Good day! I am not a full expert in that field, so I am proposing some considerations for you to think about. As for the additional suggestions on your SEM images, is it possible to evaluate thickness of oxide layer from it? Thus, it may provide some considerations on the power of the process.

As for the questions 1-3, we should consider that water is ionized and thus, some reactions will always occur. However, the reaction pathway probably will be according to the following equation: 4Fe + 3O2 = 2Fe2O3. Oxygen is saturated in water, however, as far as 2 water molecules out of 1 billion are dissociated, direct coupling 2Fe + 3O(2-) = Fe2O3 will be quite minor. As for the question 4, you should consider whether that needles are amorphous or not. Please, feel free to correct me.

It is a common knowlege that Fe anode oxidises during electrolisis of water. So, nothing unusual. But why analysis was performed only for two elements? Was it checked (with spectrum acquisition) that nothing else was present? Of couse oxides have a lot of O, which explaines results of second analysis. And I would advise to take pictures not only with BS detector, but with SE also, at lower voltages (like 5 kV). Images will be much better.

I assume that the EDS analysis was done over the area of the SEM images shown and the concentrations were obtained by pressing the “QUANT” button of the software.

The spatial resolution of EDS (laterally and in depth) is given by the beam-sample interaction (i.e. beam energy, atomic number, density) and the physics of X-ray emission.

A frequently used estimate is the Kanaya-Okayama range (K Kanaya and S Okayama, J. Phys. D: Appl. Phys. 5(1972) 43-58, DOI: 10.1088/0022-3727/5/1/308).

For pure Fe and 15 keV electrons the electron excitation range is ca. 1 µm, the size of the volume where the X-rays are from is ca. 0.7 µm for Fe K radiation.

EDS quantification assumes that the sample is sample is homogeneous, flat, and thick (compared to the excitation range).

See here: https://www.youtube.com/watch?v=et9CTYHUoos&app=desktop

Advanced Spectrum Analysis and Quantification Precision

This presentation by Dr. Jens Rafaelsen was recorded on the EDAX booth at M&M 2018

At 35:45 “If your sample doesn't meet those requirements: don't click the QUANT button: You'll get numbers, they'll be garbage!”

The “Fe-before” sample is likely Fe with a native iron oxide.

Is the sample flat? Is the sample homogeneous (laterally and in depth)? Is the oxide thick?

If the iron oxide is thin (< 0.7 µm), the volume you analyse is composed of iron oxide(s) + iron metal. The prerequisites of the quantification model are not fulfilled.

The “Fe-after” sample is likely Fe with thick iron oxide(s).

Is the sample flat? Is the sample homogeneous (laterally and in depth)?

The prerequisites of the quantification model are not fulfilled.

Is the oxide thick?

If the iron oxide is thick (> 1 µm), the volume you analyse might be composed of the iron oxide(s) formed, but the prerequisites of the quantification model are not fulfilled.

As mentioned by Vladimir Dusevich the EDS spectra should be evaluated carefully. There are small peaks around 2 keV. For “Fe-before” at < 2 keV, for “Fe-after” at ca. 2.3 keV. What’s this?

> Also, I have some questions about SEM-EDX:

Your questions 1-5 are not directly related to EDX.

The chemistry of oxidation of iron (and steels) in aqueous solutions has been studied for decades because of the importance for corrosion and power generation and the results are now textbook knowledge.

I guess for electrolysis a lot of actual work can be found.

For thicker oxides scales composed of different oxides may form. Here, preparation and analysis of cross sections may give more insight.

Preparation methods and analysis will depend on the thickness and stability of the scales (fracturing, preparation of cross section by metallographic methods, FIB cuts; EDS in SEM or TEM?)