I'm interested in a selective oxidation in an austenitic stainess steel so that Chromia rather then ferrous oxide is formed on top of my metal. I know I should have a value of pO2 around 10^-17atm. Is possible to obtain such a low value with a Nitrogen gas based atmosphere?
Thanks for yours help
You do not need such a low partial pressure of oxygen, and it is technically impossible to obtain it (10-17 atm = 10-12 Pa!). Iron stops oxidizing even at a partial pressure of a few Pascals, and at a pressure of 10-4 Pa, chromium also stops oxidizing.
For iron to stop oxidizing in a nitrogen atmosphere, nitrogen with a purity of 3.0 (99.9%) is sufficient. In such a gas, your steel will acquire a green color after the furnace. If you take nitrogen with a purity of 5.0 (99.999%), then stainless steel will remain shiny after heating in an oven.
You do not need such a low partial pressure of oxygen, and it is technically impossible to obtain it (10-17 atm = 10-12 Pa!). Iron stops oxidizing even at a partial pressure of a few Pascals, and at a pressure of 10-4 Pa, chromium also stops oxidizing.
For iron to stop oxidizing in a nitrogen atmosphere, nitrogen with a purity of 3.0 (99.9%) is sufficient. In such a gas, your steel will acquire a green color after the furnace. If you take nitrogen with a purity of 5.0 (99.999%), then stainless steel will remain shiny after heating in an oven.
Thanks so much for your answer, I will try. Do you have any suggestion about temperature and time for the soak? My goal is 1-2um of Chromia on top of it
I was thinking of that pressure of 10^-17 based on ellingham Diagram, why is this consideration wrong?
On the Ellingham diagram, the partial pressures of oxygen for the decomposition of oxides are given for reasons of theory. Therefore, there you can see such figures as 10-30 atm, which in practice are completely unattainable. However, this mainly concerns the region of low temperatures, when the stability of oxides can be considered absolute. At high temperatures, the situation changes dramatically. For example, for such oxides as FeO, NiO, CoO, CuO, WO3, MoO3 at a temperature of 700-800°C, the partial pressure of oxygen required for the decomposition of oxides is no more than 10-2 atm. Chromium oxide Cr2O3 decomposes at 800°C at an oxygen pressure of about 10-6 atm, and this pressure is quite achievable in a technical vacuum or pure nitrogen. Only the most stable oxides like Al2O3, CaO, MgO, ZrO2, etc. cannot be decomposed when heated under any pressure.
For your task, I would recommend a temperature of about 800°C, but I am not sure that the result will be as you want: an even green color of the surface with a layer of Cr2O3 a couple of microns thick. In my experience, the surface of the stainless steel will become dark and mottled, and the green chromium oxide will sit on it in a very loose layer, almost like a powder.
Thanks so much for your help. If a theoric approach is not realistic, which is the best way to proceed in this field? For trials and errors?
Federica is talking about low partial pressure (10 EXP-17)
Vadim is talking about 10 EXP+30)
Is there not a misunderstanding?
Carburizing is done in endogas atmosphere and low partial pressure can be measured and regulated by means of oxygen probe.
Company MESA or Process-Electronic have info about partial pressures
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