1. Corrosion Resistance Limits of Monel 400 in Hydrofluoric Acid
Dilute Hydrofluoric Acid (Concentration ≤ 60%) at Room Temperature
Monel 400 shows excellent corrosion resistance in this environment, with a corrosion rate of less than 0.025 mm/year. This is because the alloy forms a dense, stable copper fluoride-nickel fluoride passive film on its surface, which isolates the alloy substrate from HF erosion and prevents further chemical reactions. This makes it the primary material for equipment handling dilute HF at room temperature, such as storage tanks, pipelines, and pumps.
Concentrated Hydrofluoric Acid (Concentration > 60%) at Room Temperature
When HF concentration exceeds 60%, the corrosion rate of Monel 400 increases moderately, but it still maintains acceptable performance for industrial use (corrosion rate: 0.025–0.1 mm/year). However, if the concentration approaches anhydrous hydrofluoric acid (≥ 98%), the passive film becomes unstable, and the corrosion rate rises sharply to over 0.5 mm/year, which is not suitable for long-term service.
Key Influencing Factors on Corrosion Limits
Impurities: The presence of oxidizing impurities (e.g., Fe³⁺, Cu²⁺, HNO₃) will accelerate the corrosion of Monel 400, reducing its corrosion resistance limit by 10–20% in the same concentration and temperature conditions.
Flow Rate: High-flow-rate HF solutions will erode the passive film, leading to localized erosion-corrosion. In such cases, the applicable HF concentration needs to be reduced by 5–10% to ensure safe use.
2. Suitability of Monel 400 for High-Temperature and High-Concentration Working Conditions
The passive film on the alloy surface loses its stability and is prone to dissolution or cracking, resulting in a sharp increase in corrosion rate (e.g., in 50% HF, the corrosion rate jumps from 0.015 mm/year at 25°C to 0.8 mm/year at 80°C).
High temperatures promote the reaction between HF and the alloy, generating soluble nickel and copper fluoride complexes, which further accelerates material degradation.
For short-term intermittent operations, the maximum temperature tolerance is up to 100°C, but the service life will be significantly shortened by 70–80% compared to room temperature conditions.
A rare exception is the anhydrous HF vapor phase environment at room temperature. Monel 400 can maintain low corrosion rates in this scenario, as the vapor phase lacks sufficient moisture to break down the passive film, making it suitable for anhydrous HF vapor pipeline systems.
