Sulfuric acid (H2SO4) is the most widely produced industrial chemical, and its corrosive behavior is unique — concentration-dependent and temperature-dependent. This guide covers material selection and pump design for sulfuric acid applications.
H2SO4 Corrosion Behavior
Concentration-Dependent Corrosion
- Dilute H2SO4 (<10%): highly corrosive to steel (like other strong acids)
- Medium (10-80%): very corrosive, most metals attacked
- Concentrated (90-98%): forms protective iron sulfate film on steel
- Cast iron: suitable for 90-98% H2SO4 (passivation layer)
- Rule: corrosion rate varies non-linearly with concentration
Concentration vs Material
| Concentration | Cast Iron | 316SS | PTFE | Hastelloy |
|---|---|---|---|---|
| <10% (dilute) | ✗ Poor | △ Fair (cold) | ✓ Good | ✓ Good |
| 10-80% | ✗ Poor | ✗ Poor | ✓ Good | ✓ Good |
| 80-90% | △ Fair | ✗ Poor | ✓ Good | ✓ Good |
| 90-98% (conc) | ✓ Good | △ Fair | ✓ Good | ✓ Good |
| Fuming (oleum) | △ Fair | ✗ Poor | ✗ Not | ✓ Good |
Temperature Effects
- Cold H2SO4 (<40°C): more material options
- Hot H2SO4 (>60°C): severely limits choices, even concentrated
- Concentrated + hot: cast iron may fail (passivation breaks down)
- Dilute + hot: only PTFE, Hastelloy, or glass-lined
- Always verify: material at actual concentration + temp
Material Selection
Cast Iron (for Concentrated H2SO4)
- Suitable for 90-98% H2SO4 at ambient temp
- Mechanism: forms protective FeSO4 film (passivation)
- Advantages: low cost, readily available, good for concentrated
- Limitations: NOT for dilute (<90%), NOT for hot (>60°C), NOT for velocity >2m/s
- Use: bulk transfer of 93-98% H2SO4, storage tank circulation
PTFE/PFA (Fluoroplastic)
- Excellent all concentrations: 0-98%, up to 120-150°C
- Advantages: universal resistance, no metal contact, all concentrations
- Limitations: not for abrasive, high pressure, >150°C, oleum
- Use: dilute to concentrated, variable concentration, hot acid
- Best choice when concentration varies or is uncertain
Hastelloy Alloys
- Hastelloy C-276: good for all concentrations, high temp
- Hastelloy B-2: excellent for reducing conditions (dilute H2SO4)
- Advantages: metal strength, high temp, abrasive tolerant, high pressure
- Limitations: very high cost (5-10× 316SS)
- Use: critical service, high temp/pressure, oleum, mixed acid
Materials to Avoid
- 304/316 Stainless Steel: Pitting, SCC in H2SO4 (especially dilute, hot)
- For stainless pump: NOT for H2SO4 (except concentrated, cold, <10% time)
- Aluminum: Rapid corrosion, hydrogen gas
- Copper/bronze: Attacked (especially dilute, hot)
- PVC/PP: Only for dilute, cold (<40°C)
- Carbon steel: Only for 90-98% concentrated, cold, low velocity
Pump Design Considerations
Velocity Control
- High velocity erodes protective film (cast iron)
- Cast iron: keep velocity <1.5-2 m/s (prevent film erosion)
- PTFE: velocity <3 m/s (prevent lining damage)
- Oversize piping: reduce velocity, extend life
- Avoid: sharp bends, restrictions (high velocity, erosion)
Seal Design
- H2SO4 fumes: corrosive, toxic (SO3, SO2)
- Single seal: only for non-hazardous, low concentration
- Double seal: for concentrated, fuming, hazardous (barrier fluid)
- Magnetic drive: zero leakage (best for H2SO4 fumes)
- Seal faces: SiC/SiC (hard, chemical resistant)
Dilution Prevention
- H2SO4 + water = highly exothermic (can boil, splash)
- Never: add water to concentrated H2SO4 (violent reaction)
- System: prevent water ingress (seal flush, condensation)
- Pump: drain completely before water flushing
- Material: dilute acid is MORE corrosive than concentrated (for cast iron)
Temperature Management
- Hot H2SO4: more corrosive, material limits
- Cast iron: max 60°C (concentrated), lower for dilute
- PTFE: max 120-150°C (depending on lining)
- Hastelloy: max 200-300°C
