Lots of Fe, followed by Cr and Ni and Mn.
The Fe ends up at much higher proportion in the electrolyte than it is in the alloy.
The Fe is roughly 75% of the alloy but closer to 85-90% of the dissolved metals.

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P.E. Metallurgy, Plymouth Tube

Thank you so much for the reply, it really helped.
Regards.

I think Fe, Ni dissolved faster/easier than Cr, resulting in Cr-rich metal. And this is one of reasons electropolishing could enhance corrosion resistance.

@Ed: when you mentioned 85-90%, did you mean the metals deposited on the cathode? Fe ions concentration in electrolyte will start from 0 to some percentage, that seems not be comparable to the source metal. Also, I think 316 is about 65%Fe. There are tens of 316 versions, Fe content varies, but all of them seem unlikely go as high as 75%.

Since electropolishing produces a very smooth and level surface, it seems to me that each component of the alloy must be removed proportionally to its concentration in the alloy. There may be differences in how they plate out on the cathode.

Produced smooth surface is due to the fact rougher portion of surface get higher current and so removed more quickly. The dissolved rate for elements is probably determined by their standard electrode potentials, Cr to Cr3+ has the most negative potential and so dissolved least. While deposited on the cathode may take the opposite direction.

Compositepro,
There are two things going on in EP.
1. Asperities (high points) are removed due to higher local current density.
2. Selective dissolution of Fe from the top few atomic layers of metal.

At 16%Cr, 10%Ni, and 2%Mo you are over 70% Fe. I exaggerated a little.
So if about 1/2-2/3 of the metal dissolved is average chemistry and the rest is nearly all Fe then the dissolved metals will have Fe up in the 80+% range.

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P.E. Metallurgy, Plymouth Tube