Hello, I preparation cross-section metallographic samples with oxide layers. After polishing I use mechanical-chemical polishing by suspension OP-S fy Struers. What thickness of oxide layer you except? You can use non-destructive technique eddy current for measurement thickness. When you sent me more detail about thickness and your metallographic preparation I can sent you more information about metallographic preparation. Your speciment is not good polishing. 

Regards.

salam sister I suggest using the electrical conductivity to measure the coating thickness

There's always multiple ways to do measurements but the choice will most of the time depends on what equipment you have in your facility. if you have the facility to perform the eddy current measurement like Michaela recommendation, then it's one of the best to try. If you have X-ray fluorescence analyzer, then it's also another way. 

But if light microscope is the only mean you have, then you can try "ASTM B 681 Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and other Transparent Coatings on Opaque surfaces using the Light-Section Microscope".

In part of etching anodized Al (Al2O3), you have many option. You can also check "Etch Rates for Micromachining Processing", it has the list of etchant works for Al2O3 with the ethc rate. Working with HF is extremely dangerous. If you don't have reliable facility to use HF, maybe you can use less harmful material. But always remember that Al2O3 etchant will also etch the Al at the same time, so pay attention to the etch rate.

One non-destructive method to assess thin-film oxides is to use an ellipsometer. But looking at the surface finish of your sample this might not be possible. However, this reference might be useful for background information on your anodization method and the properties of the resulting oxide: http://sc.el.utwente.nl/~coras/pdf/2003/JAP_Galca_2003.pdf

Dear Lukman,

The reflectivity of anodized layer should be different than that of Al 5052. It means that you shoud see the anodized layer but i dont know the resolution of light microscobe is enough. Also the thickness of the anodized layer is important.

Besides, the grinding and polishing was not applied well.

Best regards.

@ Mr. Conte, Mr. Mahmood, Mr. Goodchild thank you sirs for your recommendation, but I'm afraid my only option in my lab is to use light microscope

@ Mr. Wibowo thank you very much for the recommendation and references, they are really helpful, I agree HF is really dangerous, thank you for the note. Turns out the head of the lab didn't allow students to use the microscope with HF etching, so I will try to use a simpler etch for aluminum first, see if at least the aluminum part will etch

@ Ms. Remešová and Mr. Bastan thank you for the observation, on the cross-section side, for the grinding I do it with SiC paper grit 400, 600, 800, and 1000, followed with mechanical polishing. I also realized my specimen is not well polished, thank you for the note. Is it possible that I used excessive force when pressing the specimen in the spinning plate? Aluminum is really delicate. According to the standard that I followed, the thickness that I obtained should be around 35 microns. In that photo I used an objective lense of 20x magnification

Dear Lukman,

The grinding and polishing of Al and its alloys is a little bit tricky. Hence, you should not use excessive force or time. Also please check your polishing pad, it maybe is not clean so, the particulates in it can also cause grooves.

Please check this article, it can be helpful for you.

Application of CSLM to the Surface Morphological Study of Al 5052 Alloy Anodized in Sulfuric Acid Solution

Best Regards.