- Cooling: jacketed pump or piping for hot acid
Application-Specific Design
Bulk Transfer (93-98% H2SO4)
- Concentrated, ambient temp, large flow
- Pump: sulfuric acid pump (cast iron or PTFE)
- Material: cast iron (cost-effective for concentrated)
- Design: low velocity, large piping, double seal
- Consider: rail/truck unloading, storage transfer
Acid Circulation (Dilute to Concentrated)
- Variable concentration, may be hot, process circulation
- Pump: fluoroplastic pump (PTFE)
- Material: PTFE (all concentrations, safe choice)
- Design: magnetic drive (zero leakage), SiC seal
- Consider: concentration variation, temp spikes
Acid Dosing (Dilute H2SO4)
- Precise flow, low flow, high pressure, dilute acid
- Pump: diaphragm metering pump (PTFE/PVDF)
- Material: PTFE diaphragm, PVDF/PTFE head
- Design: zero leakage, high pressure, precise control
- Use: pH control, chemical process, water treatment
Spent Acid / Mixed Acid
- Impurities, variable concentration, may contain organics/metals
- Pump: fluoroplastic pump or Hastelloy
- Material: PTFE (safe for unknown composition)
- Consider: impurities (abrasive, organic), temp
- Seal: double mechanical or magnetic (hazardous)
Safety Considerations
H2SO4 Hazards
- Corrosive: severe burns, dehydrates tissue
- Reaction with water: exothermic (can boil, splash)
- Fumes: SO3, SO2 (toxic, corrosive)
- Reaction with organics: charring, fire (strong oxidizer when hot/concentrated)
- PPE: face shield, chemical suit, gloves, respirator
System Safety
- Zero leakage: magnetic drive or double seal
- Secondary containment: dike, spill kit, neutralization (soda ash/lime)
- Ventilation: fume hood, scrubber (for fuming acid)
- Emergency: eyewash, safety shower, neutralization station
- Training: proper handling, dilution procedure (acid to water, never reverse)
Common Issues
- Cast iron in dilute H2SO4: Rapid corrosion. Use PTFE or Hastelloy for <90%.
- 316SS in H2SO4: Pitting, SCC. Use PTFE, Hastelloy, or cast iron (concentrated only).
- High velocity erosion: Protective film removed (cast iron). Keep velocity <2m/s, oversize piping.
- Water ingress: Dilution = more corrosive + exothermic. Prevent water in seal flush, condensation.
- Seal leakage: H2SO4 fumes, burns, equipment damage. Use double seal or magnetic drive.
- Hot concentrated acid: Cast iron fails >60°C. Use PTFE (up to 120°C) or Hastelloy.
- Wrong concentration data: Corrosion varies non-linearly. Verify actual concentration + temp.
- No secondary containment: Spill risk. Install dike, neutralization, emergency equipment.
Procurement Recommendations
- Provide: H2SO4 concentration, temperature, flow, head, impurities, fuming status
- For concentrated (90-98%): specify sulfuric acid pump (cast iron or PTFE)
- For dilute/variable: specify fluoroplastic self-priming pump (PTFE)
- For dosing: specify diaphragm metering pump (PTFE/PVDF)
- For hazardous/fuming: specify magnetic drive (zero leakage)
- Specify wetted material per concentration/temp (cast iron, PTFE, Hastelloy)
- Specify seal: SiC/SiC faces, double mechanical or magnetic
- Specify O-rings: EPDM or Viton (per concentration/temp), FFKM for severe
- Include velocity control (oversize piping, <2m/s for cast iron)
- Require material certificate and hydrotest
- Verify supplier has H2SO4 pump application references
Related Industries
Sulfuric acid pumping applies to chemical processing (acid production, fertilizer), metal finishing (pickling, plating), mining (leaching, ore processing), petroleum (alkylation, refining), pulp & paper (digestion, bleaching), water treatment (pH control), and semiconductor (wafer cleaning). The core principle — proper material selection for H2SO4 concentration and temperature with velocity control and zero-leakage design for safe operation — is consistent across sulfuric acid applications.