Acids are among the most corrosive and hazardous liquids encountered in pump applications, representing approximately 5% of industrial pump duties. They range from highly dilute solutions with modest corrosivity through to concentrated mineral acids that dissolve most common metals within minutes. Correct pump and material selection is critical to both safety and service life.
Common Industrial Acids
- Sulfuric acid (H₂SO₄): The most widely produced industrial chemical — dilute solutions (below 50%) are more corrosive to metals than concentrated; concentrated H₂SO₄ passivates cast iron and steel; viscous at high concentration
- Hydrochloric acid (HCl): Highly volatile; liberates HCl gas above the liquid surface; attacks almost all metals including stainless steel; requires plastics or exotic alloys
- Nitric acid (HNO₃): Strong oxidising acid — passivates stainless steel at moderate concentrations; highly destructive at high temperature; fuming at high concentration
- Phosphoric acid (H₃PO₄): Moderate corrosivity; widely used in fertiliser production and food processing; more manageable than mineral acids
- Hydrofluoric acid (HF): Extremely hazardous — attacks glass and silica; requires PTFE or high-nickel alloy wetted parts; penetrates skin and causes systemic toxicity
- Acetic and organic acids: Generally less aggressive than mineral acids; can attack mild steel; compatible with many stainless grades
Corrosivity vs. Concentration
The relationship between acid concentration and corrosivity is not linear. Sulfuric acid is most corrosive to carbon steel at intermediate concentrations (30–80%) and becomes relatively passive at concentrations above 93–95%. Stainless steel 316 tolerates dilute H₂SO₄ poorly but resists phosphoric and nitric acids well. Always consult a corrosion resistance chart specific to the acid concentration and operating temperature before selecting materials.
Key Pumping Challenges
- Corrosion rate increases with temperature: Every 10 °C rise approximately doubles the corrosion rate — pump operating temperature is as important as fluid concentration
- Vapour generation: Fuming acids (HCl above 20%, HNO₃ above 65%) release acid gas at the shaft seal — sealless pumps are strongly preferred
- Crystallisation: Some acids (HF, H₃PO₄) crystallise on cooling — pump must be flushed before shutdown in cold-climate installations
- Heat of dilution: Concentrated H₂SO₄ generates significant heat when mixed with water — dilution must always be performed by adding acid to water, never water to acid
Pump Type Selection
- General acid transfer: Centrifugal pump — sealless magnetic drive or canned motor to eliminate shaft leakage
- Fuming or highly volatile acids: Sealless centrifugal (mag-drive) or fully enclosed canned motor pump — no shaft penetration
- High-viscosity acid (concentrated H₂SO₄, H₃PO₄): Gear or screw pump with appropriate lining
- Metering and dosing: Membrane metering pump with PTFE membrane and ceramics or titanium check valves
Materials of Construction
- Stainless steel 316L: Suitable for many dilute acid services; not for HCl, HF or hot H₂SO₄
- Hastelloy C-276 / C-22: Broad acid resistance including HCl, mixed acids and reducing acids — the premium metal choice for aggressive acid service
- Titanium: Excellent for dilute H₂SO₄, HCl and HNO₃; not suitable for fuming HNO₃ or HF
- PVDF (polyvinylidene fluoride): Excellent for most acids including HF; maximum temperature 80–100 °C; not for fuming H₂SO₄ above 80%
- PTFE: Near-universal chemical resistance; used for lining pumps handling HF and fuming acids; limited pressure rating due to creep
- Alloy 20 (Carpenter 20): Specifically developed for hot sulfuric acid service
Safety Considerations
- All acid pumps must have secondary containment — drip trays, bunded floor areas or enclosed pump sumps
- Emergency eyewash and safety shower must be within 10 seconds of travel from any acid pump location
- All pipe connections must use corrosion-resistant flanges, bolts and gaskets — never use standard carbon steel fasteners
- Conduct regular inspection of shaft seal condition and pump casing for signs of pitting or weeping
- Provide ventilation above open acid sumps and pump enclosures to control acid vapour accumulation
Applications
- Fertiliser production — sulfuric, nitric and phosphoric acid handling
- Metal pickling and surface treatment — HCl and H₂SO₄ scale removal
- Chemical synthesis — acid as reagent or catalyst
- Water and wastewater pH correction — dosing of H₂SO₄ or HCl
- Semiconductor fabrication — ultra-pure HF, HCl and H₂SO₄ etching
- Electroplating and anodising — acid bath circulation
- Pharmaceutical synthesis — acid-catalysed reactions