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I have a sample of aluminium which I'm interested in finding the rough composition of.

The sample is a part of a large number of samples which I sanded down and polished with a polishing table. I cleaned the samples with acetone, washed and rinsed in ultrapure water. Area spectrum SEM EDS analyses were then ran on all the samples.

Some impurities were left on the surface of the sample as expected. All other samples show oxygen, silicon and carbon impurities assumedly left from the sanding process (silicon carbide paper), the carbon tape for the grounding and possibly carbon impurities in the SEM-chamber. The samples were left to dry overnight so assumedly the amounts of acetone and water on the surface are fairly small.

All the other samples show accuracies and composities that seem sensible to me. However, the aluminium sample that I have shows levels of oxygen much larger than I expect:

EDS results

So, three measurements of four areal spectrums each. Carbon is left out. And a mean of 17.37-at% of oxygen?

I understand that aluminium has a passive oxide layer and that EDS is a relatively surface active technique. However this paper cites passive layer thicknesses in the order of nanometers, and I understand that the EDS has a penetration thickness in the order of micrometers. A comparable steel sample that I analysed has levels of oxygen that I expected (~~0.1%)

What's going on? Can oxides precipitate inside bulk aluminium during manufacture or aging?

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  • $\begingroup$ Are you able to double check crystallography with XRD? You had the sample in an SEM: are there any unexpected microstructural features apparent in the micrographs? What does optical microscopy show? How confident is the EDS software that the peak is O and not a low energy secondary peak of something else? Is the sample an alumina reinforced aluminum composite? $\endgroup$
    – wwarriner
    Nov 28 '16 at 19:13
  • $\begingroup$ XRD is not available. There were no anomalies, optical microscopy showed only minor scratching and a few local impurities. The area scans were made from large (~tens-hundreds of square micrometers) homogenous -looking areas. They were hand picked not to contain anything that stands out in the low magnification images. Software error is quite possible. Aluminium is low Z, maybe its peak was convolved with the oxygen? The EDS 'should' be able to handle it according to the manufacturer and some work that I have seen, but maybe not? The origin of the sample is unknown. $\endgroup$
    – Tapio
    Nov 28 '16 at 21:12
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As @starrise mentioned, it is likely either a software error (secondary peak) or your sample could be aluminum reinforced with aluminum oxide. With aluminum there will always be some oxygen (even in vacuum coating we see some) but not that high from what I have seen.

Another thing to consider is if your accelerating voltage is too low, it might not go to the depth you intend it. Are you within the 15-20 kV range?

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  • $\begingroup$ The measurements were done at 15kV. $\endgroup$
    – Tapio
    Jul 19 '17 at 13:02
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I think you need to check on the grinding and polishing materials. Normal materials used are silicon carbide, alumina and diamond ( We never used quartz that I know of). Were any of these materials used and/or embedded in the samples ? Can you do an elemental display to determine how they are dispersed ?

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    $\begingroup$ Do you also have access to a sonic bath for removing excess materials? I once did work with a material which had large amounts of void space which would grab onto polishing media. We actually started suspending the imaged surface upside-down inside of the sonic bath which solved our issue. Still I would expect the media would show up on his images. I also seem to recall that EDS was not particularly effective in finding elements with low atomic weights (1st and 2nd rows roughly). We didn't rely upon it much so I could be mis-remembering. $\endgroup$
    – Sully45
    Jul 10 '17 at 13:13
  • $\begingroup$ 25 years ago our unit did OK with C, N, And O. $\endgroup$ Jul 11 '17 at 14:19
  • $\begingroup$ You are right, the paper was silicon carbide. I have no idea where I got the idea that it would be quartz. The samples are embedded in a beer can sized blob of epoxy and the results are buried deep in the appendix of my thesis, no further processing is possible. $\endgroup$
    – Tapio
    Jul 19 '17 at 13:08

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