Question

Which of the following is a suitable method to remove hydrogen from molten aluminium?

• A. Cover melt surface with a flux
• B. Increase melt temperature
• C. Expose flowing melt to vacuum
• D. Bubble humidified argon gas through the melt

Hint:

Hydrogen in molten aluminium is best removed by \textbf{argon degassing} — a standard practice in foundries to prevent porosity.

Answer 1

The Correct Option is D

Step 1: Hydrogen Problem in Aluminium
Hydrogen is the only gas that is significantly soluble in molten aluminium. If not removed, it forms pores in solidified metal, degrading mechanical properties and surface finish. Step 2: Removal via Degassing
The most effective industrial method is to bubble dry or humidified inert gas (commonly argon or nitrogen) through the molten aluminium. This works on the principle of partial pressure difference, helping hydrogen diffuse out of the melt. \[ \text{H}_{\text{dissolved}} \rightarrow \text{H}_{\text{gas bubbles}} \] Step 3: Why Argon?
Argon is chemically inert and does not react with the aluminium melt. Bubbling it through the melt provides nucleation sites for hydrogen to escape and is safer and more efficient than vacuum or flux methods for hydrogen. Step 4: Eliminating Other Options
- Flux covering helps prevent oxidation but doesn't remove dissolved hydrogen.
- Increasing temperature can increase hydrogen solubility — worsening the problem.
- Vacuum degassing is less effective and costly for aluminium compared to argon purging. Conclusion: The best method to remove hydrogen from molten aluminium is to bubble humidified argon gas through the melt.