[NAC2019]

Hi Guys,
I am dissolving steel sample (containing Fe, Si, and ...) in HCL acid in a 3-neck flask with N2 gas, condenser, and thermometer connected to it. I don't want O2 to get into the system and form SiO2, so I need it to be sealed temporarily. The necks are closed with stopper or, in case of condenser, ground glass joint. I'd rather not to use high vacuum grease since it contains SiO2. Is there anything else that I could use to cover the neck and make them as much airtight as possible? I appreciate your help.

[clarkstill]

How are you adding the N2 gas? The best way would be to have nitrogen being continuously added via a bubbler. Since your reaction will presumably produce H2 gas, this will allow the pressure to be relieved, and will also ensure there is a positive internal pressure of nitrogen at all times. This means that even if the ground glass joints arent perfectly air-tight there is always a slight outward flow of N2 that prevents any air/oxygen from making its way inside.

[Enthalpy]

I expect Si to find its oxygen in the water. In steel, Si is distributed very uniformly, so the native oxide layer will eat much or all of the grain radius. Is the goal to obtain a powder or sponge of reduced Si?

Compare the heat of formation, kJ/mol:
-911  SiO₂, or -455 per mol of O atoms
-286  H₂O

[wildfyr]

Uh, if youre very sensitive to silicon, maybe using SiO₂ glassware isnt the best choice. If its just grease and stoppers, use teflon stoppers/adaptors and definitely do this under positive nitrogen pressure.

[NAC2019]

How are you adding the N2 gas? The best way would be to have nitrogen being continuously added via a bubbler. Since your reaction will presumably produce H2 gas, this will allow the pressure to be relieved, and will also ensure there is a positive internal pressure of nitrogen at all times. This means that even if the ground glass joints arent perfectly air-tight there is always a slight outward flow of N2 that prevents any air/oxygen from making its way inside.

Thank you for the reply. I am attaching a figure showing the setup. I guess the flask on the right acts as bubbler. I make sure there is approximately 1 bubbles per 5 seconds there. About the hydrogen gas formation, you're right. At 90 C, the dissolution is fast and generates lots of H2.

[NAC2019]

I expect Si to find its oxygen in the water. In steel, Si is distributed very uniformly, so the native oxide layer will eat much or all of the grain radius. Is the goal to obtain a powder or sponge of reduced Si?

Compare the heat of formation, kJ/mol:
-911  SiO₂, or -455 per mol of O atoms
-286  H₂O

Thank you for the reply.
No, I am not interested in Si. I am trying to extract powders of NbC and TiN which are dispersed inside the sample. And I'd like to keep them as pure as possible without any SiO2 contamination since SiO2 will be amorphous and make some problem in XRD data.
At room temperature, the concentration of O2 in water is around 8 mg/L (I am not sure if it's the same for acidic solutions though). I am trying to remove the dissolved oxygen through sparging for an hour. However, I am not sure How long I should continue to sparge before inserting the sample inside the 3-neck flask. Is there a rule of thumb for this method of deoxygenation? I mean flow rate and sparging time?
Also, do you mean the Si will get the oxygen from decomposing H2O? And I did not understand the part about grain radius.

[NAC2019]

Uh, if youre very sensitive to silicon, maybe using SiO₂ glassware isnt the best choice. If its just grease and stoppers, use teflon stoppers/adaptors and definitely do this under positive nitrogen pressure.

Is there plastic 3-neck flask? I need it to be clear so that I could see when the sample is completely dissolved. Thank you for the suggestions. I will use Teflon ribbed sleeve for ground glass joint and Teflon adapter for the thermocouple and gas line connections.

[Enthalpy]

Acids corrode AlN, and titanium is about as reactive as aluminium. Shall the fine TiN powder survive warm HCl?

Si is already partly oxidised in steel. The oxygen remaining in the melt combines with the dissolved silicon preferentially to iron. It is one function of silicon in transformer steel, to deoxidize the iron. When silicon isn't added intentionally, its source (like 0.3%) is the silicates that insulate the furnace.

Native fine powder of Fe_xSi_y should react with water, whether oxygen is dissolved or not. Reduced Si reacts immediately to create its silica layer.
450kJ/mol  O-Si single bond
430kJ/mol  H-O
I expect reduced silicon to decompose water like a reactive metal does. Not noticed with massive silicon, as a thin silica layer stops the reaction. How small do you expect the Fe_xSi_y particles to be?

Even if Fe_xSi_y made some Si-Cl compound in the warm acid, they decompose in water to make silica
https://en.wikipedia.org/wiki/Silicon_tetrachloride

[NAC2019]

Acids corrode AlN, and titanium is about as reactive as aluminium. Shall the fine TiN powder survive warm HCl?

Si is already partly oxidised in steel. The oxygen remaining in the melt combines with the dissolved silicon preferentially to iron. It is one function of silicon in transformer steel, to deoxidize the iron. When silicon isn't added intentionally, its source (like 0.3%) is the silicates that insulate the furnace.

Native fine powder of Fe_xSi_y should react with water, whether oxygen is dissolved or not. Reduced Si reacts immediately to create its silica layer.
450kJ/mol  O-Si single bond
430kJ/mol  H-O
I expect reduced silicon to decompose water like a reactive metal does. Not noticed with massive silicon, as a thin silica layer stops the reaction. How small do you expect the Fe_xSi_y particles to be?

Even if Fe_xSi_y made some Si-Cl compound in the warm acid, they decompose in water to make silica
https://en.wikipedia.org/wiki/Silicon_tetrachloride

TiN will dissolve if it remains in HCl acid for a long time. However, I stop the process after 4 or 5 days, and by the end of dissolution process, a significant amount of TiN is still in residues.

The amount of Si in steel is around 0.3 weight percent. I usually dissolve 10 gr of steel in 500 ml HCl acid (around 2.4 N). With my calculations, if the entire Si becomes SiO2, then I'll be getting around 64 mg of SiO2. However, in my experiments, I usually extract 5 to 10 mg SiO2 and around 10 mg of other compounds. That's why I am skeptical about kinetics of SiO2 formation due to H2O decomposition. It is also possible I am not extracting all SiO2, probably because it takes sometime for SiO2 to polymerize and settle down.

How about using HCl in ethanol instead of water? Would it resolve the issue?

[Enthalpy]

Possibly the TiN and Fe_xSi_y compounds have grains big enough that most material survives the aqueous acid.

Several days to attack steel with warm HCl, that's slow. Could you make much finer grain of the steel, by milling, filing...? Even grinding if the chips stay cold. If the reaction happens in a quarter hour instead, but the grains of TiN and Fe_xSi_y keep the same size, you should degrade these less over the shorter time.