses and ceramics. Often trying to save time by speeding the process is counterproductive.

The slurries are applied drop by drop and the smple is holded in a heavy holder that freely rotate in the big base plate. It is important to avoid that the contact surfaces get dry, that will cause scratches.

The holes could also be part of the polishing process, if some grain in your sample is comparatively hard could be removed leaving the holes. Another possible origin could be that the holes where already in your sample as bubbles or like intersticial holes.

Hope it helps.

Manuel Gómez

Thanks for your advice. I polished it by my hands, maybe I should use less strength while polishing.

Hi Cheng-Yu Chiang , you´re welcome.

May be you could ask at the university central facilities or in other research groups for some polishing machine. The one we have is a really old model of Logitech Ltd. the model PM2. In the following link you can see the working process I tried to describe earlier. May be some group in the petrographic department, or may be at the microscopy facilities they have something similar and experienced people that could help you. These machines allow to obtain optical quality finish, so free of scratches. Here the links to the video and the manufacturer website.

https://www.youtube.com/watch?v=32iw06KXoBo

https://logitech.uk.com

Hope it helps, Good luck with your research.