How Corrosive Is Aqua Regia? Fluororubber vs. Perfluoroelastomer: Which Resists Acid Better?

Understanding Aqua Regia and Its Corrosive Power

Aqua regia, known as the "king of acids," is a highly corrosive mixture of concentrated hydrochloric acid and nitric acid in a 3:1 ratio. Famous for its ability to dissolve noble metals like gold and platinum, aqua regia presents an extreme challenge for most materials. But which elastomer can withstand its aggressive nature? Let's compare fluororubber (FKM) and perfluoroelastomer (FFKM) to determine which performs better in acid resistance.

Fluororubber (FKM): Tough and Acid-Resistant

Fluororubber is well-regarded for its resilience against strong acids, making it a widely used material in harsh chemical environments.

Key Acid Resistance Properties:

• Hydrochloric Acid (HCl, 36%): While most materials degrade, fluororubber remains stable.

• Sulfuric Acid (H₂SO₄, 10%-98%): From dilute to concentrated solutions, fluororubber holds up exceptionally well.

• Nitric Acid (HNO₃, 10%-50%): Strong resistance to nitric acid, retaining its integrity.

• High-Temperature Durability: Soaked in 67% sulfuric acid at 140°C, fluororubber maintains its structural stability.

Type 23 Fluororubber: Enhanced Performance

A specialized variant, Type 23 fluororubber, offers superior acid resistance. When immersed in 98% nitric acid for 27 days, it experiences only 13%-15% volume swelling. It also withstands fuming nitric acid and concentrated sulfuric acid. Against aqua regia, its resistance rating is "2", meaning it can endure for a limited time but is not fully impervious.

Perfluoroelastomer (FFKM): The Ultimate Acid-Resistant Material

Perfluoroelastomer (FFKM) is considered the gold standard for extreme chemical resistance, making it a top choice for environments exposed to aqua regia.

What Makes FFKM Superior?

FFKM’s molecular structure replaces nearly all hydrogen atoms with fluorine, forming robust C-F bonds (bond energy ~485 kJ/mol), resulting in three key advantages:

1. Ultra-Low Surface Energy (15-20 mN/m): Ensures anti-stick and self-cleaning properties, ideal for high-purity applications.

2. Exceptional Chemical Stability: Resists highly concentrated acids, bases, and strong oxidizers, making it far superior to standard elastomers.

3. Extreme Durability: Performs reliably in harsh petrochemical environments, resisting high heat, pressure, and corrosion over extended periods.

Applications: Choosing the Right Elastomer

• Fluororubber (FKM): Best suited for general acid-resistant applications where exposure is limited.

• Perfluoroelastomer (FFKM): The preferred choice for extreme chemical environments, including aqua regia exposure.

Both materials are widely used in chemical processing, pipelines, and sealing applications. As material science advances, even more acid-resistant elastomers may emerge, offering enhanced durability and performance.

Quick Q&A

Q: Can aqua regia corrode all rubber materials?
A: Not entirely. Fluororubber provides moderate resistance, while perfluoroelastomer offers significantly better protection against corrosion.

Q: Is perfluoroelastomer expensive?
A: Yes. Its superior chemical resistance and durability come at a higher cost compared to fluororubber.

Conclusion

For applications requiring resistance to aqua regia and other aggressive acids, FFKM is the superior choice, while FKM serves as a cost-effective alternative for general acid resistance. Choosing the right material depends on the severity of the chemical exposure, operational temperature, and budget considerations.

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