Selecting the proper electrode and liner materials is critical to ensure accuracy, reliability, and long service life of electromagnetic flow meters.
Material choices must be made according to fluid properties such as corrosion, abrasiveness, temperature, and pressure.
1. Electrode Material Selection Principles
Electrodes are in direct contact with the process fluid and serve as the signal pickup interface.
Therefore, electrode materials must provide excellent corrosion resistance, sufficient conductivity, and mechanical durability.
1.1 Based on Corrosiveness of the Medium
✔ Neutral water / general utility liquids (pH 5–9)
316L Stainless Steel
Low cost, good chemical resistance, and widely applicable.
✔ Acidic solutions (HCl, H₂SO₄, HNO₃)
Dilute acids (<30%) → Hastelloy C
Concentrated nitric acid (>65%) → Titanium (Ti)
Hydrofluoric acid (HF) → Tantalum (Ta) or Platinum (Pt)
Note: HF attacks most metals except Ta; Pt optional for ultra-high purity.
✔ Alkaline solutions (NaOH, NH₄OH)
Low concentration alkali → 316L or Hastelloy B
Strong alkali or molten caustic → Nickel (Ni)
Nickel performs well in caustic media but is not acid resistant.
✔ Chloride-containing media (seawater, NaClO)
Avoid 316L (risk of stress corrosion cracking).
Prefer Ti or Hastelloy C-276.
1.2 Based on Abrasiveness of Medium
Suspended particles, fibers, or slurry
→ Tungsten Carbide (WC) or Platinum-Iridium Alloy (Pt-Ir)
Provides superior hardness and wear resistance compared with conventional metals.
1.3 Special Use Scenarios
Food/Pharmaceutical → 316L or Pt, hygienic grade
High temperature → choose alloys with 150–200°C capability (Ti, Hastelloy)
2. Liner Material Selection Principles
The liner provides electrical insulation and protects the measuring tube from corrosion and erosion.
Proper selection follows chemical compatibility, temperature rating, abrasion resistance, and sealing performance.
2.1 Based on Chemical Compatibility
✔ Strong acids, strong alkalis, organic chemicals
PTFE (Teflon)
Excellent chemical inertness, pH 0–14, suitable for almost all corrosive fluids.
✔ Weak acids, weak alkalis, saline solutions
FEP
Similar corrosion resistance to PTFE, but more flexible and suitable where vibration exists.
✔ Non-corrosive viscous fluids (oil, syrup, wastewater)
CR (Chloroprene Rubber) or NBR (Nitrile Rubber)
Cost-effective, smooth surfaces with low adhesion tendency.
2.2 Based on Temperature and Pressure
| Liner | Temperature Limit | Notes |
|---|---|---|
| CR / NBR | -20 ~ 80°C | Economical but aging at high temp |
| FEP | -20 ~ 150°C | Good flexibility |
| PTFE | -20 ~ 200°C | Best for harsh chemical & high temp |
| Reinforced PTFE | -20 ~ 200°C | Suitable for higher pressure (>1.6 MPa) |
2.3 Based on Abrasiveness
High solids, slurry, mineral process water
→ Polyurethane (PU) or reinforced PTFE
Superior wear resistance compared with standard rubber.
3. Key Considerations and Common Pitfalls
3.1 Material Compatibility
Electrode and liner must support the same fluid condition:
HF acid → Ta electrode + PTFE liner
High-temperature hot water → Ti electrode + PTFE liner
3.2 Avoid Misapplications
Titanium is not a universal choice — avoid in acidic fluoride environments
Rubber liners dissolve in organic solvents (fuel, alcohols)
316L is unreliable for chloride-rich seawater
3.3 Cost and Economics
Use premium alloys/rare metals only when required
Reduce cost for small sizes (DN <50) using FEP + 316L, if compatible
3.4 Installation and Maintenance
Rubber liners are prone to mechanical cuts during installation
PTFE cracking risk increases with flange misalignment
Long shutdowns → drain fluid to prevent chemical soaking and liner swelling
3.5 Verification for Special Fluids
Where compatibility is uncertain:
Conduct bench corrosion tests
Minimum immersion period: 72 hours on candidate materials
4. Conclusion
Electrode and liner selection for electromagnetic flow meters should always follow a fluid-driven approach:
Evaluate chemical composition
Define temperature & pressure conditions
Consider solids and abrasion
Balance technical suitability and cost
Choosing the correct combination ensures:
Long-term stability
Accurate measurement
Lower maintenance cost
Avoidance of premature instrument failure